A&A 365, L67-L73 (2001)
M. Arnaud 1 -
N. Aghanim2 -
R. Gastaud3 -
D. M. Neumann1 -
D. Lumb 4 -
U. Briel 5 -
B. Altieri6 -
S. Ghizzardi7 -
J. Mittaz8 -
T. P. Sasseen9 -
W. T. Vestrand10
Send offprint request: M. Arnaud
1 - CEA/DSM/DAPNIA Saclay, Service d'Astrophysique, L'Orme des
Merisiers, Bât. 709,
91191 Gif-sur-Yvette, France
2 - IAS-CNRS,
Université Paris Sud, Bât. 121, 91405 Orsay Cedex, France
3 - CEA/DSM/DAPNIA Saclay, Service d'Électronique
et d'Informatique, 91191 Gif-sur-Yvette, France
4 - Space Science Dept., European Space Agency, ESTEC Postbus 299,
2200 AG Noordwijk, The Netherlands
5 - Max-Planck-Institut für
extraterrestrische Physik, 85740 Garching, Germany
6 - XMM-NewtonScience Operations Centre, ESA Space Science Department, PO Box
50727,
28080 Madrid, Spain
7 - IFC/CNR, Via Bassini 15,
20133 Milano, Italy
8 - Department of Space and Climate Physics,
UCL, Mullard Space Science Laboratory, Holmbury St. Mary, Surrey, UK
9 - University of California, Santa Barbara, CA 93110, USA
10 -
NIS-2, MS D436, Los Alamos National Laboratory Los Alamos, NM 87545,
USA
Received 2 October 2000 / Accepted 2 November 2000
Abstract
We present a temperature map and a temperature profile of
the central part (r < 20' or 1/4 virial radius) of the Coma cluster.
We combined 5 overlapping pointings made with XMM/EPIC/MOS and
extracted spectra in boxes of
.
The temperature
distribution around the two central galaxies is remarkably homogeneous
(r<10'), contrary to previous ASCA results, suggesting that the core
is actually in a relaxed state. At larger distance from the cluster
center we do see evidence for recent matter accretion. We confirm the
cool area in the direction of NGC 4921, probably due to gas stripped
from an infalling group. We find indications of a hot front in the
South West, in the direction of NGC 4839, probably due to an adiabatic
compression.
Key words: galaxies: intergalactic medium - Cosmology:
observations - Cosmology: dark matter -
Cosmology: large-scale
structure of the Universe - X-rays: general
Author for correspondance: marnaud@discovery.saclay.cea.fr
Obs. | Rev. | RA | DEC. | MOS1 | MOS2 | MOS1&2 counts | |||
(J2000.0) | (J2000.0) | Exp. | Exp. | [0.3-10] keV | [5-10] keV | ||||
(ksec) | (ksec) | Source | Bkgd | Source | Bkgd | ||||
Pc | 86 |
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16.4 | 16.3 | 7.70 105 | 4.2 104 | 3.52 104 | 1.00 104 |
P5 | 86 |
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20.9 | 21.4 | 6.63 105 | 5.5 104 | 3.23 104 | 1.30 104 |
P6 | 93 |
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7.4 | 7.3 | 1.81 105 | 1.9 10 4 | 8.53 103 | 4.52 103 |
P9 | 93 |
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20.8 | 21.0 | 7.03 105 | 5.4 104 | 3.08 104 | 1.29 104 |
P10 | 98 |
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20.6 | 20.9 | 5.26 105 | 5.4 104 | 2.43 104 | 1.27 104 |
Recent studies of the Coma cluster with the ASCA satellite (Donnelly et al. 1999; Watanabe et al. 1999) revealed complex temperature variations in this massive cluster. They were interpreted as indicative of recent mergers, confirming earlier evidence based on optical dynamical studies (Colless & Dunn 1996 and references therein) and X-ray morphological analysis (Briel et al. 1992; White et al. 1993; Vikhlinin et al. 1994, 1997). ASCA covered a broad energy band, which is essential for precise temperature estimate, but the observations suffered from a relatively large energy dependent PSF. Therefore temperature structure determination with ASCA might have been subject to systematic errors. Furthermore the spatial resolution was insufficient to resolve precisely the temperature radial profile in the very core of clusters.
