Issue |
A&A
Volume 517, July 2010
|
|
---|---|---|
Article Number | A65 | |
Number of page(s) | 14 | |
Section | Cosmology (including clusters of galaxies) | |
DOI | https://doi.org/10.1051/0004-6361/201014116 | |
Published online | 06 August 2010 |
Internal dynamics of Abell 2294: a massive, likely merging cluster
M. Girardi1,2 - W. Boschin3 - R. Barrena4,5
1 - Dipartimento di Fisica dell' Università degli
Studi di Trieste - Sezione di Astronomia, via Tiepolo 11, 34143
Trieste, Italy
2 - INAF - Osservatorio Astronomico di Trieste, via Tiepolo 11, 34143 Trieste, Italy
3 - Fundación Galileo Galilei - INAF, Rambla José Ana Fernández Perez 7, 38712
Breña Baja (La Palma), Canary Islands, Spain
4 - Instituto de Astrofísica de Canarias, C/Vía Láctea s/n, 38205 La
Laguna (Tenerife), Canary Islands, Spain
5 - Departamento de Astrofísica, Universidad de La Laguna, Av. del
Astrofísico Franciso Sánchez s/n, 38205 La Laguna
(Tenerife), Canary Islands, Spain
Received 22 January 2010 / Accepted 18 April 2010
Abstract
Context. The mechanisms giving rise to diffuse radio
emission in galaxy clusters, and in particular their connection with
cluster mergers, are still debated.
Aims. We seek to explore the internal dynamics of the cluster Abell 2294, which has been shown to host a radio halo.
Methods. Our analysis is mainly based on redshift data for 88
galaxies acquired at the Telescopio Nazionale Galileo. We combine
galaxy velocities and positions to select 78 cluster galaxies and
analyze its internal dynamics. We also use both photometric data
acquired at the Isaac Newton Telescope and X-ray data from the Chandra
archive.
Results. We re-estimate the redshift of the large, brightest cluster galaxy (BCG) obtaining
,
which closely agrees with the mean cluster redshift. We estimate a quite large line-of-sight (LOS) velocity dispersion
km s
and X-ray temperature
keV.
Our optical and X-ray analyses detect substructure. Our results imply
that the cluster is composed of two massive subclusters separated by a
LOS rest frame velocity difference
km s-1,
very closely projected in the plane of sky along the SE-NW direction.
This observational picture, interpreted in terms of the analytical
two-body model, suggests that Abell 2294 is a cluster merger elongated
mainly in the LOS direction and captured during the bound outgoing
phase, a few fractions of Gyr after the core crossing. We find that
Abell 2294 is a very massive cluster with a range of M=2-4
,
depending on the adopted model. In contrast to previous findings, we find no evidence of H
emission in the spectrum of the BCG galaxy.
Conclusions. The emerging picture of Abell 2294 is that of a
massive, quite ``normal'' merging cluster, like many clusters hosting
diffuse radio sources. However, perhaps because of its particular
geometry, more data are needed for reach a definitive, more
quantitative conclusion.
Key words: galaxies: clusters: individual: Abell 2294 - galaxies: clusters: general - galaxies: kinematics and dynamics
1 Introduction
Merging processes constitute an essential ingredient of the evolution
of galaxy clusters (see Feretti et al. 2002b, for a review). An
interesting aspect of these phenomena is the possible connection
between cluster mergers and extended, diffuse radio sources: halos and
relics. The synchrotron radio emission of these sources demonstrates
the existence of large-scale cluster magnetic fields and of
widespread relativistic particles. Cluster mergers have been proposed
to provide the large amount of energy necessary for electron
reacceleration to relativistic energies and for magnetic field
amplification (Tribble 1993; Feretti 1999, 2002a; Sarazin 2002). Radio relics (``radio gischts'' as referred to by Kempner et al. 2004), which are polarized
and elongated radio sources located in the cluster peripheral regions,
seem to be directly associated with merger shocks (e.g., Ensslin et al. 1998; Roettiger et al. 1999; Ensslin &
Gopal-Krishna 2001; Hoeft et al. 2004). Radio halos,
unpolarized sources that permeate the cluster volume in a similar way
to the X-ray emitting gas (intracluster medium, hereafter ICM), are
more likely to be associated with the turbulence following a cluster
merger (Cassano & Brunetti 2005; Brunetti et al. 2009). However, the precise radio halos/relics formation
scenario remains unclear because diffuse radio sources are quite
uncommon and one has been able to study these phenomena only recently
on the basis of a sufficient statistics (few dozen clusters up to
,
e.g., Giovannini et al. 1999; see also Giovannini
& Feretti 2002; Feretti 2005; Giovannini et al. 2009) and attempt a classification (e.g., Kempner et al. 2004; Ferrari et al. 2008). It is expected that new telescopes will largely increase the statistics of diffuse sources
(e.g., LOFAR, Cassano et al. 2009).
![]() |
Figure 1: INT R-band image of the cluster A2294 (north at the top and east to the left) with, superimposed, the contour levels of the Chandra archival image ID 3246 (thick contours; photons in the energy range 0.5-2 keV) and the contour levels of a VLA radio image at 1.4 GHz (thin contours, see Giovannini et al. 2009). Labels and arrows highlight the positions of radio sources listed by Rizza et al. (2003). |
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From the observational point of view, there is growing evidence of the connection between diffuse radio emission and cluster merging, since up to now diffuse radio sources have been detected only in merging systems. In several cases the cluster dynamical state has been derived from X-ray observations (see Buote 2002; Feretti 2008, 2006, and refs. therein). Optical data are a powerful way to investigate the presence and the dynamics of cluster mergers (e.g., Girardi & Biviano 2002), too. The spatial and kinematical analysis of member galaxies allow us to detect and measure the amount of substructure, and both identify and analyze possible pre-merging clumps or merger remnants. This optical information is really complementary to X-ray information since galaxies and the intra-cluster medium react on different timescales during a merger (see, e.g., numerical simulations by Roettiger et al. 1997).
In this context, we are conducting an intensive observational and data
analysis program to study the internal dynamics of clusters with
diffuse radio emission by using member galaxies (Girardi et al. 2007). Most clusters
exhibiting diffuse radio emission have a relatively high gravitational
mass (higher than
within 2
h70-1 Mpc; see Giovannini
& Feretti 2002) and, indeed, most clusters we analyzed are
very massive clusters with few exceptions (Boschin et al. 2008).
During our observational program, we have conducted an intensive study
of the cluster Abell 2294 (hereafter A2294).
A2294 is a very rich, X-ray luminous, and hot Abell cluster: Abell
richness class =2 (Abell et al. 1989); (0.1-2.4 keV) =
erg s-1;
-9 keV recovered from ROSAT and Chandra data
(Ebeling et al. 1998; Rizza et al. 1998;
Maughan et al. 2008). Optically, the cluster is classified as Bautz-Morgan class II (Abell et al. 1989) and is dominated by a central, large brightest
cluster galaxy (BCG, see Fig. 1).
According to both ROSAT and Chandra data, A2294 is known to have no cool core (Rizza et al. 1998; Bauer et al. 2005). As for the presence of possible substructure, using ROSAT data, Rizza et al. (1998) found evidence of a ``centroid shift'' and detected a southern excess in the X-ray emission. Using Chandra data, Hashimoto et al. (2007) classified A2294 as a ``distorted'' cluster because of the large value of its asymmetry parameter. Indeed, A2294 is a very peculiar cluster since, in contrast to the absence of a cooling core, it is very compact in its X-ray appearance (see Fig. 3 of Bauer et al. 2005). Among a sample of 115 clusters analyzed using Chandra data, A2294 has the smallest ellipticity, and exhibits a small, but highly significant, centroid shift (Maughan et al. 2008).
In another respect, A2294 is also peculiar. Out of a sample of 13 clusters at
showing evidence for H
emission in their BCG spectrum, it is the only one that does not appear
to have a cool core (see Fig. 5 of Bauer et al. 2005). The correlation
between BCG H
emission and the presence of a cool core is also
found for nearby clusters where ``H
luminous galaxies lie at
the center of large cool cores, although this special cluster
environment does not guarantee the emission-line nebulosity in its
BCG'' (Peres et al. 1998). More recent observations also
agree that H
emission is more typical of cool core clusters
than of non-cool core clusters (
compared to
,
Edwards et al. 2007).
As for the diffuse radio emission, Owen et al. (1999) first
reported the existence of a detectable diffuse radio source in this
cluster. Despite the presence of some disturbing point-like sources
in the central region of the cluster, Giovannini et al. (2009)
were able to detect a radio-halo 3
in size. In particular, the position
of A2294 in the
(radio power at 1.4 GHz) -
plane is consistent with that of all other radio-halo clusters
(see Fig. 17 of Giovannini et al. 2009).
To date, only a small amount of optical data have been available. The
cluster redshift reported in the literature (z=0.178) is based only
on the BCG H
emission line (Crawford et al. 1995). Instead, the true cluster redshift, as estimated in this paper, is rather
fully consistent with
that measured for the BCG on the basis of our data, which, indeed, do not
show any evidence of H
emission (see Sect. 2).
