A&A 467, 459-463 (2007)
DOI: 10.1051/0004-6361:20065740
C. Adami1 - D. Russeil1 - F. Durret2,3
1 - LAM, Traverse du Siphon, 13012 Marseille, 2 place Le Verrier, 13248 Marseille,
France
2 -
Institut d'Astrophysique de Paris, CNRS, UMR 7095, Université Pierre et
Marie Curie, 98bis Bd Arago, 75014 Paris, France
3 -
Observatoire de Paris, LERMA, 61 Av. de l'Observatoire, 75014 Paris, France
Received 1 June 2006 / Accepted 13 March 2007
Abstract
Context. Fossil groups are galaxy structures that probably underwent a nearly complete fusion of all intermediate magnitude galaxies into a single large central dominant galaxy. However, the formation and evolution processes of these structures are still not well understood.
Aims. In order to test this scenario and its implications we studied the fossil group RX J1119.7+2126, based on available spectroscopy of the galaxies in the low-density, large-scale region around the fossil group and deep B and R band imaging of its close vicinity and three comparison fields.
Methods. We used spectroscopic data to investigate the degree of isolation of RX J1119.7+2126 in terms of bright neighbor galaxies. The imaging data were used to derive the color-magnitude relation and select faint galaxies statistically belonging to this structure.
Results. The structure appears as a very isolated group exhibiting a red sequence in the color magnitude relation with characteristics close to the red sequences already observed for other fossil groups.
Conclusions. All these results can be interpreted consistently in the framework of the building-up process generally proposed for fossil groups.
Key words: galaxies: clusters: individual: RX J1119.7+2126
Hierarchical building of structures is a key ingredient for cosmological models, since galaxy structures such as groups and clusters are expected to be located at the intersections of cosmic filaments (e.g. Lanzoni et al. 2005, and references therein). These structures are then continuously fed by infalling field matter (galaxies and gas). However, a peculiar class of structures seems not to follow this general behaviour: fossil groups (e.g. Ulmer et al. 2005; Mendes de Oliveira et al. 2006, and references therein).
These structures are considered as the ultimate stage of group
evolution: the nearly complete fusion of all the bright and
intermediate magnitude galaxies of the group into a single bright
galaxy. This resulting galaxy is brighter than the second remaining
group galaxy (within half the projected virial radius) by at least 2
magnitudes (in the R band). However, the extended X-ray gas envelope
is still present
and more luminous than 10
42 h50-2 erg s-1 (Jones et
al. 2003). Recent numerical simulations by D'Onghia et al. (2005) have
indeed shown that fossil groups have already assembled 50% of their
final dark matter mass at
and that they subsequently grow
by minor merging. D'Onghia et al. (2005) have also shown that they are
expected to be overluminous in X-rays relatively to non-fossil
groups. It was also suggested in this paper that fossil groups exist
only because infall of
galaxies happens along filaments
with small impact parameters.
We can also explain this lack of bright galaxies if fossil groups are isolated from any surrounding large scale cosmic structure during a time typically longer than the galaxy crossing time in the considered group (e.g. Sarazin 1986). In order to test this hypothesis, we carried out a detailed study of the fossil group RX J1119.7+2126 (RX J1119 hereafter).
RX J1119 is one of the less massive known fossil groups (Jones et al.
2003). It is located at
,
(J2000.0) and at a redshift of 0.061. This
low redshift value enables it to be studied with moderately sized
telescopes and its coordinates are outside any major recent survey
such as the SDSS or 2dF. It is located on the same line of sight
but significantly beyond, the well-known filaments embedding the Coma
cluster. Its field of view is therefore heavily polluted by
foreground galaxies.
In Sect. 2, we present our data and the characteristics deduced for
this structure in terms of bright neighboring galaxies. Section 3
concerns the study of the faint galaxy population of RX J1119. Section 4 is the conclusion. In this paper we assume H0 = 75 km s-1 Mpc-1,
and
.
All
magnitudes are given in the Vega system.
The first goal of this paper is to investigate the degree of isolation of RX J1119 and the surrounding Large Scale Structure (LSS hereafter). The degree of isolation can be understood in terms of bright galaxies. In the model of a nearly complete fusion of the intermediate magnitude galaxies into a single bright galaxy, this galaxy is expected to be the dominant one of the neighboring space. The minimal size of the space to be considered is imposed by the fact that RX J1119 has to be isolated and therefore probably located in a void. The average size of the known voids is close to 40 Mpc in diameter (Hoyle & Vogeley 2004). We therefore tried to detect if there are brighter galaxies than the central RX J1119 galaxy in similar areas around the fossil group. In other words, is the RX J1119 central galaxy the brightest of its bubble (if located in such a bubble)?