The EPIC instrument (Turner et al. 2001) on board XMM (Jansen
et al. 2001) combines a high sensitivity with good spatial and
spectral resolution, on a wide energy range. In this paper, we use
this unique capability to study the temperature structure in the
central (
Mpc) region of Coma. We present further
XMM results in two other papers of this issue: the large scale
morphology of Coma (Briel et al. 2001) and the dynamics of the
infalling NGC 4839 group (Neumann et al. 2001). In the
following, we assume
and q0=0.5.
The central part of Coma was observed with 5 overlapping pointings in Full Frame mode with the EPIC/MOS camera (medium filter) and in extended Full Frame mode with the pn camera. As CTE correction in this pn mode is still being studied, we considered only MOS data in the present spectroscopic analysis.
We generated calibrated event files with SASv4.1, except for the gain correction. Correct PI channels are obtained by interpolating gain values obtained from the nearest observations of the on board calibration source. Data were also checked to remove any remaining bright pixels. We excluded periods of high background induced by solar flare protons. We discarded all frames corresponding to a count rate greater than 15 ct/100 s in the [10-12] keV band, where the emission is dominated by the particle induced background. Finally, only events in the nominal FOV are considered.
The central position of each pointed observation is listed in Table 1, together with the revolution number and remaining observing time after cleaning.
Spectra in various regions were considered to study temperature variations. Each pointing and each MOS camera are first treated separately.
The effective area at a given energy depends on position. When
extracting the spectrum of a region, we weight each photon falling at
position
(xj,yj) of the detector and of energy Ej by the
ratio of the effective area at that position, to the central effective
area (for this energy). This weighting is taken into account in the
error estimate. The ``corrected'' spectrum obtained is an estimate of
the spectrum one would get if the detector was flat. A detailed
description of the method and of the vignetting calibration data used
are given in Arnaud et al. (2001). Note that the vignetting
due to the RGA is included, but is assumed to be energy independent
(the variations are less than
below 6 keV).
We generated EPIC MOS background event files (one for each MOS camera)
by combining several high galactic latitude pointings. The data are
cleaned for bright pixels, background flares and regions corresponding
to bright point sources are excluded. The integrated exposure time is
94.3 ksec for MOS1 and 78.9 ksec for MOS2. These event files can be
used for a proper estimate of the cosmic ray (CR) induced background
but not the X-ray background, which depends on pointing position and
filter. However, bright cluster emission, like the one observed in
the Coma center, usually dominates the background except at high
energy (see also Arnaud et al. 2001). We thus are mostly
sensitive to CR induced background. Note also that the offset
pointings considered here always include the cluster peak emission, so
scattered light is not a problem. The total estimated number of
source and background photons, in the
and
energy ranges, are listed in Table 1 for each
pointing. Note the degradation of the S/N ratio at high energies as a
consequence of the very hard CR induced background.
It is known that the CR background changes slightly in the FOV. It is thus better to consider the same extraction regions in detector coordinates for the source and the background. Furthermore, if one wants to combine spectra of a given physical Coma region obtained from different pointings, it is mandatory to define extraction regions in sky coordinates. To alleviate this practical problem, we simply generated a specific background event file for each Coma pointing and camera by modifying the sky coordinates of the background event file using the aspect solution of the considered Coma observation.
For consistency, the background spectra were obtained using the same correction method as used for the source. The background component induced by CR is not vignetted, but as we extract the background and source spectra from the same region in detector coordinates the correction factor is the same and does not introduces bias.
Further details on the characteristic of the EPIC/MOS background and subtraction method can be found in Arnaud et al. (2001).
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Figure 1:
The EPIC/MOS mosaic image of the central region of
Coma (5 overlapping pointings) in the [0.3-2] energy band. The
iso contours are the residuals (in ![]() ![]() ![]() ![]() |
Open with DEXTER |
The source and background spectra of a given region (defined in sky
coordinates) are first extracted separately for each MOS camera and
each pointing, using the method described above. As the spectra are
corrected for vignetting, the spectra of the same physical region
observed with different off-axis angle (from different pointings) and
different camera can be simply added to maximize the signal to noise
ratio. The errors are propagated using quadratic summation.