Our new spectroscopic and photometric data come from the Telescopio Nazionale Galileo (TNG) and the Isaac Newton Telescope (INT), respectively. Our present analysis is based on these optical data and X-ray Chandra archival data.
This paper is organized as follows. We present our new optical data and the cluster catalog in Sect. 2. We present our results about the cluster structure based on optical and X-ray data in Sects. 3 and 4, respectively. We briefly discuss our results and present our conclusions in Sect. 5.
Unless otherwise stated, we indicate errors at the 68% confidence level
(hereafter c.l.). Throughout this paper, we use H0=70 km s-1 Mpc-1 in a
flat cosmology with
and
.
In the
adopted cosmology, 1
corresponds to
173
h70-1 kpc
at the
cluster redshift.
2 New data and galaxy catalog
Multi-object spectroscopic observations of A2294 were carried out at
the TNG telescope in December 2007 and August 2008. We used
DOLORES/MOS with the LR-B Grism 1, yielding a dispersion of 187 Å/mm. We used the new
pixels E2V CCD, with a pixel
size of 13.5
m. In total, we observed 4 MOS masks (3 in 2007 and
1 in 2008) for a total of 124 slits. We acquired three exposures of
1800 s for each mask. Wavelength calibration was performed using
helium-argon lamps. Reduction of spectroscopic data was carried out
using the IRAF
package. Radial
velocities were determined using the cross-correlation technique
(Tonry & Davis 1979) implemented in the RVSAO package
(developed at the Smithsonian Astrophysical Observatory Telescope Data
Center). Each spectrum was correlated against six templates for a
variety of galaxy spectral types: E, S0, Sa, Sb, Sc, and Ir (Kennicutt
1992). The template producing the highest value of
,
i.e., the parameter given by RVSAO and related to the
signal-to-noise ratio of the correlation peak, was chosen. Moreover,
all spectra and their best correlation functions
were examined visually to verify the redshift determination. In six
cases (IDs. 5, 13, 15, 60, 81, and 82; see Table 1),
we assumed the EMSAO redshift to be a reliable estimate of the
redshift.
Our spectroscopic catalog lists 88 galaxies in the field of A2294.
Table 1: Velocity catalog of 88 spectroscopically measured galaxies in the field of the cluster A2294.
The nominal errors as given by the cross-correlation are known to be smaller than the true errors (e.g., Malumuth et al. 1992; Bardelli et al. 1994; Ellingson & Yee 1994; Quintana et al. 2000). Duplicate observations for the same galaxy allowed us to estimate the true intrinsic errors in data of the same quality taken with the same instrument (e.g. Barrena et al. 2007a,b). Here we have a limited number of double determinations (i.e., five galaxies from four different masks), thus we decided to apply the correction that had already been applied in above studies. Hereafter we assume that true errors are larger than nominal cross-correlation errors by a factor of 1.4. For the five galaxies with two redshift estimates, we used the weighted mean of the two measurements and the corresponding errors. The median error in cz is 71 km s-1.
Our photometric observations were carried out with the Wide Field
Camera (WFC), mounted at the prime focus of the 2.5 m INT telescope. We
observed A2294 in May 18th 2007 with filters
and
in photometric conditions and a seeing of
1.5
.
The WFC consists of a four-CCD mosaic covering a
field of view, with only a 20% marginally
vignetted area. We took nine exposures of 720 s in
and 360 s in
Harris filters (a total of 6480 s and 3240 s in each
band) developing a dithering pattern of nine positions. This observing
mode allowed us to build a ``supersky'' frame that was used to correct
our images for fringing patterns (Gullixson 1992). In
addition, the dithering helped us to clean cosmic rays and avoid the
effects of gaps between the CCDs in the final images. Another problem
associated with the wide field frames is the distortion of the
field. To match the photometry of several filters, a good astrometric
solution is needed to take into account these distortions. Using the
imcoords IRAF tasks and taking as a reference the USNO B1.0 catalog,
we were able to find an accurate astrometric solution (rms
0.4
)
across the full frame. The photometric calibration was performed by observing standard Landolt fields (Landolt 1992).
We finally identified galaxies in our
and
images and measured their magnitudes with the SExtractor package
(Bertin & Arnouts 1996) and AUTOMAG procedure. In a few cases
(e.g., close companion galaxies, galaxies close to defects of the CCD)
the standard SExtractor photometric procedure failed. In these cases,
we computed magnitudes by hand. This method consisted of assuming a
galaxy profile of a typical elliptical galaxy and scaling it to the
maximum observed value. The integration of this profile provided an
estimate of the magnitude. This method is similar to PSF photometry,
but assumes a galaxy profile, which is more appropriate in this case.
We transformed all magnitudes into the Johnson-Cousins system
(Johnson & Morgan 1953; Cousins 1976). We used
and
as derived from the Harris filter
characterization
and
assuming a
for E-type galaxies (Poggianti
1997). As a final step, we estimated and corrected the
Galactic extinction
,
using Burstein &
Heiles's (1982) reddening maps. These values are especially
high because A2294 is immersed in a diffuse dust cloud soaring high
above the plane of our Milky Way Galaxy, and known as the Polaris Dust
Nebula. We estimated that our photometric sample is complete down to
R=22.0 (23.0) and B=23.5 (24.5) for S/N=5 (3) within the
observed field.
![]() |
Figure 2:
INT R-band image of the cluster A2294 (north at the top
and east to the left). Circles and squares indicate cluster members
and non-members, respectively (see Table 1). Solid
circle in the center highlights the position of the BCG galaxy.
Annuli and box annuli show member and non-member emission line
galaxies, respectively. Labels indicate the IDs of cluster
galaxies cited in the text. A diamond at the right border of the
image highlights a QSO at |
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We assigned magnitudes to all galaxies of our spectroscopic catalog.
Table 1 lists the velocity catalog (see also
Fig. 2): identification number of each galaxy, ID
(Col. 1); right ascension and declination,
and
(J2000, Col. 2); B and R magnitudes (Cols. 3 and 4); heliocentric
radial velocities,
(Col. 5) with errors,
(Col. 6); and emission lines detected in the spectra (Col. 7).
The brightest galaxy of A2294 (ID. 46 in Table 1,
hereafter BCG) is probably the dominant galaxy, 1.6 R-magnitudes
more luminous than other cluster members. The measured redshift is
,
different from that reported by Crawford et al. (1995), z=0.178, using INT data and measured on the H
emission line only. This discrepancy prompted us to acquire additional
data for this galaxy. In August 2009, we acquired two 900 s
exposure long-slit spectra of the BCG. We used the LR-R grism,
covering the wavelength range
4500-10 000 Å. The target was
positioned in two slightly different positions along the slit to
perform an optimal sky subtraction with a technique commonly used to
reduce spectroscopic data in the near-infrared. Our reduced spectrum
(see Fig. 3) does not show any evidence of the H
emission. We note that the H
emission reported by Crawford et al. (1995) is very strong
(see
for comparison the spectrum of A291 having
in their Fig. 1) and thus its presence would be just striking in
our spectrum. Indeed, we strongly suspect that their detection is
caused by some problem in data reduction, e.g., a cosmic ray or sky
subtraction, as also suggested by the incorrect measure of the galaxy redshift.
![]() |
Figure 3:
Top panel: 2D spectrum of the BCG galaxy taken with the grism
LR-R mounted on DOLORES in the wavelength range |
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Other cluster members are far less luminous than the BCG: the brightest ones lie in the central cluster region (IDs. 28, 39 and 18) with the exception of ID. 17 (hereafter R6) which lies in the northern region and is very radio luminous (>1024 W Hz-1, No. 6 of Rizza et al. 2003). Out of radio galaxies listed by Rizza et al. (2003), we also acquired a redshift for their No. 2 (ID. 47, labelled as R2 in Fig. 1), confirming its membership to the cluster.
3 Analysis of the spectroscopic sample
3.1 Member selection
To select cluster members among the 88 galaxies with redshifts, we
follow a two-step procedure. We first perform the 1D adaptive-kernel method (hereafter DEDICA, Pisani 1993, 1996; see also Fadda et al. 1996; Girardi et al. 1996). We search for significant peaks in the velocity distribution at >99% c.l. This procedure detects A2294 as a peak at
populated by 80 galaxies considered as candidate cluster members (in the range
km s-1, see Fig. 4). Out of eight non-members, three and five are foreground and background galaxies, respectively.
![]() |
Figure 4: Redshift galaxy distribution. The solid line histogram refers to the 80 galaxies assigned to the A2294 complex according to the DEDICA reconstruction method. The number-galaxy density in the redshift space, as provided by the adaptive kernel reconstruction method is superimposed on the histogram. |
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All the galaxies assigned to the cluster peak are analyzed in the
second step, which uses the combination of position and velocity
information, i.e., the ``shifting gapper'' method by Fadda et al. (1996).