In order to estimate (in a deg2 area or
Mpc2, see below) the expected number
of galaxies at
brighter
than the RX J1119 central galaxy (
in our cosmology:
Jones et al. 2003) and twice as bright as the RX J1119 central galaxy
(
), we used the field luminosity functions of Ilbert et al. (2005) and the cluster luminosity functions of Popesso et al. (2005). On the one hand, without any rich galaxy structures in the
bubble, the field luminosity functions predict about
50 galaxies per magnitude bin brighter than
in our
sampled volume and less than 5 galaxies per magnitude bin brighter
than
,
respectively. On the other hand, if one or several rich
clusters are present in the area, these numbers increase significantly:
several dozen galaxies brighter than
would be expected
per magnitude bin.
In order to compare observations with these predictions, we compiled (from NED) all
the redshifts known within a
deg2 area (
Mpc2) whatever their magnitude. We limited the searched redshift
range to [0.051, 0.071]. This range corresponds to a physical size of
90 Mpc and
3 times the galaxy velocity dispersion of a
massive cluster. In this range, there are 20 galaxies in a
deg2 area (the area in which RX J1119 seems isolated,
i.e.
Mpc2) and 143 in the
deg2area. The closest known cluster is Abell 1145, only included in the
outer region. We show these positions in Fig. 1 along
with the two searched areas. Now, restricting this list to galaxies
brighter than the central RX J1119 galaxy (magnitudes taken from the
NED database), in the
deg2 area, there is only one
galaxy (as compared to 50 from the expected numbers even without any
rich galaxy structure in the field). This galaxy (also brighter than
)
is, moreover, quite distant from the fossil
group (about 12 Mpc or slightly more than 3 deg). The redshift catalog
is however not complete down to
and we cannot
exclude that a significant number of galaxies with unknown redshift
maybe within the considered redshift range.
We then measured additional redshifts for 20 of the 23 galaxies
extracted from NED (without the redshift information) in the largest
area down to
(plus one fainter galaxy that we
caught at the same time as a brighter target). This produced a
spectroscopic catalog
100
complete down to this magnitude.
These spectroscopic observations were made at the Observatoire de
Haute Provence with the 193 cm telescope and the Carelec
spectrograph. We used the 133 Å/mm configuration (resolution R=900)
and the useful spectral range was 3700-6700 Å. This instrument is a
single-slit device well adapted to such a survey: the target density
is 0.16 per deg2 making high multiplex multi-object spectrographs
inefficient. Results are given in Table 1.
None of these additional galaxies is in the [0.051, 0.071] redshift
range: most are located around z=0.023, and 4 are around
z=0.035. Therefore, there is only one galaxy with
galaxies at the RX J1119 redshift and in the
deg2area (assuming that the three galaxies with unknown redshift are not at
z=0.061). This is clearly lower than the several dozen of galaxies
expected if rich galaxy structures were present. This is even lower
than the expected galaxy number assuming a "normal'' field galaxy
population.
RX J1119 therefore seems to be very isolated in terms of bright galaxies
and it appears to be the dominant structure of its
bubble. This bubble would have a typical diameter of 28 Mpc, close to
the known void dimensions (Hoyle & Vogeley 2004). However, this
remains to be confirmed for faint galaxies with a redshift catalog complete
down to fainter magnitudes than the one we have. For example, only less than
10
of all
galaxies known in the
deg2 area (from NED) have a measured
redshift.
![]() |
Figure 1:
Map of the
![]() ![]() ![]() |
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Table 1: Column 1: NED galaxy name; Col. 2: measured redshift; Col. 3: nature of the spectrum: emission lines (Em), absorption lines (Abs). The typical redshift uncertainty for these galaxies is 0.001.