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Figure 2:
EPIC/MOS1 (green) and EPIC/MOS2 (blue) spectra
extracted from within 10' in radius of the galaxy NGC 4874.
Red line: best fit redshifted isothermal model:
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Open with DEXTER |
Before model fitting, the source spectra are binned so that the S/N
ratio is greater than 3
in each bin after background
subtraction. The spectra are fitted with XSPEC using isothermal mekal
models (with fixed redshift z=0.0231). Although the thermodynamic
state of the plasma could be more complex (e.g. see the isobaric
multiphase model of Nagai et al. 2000), this is adequate to
study temperature spatial variations, the derived best fit temperature
being an estimate of the mean temperature in each projected region
considered. Since the spectra are ``corrected" for vignetting effects
we can use the on axis MOS response file, which is considered to be
the same for MOS1 and MOS2 (version v3.15). Only data above 0.3 keV are
considered due to remaining uncertainties in the MOS detector
response below this energy. Unless otherwise stated errors are with a
confidence level.
The MOS mosaic image in the
energy band is presented in
Fig. 1. The count images for each camera and each
pointing are extracted using the weighting procedure described above
to correct for vignetting. They are then projected on a common sky
reference axis, summed and divided by the mosaic exposure map (which
takes into account the exposure time and FOV coverage for each
pointing). The images are not background subtracted. In that energy
range and in this central area of the cluster, the particle background
is negligible for MOS.
To identify significant substructures in the core, we fitted the MOS
image with a 2-D ellipsoidal
model plus background and built
up the map of the residuals of the data over the best fit model. The
method is discussed in detail in Neumann & Böhringer
(1997) and Neumann (1999). The iso-contours of
significance of the excess (number of
over background plus
cluster model) are overlaid on the MOS image in Fig. 1.
We unambiguously confirm the excess emission around the two central galaxies (NGC 4874 and NGC 4889) and the tail of gas in the direction of NGC 4911, revealed by the wavelet analysis of Vikhlinin et al. (1997). However, contrary to Vikhlinin et al. result, this filamentary structure does not seem to be directly connected to the Coma center, but originates 0.5 Mpc South of it. Note also that part of the excess is due to resolved galaxy emission (NGC 4911, NGC 4921, QSO 1259+281, and 5 other point sources).
Diffuse excess emission is clearly detected in the South-West in particular in the direction of the NGC 4839 group (see Briel et al. 2001, for full discussion).
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Figure 3:
Position of the
![]() ![]() |
Open with DEXTER |
To compare with results obtained with other satellites we extracted
the overall MOS1 and MOS2 spectra from a circular region of 10 arcmin
in radius centered on NGC 4874. Only data from the central pointing
are used. The data are fitted in the
range, with
independent normalizations for MOS1 and MOS2, and common temperatures
and abundances. The spectra are plotted in Fig. 2,
the bottom panel gives the residuals. Fixing the
value
to the 21 cm value (
,
Dickey &
Lockman 1990), we obtain a best fit temperature of
and an abundance of
.
The reduced
is 1.38 (
for 1058 d.o.f). The fit is
satisfactory. The residuals are concentrated around the instrument
edges, with residual ratios between data and model of about
,
consistent with our present knowledge of the instrument response
(Fig. 2). If we let the
value free we get
,
an abundance of 0.25 and
,
in agreement with the 21 cm value and
is unchanged. In the following we thus fix the
value to the 21 cm value.
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Figure 4:
Temperature determined from the offset pointings
(P5, P6, P9 and/or P10) versus the temperature obtained, for the same
region, using the central pointing (Pc) data. Errors are at
![]() ![]() |
Open with DEXTER |
When fitted separately the MOS1 and MOS2 temperatures are consistent and the relative normalization is 1.05. In the following analysis we will thus sum MOS1 and MOS2 spectra.
The best fit overall temperature in this central (R<10') region is
in remarkable agreement with the overall GINGA value of
(Hughes et al. 1993) and is marginally
consistent with the ASCA value of
(Donnelly et al. 1999) obtained for the central (R<9') region. Note that
the hard excess seen by SAX (Fusco-Femiano et al. 1999)
could not be seen by XMM, since it appears above 20 keV.
