744 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
The X-ray Evolution of Merging Galaxies
We present here the first study of the X-ray properties of an evolutionary
sample of merging galaxies. Both ROSAT PSPC and HRI data are presented for a
sample of eight interacting galaxy systems, each believed to involve a similar
encounter between two spiral discs of approximately equal size. The mergers
span a large range in age, from completely detached to fully merged systems. A
great deal of interesting X-ray structure is seen, and the X-ray properties of
each individual system are discussed in detail. Along the merging sequence,
several trends are evident: in the case of several of the infrared bright
systems, the diffuse emission is very extended, and appears to arise from
material ejected from the galaxies. The onset of this process seems to occur
very soon after the galaxies first encounter one another, and these ejections
soon evolve into distorted flows. More massive extensions (perhaps involving up
to 1e10 solar masses of hot gas) are seen at the `ultraluminous' peak of the
interaction, as the galactic nuclei coalesce. The amplitude of the evolution of
the X-ray emission through a merger is markedly different from that of the
infrared and radio emission however, and this, we believe, may well be linked
with the large extensions of hot gas observed. The late, relaxed remnants,
appear relatively devoid of gas, and possess an X-ray halo very different from
that of typical ellipticals, a problem for the `merger hypothesis', whereby the
merger of two disc galaxies results in an elliptical galaxy. However, these
systems are still relatively young in terms of total merger lifetime, and they
may still have a few Gyr of evolution to go through, before they resemble
typical elliptical galaxies.Comment: 30 pages, 15 figures, accepted by MNRA
The intragroup medium in loose groups of galaxies
We have used the ROSAT PSPC to study the properties of a sample of 24 X-ray
bright galaxy groups, representing the largest sample examined in detail to
date. Hot plasma models are fitted to the spectral data to derive temperatures,
and modified King models are used to characterise the surface brightness
profiles. In agreement with previous work, we find evidence for the presence of
two components in the surface brightness profiles. The extended component is
generally found to be much flatter than that observed in galaxy clusters, and
there is evidence that the profiles follow a trend with system mass. We derive
relationships between X-ray luminosity, temperature and optical velocity
dispersion. The relation between X-ray luminosity and temperature is found to
be L_X \propto T^{4.9}, which is significantly steeper than the same relation
in galaxy clusters. These results are in good agreement with preheating models,
in which galaxy winds raise the internal energy of the gas, inhibiting its
collapse into the shallow potential wells of poor systems.Comment: 17 pages, 10 figures. Accepted for publication in MNRA
The Properties of the Hot Gas in Galaxy Groups and Clusters from 1-D Hydrodynamical Simulations -- I. Cosmological Infall Models
We report the results of 1-D hydrodynamical modelling of the evolution of gas
in galaxy clusters. We have incorporated many of the effects missing from
earlier 1-D treatments: improved modelling of the dark matter and galaxy
distributions, cosmologically realistic evolution of the cluster potential, and
the effects of a multiphase cooling flow. The model utilises a fairly standard
1-D Lagrangian hydrodynamical code to calculate the evolution of the
intracluster gas. This is coupled to a theoretical model for the growth of dark
matter density perturbations. The main advantages of this treatment over 3-D
codes are (1) improved spatial resolution within the cooling flow region, (2)
much faster execution time, allowing a fuller exploration of parameter space,
and (3) the inclusion of additional physics.
In the present paper, we explore the development of infall models -- in which
gas relaxes into a deepening potential well -- covering a wide range of cluster
mass scales. We find that such simple models reproduce many of the global
properties of observed clusters. Very strong cooling flows develop in these 1-D
cluster models. In practice, disruption by major mergers probably reduces the
cooling rate in most clusters. The models overpredict the gas fraction in low
mass systems, indicating the need for additional physical processes, such as
preheating or galaxy winds, which become important on small mass scales.Comment: 38 pages, 21 encapsulated postscript figures, accepted for
publication in MNRA
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
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