This procedure rejects galaxies that are too far in velocity from the
main body of galaxies within a fixed bin that shifts along the distance
from the cluster center. The procedure is iterated until the number of
cluster members converges to a stable
value. Following Fadda et al. (1996), we use a gap of 1000 km s- in the cluster rest-frame - and a bin of 0.6
h70-1 Mpc, or large
enough to include 15 galaxies. As for the center of A2294, we adopt the
position of the BCG [RA =
,
Dec =
(J2000.0)], which is almost
coincident with the X-ray centroid obtained in this paper using Chandra
data (see Sect. 4). The ``shifting gapper'' procedure rejects
another two obvious interlopers very far from the main body (>2000 km s-1) but that survived the first step of our member selection procedure. We obtain a sample of 78 fiducial members (see
Fig. 5).
![]() |
Figure 5: The 78 galaxies assigned to the cluster. Upper panel: velocity distribution. The arrows indicate the velocities of the five brightest galaxies, in particular we indicate the brightest cluster galaxy ``BCG'', the bright radio galaxy ``R6''. Lower panel: stripe density plot where the arrows indicate the positions of the significant gaps. |
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The five member galaxies exhibiting emission lines (ELGs) are
preferentially found in the external cluster regions (see
Fig. 2). The only ELG close to the cluster center
(ID. 24) lies in the high tail of the velocity distribution, far more
than cz=2500 km sfrom the mean cluster velocity, as expected
e.g., in the case of a very radial orbit. These findings are in
general agreement with large statistical analyses of ELGs in
clusters (see Biviano et al. 1997, and refs. therein).
3.2 Global cluster properties
By applying the biweight estimator to the 78 cluster members (Beers et al. 1990, ROSTAT software), we compute a mean cluster redshift
of
,
i.e.
) km s-1. We estimate the LOS
velocity dispersion,
,
by using the biweight estimator
and applying the cosmological correction and the standard correction
for velocity errors (Danese et al. 1980). We obtain
km s-1, where errors are estimated
through a bootstrap technique.
To evaluate the robustness of the
estimate, we analyze
the velocity dispersion profile (Fig. 6). The integral
profile rises out to
h70-1 Mpc
and then flattens suggesting
that a robust value of
is asymptotically reached in
the external cluster regions, as found for most nearby clusters (e.g.,
Fadda et al. 1996; Girardi et al. 1996).
![]() |
Figure 6:
Top panel: rest-frame velocity vs. projected distance from the
cluster center. Squares indicate the five brightest galaxies.
Middle and bottom panels: differential (big circles) and integral
(small points) profiles of mean velocity and LOS velocity
dispersion, respectively. For the differential profiles, we plot the
values for five annuli from the center of the cluster, each of 0.25
h70-1 Mpc |
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In the framework of usual assumptions (cluster sphericity, dynamical
equilibrium, coincidence of the galaxy and mass distributions), one
can compute virial global quantities. Following the prescriptions of
Girardi & Mezzetti (2001), we assume for the radius of the
quasi-virialized region is
h70-1 Mpc
from their Eq. (1) with the scaling of H(z)(see also Eq. 8 of Carlberg et al. 1997 for R200).
We compute the virial mass (Limber & Mathews 1960; see also,
e.g., Girardi et al. 1998)
![]() |
(1) |
where SPT is the surface pressure term correction (The & White 1986) and

The estimate of
is robust when computed within a
large cluster region (see Fig. 6). The value of
depends on the size of the sampled region and possibly on the quality of the spatial sampling (e.g., whether the cluster is
uniformly sampled or not). Since in A2294 we sampled only a
fraction of
,
we have to use an alternative
estimate of
on the basis of the knowledge of the
galaxy distribution. Following Girardi et al. (1998; see also
Girardi & Mezzetti 2001), we assume a King-like distribution
with parameters typical of nearby/medium-redshift clusters: a core
radius
and a
slope-parameter
,
i.e. the volume galaxy density
at large radii goes as
.
We obtain
(<
h70-1 Mpc, where a
error is
expected (Girardi et al. 1998, see also the approximation
given by their Eq. (13) when
). The value of SPT
depends strongly on the radial component of the velocity dispersion at the
radius of the sampled region and could be obtained by analyzing the
(differential) velocity dispersion profile, although this procedure
would require several hundreds of galaxies. We decide to assume a
SPT correction as obtained in the literature by combining data on many
clusters sampled out to about
(Carlberg et al. 1997; Girardi et al. 1998). We compute M(<
.
3.3 Velocity distribution
We analyze the velocity distribution to search for possible deviations from Gaussianity that might provide important signatures of complex dynamics. For the following tests, the null hypothesis is that the velocity distribution is a single Gaussian.
We estimate three shape estimators, i.e., the kurtosis, the skewness, and the scaled tail index (see, e.g., Bird & Beers 1993). We find no evidence that the velocity distribution departs from Gaussianity.
We then investigate the presence of gaps in the velocity distribution.
We follow the weighted gap analysis presented by Beers et al. (1991, 1992; ROSTAT software). We look for
normalized gaps larger than 2.25 since in random draws of a Gaussian
distribution they arise at most in about
of the cases,
independent of the sample size (Wainer & Schacht 1978). We
detect two significant gaps (at the
and
c.l.s), which
divide the cluster into three groups of 28, 14, and 36 galaxies from
low to high velocities (hereafter GV1, GV2, and GV3, see
Fig. 5). The BCG is assigned to the GV2 peak. Among
other luminous galaxies, three galaxies (R6, ID. 28, and ID. 39)
are assigned to the GV1 peak and one galaxy (ID. 18) is assigned to
the GV3 peak.
Following Ashman et al. (1994), we also apply the Kaye's mixture model (KMM) algorithm. This test does not find a three-groups partition, which provides a significantly more accurate description of the velocity distribution than a single Gaussian.
3.4 3D-analysis
The existence of correlations between positions and velocities of cluster galaxies is a characteristic of true substructures. Here we use three different approaches to analyze the structure of A2294 combining position and velocity information.
To search for a possible physical meaning of the three subclusters
determined by the two weighted gaps, we compare two by two the spatial
galaxy distributions of GV1, GV2, and GV3. We find that the GV1 and
GV3 groups differ in the distributions of the
clustercentric distances of member galaxies at the
c.l.
(according to the the 1D Kolmogorov-Smirnov test; hereafter
1DKS-test, see e.g., Press et al. 1992). The GV1 galaxies
are, on average, closer to the cluster center than the GV3 galaxies
(see Fig. 7).
![]() |
Figure 7: Spatial distribution on the sky of the cluster galaxies showing the three groups recovered by the weighted gap analysis. Solid circles, crosses, and open circles indicate the galaxies of GV1, GV2, and GV3, respectively. The BCG is taken as the cluster center. Large squares indicate the five brightest cluster members. Among them, the BCG and R6 are indicated by the two largest squares. |
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We analyze the presence of a velocity gradient performing a multiple
linear regression fit to the observed velocities with respect to the
galaxy positions in the plane of the sky (e.g., Boschin et al. 2004, and refs. therein). We find a position angle on the celestial sphere of
degrees (measured
counter-clock wise from north), i.e. higher-velocity galaxies lie
in the SSW region of the cluster. To assess the significance of this
velocity gradient, we perform 1000 Monte Carlo simulations by randomly
shuffling the galaxy velocities and for each simulation we determine
the coefficient of multiple determination (RC2, see e.g., NAG
Fortran Workstation Handbook 1986). We define the
significance of the velocity gradient as the fraction of times in
which the RC2 of the simulated data is smaller than the observed
RC2. We find that the velocity gradient is marginally significant
at the
c.l.
We also combine galaxy velocity and position information to compute
the -statistics devised by Dressler & Schectman
(1988; see also e.g., Boschin et al. 2006, for a recent
application). We find no significant indication of substructure.
3.5 Kinematics of more luminous galaxies
The presence of velocity segregation of galaxies with respect to their colors, luminosities, and morphologies is often taken as evidence of advanced dynamical evolution of the parent cluster (e.g. Biviano et al. 1992; Fusco-Femiano & Menci 1998). Here we check for possible luminosity segregation of galaxies in the velocity space.
We find no significant correlation between the absolute velocity |v| and R-magnitude. We also divide the sample into a low and a high-luminosity subsamples by using the median R-magnitude =18.145. The two subsamples do not differ in their velocity distribution as we verify with the standard means-test and F-test (e.g., Press et al. 1992) applied to the means and variances of velocities and with the 1DKS-test applied to the whole velocity distributions. This agrees with the very small range of action of velocity segregation in galaxy clusters, i.e. typically only the three most luminous galaxies (Biviano et al. 1992; see also Goto 2005).
Examining the velocity distributions of the two subsamples in more
detail, we find that the distribution of luminous galaxies is found to
be non-Gaussian according to the scale tail index (at the c.l.) and that, according to the 1D-DEDICA technique, it is more
accurately described by a bimodal distribution (see
Fig. 8). The two peaks of this distribution, of 20
and 19 galaxies at
49 700 and 51 950 km s
respectively, are
separated by
2000 km s
in the rest cluster frame and appear
to overlap, i.e. 15 galaxies have a non-null probability of
belonging to both the peaks. The BCG is assigned to the low-velocity
peak, but has a high probability of belonging to the other peak. Among
other luminous galaxies, R6, IDs. 28 and 39 are assigned to the
low-velocity peak and ID. 18 to the high-velocity peak.