To study the cluster temperature structure, we next extracted spectra
in
contiguous regions in sky coordinates. The box size
was chosen so that the two central galaxies fall approximately in the
center of a box, and that a sufficient S/N ratio is reached for each
box. Circular regions (20'' in radius) around bright sources (in
particular NGC 4889 and NGC 4911) are excised from the boxes. The
overall region considered for this spatially resolved spectroscopic
analysis is about 20' in radius. We only considered boxes at off-axis
angles smaller than 10' in each pointing (the
vignetting factor being more uncertain beyond this radius). The
central
region of each pointing, delineated as circle,
and the central position of the various boxes (95 in total) are
plotted in Fig. 3.
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Figure 5:
Color coded temperature map. Note the hot front in
the south-west (white) and the cold region in the South-East
(brown/dark red). The isocontours of the PN image in the
![]() ![]() ![]() |
Open with DEXTER |
The validity of our vignetting correction can be assessed by comparing
the fitted temperature of the same region in various pointings. The
vignetting effect (decrease of effective area with off-axis angle)
increases with energy. An understimate (overestimate) of this energy
dependence would yield to underestimate (overestimate) of the
temperature. Since the various pointings of the same region
correspond to different off-axis angles, an improper vignetting
correction would translate in systematic differences between
temperature estimates for the same region. Figure 4
shows the temperatures determined from the offset pointings
versus the temperature obtained, for the same region, using the
central pointing
(errors are at
-level). The
insert shows the histogram of the differences in term of
computed for each pair of measurements. Three outliers
(
or more than
discrepancy in estimates) are
clearly apparent. They correspond to the points at
(6.7, 8.4), (6.5, 7.3) and (7.6, 8.5). We do not see any particular
problem in the corresponding spectra (the statistic and fit are good)
and failed to find any obvious reason for the discrepancy. However,
these outliers correspond to isolated regions (the agreement between
estimates is good in adjacent regions) and the overall
for 36 d.o.f is satisfactory when these outliers are excluded. This
suggests that the vignetting correction is basically correct. We thus
sum all spectra obtained for a given physical region and build up the
temperature map presented in Fig. 5.
![]() |
Figure 6:
Excess emission over a ![]() |
Open with DEXTER |
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Figure 7:
Temperature of each region of the temperature map
with ![]() |
Open with DEXTER |
There is no strong evidence of temperature variations, except for a
cold area in the South-East (contiguous regions colder than average)
and a hot area in the South-West. It is instructive to compare these
temperature features with the X-ray image substructures
(Fig. 6). The S/E cold region in the temperature map
generally coincides with the filamentary substructure originating near
NGC 4911 and NGC 4921. It includes the cold regions put into evidence
by Donnelly et al. (1999) in that area (region 1 and part of
region 20 in their Fig. 2), that we thus confirm. The hot regions in
the S/W appear as a hot front perpendicular to the direction
connecting the cluster center to the NGC 4839 group, just ahead of the
excess emission in that direction. This excess emission extends
somewhat further to the North, where no specific temperature feature
is apparent. However, the temperature map is specially noisy in that
direction. The statistical significance of the temperature variations
can be seen in Fig. 7 where we plotted the temperature
of the various boxes versus their distance to NGC 4874. We split the
data in three subsamples: i) one S-E sector encompassing the
filamentary structure towards NGC 4911 ii) one S-W sector along the
direction towards NGC 4839 group iii) the rest of the sample. The hot
front (
)
in the S-W located at about 15' from the
center clearly stands out, as well as the colder region (
)
beyond 10' in the S-E sector. Otherwise the temperature does
not deviate significantly from the
temperature range
(less than
variation). In particular, we see no evidence of
the hot spot seen by Donnelly et al. (1999) 3' north of the
NGC 4874 galaxy. We further extracted the spectrum (using the central
pointing) corresponding to this hot ASCA region: from Fig. 1 of
Donnelly et al. (1999) we considered a rectangular region of
size
centered on
.
We get
consistent with the mean value and
inconsistent with the ASCA value of Box 11 (
).
