193 research outputs found
XMM-Newton observations of Abell 2255 : a test case of a merger after `core-crossing'
It has been known that Abell 2255 is not a relaxed cluster, but it is
undergoing a merger. Here, we report on the analysis of the XMM-Newton
observations of this cluster. The X-ray data give us the opportunity to reveal
the complexity of the cluster, especially its temperature distribution. The
integrated spectrum is well fitted by a single temperature thermal model,
indicating a mean temperature of ~7 keV. However, the cluster is not isothermal
at this temperature: its eastern regions are significantly cooler, at ~5.5 keV,
whilst towards the West the temperature reaches ~8.5 keV.
These temperature asymmetries can be explained if Abell 2255 has been
assembled recently by the merging of smaller subunits. It is now in the phase
after the cores of these subunits have collided (the `core-crossing' phase)
some 0.1-0.2 Gyr ago. A comparison with numerical simulations suggests that it
will settle down into a single relaxed cluster in ~(2-3) Gyr.Comment: 9 pages, 6 figures, accepted by MNRA
XMM-Newton observations of the binary cluster system Abell 399/401
Abell 399 and Abell 401 are both rich clusters of galaxies, at temperatures
7.2keV and 8.5keV respectively. They lie at a projected separation of ~3Mpc,
forming a close pair. We have observed the system with the XMM-Newton
satellite. The data of each cluster show significant departures from our
idealised picture of relaxed rich clusters. There is also evidence for enhanced
X-ray flux in the region between the two, where the temperature is higher than
our expectations.
Although tidal or compression effects might affect the large scale structure
of the two clusters, we show that these cannot account for the distortions seen
in the inner regions. We argue that the reasonably relaxed morphology of the
clusters, and the absence of major temperature anomalies, argues against models
in which the two have already experienced a close encounter. The properties of
the intermediate region suggests that they are at an early stage of merging,
and are currently interacting mildly, because their separation is still too
large for more dramatic effects. The substructure we find in their inner
regions seems to point to their individual merging histories. It seems likely
that in the Abell 399/401 system, we are witnessing two merger remnants, just
before they merge together to form a single rich cluster. This picture is
consistent with recent numerical simulations of cluster formation. (abridged)Comment: 20 pages, 14 figures, to be published in MNRA
The Chandra Deep Group Survey -- cool core evolution in groups and clusters of galaxies
We report the results of a study which assembles deep observations with the
ACIS-I instrument on the Chandra Observatory to study the evolution in the core
properties of a sample of galaxy groups and clusters out to redshifts . A search for extended objects within these fields yields a total of 62
systems for which redshifts are available, and we added a further 24
non-X-ray-selected clusters, to investigate the impact of selection effects and
improve our statistics at high redshift. Six different estimators of cool core
strength are applied to these data: the entropy (K) and cooling time
() within the cluster core, the cooling time as a fraction of the age
of the Universe (), and three estimators based on the
cuspiness of the X-ray surface brightness profile. A variety of statistical
tests are used to quantify evolutionary trends in these cool core indicators.
In agreement with some previous studies, we find that there is significant
evolution in , but little evolution in , suggesting
that gas is accumulating within the core, but that the cooling time deep in the
core is controlled by AGN feedback. We show that this result extends down to
the group regime and appears to be robust against a variety of selection biases
(detection bias, archival biases and biases due to the presence of central
X-ray AGN) which we consider.Comment: Accepted by MNRAS, 24 pages, 11 figure
X-ray bright groups and their galaxies
Combining X-ray data from the ROSAT PSPC and optical data drawn from the
literature, we examine in detail the relationship between the X-ray and optical
properties of X-ray bright galaxy groups. We find a relationship between
optical luminosity and X-ray temperature consistent with that expected from
self-similar scaling of galaxy systems, L_B \propto T^{1.6 +/- 0.2}. The
self-similar form and continuity of the L_B : T relation from clusters to
groups and the limited scatter seen in this relation, implies that the star
formation efficiency is rather similar in all these systems. We find that the
bright extended X-ray components associated with many central galaxies in
groups appear to be more closely related to the group than the galaxy itself,
and we suggest that these are group cooling flows rather than galaxy halos. In
addition we find that the optical light in these groups appears to be more
centrally concentrated than the light in clusters. We also use the optical and
X-ray data to investigate whether early or late type galaxies are primarily
responsible for preheating in groups. Using three different methods, we
conclude that spiral galaxies appear to play a comparable role to early types
in the preheating of the intragroup medium. This tends to favour models in
which the preheating arises primarily from galaxy winds rather than AGN, and
implies that spirals have played a significant role in the metal enrichment of
the intragroup medium.Comment: 17 pages, accepted for publication in MNRA
An old galaxy group: Chandra X-ray observations of the nearby fossil group NGC 6482
We present the first detailed X-ray observations, using Chandra, of NGC 6482
- the nearest known `fossil group'. The group is dominated by an optically
luminous giant elliptical galaxy and all other known group members are at least
two magnitudes fainter. The global X-ray properties (luminosity, temperature,
extent) of NGC 6482 fall within the range of other groups, but the detailed
properties show interesting differences. We derive the gas temperature and
total mass profiles for the central 30 kpc using ACIS spatially resolved
spectroscopy. The temperature profile shows a continuous decrease outward,
dropping to 0.63 of its central value at 0.1 r_200. The derived total mass
profile is strongly centrally peaked, suggesting an early formation epoch.