![]() |
Figure 8: Velocity galaxy distribution of more and less luminous cluster members (solid and dashed lines, respectively). The arrows indicate the velocities of the five brightest galaxies, in particular ``BCG'' indicates the bright, central galaxy and ``R6'' indicates the bright radio-galaxy. The number-galaxy densities, as provided by the adaptive kernel reconstruction method are superimposed on the histograms. |
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According to the DEDICA assignment, we estimate
and
640 km s
for the low and high-velocity groups,
respectively. However, since there is a wide velocity-range where
galaxies have a non-zero probability of belonging to both the clumps,
DEDICA membership assignment leads to an artificial truncation of the
tails of the distributions. This truncation may lead to an
underestimate of velocity dispersion for the subclusters. Thus, we
prefer to rely on the estimates obtained through the KMM approach even
if the tightest bimodal fit does not represent a significant
improvement on that of the single Gaussian according to the likelihood
ratio test. The low- and high-velocity groups given by the best KMM
bimodal fit have mean velocities
and
52 230 km s-1, in good agreement with the peak velocities reported above
and
and
670 km s-1.
3.6 Analysis of the photometric sample
By applying the 2D adaptive-kernel method to the positions of A2294
galaxy members, we identify only one significant peak. However, our
spectroscopic data do not cover the entire cluster field and are affected
by magnitude incompleteness. To overcome these problems, from our
photometric catalog we select likely members on the basis of the
color-magnitude relation (hereafter CMR), which indicates the
early-type galaxy locus. To determine the CMR, we fix the slope
according to López-Cruz et al. (2004, see their Fig. 3) and
apply the two-sigma-clipping fitting procedure to the cluster
members obtaining B-
(see Fig. 9).
From our photometric catalog, we consider as likely cluster members
those objects with a SExtractor stellar index
0.9 lying within
0.25 mag of the CMR. To avoid contamination by field galaxies, we do
not show results for galaxies fainter than 21 mag (in R-band).
Figure 10 shows the contour map for the likely cluster
members with
:
we find again that A2294 is well described
by only one peak centered on the BCG galaxy.
![]() |
Figure 9:
B-R vs. R diagram for galaxies with available spectroscopy is shown
by circles and crosses (cluster and field members, respectively).
The solid line gives the best-fit color-magnitude relation as
determined for member galaxies; the dashed lines are drawn at |
Open with DEXTER |
![]() |
Figure 10:
Spatial distribution on the sky and relative isodensity contour map
of likely cluster members extracted from our photometric catalog
with |
Open with DEXTER |
4 X-ray analysis
The X-ray analysis of A2294 is performed on the archival data of the
Chandra ACIS-I observation 800 246 (exposure ID #3246). The pointing
has an exposure time of 10 ks. Data reduction is performed using the
package CIAO (Chandra Interactive Analysis of
Observations, ver. 3.3 with CALDB ver. 3.2.1) on chips I0, I1,
I2, and I3 (field of view
). First, we
remove events from the level 2 event list with a status not equal to
zero and with grades one, five, and seven. Then, we select all events
with energy between 0.3 and 10 keV. In addition, we clean bad offsets
and examine the data, filtering out bad columns and removing times
when the count rate exceeds three standard deviations from the mean
count rate per 3.3 s interval. We then clean the four chips for
flickering pixels, i.e., times where a pixel has events in two
sequential 3.3 s intervals. The resulting exposure time for the
reduced data is 9.9 ks.
A quick look at the reduced image is sufficient to reveal the regular
morphology of the extended X-ray emission of this cluster (see
Fig. 11). The low values of the
power ratios
found by Bauer et al. (2005) quantitatively support this
feeling. The absence of multiple clumps in the ICM is confirmed by
performing a wavelet multiscale analysis on the chip I3: the task
CIAO/Wavdetect identifies A2294 as a single extended X-ray source.
To more accurately characterize the X-ray morphology of the cluster, by using
the CIAO package Sherpa we fit a simple Beta model to the 2D X-ray
photon distribution on the chip I3. The model is defined
to be (Cavaliere & Fusco-Femiano 1976)
![]() |
(2) |
where









![]() |
Figure 11:
|
Open with DEXTER |
The above model provides an adequate description fit to the data (the reduced CSTAT statistic is 1.04; Cash 1979). However, we check for possible departures of the X-ray surface brightness, and thus of the gas density distribution, from the Beta model fit by investigating the Beta model residuals. The residuals show a deficit of X-ray emitting gas in a region extending along the NE-SW direction, with a negative peak in the very central cluster region (see Fig. 12). Around the cluster center, elongated in the direction SE-NW, there are also regions that have an excess of positive residuals.
![]() |
Figure 12: The smoothed X-ray emission in the right-upper quadrant of Fig. 11 with, superimposed, the contour levels of the positive (black) and negative (white) smoothed Beta model residuals (North at the top and East to the left). |
Open with DEXTER |
As for the spectral properties of the cluster X-ray photons, we
compute a global estimate of the ICM temperature. The temperature is
computed from the X-ray spectrum of the cluster within a circular
aperture of 173
radius (0.5
h70-1 Mpc
at the cluster redshift)
around the centroid of the X-ray emission. Fixing the absorbing
galactic hydrogen column density at
cm-2,
computed from the HI maps by Dickey & Lockman (1990), we fit
a Raymond-Smith (1977) spectrum using the CIAO package Sherpa
with a
statistics and assuming a metal abundance of 0.3 in
solar units. We find a best-fit temperature of
keV.
A detailed temperature and metallicity map would be highly desirable
to more accurately describe the properties of the ICM, but due to the
relatively low exposure times, the photon statistics are insufficient
for this aim. However, in a low signal-to-noise situation, possible
temperature gradients in the ICM might be detected using the
``hardness'' (or ``softness'') map of the cluster. We
create two images in the energy bands 0.5-2 keV (soft band) and
2-7 keV (hard band), subtracting a constant background level in
each energy band. Computing counts in the soft (S) and in the hard (H) band, we define the quantity ``softness ratio'' as
SR=(S-H)/(S+H). Both images are exposure-corrected with their
corresponding exposure maps. Because of the low number of photons
available, we have to choose a large pixel size to obtain a high
quality count statistic per pixel, so the resolution of the soft and
hard images is low (182 kpc pix-1). A 3D graph of the SR values
in the pixels within a radius of 0.5
h70-1 Mpcfrom the cluster center is
reported in Fig. 13. Typical errors are
,
while
the median value of SR is 0.25. According to the PIMMS tool, this
value corresponds to a temperature of
9.7 keV, in good
agreement with the global temperature reported above. The SR 3D
graph does not show any evidence of significant temperature gradients.
![]() |
Figure 13: Softness ratio 3D map of A2294 around the centroid of the X-ray distribution. The plane at the median SR value (0.25) is drawn. |
Open with DEXTER |
5 Discussion and conclusions
Our estimate of the cluster redshift is
0.0005 and the BCG is well at rest within the cluster (cf. our
z=0.1690 with z=0.178 by Crawford et al. 1995).
For the first time, the internal dynamics of A2294 have been analyzed.
The high values of the velocity dispersion
km s
and X-ray temperature
keV are comparable to the highest values found in
typical clusters (see Mushotzky & Scharf 1997; Girardi &
Mezzetti 2001; Leccardi & Molendi 2008). Our
estimates of
and
are fully consistent
when assuming the equipartition of energy density between ICM and
galaxies. We obtain
to be
compared with
,
where
with
the mean
molecular weight and
the proton mass (see also
Fig. 6). Taking this result at face value one might think
that A2294 is not far from dynamical equilibrium and the virial mass
estimate M(<
4
computed in
Sect. 3.2 to be quite reliable.
5.1 Internal structure
However, our analysis indicates that this cluster is not so relaxed as one may interpret at first glance. Evidence of this comes from both optical and X-ray analyses.
First of all, both the integral and differential velocity dispersion
profiles rise in the central region out to 0.2
h70-1 Mpc
(Fig. 6, middle and bottom panels). As for the velocity dispersion, this behavior might be a signature of a relaxed cluster
exhibiting circular velocities and galaxy merger phenomena in the central
cluster region (e.g., Merritt 1988; Menci & Fusco Femiano 1996; Girardi et al. 1998).
Alternatively, it might be the signature of subclumps of different mean
velocities (see, e.g., Abell 3391-3395 in Girardi et al. 1996; Abell 115 in Barrena et al. 2007b). The latter hypothesis is supported by the behavior of the mean velocity
profile and by the plot of velocity versus projected clustercentric
distance, where the central region is more populated by low velocity
galaxies than high velocity galaxies. This suggests the presence of
substructure in the cluster core.
Second, the two gaps found in the velocity distribution indicate that there are three subclumps with the BCG in the middle velocity subclump. Although the presence of the three groups (GV1, GV2, GV3) is not very strongly significant on the basis of only velocity data, the existence of a spatial segregation between GV1 and GV3 groups is a indicator of true substructures. We also find a (marginally significant) velocity gradient toward the SSW direction.