We investigate in this section what happens at smaller scales,
as the second goal of this research is to study the faint galaxy
population in RX J1119 itself. Little is known of the galaxy
populations in fossil groups beyond the spectroscopic limit and the
second brightest galaxy for most of these structures (see however
Mendes de Oliveira et al. 2006). Rather than measuring redshifts of
galaxies of this population, we chose to obtain deep B and R band
imaging data in order to draw a color magnitude relation (CMR
hereafter). This was done at the OHP 120 cm telescope. A CCD camera
with a
arcmin2 field (
pixels with a pixel size of
arcsec2) is mounted on
this telescope. We used Johnson B and R filters and we observed RX J1119
itself as well as three comparison fields (see
Table 2). The comparison fields (C1, C2 and C3) were
chosen in order to have a Galactic extinction similar to that of RX J1119 (from
the Schlegel et al. 1998 maps) and not to include nearby known galaxy
structures. Data were acquired under photometric conditions and
seeing of the order of 2.5 arcsec. The final useful area for
RX J1119 with both B and R data available was
arcmin2. The three comparison fields cover 336 arcmin2in total and are distributed over a region of
deg2 on
the sky. The images
were calibrated using Landolt (1992) standard stars. We extracted
catalogs of objects from these images using the SExtractor package
(Bertin & Arnouts 1996). An R image of the RX J1119 field is
displayed in Fig. 2 and the magnitude histograms are
given in Fig. 3.
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Figure 2:
R band
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Table 2: Column 1: observed field; Col. 2: coordinates (2000); Col. 3: R exposure time; Col. 4: B exposure time; Col. 5: E(B-V) following Schlegel et al. (1998).
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Figure 3: Magnitude histograms of galaxies along the RX J1119 line of sight. Top: B band, bottom: R band. |
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It is now important to perform a star-galaxy separation in order to avoid star pollution and to take into account the fact that the four observed fields do not have the same Galactic latitude. Seeing conditions are not good enough to use the classical SExtractor star-galaxy separation flag. Instead, we used a total magnitude vs. central surface brightness plot (e.g. Adami et al. 2006) based on the deepest band data (the R band). Results are illustrated in Fig. 4 for RX J1119, where we clearly see the star sequence. We were therefore able to make an efficient star-galaxy separation down to R=20 for the four observed fields. For fainter magnitudes, the surface-brightness profile of all objects is dominated by seeing effects and any star-galaxy separation would be very uncertain. However, the number of stars fainter than R=20 is small (Gazelle et al. 1995) and we decided to assume that all objects fainter than this limit are galaxies.
In order to test the separation down to R=20 for RX J1119, we retrieved in NED the 8 galaxies in the imaging field with a known redshift. We see that they are all classified as galaxies by our method in Fig. 4, including a relatively compact object.
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Figure 4: R central surface brightness versus total Rmagnitude for the RX J1119 field of view. Objects classified as stars are circled dots. We can see the saturation of the brightest stars. Crossed dots are objects classified as galaxies using spectroscopy. |
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We compute here the galaxy distribution in the R/B-R plane for
the RX J1119 structure. In order to achieve this goal, we have to
subtract the fore and background contributions along the line of
sight to RX J1119. We used a technique similar to that described in
Adami et al. (2007). Briefly, the fore and background RX J1119
line of sight galaxy populations were estimated using the three
comparison fields. For a given R and B-R, the number of galaxies per
deg2 in the RX J1119 structure (
)
was computed with the
number of galaxies per deg2 along the RX J1119 line of sight
(
)
and the numbers of galaxies per deg2 along the
C1, C2 and C2 lines of sight (
)
as:
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Figure 5:
Total R magnitude versus B-R color for galaxies
statistically inside the RX J1119 structure. Galaxies with a known
redshift are circles (red: inside the structure, green:
outside). Using an adaptative kernel technique, we overplotted the
density contours in the R/B-R space. The green line has been
normalized to fit the RX J1552 and RX J1416 galaxies in the B-R/R space
and to have the Coma cluster slope. Blue dots are galaxies with
spectroscopy from the RX J1552 and RX J1416 fossil groups (Mendes de Oliveira et al. 2006, Cypriano et al. 2006) with their magnitudes and
colors translated into our system. The blue-delimited area corresponds
to our modeling of the positive loci of the density contours. We
limit the graph at ![]() |
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Finally, we divided the
map by the
map to
generate a significance map.
The resulting CMR is shown in Fig. 5. We only plotted
contours of the areas where
was greater than
,
i.e., the places where galaxies are statistically present
inside the RX J1119 structure.
First, this figure shows that RX J1119 is indeed a fossil group
(besides the fact that its X-ray luminosity estimated by Jones et al. (2003) falls within the allowed range for such groups): the magnitude
difference between the brightest and second brightest galaxies in the
structure is greater than two magnitudes. The magnitude difference
between the brightest galaxy and the bright galaxy peak (at and
)
in Fig. 4 is also greater than two
magnitudes. The only galaxy with a magnitude difference less than two
is at a redshift of
0.0825, and does not belong to the
structure.
