20 research outputs found
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
Chandra Observations of low velocity dispersion groups
Deviations of galaxy groups from cluster scaling relations can be understood
in terms of an excess of entropy in groups. The main effect of this excess is
to reduce the density and thus luminosity of the intragroup gas. Given this,
groups should also should show a steep relationship between X-ray luminosity
and velocity dispersion. However, previous work suggests that this is not the
case with many measuring slopes flatter than the cluster relation.
Examining the group L_X:\sigma relation shows that much of the flattening is
caused by a small subset of groups which show very high X-ray luminosities for
their velocity dispersions (or vice versa).
Detailed Chandra study of two such groups shows that earlier ROSAT results
were subject to significant (~30-40%) point source contamination, but confirm
that a significant hot IGM is present in these groups, although these are two
of the coolest systems in which intergalactic X-ray emission has been detected.
Their X-ray properties are shown to be broadly consistent with those of other
galaxy groups, although the gas entropy in NGC 1587 is unusually low, and its
X-ray luminosity correspondingly high for its temperature, compared to most
groups.
This leads us to suggest that the velocity dispersion in these systems has
been reduced in some way, and we consider how this might have come about.Comment: Accepted for publication in Ap
A New Window of Exploration in the Mass Spectrum: Strong Lensing by Galaxy Groups in the SL2S
The existence of strong lensing systems with Einstein radii (Re) covering the
full mass spectrum, from ~1-2" (produced by galaxy scale dark matter haloes) to
>10" (produced by galaxy cluster scale haloes) have long been predicted. Many
lenses with Re around 1-2" and above 10" have been reported but very few in
between. In this article, we present a sample of 13 strong lensing systems with
Re in the range 3"- 8", i.e. systems produced by galaxy group scale dark matter
haloes, spanning a redshift range from 0.3 to 0.8. This opens a new window of
exploration in the mass spectrum, around 10^{13}- 10^{14} M_{sun}, which is a
crucial range for understanding the transition between galaxies and galaxy
clusters. Our analysis is based on multi-colour CFHTLS images complemented with
HST imaging and ground based spectroscopy. Large scale properties are derived
from both the light distribution of the elliptical galaxies group members and
weak lensing of the faint background galaxy population. On small scales, the
strong lensing analysis yields Einstein radii between 2.5" and 8". On larger
scales, the strong lenses coincide with the peak of the light distribution,
suggesting that mass is traced by light. Most of the luminosity maps have
complicated shapes, indicating that these intermediate mass structures are
dynamically young. Fitting the reduced shear with a Singular Isothermal Sphere,
we find sigma ~ 500 km/s and an upper limit of ~900 km/s for the whole sample.
The mass to light ratio for the sample is found to be M/L_i ~ 250 (solar units,
corrected for evolution), with an upper limit of 500. This can be compared to
mass to light ratios of small groups (with sigma ~ 300 km/s and galaxy clusters
with sigma > 1000 km/s, thus bridging the gap between these mass scales.Comment: A&A Accepted. Draft with Appendix images can be found at
http://www.dark-cosmology.dk/~marceau/groups_sl2s.pd
Groups of Galaxies in AEGIS: The 200 ksec Chandra Extended X-ray Source catalogue
We present the discovery of seven X-ray emitting groups of galaxies selected
as extended X-ray sources in the 200 ksec Chandra coverage of the
All-wavelength Extended Groth Strip International Survey (AEGIS). In addition,
we report on AGN activity associated to these systems. Using the DEEP2 Galaxy
Redshift Survey coverage, we identify optical counterparts and determine
velocity dispersions. In particular, we find three massive high-redshift groups
at z>0.7, one of which is at z=1.13, the first X-ray detections of
spectroscopically selected DEEP2 groups. We also present a first look at the
the L_X-T, L_X-sigma, and sigma-T scaling relations for high-redshift massive
groups. We find that the properties of these X-ray selected systems agree well
with the scaling relations of similar systems at low redshift, although there
are X-ray undetected groups in the DEEP2 catalogue with similar velocity
dispersions. The other three X-ray groups with identified redshifts are
associated with lower mass groups at z~0.07 and together form part of a large
structure or "supergroup" in the southern portion of the AEGIS field. All of
the low-redshift systems are centred on massive elliptical galaxies, and all of
the high-redshift groups have likely central galaxies or galaxy pairs. All of
the central group galaxies host X-ray point sources, radio sources, and/or show
optical AGN emission. Particularly interesting examples of central AGN activity
include a bent-double radio source plus X-ray point source at the center of a
group at z=0.74, extended radio and double X-ray point sources associated to
the central galaxy in the lowest-redshift group at z=0.066, and a bright green
valley galaxy (part of a pair) in the z=1.13 group which shows optical AGN
emission lines.Comment: accepted to MNRAS, 15 pages, 11 figures, for version with full
resolution figures see http://www.ucolick.org/~tesla/aegis_groups.ps.g
Hot Gas in Galaxy Groups: Recent Observations
Galaxy groups are the least massive systems where the bulk of baryons begin
to be accounted for. Not simply the scaled-down versions of rich clusters
following self-similar relations, galaxy groups are ideal systems to study
baryon physics, which is important for both cluster cosmology and galaxy
formation. We review the recent observational results on the hot gas in galaxy
groups. The first part of the paper is on the scaling relations, including
X-ray luminosity, entropy, gas fraction, baryon fraction and metal abundance.