Third, the high-luminosity galaxy subsample shows two peaks (largely
overlapping) in the velocity distribution, with the BCG being
somewhere in-between. This result is very interesting, implying that
galaxies of different luminosity may trace the dynamics of cluster
mergers in a different way. A noticeable example was reported by
Biviano et al. (1996): they found that the two central
dominant galaxies of the Coma cluster are surrounded by luminous
galaxies, accompanied by the two main X-ray peaks, while the
distribution of faint galaxies does not appear to be centered on one
of the two dominant galaxies, but is rather coincident with a
secondary peak detected in the X-ray image. Biviano et al. speculate that
the merging of Coma is in an advanced phase, where faint galaxies
trace the forming structure of the cluster, while the most luminous
galaxies still trace the remnant of the core-halo structure of a
pre-merging clump, which may be sufficiently dense to survive for a
long time after the merging (as suggested by numerical simulations,
e.g. González-Casado et al. 1994). In A2294, luminous
galaxies may trace the remnants of two merging subclusters
characterized by an impact velocity 2000 km s-1. Assuming the
dynamical equilibrium for each of the two individual subclusters, from
the values of
of the two subclusters we obtain a
virial mass of 1.8
and 0.5
for the low and high velocity
subclusters. The total mass
is lower than the
global virial value computed in Sect. 3.2, but is still a high value.
As for the X-ray data, we find no evidence of obvious substructure. Our multiscale wavelet analysis of the Chandra image does not identify any subclumps in the X-ray photon distribution. We also confirm the absence of a significant, macroscopic cluster ellipticity (see also Hashimoto et al. 2007; Maughan et al. 2008).
As for the Beta model we fit, the value of the core radius
h70-1 kpc
and the value of the slope parameter
agree well with those computed by
Hart in his PhD thesis (2008;
h70-1 kpc;
). The value of the core radius
agrees with that expected from the relation between surface brightness
concentration index and core radius shown by Hashimoto et al. (2007, see their Fig. 8 and the value of concentration in
their Table 2). The values of
and
lie on the low end of the parabolic relation found between these two
parameters (Neumann & Arnaud 1999). On the other hand, the
value of
might be somewhat small
compared to the typical values for very rich/hot clusters (e.g., Jones
& Forman 1999; Vikhlinin et al. 1999). However, we
note that most of our signal comes from the region with a radius of
0.5
h70-1 Mpc
(
)
and that there are
indications of a continuous steepening of the X-ray brightness
profiles with increasing radius (e.g., Vikhlinin et al. 1999;
Neumann 2005). This steepening is the most likely cause of
offsets between different cluster samples (see Vikhlinin et al. 1999 where
vs. Jones & Forman 1999 where
)
and
of apparent discrepancies between fit parameters obtained for the same
clusters (e.g. Buote et al. 2005). Indeed, the most
appropriate way to compare different clusters it seems is to consider the
measure of the local slope of the surface brightness at a certain,
rescaled radius (see Croston et al. 2008 for variation of
this parameter with radius). As for A2294, Maughan et
al. (2008) computed the slope
at a radius of
R500=1.3
h70-1 Mpc,
using the data in the radial range 0.7R500-1.3R500,
i.e. well outside the region we analyze. This value is in agreement
with that expected for very hot clusters at z<0.5 (see their
Fig. 11). Finally, we note that, in the case of a cluster merger,
numerical simulations predict a clear expansion of the gas core and
a steepening of the slope (Roettiger et al. 1996, see their
Fig. 3). This agrees with the results of Jones & Forman (1999) to explain the large core radii found for a few observed clusters, but we refer to Neumann & Arnaud (1999) for no
link between
value and cluster dynamical
status. To summarize, our small values of
and
are not indicative of substructure. Direct
evidence of cluster substructure comes from the 2D image of the Beta
model residuals, which shows positive residuals in the X-ray
emission along the SE-NW direction (see Fig. 12).
To interpret the residual image, we simulate two systems, both having
an X-ray surface brightness profile following a Beta model with the
same
,
but different
and S0, with
the centers separated by a distance of the order of the two adopted
core radii. The surface brightness profile of the composed system has
a single peak, as in the case of A2294 (see Fig. 14,
upper panel). The fit with a single Beta model provides a value for
and
larger than the two adopted core
radii, respectively. Instead,
is
larger than the adopted value. The appearance of the 2D image of the
residuals (Fig. 14, lower panel) is roughly similar to
that obtained for A2294, with a two-clump surplus of X-ray photons
(with the left clump being the most evident) in the line defined by
the centers of the subsystems and a deficit in the perpendicular
direction (cf. Fig. 14 with Fig. 12). Thus
the residual image of A2294 data might be explained by two very close
(or very closely projected) systems along the SE-NW direction. In
particular, we find that an asymmetry between the two components might
explain the more prominent excess of the SE structure in the residual
image. Some pieces of observational evidence found in the literature,
i.e. the presence of a centroid shift (Rizza et al. 1998;
Maughan et al. 2008, but see Hashimoto et al. 2007) or
a certain degree of asymmetry of the X-ray profile around the centroid
of the photon distribution (Hashimoto et al. 2007) are
likely to be connected with the substructure we detect. Our above
bimodal model is obviously a very simplified scenario in the case of a
close interaction between two galaxy systems - as we discuss below
- and we do not attempt to go further in interpreting the observed
data, e.g., exploring in more detail the quantitative parameters.
![]() |
Figure 14: Upper panel: integrated X-ray profile of the simulated system composed of two subsystems, both following a Beta model profile but with different model parameters (see text). Lower panel: surface brightness distribution of the simulated system with, superimposed, the contour levels of the Beta model residuals. White (dark/gray) contours represent negative (positive) residuals. As a reference, the size of the fitted core radius is drawn. |
Open with DEXTER |
5.2 Investigating the likely merger cluster
The absence of a macroscopic elongation of the galaxy and ICM distributions and the poor significance of the velocity gradient suggests that the evidence of substructure we detect is a trace of minor/old accretion phenomena or that the direction of the cluster merger is aligned along the LOS. The LOS direction might explain the difficulty of the analysis of the cluster internal dynamics. Another example of a cluster merger along the LOS is the galaxy cluster CL 0024+17, an apparently relaxed system, which is actually a collision between two clusters, the interaction occurring along our LOS, as demonstrated by Czoske et al. (2002) using about 300 galaxies with redshifts in the cluster field.
The cluster merger scenario is generally consistent with the absence
of the cool core. Although simulations yield ambivalent results about
the role of mergers in destroying cool cores (Poole et al. 2006; Burns et al. 2008), observations seem to favor cool core destruction by means of cluster mergers (Allen et al. 2001; Sanderson et al. 2006). In particular, the
LOS merging direction might explain the high compactness of A2294 with
respect to other non-cool core clusters (Bauer et al. 2005,
see their Fig. 3). We note that our new data for the BCG exclude the
presence of H
emission, which had been previously reported by
Crawford et al. (1995), thus reclassifying A2294 as a quite
``normal'' non-cool core.
In the framework of a cluster merger where the two subclusters are
well traced by the luminous galaxies (for the non-collisional part,
i.e., dark matter and galaxies) and the residual image (for the
collisional part, i.e., the gas), we may also obtain some information
about the evolutionary stage of the merger. Assuming that
for each of the two subclusters, from the values of
we obtain the X-ray temperatures
and
2.8 keV. The observed X-ray temperature is thus
1.4 times
that of the main subcluster. While the observed X-ray temperature of
the merging simulated clusters is still not clear at later times
(e.g. 2-3 Gyr after the collision, see ZuHone et al. 2009 and
refs. therein), numerical simulations agree in finding enhancements of
the X-ray temperature around the time of the
core-crossing. After a very sharp rise, the temperature peaks
either during the core-crossing or just after and then declines
(Ricker & Sarazin 2001; Mastropietro & Burkert
2008). Since we see no evidence of a very hot, arc-shaped
feature at the cluster center, we assume that the merger is being
captured after the core-crossing, i.e. during the outgoing phase.
For the case of a 1:3 mass ratio, Fig. 8 of Ricker & Sarazin
(2001) suggests a time
0.5 Gyr after the core-crossing.
At this point, we have the minimum amount of observation-based
information to be able to apply the two-body model (Beers et al. 1982; Thompson 1982) following the methodology
outlined for, e.g., Abell 1240 (Barrena et al. 2009). This
simple model assumes radial orbits for the clumps with no shear or net
rotation of the system. According to the boundary conditions usually
considered, the clumps are assumed to begin their evolution at time
t0=0 with a separation d0=0, and are now moving apart or coming
together for the first time in their history. In the case of a
collision, we assume that the time t0=0 with separation d0=0 is
the time of their core-crossing and that we are looking at the system
after a time t. The values of the relevant parameters for the
two-subcluster system are: Gyr, the relative LOS velocity
in the rest-frame,
km s-1, and the projected
linear distance between the two clumps,
h70-1 Mpc. The last
parameter is deduced from the residual image and therefore might be an
underestimate of the non-collisional component.