These results support a picture in which fossil groups are old, giving time for
the most massive galaxies to have merged to produce a central giant elliptical
galaxy.
Although the cooling time within 0.1 r_200 is less than a Hubble time, no
decrease in central temperature is detected. The entropy of the system lies
toward the low side of the distribution seen in poor groups, and it drops all
the way into the centre of the system, reaching very low values. No isentropic
core, such as those predicted in simple preheating models, is present. Given
the lack of any central temperature drop in the system, it seems unlikely that
radiative cooling can be invoked to explain this low central entropy. We find
that the centrally peaked temperature profile is consistent with a steady-state
cooling flow solution with an accretion rate of 2 solar mass per year, given
the large PdV work arising from the cuspy mass profile. However, solutions
involving distributed or non-steady heating cannot be ruled out.Comment: 11 pages, 12 postscript figures. Accepted for publication in MNRA
Evolution of the X-ray Profiles of Poor Clusters from the XMM-LSS Survey
A sample consisting of 27 X-ray selected galaxy clusters from the XMM-LSS
survey is used to study the evolution in the X-ray surface brightness profiles
of the hot intracluster plasma. These systems are mostly groups and poor
clusters, with temperatures 0.6-4.8 keV, spanning the redshift range 0.05 to
1.05. Comparing the profiles with a standard beta-model motivated by studies of
low redshift groups, we find 54% of our systems to possess a central excess,
which we identify with a cuspy cool core. Fitting beta-model profiles, allowing
for blurring by the XMM point spread function, we investigate trends with both
temperature and redshift in the outer slope (beta) of the X-ray surface
brightness, and in the incidence of cuspy cores. Fits to individual cluster
profiles and to profiles stacked in bands of redshift and temperature indicate
that the incidence of cuspy cores does not decline at high redshifts, as has
been reported in rich clusters. Rather such cores become more prominent with
increasing redshift. Beta shows a positive correlation with both redshift and
temperature. Given the beta-T trend seen in local systems, we assume that
temperature is the primary driver for this trend. Our results then demonstrate
that this correlation is still present at z~0.3, where most of our clusters
reside.Comment: Accepted for publication in MNRAS. 15 pages, 12 figure
First results of the XI Groups Project: Studying an unbiased sample of galaxy groups
X-ray observations of hot, intergalactic gas in galaxy groups provide a
useful means of characterizing the global properties of groups. However, X-ray
studies of large group samples have typically involved very shallow X-ray
exposures or have been based on rather heterogeneous samples. Here we present
the first results of the XI (XMM/IMACS) Groups Project, a study targeting, for
the first time, a redshift-selected, statistically unbiased sample of galaxy
groups using deep X-ray data. Combining this with radio observations of cold
gas and optical imaging and spectroscopy of the galaxy population, the project
aims to advance the understanding of how the properties and dynamics of group
galaxies relate to global group properties. Here, X-ray and optical data of the
first four galaxy groups observed as part of the project are presented. In two
of the groups we detect diffuse emission with a luminosity of L_X ~ 10^41
erg/s, among the lowest found for any X-ray detected group thus far, with a
comparable upper limit for the other two. Compared to typical X-ray selected
groups of similar velocity dispersion, these four systems are all surprisingly
X-ray faint. We discuss possible explanations for the lack of significant X-ray
emission in the groups, concluding that these systems are most likely
collapsing for the first time. Our results strongly suggest that, unlike our
current optically selected sample, previous X-ray selected group samples
represented a biased picture of the group population. This underlines the
necessity of a study of this kind, if one is to reach an unbiased census of the
properties of galaxy groups and the distribution of baryons in the Universe.Comment: 14 pages, 8 figures, accepted for publication in MNRA
- …