334 research outputs found
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
The Birmingham-CfA cluster scaling project - II. Mass composition and distribution
We investigate the spatial distribution of the baryonic and non-baryonic mass
components in a sample of 66 virialized systems. We have used X-ray
measurements to determine the deprojected temperature and density structure of
the intergalactic medium and have employed these to map the underlying
gravitational potential. In addition, we have measured the deprojected spatial
distribution of galaxy luminosity for a subset of this sample, spanning over 2
decades in mass. With this combined X-ray/optical study we examine the scaling
properties of the baryons and address the issue of mass-to-light (M/L) ratio in
groups and clusters of galaxies. We measure a median mass-to-light ratio of 224
h70 M/L (solar) in the rest frame B_j band, in good agreement with other
measurements based on X-ray determined masses. There is no trend in M/L with
X-ray temperature and no significant trend for mass to increase faster than
luminosity: M \propto \L_{B,j}^{1.08 +/- 0.12}. This implied lack of
significant variation in star formation efficiency suggests that gas cooling
cannot be greatly enhanced in groups, unless it drops out to form baryonic dark
matter. Correspondingly, our results indicate that non-gravitational heating
must have played a significant role in establishing the observed departure from
self-similarity in low mass systems. The median baryon fraction for our sample
is 0.162 h70^{-3/2}, which allows us to place an upper limit on the
cosmological matter density, Omega_m <= 0.27 h70^{-1}, in good agreement with
the latest results from WMAP. We find evidence of a systematic trend towards
higher central density concentration in the coolest haloes, indicative of an
early formation epoch and consistent with hierarchical formation models.Comment: 14 pages, 11 figures; published in MNRAS. Corrected mistake in
photometric conversion (equation 2): Bj luminosities increased for A2218,
N2563 & N5846. Conclusions unchange
The X-ray Evolution of Merging Galaxies
From a Chandra survey of nine interacting galaxy systems the evolution of
X-ray emission during the merger process has been investigated. From comparing
Lx/Lk and Lfir/Lb it is found that the X-ray luminosity peaks around 300 Myr
before nuclear coalescence, even though we know that rapid and increasing star
formation is still taking place at this time. It is likely that this drop in
X-ray luminosity is a consequence of outflows breaking out of the galactic
discs of these systems. At a time around 1 Gyr after coalescence, the
merger-remnants in our sample are X-ray dim when compared to typical X-ray
luminosities of mature elliptical galaxies. However, we do see evidence that
these systems will start to resemble typical elliptical galaxies at a greater
dynamical age, given the properties of the 3 Gyr system within our sample,
indicating that halo regeneration will take place within low Lx
merger-remnants.Comment: 4 pages, 1 figure, to appear in the Proceedings of the IAU Symposium
No. 23
The mass and dynamical state of Abell 2218
Abell 2218 is one of a handful of clusters in which X-ray and lensing
analyses of the cluster mass are in strong disagreement. It is also a system
for which X-ray data and radio measurements of the Sunyaev-Zel'dovich decrement
have been combined in an attempt to constrain the Hubble constant. However, in
the absence of reliable information on the temperature structure of the
intracluster gas, most analyses have been carried out under the assumption of
isothermality. We combine X-ray data from the ROSAT PSPC and the ASCA GIS
instruments, enabling us to fit non-isothermal models, and investigate the
impact that this has on the X-ray derived mass and the predicted
Sunyaev-Zel'dovich effect.
We find that a strongly non-isothermal model for the intracluster gas, which
implies a central cusp in the cluster mass distribution, is consistent with the
available X-ray data and compatible with the lensing results. At r<1 arcmin,
there is strong evidence to suggest that the cluster departs from a simple
relaxed model. We analyse the dynamics of the galaxies and find that the
central galaxy velocity dispersion is too high to allow a physical solution for
the galaxy orbits. The quality of the radio and X-ray data do not at present
allow very restrictive constraints to be placed on H_0. It is apparent that
earlier analyses have under-estimated the uncertainties involved. However,
values greater than 50 km/s/Mpc are preferred when lensing constraints are
taken into account.Comment: 16 pages, 9 postscript figures, accepted for publication in MNRA
The X-ray properties of the merging galaxy pair NGC 4038/9 - the Antennae
We report the results of an X-ray spectral imaging observation of the
Antennae with the ROSAT PSPC. 55% of the soft X-ray flux from the system is
resolved into discrete sources, including components identified with the
galactic nuclei and large HII regions, whilst the remainder appears to be
predominantly genuinely diffuse emission from gas at a temperature ~4x10^6 K.
The morphology of the emission is unusual, combining a halo which envelopes the
galactic discs, with what appears to be a distorted, but well-collimated
bipolar outflow. We derive physical parameters for the hot gas in both diffuse
components, which are of some interest, given that the Antennae probably
represents an elliptical galaxy in the making.Comment: 15 pages plus 9 figures, uuencoded encapsulated postscript file.
Accepted for publication in MNRA
The Birmingham-CfA cluster scaling project - III: entropy and similarity in galaxy systems
We examine profiles and scaling properties of the entropy of the
intergalactic gas in a sample of 66 virialized systems, ranging in mass from
single elliptical galaxies to rich clusters, for which we have resolved X-ray
temperature profiles. Some of the properties we derive appear to be
inconsistent with any of the models put forward to explain the breaking of
self-similarity in the baryon content of clusters. In particular, the entropy
profiles, scaled to the virial radius, are broadly similar in form across the
sample, apart from a normalization factor which differs from the simple
self-similar scaling with temperature. Low mass systems do not show the large
isentropic cores predicted by preheating models, and the high entropy excesses
reported at large radii in groups by Finoguenov et al (2002) are confirmed, and
found to extend even to moderately rich clusters. We discuss the implications
of these results for the evolutionary history of the hot gas in clusters, and
suggest that preheating may affect the entropy of intracluster gas primarily by
reducing the density of material accreting into groups and clusters along
cosmic filaments.Comment: 13 pages, 8 figures - accepted for publication in MNRA
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