![]() |
Figure 15:
System mass vs. projection angle for bound and unbound solutions
(thick solid and thick dashed curves, respectively) of the two-body
model applied to the low and high velocity subclusters. Labels
BI |
Open with DEXTER |
The bimodal model solution gives the total system mass
,
i.e. the sum of the masses of the two subclusters, as a function of
,
where
is the projection angle between the plane of
the sky and the line connecting the centers of the two clumps (e.g.,
Gregory & Thompson 1984). Figure 15 compares the
bimodal-model solutions with the observed mass of the system
considering a 50% uncertainty band. Among other
solutions, we find the bound outgoing solution (BO) with
,
i.e. the cluster merger is occurring largely in the LOS
direction, in agreement with our expectations. In the framework of
this solution the SE clump, which is the more X-ray luminous and thus
probably the more massive, is moving towards SE in the direction of
the observer, while the less X-ray luminous and massive NW subcluster
is moving towards NW in the opposite direction with respect to the
observer. The true spatial distance between the two subclumps is
h70-1 Mpc
and the real, i.e. deprojected, velocity
difference is
km s-1. In this scenario, we
expect the presence of a gas shock (Bykov et al. 2008, and
refs. therein). We can estimate the Mach number of the shock to be
,
where
is the
velocity of the shock and
is the sound speed in the
pre-shock gas (see e.g., Sarazin 2002 for a review). The
value of
can be obtained from our estimate of
km s
for the most massive subcluster. For the value of
,
we use
km s-1, since after the core crossing the shock velocity is larger than
the subcluster velocity (see Fig. 4 of Springel & Farrar 2007
and Fig. 14 of Mastropietro & Burkert 2008). This leads to
,
in agreement with the moderate Mach numbers
expected for shocks due to the cluster merging.
In conclusion, present observational evidence is consistent with A2294 being a very massive cluster that has just formed or is in the process of forming by means of a merger, i.e. similar to most DARC clusters we previously analyzed (e.g. Boschin et al. 2006; Barrena et al. 2007a; Girardi et al. 2008). The timescale of a few fractions of Gyr agrees both with the results of other merging clusters showing radio halos/relics (e.g., Markevitch et al. 2002; Girardi et al. 2008; Barrena et al. 2009) and with theoretical expectations for radio halos (Brunetti et al. 2009). The morphology of the A2294 radio halo is somewhat intriguing. After the subtraction of discrete sources, the radio halo of A2294 appears quite elongated along the EW direction as shown by Giovannini et al. (2009; their Fig. 10 on the left), while the radio halos of other DARC clusters have either a round structure or a structure elongated in the direction of the merger (Abell 697, Girardi et al. 2006; Abell 520, Girardi et al. 2008). This appearent misalignment with the projected merging direction (SE-NW) deserves further investigation.
In general, to verify our hypothesis of a cluster merger in A2294 and more accurately quantify the merging framework we suggest both the acquisition of many more redshifts in the cluster field and/or deeper X-ray observations. In particular, deeper X-ray data would allow us to confirm the temporal phase of the merger, although the LOS geometry of the merger implies that the direct observation of the shock would be difficult (e.g., Markevitch et al. 2005). The acquisition of more redshifts might allow us to more accurately determine the non-collisional components of the merging subclusters.
AcknowledgementsWe are in debt with Gabriele Giovannini for the VLA radio image and his useful comments. M.G. acknowledges financial support from ASI-INAF I/088/06/0 grant. We thank the anonymous referee for his/her very stimulating suggestions. This publication is based on observations made on the island of La Palma with the Italian Telescopio Nazionale Galileo (TNG) and the Isaac Newton Telescope (INT). The TNG is operated by the Fundación Galileo Galilei - INAF (Istituto Nazionale di Astrofisica). The INT is operated by the Isaac Newton Group. Both telescopes are located in the Spanish Observatorio of the Roque de Los Muchachos of the Instituto de Astrofisica de Canarias. This research has made use of the NASA/IPAC Extragalactic Database (NED), which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.
References
- Abell, G. O., Corwin, H. G. Jr., & Olowin, R. P. 1989, ApJS, 70, 1 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
- Allen, S. W., Ettori, S., & Fabian, A. C. 2001, MNRAS, 324, 877 [NASA ADS] [CrossRef] [Google Scholar]
- Ashman, K. M., Bird, C. M., & Zepf, S. E. 1994, AJ, 108, 2348 [NASA ADS] [CrossRef] [Google Scholar]
- Bardelli, S., Zucca, E., Vettolani, G., et al. 1994, MNRAS, 267, 665 [Google Scholar]
- Barrena, R., Boschin, W., Girardi, M., & Spolaor, M. 2007a, A&A, 467, 37 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
- Barrena, R., Boschin, W., Girardi, M., & Spolaor, M. 2007b, A&A, 469, 861 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
- Barrena, R., Girardi, M., Boschin, & Dasí, M. 2009, A&A, 503, 357 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
- Bauer, F. E., Fabian, A. C., Sanders, J. S., Allen, S. W., & Johnstone, R. M. 2005, MNRAS, 359, 1481 [NASA ADS] [CrossRef] [Google Scholar]
- Beers, T. C., Geller, M. J., & Huchra, J. P. 1982, ApJ, 257, 23 [NASA ADS] [CrossRef] [Google Scholar]
- Beers, T. C., Flynn, K., & Gebhardt, K. 1990, AJ, 100, 32 [NASA ADS] [CrossRef] [Google Scholar]
- Beers, T. C., Forman, W., Huchra, J. P., Jones, C., & Gebhardt, K. 1991, AJ, 102, 1581 [NASA ADS] [CrossRef] [Google Scholar]
- Beers, T. C., Gebhardt, K., Huchra, J. P., et al. 1992, ApJ, 400, 410 [NASA ADS] [CrossRef] [Google Scholar]
- Bertin, E., & Arnouts, S. 1996, A&AS, 117, 393 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
- Bird, C. M., & Beers, T. C., 1993, AJ, 105, 1596 [NASA ADS] [CrossRef] [Google Scholar]
- Biviano, A., Durret, F., Gerbal, D., et al. 1996, A&A, 311, 95 [NASA ADS] [Google Scholar]
- Biviano, A., Girardi, M., Giuricin, G., Mardirossian, F., & Mezzetti, M. 1992, ApJ, 396, 35 [CrossRef] [Google Scholar]
- Biviano, A., Katgert, P., Mazure, A., et al. 1997, A&A, 321, 84 [NASA ADS] [Google Scholar]
- Boschin, W., Girardi, M., Barrena, R., et al. 2004, A&A, 416, 839 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
- Boschin, W., Girardi, M., Spolaor, M., & Barrena, R. 2006, A&A, 449, 461 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
- Brunetti, G., Cassano, R., Dolag, K., & Setti, G. 2009, A&A, 507, 661 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
- Boschin, W., Barrena, R., Girardi, M., & Spolaor, M. 2008, A&A, 487, 33 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
- Buote, D. A. 2002, in Merging Processes in Galaxy Clusters, ed. L. Feretti, I. M. Gioia, & G. Giovannini (The Netherlands: Kluwer Ac. Pub.) Optcial Analysis of Cluster Mergers [Google Scholar]
- Buote, D. A., Humphrey, P. J., & Stocke, T. 2005, ApJ, 630, 750 [NASA ADS] [CrossRef] [Google Scholar]
- Burns, J. O., Hallman, E. J., Gantner, B., Motl, P. M., & Norman, M. L. 2008, ApJ, 675, 1125 [NASA ADS] [CrossRef] [Google Scholar]
- Burstein, D., & Heiles, C. 1982, AJ, 87, 1165 [NASA ADS] [CrossRef] [Google Scholar]
- Bykov, A. M., Dolag, K., & Durret, F. 2008, Space Sci. Rev., 134, 119 [NASA ADS] [CrossRef] [Google Scholar]
- Carlberg, R. G., Yee, H. K. C., & Ellingson, E. 1997, ApJ, 478, 462 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
- Cash, W. 1979, ApJ, 228, 939 [NASA ADS] [CrossRef] [Google Scholar]
- Cassano, R., & Brunetti, G. 2005, MNRAS, 357, 1313 [NASA ADS] [CrossRef] [Google Scholar]