![]() |
Figure 8: Radial temperature (top panel) and abundance(bottom panel) profiles. The rings are centered on NGC 4874 |
Open with DEXTER |
We further extracted spectra in concentric rings centered on NGC 4874 (all the data are summed). The region around NGC 4889 (40'' in radius) and bright point sources were excluded. The temperature and abundance profiles are shown in Fig. 8.
The dynamical state of the core of Coma has been much debated, in
particular the nature of the merging unit(s) and their link with the
dominant cluster galaxies. If there is a consensus that a merging
group is associated with NGC 4889, the situation of NGC 4874 is less
clear (see in particular Colless & Dunn 1996; Donnelly
et al. 1999). As already proposed by Colless & Dunn
(1996), our data suggest that NGC 4874 is simply the central
galaxy of the main Coma cluster, rather than being associated with a
second subgroup in an early merging stage with the main cluster (as
proposed by Donnelly et al. 1999). Several facts support
this picture. First the remarkably homogeneous temperature
distribution within the central
region suggests the gas
in that region is basically in a relaxed state. Second, there is no
obvious evidence of a third peak in between NGC 4889 and NGC 4874, which
would be associated with the ``true'' cluster center. Actually, apart
from the excess around NGC 4889, the X-ray morphology can be
classified as an offset-center cluster morphology (variation of
isophote centroid with scale). The excess (size
3' in radius)
around NGC 4874, when subtracting a
model (representative of
the large scale morphology) is a natural consequence for this type of
morphology. Moreover, part of the excess is certainly due to the
contribution of the halo of the galaxy itself. The very significant
drop in temperature within
of NGC 4874
(Fig. 8) is natural in that context, as well as the
increase in abundance, which could be due to an enriched ISM.
It might be surprising that substructures survive in the gas density distribution (excess around NGC 4889 and centroid shift for the main cluster) while the temperature distribution appears homogeneous. We must first emphasize that the spatial resolution and accuracy of the temperature and imaging data is not comparable. However, our results may also indicate that the gas simply follows the dark matter distribution. Dark matter substructures can survive for a long time after mergers, as indicated by high resolution simulations (Moore et al. 1998). High resolution hydrodynamic simulation, and more sophisticated morphology analysis, are essential to better understand this issue.
At larger scale we do see evidence of recent merger activity. The cold filamentary structure in the South-East can be naturally explained by a merging group (see Vikhlinin et al. 1997; Donnelly et al. 1999). The position and extent of the cold substructure and the core properties outlined above suggest that the merging group is associated with NGC 4911 and NGC 4921, rather than being due to gas stripped from a group centered on NGC 4874 as proposed by Donnelly et al. (1999). Note that an excess in the galaxy distribution is also observed around NGC 4921/NGC 4911 (Mellier et al. 1988).
Our analysis revealed for the first time a hot front in the South-West, just ahead of the excess emission that we see at the edge of the MOS mosaic and which extends further away towards NGC 4839 (see Briel et al. 2001). It is situated roughly at the boundary of the group associated with this galaxy, as defined from the optical (Colless & Dunn 1996, Fig. 9) and is perpendicular to the direction connecting the center of Coma and NGC 4839. This temperature structure is likely to be due to adiabatic compression, caused by the infall of matter associated with the NGC 4839 group. The feature we find is indeed very similar to the one displayed in Fig. 6c of Schindler & Müller (1993), where at this time of the merger event no accretion shock has yet formed. To definitively characterise the feature we need to know the temperature structure at larger radii, to quantify the transition conditions. It has already been noted that the NGC 4839 group is located along the large scale filament connecting Coma and A1367 (e.g. West et al. 1995). It is commonly thought that clusters form preferentially through anisotropic accretion of sub-clusters along large scale filaments. Our finding supports this scenario. The merger activity in that direction, particularly interesting for our understanding of cluster formation, is further discussed in Briel et al. (2001) and Neumann et al. (2001).
Except for the very center as discussed above, the abundance is constant and the temperature radial profile is very weakly decreasing with radius. The slight drop beyond 10' is likely to be due to the cold S/W structure. The implications of this profile for the distribution of dark matter in the core will be studied in a forthcoming paper.
Acknowledgements
We would like to thank J. Ballet for support concerning the SAS software and J.-L. Sauvageot for providing the gain correction. We thank S. Schindler and the anonymous referee for useful comments, which improved the paper.