Second, the presence of a Red Sequence (RS hereafter) in the CMR is a
well-known characteristic of massive galaxy systems (see e.g. Godwin &
Peach 1977 or Mazure et al. 1988). The usual negative slope of this RS is a
metallicity effect (Kodama & Arimoto 1997) originating from the
higher ability of massive early type galaxies (as opposed to low mass
objects) to keep metals against dissipative processes as supernovae,
and then to form redder stars. However, only little is known on a
possible RS in fossil groups and especially in RX J1119.
Figure 5 shows concentrations of galaxies that are similar
to a RS. The green-shaded areas are ones where the RX J1119 structure counts
are significant between the 2
and 3
levels. The red-shaded
areas are significant above the 3
level.
In order to compare with other fossil groups, we superposed on this figure the CMR of RX J1552.2+2013 (z=0.136, RX J1552 hereafter) and RX J1416.4+2315 (z=0.137, RX J1416 hereafter, Mendes de Oliveira et al. 2006 and Cypriano et al. 2006). We used Fukugita et al. (1995) to take into account the redshift and filter differences and translated their AB magnitudes into the Vega system. The green line shown in Fig. 5 has been chosen to fit the RX J1552 and RX J1416 galaxies in the B-R/R space and to have a slope of -0.045 (see Adami et al. 2007). The RX J1119 overall RS is in good agreement with those of RX J1552 and RX J1416. A remarkable characteristic of RX J1119 compared to RX J1552 and RX J1416 is that the population of galaxies is very poor. Translated to z=0.061, the RX J1416 and RX J1552 fields show a significant population of galaxies between R=16 and R=19while the first significant RX J1119 galaxy concentrations occur for Rfainter than 18.5. This is probably related to the fact that RX J1416 and RX J1552 are much more massive than RX J1119 (from the X-ray luminosities given by Jones et al. 2003) and probably underwent a larger field galaxy infall, which compensated more efficiently the galaxy structure depopulation by merging or disruptions. We also note that the brightest galaxy of RX J1119 is bluer than the mean RS, as already observed in RX J1552 and RX J1416 (Mendes de Oliveira et al. 2006; Cypriano et al. 2006).
We can now use the RS in the RX J1119 CMR to select galaxies possibly in RX J1119 and to compute the galaxy density map of the structure. This is done by selecting galaxies inside the blue-delimited region (arbitrarily defined to encompass the positive areas) in Fig. 5. This does not totally prevent us from selecting background galaxies with the same color/magnitude combination, but the contrast between structure and field galaxies is increased roughly threefold. We computed such a galaxy density map in Fig. 6. This figure shows that there is a galaxy concentration close to the X-ray position of RX J1119 (but not centered on it) and with an extension larger than the X-ray halo (see Jones et al. 2003). In order to encourage a possible spectroscopic survey of these faint galaxies, we list them in Table 3.
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Figure 6: Adaptative kernel galaxy density map (in pixels) for galaxies inside the blue-delimited area in Fig. 5. The X-ray position of RX J1119 is shown by the filled circle. |
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This concentration is isolated from several other concentrations of galaxies
sharing the same colors. Assuming that these galaxies are part of the RX J1119
structure, it therefore seems that the inner core of RX J1119
is also isolated in terms of faint galaxies. The closer external galaxy
concentrations are located at a mean distance of 150 kpc from
the central structure.
Using a bidimensional Kolmogorov-Smirnov test, we show that the
galaxies inside and outside the central galaxy concentration of
Fig. 6 (and inside the blue-delimited area of
Fig. 5) do not have a different distribution inside the
R/B-R space (at the 99
level). Therefore they probably followed
similar evolutionary paths. The galaxies very close to the RX J1119
center and at the edges of the structure can therefore be considered
as similar.
Let us summarize our main findings:
Table 3: Coordinates, R band magnitude and B-R colors of objects included in the central concentration around RX J1119. They were previously uncataloged in NED and there is no redshift available for these objects.
Acknowledgements
The authors thank the referee for very useful and constructive comments. The authors gratefully acknowledge the contributions of the OHP team and the students of the 2005/2006 class of the Aix-Marseille I Rayonnement, Plasmas et Astrophysique M2.