- Cassano, R., Brunetti, G., Röttgering, H. J. A., & Brüggen, M. 2010, A&A, 509, A68 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
- Cavaliere, A., & Fusco-Femiano, R. 1976, A&A, 49, 137 [NASA ADS] [Google Scholar]
- Cousins, A. W. J. 1976, Mem. R. Astr. Soc, 81, 25 [Google Scholar]
- Crawford, C. S., Edge, A. C., Fabian, A. C., et al. 1995, MNRAS, 274, 75 [NASA ADS] [CrossRef] [Google Scholar]
- Croston, J. H., Pratt, G. W., Böhringer, et al. 2008, A&A, 431, 443 [Google Scholar]
- Czoske, O., Moore, B., Kneib, J.-P., & Soucail, G. 2002, A&A, 386, 31 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
- Danese, L., De Zotti, C., & di Tullio, G. 1980, A&A, 82, 322 [NASA ADS] [Google Scholar]
- Dickey, J. M., & Lockman, F. J. 1990, ARA&A, 28, 215 [NASA ADS] [CrossRef] [MathSciNet] [Google Scholar]
- Dressler, A., & Shectman, S. A. 1988, AJ, 95, 985 [NASA ADS] [CrossRef] [Google Scholar]
- Ebeling, H., Edge, A. C., Böhringer, H., et al. 1998, MNRAS, 301, 881 [NASA ADS] [CrossRef] [Google Scholar]
- Edwards, L. O. V., Hudson, M. J., Balogh, M. L., & Smith, R. J. 2007, MNRAS, 379, 100 [NASA ADS] [CrossRef] [Google Scholar]
- Ellingson, E., & Yee, H. K. C. 1994, ApJS, 92, 33 [NASA ADS] [CrossRef] [Google Scholar]
- Ensslin, T. A., Biermann, P. L., Klein, U., & Kohle, S. 1998, A&A, 332, 395 [NASA ADS] [Google Scholar]
- Ensslin, T. A., & Gopal-Krishna 2001, A&A, 366, 26 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
- Fadda, D., Girardi, M., Giuricin, G., Mardirossian, F., & Mezzetti, M. 1996, ApJ, 473, 670 [NASA ADS] [CrossRef] [Google Scholar]
- Feretti, L. 1999, MPE Report No. 271 [Google Scholar]
- Feretti, L. 2002a, The Universe at Low Radio Frequencies, Proceedings of IAU Symposium 199, held 30 Nov-4 Dec 1999, Pune, India, ed. A. Pramesh Rao, G. Swarup, & Gopal-Krishna, 2002., 133 [Google Scholar]
- Feretti, L. 2005, X-Ray and Radio Connections, ed. L. O. Sjouwerman, & K. K. Dyer. Published electronically by NRAO, http://www.aoc.nrao.edu/events/xraydio. Held 3-6 February 2004 in Santa Fe, New Mexico, USA [Google Scholar]
- Feretti, L. 2006, Proceedings of the XLIst Rencontres de Moriond, XXVIth Astrophysics Moriond Meeting: From dark halos to light, ed. L. Tresse, S. Maurogordato, & J. Tran Thanh Van, [astro-ph/0612185] [Google Scholar]
- Feretti, L. 2008, Mem. SAIt, 79, 176 [NASA ADS] [Google Scholar]
- Feretti, L., Gioia I. M., & Giovannini G. 2002b, Astrophysics and Space Science Library, Merging Processes in Galaxy Clusters (The Netherlands: Kluwer Academic Publisher), 272 [Google Scholar]
- Ferrari, C., Govoni, F., Schindler, S., Bykov, A. M., & Rephaeli, Y. 2008, Space Sci. Rev., 134, 93 [NASA ADS] [CrossRef] [Google Scholar]
- Fusco-Femiano, R., & Menci, N. 1998, ApJ, 498, 95 [NASA ADS] [CrossRef] [Google Scholar]
- Giovannini, G., Bonafede, A., Feretti, L., et al. 2009, A&A, 507, 1257 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
- Giovannini, G., & Feretti, L. 2002, in Merging Processes in Galaxy Clusters, ed. L. Feretti, I. M. Gioia, & G. Giovannini (The Netherlands: Kluwer Ac. Pub.): Diffuse Radio Sources and Cluster Mergers [Google Scholar]
- Giovannini, G., Tordi, M., & Feretti, L. 1999, New Astron., 4, 141 [NASA ADS] [CrossRef] [Google Scholar]
- Girardi, M., & Mezzetti, M. 2001, ApJ, 548, 79 [NASA ADS] [CrossRef] [Google Scholar]
- Girardi, M., & Biviano, A. 2002, in Merging Processes in Galaxy Clusters, ed. L. Feretti, I. M. Gioia, & G. Giovannini (The Netherlands: Kluwer Ac. Pub.): Optical Analysis of Cluster Mergers [Google Scholar]
- Girardi, M., Fadda, D., Giuricin, G., et al. 1996, ApJ, 457, 61 [NASA ADS] [CrossRef] [Google Scholar]
- Girardi, M., Giuricin, G., Mardirossian, F., Mezzetti, M., & Boschin, W. 1998, ApJ, 505, 74 [NASA ADS] [CrossRef] [Google Scholar]
- Girardi, M., Boschin, W., & Barrena, R. 2006, A&A, 455, 45 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
- Girardi, M., Barrena, R., & Boschin, W. 2007, Contribution to Tracing Cosmic Evolution with Clusters of Galaxies: Six Years Later, conference - http://www.si.inaf.it/sesto2007/contributions/Girardi.pdf [Google Scholar]
- Girardi, M., Barrena, R., Boschin, W., & Ellingson, E. 2008, A&A, 491, 379 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
- Goto, T. 2005, MNRAS, 359, 141 [Google Scholar]
- González-Casado, G., Mamon, G. A., & Salvador-Solé, E. 1994, ApJ, 433, 61 [Google Scholar]
- Gregory, S. A., & Thompson, L. A. 1984, ApJ, 286, 422 [NASA ADS] [CrossRef] [Google Scholar]
- Gullixson, C. A. 1992, in Astronomical CCD Observing and Reduction techniques, ed. S. B. Howell, ASP Conf. Ser., 23, 130 [Google Scholar]
- Hart, B. C. 2008, Evolution of substructure in galaxy clusters as observed in X-rays, PhDT [arXiv:0801.4093] [Google Scholar]
- Hashimoto, Y., Böhringer, H., Henry, J. P., Hasinger, G., & Szokoly, G. 2007, A&A, 467, 485 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
- Hoeft, M., Brüggen, M., & Yepes, G. 2004, MNRAS, 347, 389 [NASA ADS] [CrossRef] [Google Scholar]
- Johnson, H. L., & Morgan, W. W. 1953, ApJ, 117, 313 [NASA ADS] [CrossRef] [Google Scholar]
- Jones, C., & Forman, W. 1999, ApJ, 511, 65 [NASA ADS] [CrossRef] [Google Scholar]
- Kempner, J. C., Blanton, E. L., Clarke, T. E. et al. 2004, Proceedings of the conference, The Riddle of Cooling Flows in Galaxies and Clusters of Galaxies, ed. T. H. Reiprich, J. C. Kempner, & N. Soker, [astro-ph/0310263] [Google Scholar]
- Kennicutt, R. C. 1992, ApJS, 79, 225 [Google Scholar]
- Landolt, A. U. 1992, AJ, 104, 340 [NASA ADS] [CrossRef] [Google Scholar]
- Leccardi, A., & Molendi, S. 2008, A&A, 486, 359 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
- Limber, D. N., & Mathews, W. G. 1960, ApJ, 132, 286 [NASA ADS] [CrossRef] [Google Scholar]
- López-Cruz, O., Barkhouse, W. A., & Yee, H. K. C. 2004, ApJ, 614, 679 [NASA ADS] [CrossRef] [Google Scholar]
- Malumuth, E. M., Kriss, G. A., Dixon, W. V. D., Ferguson, H. C., & Ritchie, C. 1992, AJ, 104, 495 [NASA ADS] [CrossRef] [Google Scholar]
- Markevitch, M., Gonzalez, A. H., David, L., et al. 2002, ApJ, 567, 27 [Google Scholar]
- Markevitch, M., Govoni, F., Brunetti, G, & Jerius, D. 2005, ApJ, 627, 733 [NASA ADS] [CrossRef] [Google Scholar]
- Mastropietro, C., & Burkert, A. 2008, MNRAS, 389, 967 [NASA ADS] [CrossRef] [Google Scholar]
- Maughan, B. J., Jones, C., Forman, W., & Van Speybroeck, L. 2008, ApJS, 174, 117 [NASA ADS] [CrossRef] [Google Scholar]
- Merritt, D. 1988, in The Minneosota lectures on clusters of galaxies and large-scale structure (A90-36758 15-90). San Francisco, CA, Astronomical Society of the Pacific, 1988, 175 [Google Scholar]
- Menci, N., & Fusco-Femiano, R. 1996, ApJ, 472, 46 [NASA ADS] [CrossRef] [Google Scholar]
- Mushotzky, R. F., & Scharf, C. A. 1997, ApJ, 482, L13 [NASA ADS] [CrossRef] [Google Scholar]
- NAG Fortran Workstation Handbook, 1986 (Downers Grove, IL: Numerical Algorithms Group) [Google Scholar]
- Neumann, D. M., & Arnaud, M. 1999, A&A, 348, 711 [NASA ADS] [Google Scholar]
- Neumann, D. M., & Arnaud, M. 2005, A&A, 439, 465 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
- Ota, N., Pointecouteau, E., Hattori, M., & Mitsuda, K. 2004, ApJ, 601, 1200 [NASA ADS] [CrossRef] [Google Scholar]
- Owen, F., Morrison, G., & Voges, W. 1999, proceedings of the workshop, Diffuse Thermal and Relativistic Plasma in Galaxy Clusters, ed. H. Böhringer, L. Feretti, & P. Schuecker, MPE Report 271, 9 [Google Scholar]
- Peres, C. B, Fabian, A. C., Edge, A. C., et al. 1998, MNRAS, 298, 416 [NASA ADS] [CrossRef] [Google Scholar]
- Pisani, A. 1993, MNRAS, 265, 706 [NASA ADS] [Google Scholar]
- Pisani, A. 1996, MNRAS, 278, 697 [NASA ADS] [CrossRef] [Google Scholar]
- Poggianti, B. M. 1997, A&AS, 122, 399 [Google Scholar]
- Poole, G. B., Fardal, M. A., Babul, A., et al. 2006, MNRAS, 373, 881 [NASA ADS] [CrossRef] [Google Scholar]
- Press, W. H., Teukolsky, S. A., Vetterling, W. T., & Flannery, B. P. 1992, in Numerical Recipes 2nd edn. (Cambridge University Press) [Google Scholar]
- Quintana, H., Carrasco, E. R., & Reisenegger, A. 2000, AJ, 120, 511 [NASA ADS] [CrossRef] [Google Scholar]
- Raymond, J. C., & Smith, B. W. 1977, ApJS, 35, 419 [NASA ADS] [CrossRef] [Google Scholar]
- Ricker, P. M., & Sarazin, C. L. 2001, ApJ, 561, 621 [NASA ADS] [CrossRef] [Google Scholar]
- Rizza, E., Burns, J. O., Ledlow, M. J., et al. 1998, MNRAS, 301, 328 [NASA ADS] [CrossRef] [Google Scholar]
- Rizza, E., Morrison, G. E., Owen, F. N., et al. 2003, AJ, 126, 119 [Google Scholar]
- Roettiger, K., Burns, J. O., & Loken, C. 1996, ApJ, 473, 651 [NASA ADS] [CrossRef] [Google Scholar]
- Roettiger, K., Loken, C., & Burns, J. O. 1997, ApJS, 109, 307 [NASA ADS] [CrossRef] [Google Scholar]
- Roettiger, K., Burns, J. O., & Stone, J. M. 1999, ApJ, 518, 603 [NASA ADS] [CrossRef] [Google Scholar]
- Sanderson, A. J. R., Ponman, T. J., & O'Sullivan, E. 2006, MNRAS, 372, 1496 [NASA ADS] [CrossRef] [Google Scholar]
- Sarazin, C. L. 2002, in Merging Processes in Galaxy Clusters, ed. L. Feretti, I. M. Gioia, & G. Giovannini (The Netherlands: Kluwer Ac. Pub.): The Physics of Cluster Mergers [Google Scholar]
- Springel, V., & Farrar, G. R. 2007, MNRAS, 380, 911 [NASA ADS] [CrossRef] [Google Scholar]
- The, L. S., & White, S. D. M. 1986, AJ, 92, 1248 [NASA ADS] [CrossRef] [Google Scholar]
- Thompson, L. A. 1982, in Early Evolution of the Universe and the Present Structure, ed. G.O. Abell, & G. Chincarini (Dordrecht: Reidel), IAU Symposium, 104 [Google Scholar]
- Tonry, J., & Davis, M. 1979, ApJ, 84, 1511 [Google Scholar]
- Tribble, P. C. 1993, MNRAS, 261, 57 [NASA ADS] [CrossRef] [Google Scholar]
- Vikhlinin, A., Forman, W., & Jones, C. 1999, ApJ, 525, 47 [NASA ADS] [CrossRef] [Google Scholar]
- Wainer, H., & Schacht, S. 1978, Psychometrika, 43, 203 [CrossRef] [Google Scholar]
- ZuHone, J. A., Ricker, P. M., Lamb, D. Q., & Karen Yang, H.-Y. 2009, ApJ, 699, 1004 [NASA ADS] [CrossRef] [Google Scholar]
Footnotes
- ...2007
- see also the web site of the DARC (Dynamical Analysis of Radio Clusters) project: http://adlibitum.oat.ts.astro.it/girardi/darc.
- ... IRAF
- IRAF is distributed by the National Optical Astronomy Observatories, which are operated by the Association of Universities for Research in Astronomy, Inc., under cooperative agreement with the National Science Foundation.
- ...
characterization
- we refer to http://www.ast.cam.ac.uk/ wfcsur/technical/photom/colours/
- ... CIAO
- CIAO is freely available at http://asc.harvard.edu/ciao/
All Tables
Table 1: Velocity catalog of 88 spectroscopically measured galaxies in the field of the cluster A2294.
All Figures
![]() |
Figure 1: INT R-band image of the cluster A2294 (north at the top and east to the left) with, superimposed, the contour levels of the Chandra archival image ID 3246 (thick contours; photons in the energy range 0.5-2 keV) and the contour levels of a VLA radio image at 1.4 GHz (thin contours, see Giovannini et al. 2009). Labels and arrows highlight the positions of radio sources listed by Rizza et al. (2003). |
Open with DEXTER | |
In the text |
![]() |
Figure 2:
INT R-band image of the cluster A2294 (north at the top
and east to the left). Circles and squares indicate cluster members
and non-members, respectively (see Table 1). Solid
circle in the center highlights the position of the BCG galaxy.
Annuli and box annuli show member and non-member emission line
galaxies, respectively. Labels indicate the IDs of cluster
galaxies cited in the text. A diamond at the right border of the
image highlights a QSO at |
Open with DEXTER | |
In the text |
![]() |
Figure 3:
Top panel: 2D spectrum of the BCG galaxy taken with the grism
LR-R mounted on DOLORES in the wavelength range |
Open with DEXTER | |
In the text |
![]() |
Figure 4: Redshift galaxy distribution. The solid line histogram refers to the 80 galaxies assigned to the A2294 complex according to the DEDICA reconstruction method. The number-galaxy density in the redshift space, as provided by the adaptive kernel reconstruction method is superimposed on the histogram. |
Open with DEXTER | |
In the text |
![]() |
Figure 5: The 78 galaxies assigned to the cluster. Upper panel: velocity distribution. The arrows indicate the velocities of the five brightest galaxies, in particular we indicate the brightest cluster galaxy ``BCG'', the bright radio galaxy ``R6''. Lower panel: stripe density plot where the arrows indicate the positions of the significant gaps. |
Open with DEXTER | |
In the text |
![]() |
Figure 6:
Top panel: rest-frame velocity vs. projected distance from the
cluster center. Squares indicate the five brightest galaxies.
Middle and bottom panels: differential (big circles) and integral
(small points) profiles of mean velocity and LOS velocity
dispersion, respectively. For the differential profiles, we plot the
values for five annuli from the center of the cluster, each of 0.25
h70-1 Mpc |
Open with DEXTER | |
In the text |
![]() |
Figure 7: Spatial distribution on the sky of the cluster galaxies showing the three groups recovered by the weighted gap analysis. Solid circles, crosses, and open circles indicate the galaxies of GV1, GV2, and GV3, respectively. The BCG is taken as the cluster center. Large squares indicate the five brightest cluster members. Among them, the BCG and R6 are indicated by the two largest squares. |
Open with DEXTER | |
In the text |
![]() |
Figure 8: Velocity galaxy distribution of more and less luminous cluster members (solid and dashed lines, respectively). The arrows indicate the velocities of the five brightest galaxies, in particular ``BCG'' indicates the bright, central galaxy and ``R6'' indicates the bright radio-galaxy. The number-galaxy densities, as provided by the adaptive kernel reconstruction method are superimposed on the histograms. |
Open with DEXTER | |
In the text |
![]() |
Figure 9:
B-R vs. R diagram for galaxies with available spectroscopy is shown
by circles and crosses (cluster and field members, respectively).
The solid line gives the best-fit color-magnitude relation as
determined for member galaxies; the dashed lines are drawn at |
Open with DEXTER | |
In the text |
![]() |
Figure 10:
Spatial distribution on the sky and relative isodensity contour map
of likely cluster members extracted from our photometric catalog
with |
Open with DEXTER | |
In the text |
![]() |
Figure 11:
|
Open with DEXTER | |
In the text |
![]() |
Figure 12: The smoothed X-ray emission in the right-upper quadrant of Fig. 11 with, superimposed, the contour levels of the positive (black) and negative (white) smoothed Beta model residuals (North at the top and East to the left). |
Open with DEXTER | |
In the text |
![]() |
Figure 13: Softness ratio 3D map of A2294 around the centroid of the X-ray distribution. The plane at the median SR value (0.25) is drawn. |
Open with DEXTER | |
In the text |
![]() |
Figure 14: Upper panel: integrated X-ray profile of the simulated system composed of two subsystems, both following a Beta model profile but with different model parameters (see text). Lower panel: surface brightness distribution of the simulated system with, superimposed, the contour levels of the Beta model residuals. White (dark/gray) contours represent negative (positive) residuals. As a reference, the size of the fitted core radius is drawn. |
Open with DEXTER | |
In the text |
![]() |
Figure 15:
System mass vs. projection angle for bound and unbound solutions
(thick solid and thick dashed curves, respectively) of the two-body
model applied to the low and high velocity subclusters. Labels
BI |
Open with DEXTER | |
In the text |
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