Compared to clusters, groups have a lower fraction of hot gas around the center
(e.g., r < r_2500), but may have a comparable gas fraction at large radii
(e.g., r_2500 < r < r_500). Better constraints on the group gas and baryon
fractions require sample studies with different selection functions and deep
observations at r > r_500 regions. The hot gas in groups is also iron poor at
large radii (0.3 r_500 - 0.7 r_500). The iron content of the hot gas within the
central regions (r < 0.3 r_500) correlates with the group mass, in contrast to
the trend of the stellar mass fraction. It remains to be seen where the missing
iron in low-mass groups is. In the second part, we discuss several aspects of
X-ray cool cores in galaxy groups, including their difference from cluster cool
cores, radio AGN heating in groups and the cold gas in group cool cores.
Because of the vulnerability of the group cool cores to radio AGN heating and
the weak heat conduction in groups, group cool cores are important systems to
test the AGN feedback models and the multiphase cool core models. At the end of
the paper, some outstanding questions are listed.Comment: 31 pages, 9 figures, to appear in the focus issue on "Galaxy
Clusters", New Journal of Physics,
http://iopscience.iop.org/1367-2630/focus/Focus%20on%20Galaxy%20Cluster
The evolution of galaxy groups and of galaxies therein
Properties of groups of galaxies depend sensitively on the algorithm for
group selection, and even the most recent catalogs of groups built from
redshift-space selection should suffer from projections and infalling galaxies.
The cosmo-dynamical evolution of groups from initial Hubble expansion to
collapse and virialization leads to a fundamental track (FT) in
virial-theorem-M/L vs crossing time. The increased rates of mergers, both
direct and after dynamical friction, in groups relative to clusters, explain
the higher fraction of elliptical galaxies at given local number density in
X-ray selected groups, relative to clusters, even when the hierarchical
evolution of groups is considered. Galaxies falling into groups and clusters
should later travel outwards to typically 2 virial radii, which is somewhat
less than the outermost radius where observed galaxy star formation
efficiencies are enhanced relative to field galaxies of same morphological
type. An ongoing analysis of the internal kinematics of X-ray selected groups
suggests that the radial profiles of line of sight velocity dispersion are
consistent with isotropic NFW distributions for the total mass density, with
higher (lower) concentrations than LambdaCDM predictions in groups of high
(low) mass. The critical mass, at M200 ~ 10^13 M_sun is consistent with
possible breaks in the X-ray luminosity-temperature and Fundamental Plane
relations. The internal kinematics of groups indicate that the M-T relation of
groups should agree with that extrapolated from clusters with no break at the
group scale. The analyses of observed velocity dispersion profiles and of the
FT both suggest that low velocity dispersion groups (compact and loose, X-ray
emitting or undetected) are quite contaminated by chance projections.Comment: Invited review, ESO workshop "Groups of Galaxies in the Nearby
Universe", held in Santiago, Chile, 5-9 December 2005, ed. I. Saviane, V.
Ivanov & J. Borissova, 16 page
The Brera Multi-scale Wavelet HRI Cluster Survey: I Selection of the Sample and Number Counts
We describe the construction of the Brera Multi-scale Wavelet (BMW) HRI
Cluster Survey, a deep sample of serendipitous X-ray selected clusters of
galaxies based on the ROSAT HRI archive. This is the first cluster catalog
exploiting the high angular resolution of this instrument. Cluster candidates
are selected on the basis of their X-ray extension only, a parameter which is
well measured by the BMW wavelet detection algorithm. The survey includes 154
candidates over a total solid angle of ~160 deg2 at 10^{-12}erg s^{-1} cm^{-2}
and ~80 deg^2 at 1.8*10^{-13} erg s^{-1}$ cm^{-2}. At the same time, a fairly
good sky coverage in the faintest flux bins (3-5*10^{-14}erg s^{-1} cm^{-2})
gives this survey the capability to detect a few clusters with z\sim 1-1.2,
depending on evolution. We present the results of extensive Monte Carlo
simulations, providing a complete statistical characterization of the survey
selection function and contamination level. We also present a new estimate of
the surface density of clusters of galaxies down to a flux of 3*10^{-14}erg
s^{-1} cm^{-2}, which is consistent with previous measurements from PSPC-based
samples. Several clusters with redshifts up to z=0.92 have already been
confirmed, either by cross-correlation with existing PSPC surveys or from early
results of an ongoing follow-up campaign. Overall, these results indicate that
the excellent HRI PSF (5 arcsec ~FWHM on axis) more than compensates for the
negative effect of the higher instrumental background on the detection of
high-redshift clusters. In addition, it allows us to detect compact clusters
that could be lost at lower resolution, thus potentially providing an important
new insight into cluster evolution.Comment: 18 pages, 16 figures, accepted by A&A. Final version with the editor
correction
X-ray spectra of sources in the 13H XMM-Newton / Chandra deep field
We present the X-ray spectra of 86 optically-identified sources in the 13H
XMM-Newton/Chandra deep field which have >70 X-ray counts. The sample consists
of 50 broad line AGN, 25 narrow emission line galaxies, 6 absorption line
galaxies, and 5 Galactic stars. The majority (42/50) of the broad line AGN have
X-ray spectra which are consistent with a power law shape. They have a mean
photon index of Gamma = 2.0 +- 0.1 and an intrinsic dispersion sigma = 0.4 +-
0.1. Five BLAGN show a deficit of soft X-rays, indicating absorption.
Significant absorption is more common in the narrow emission line galaxies
(13/25) and absorption line galaxies (2/6) than in the broad line AGN (5/50),
but is not universal in any of these classes of object. The majority of the 20
absorbed sources have X-ray spectra consistent with a simple cold photoelectric
absorber, but 6/20 require more complex models with either an additional
component of soft X-ray emitting plasma, or an ionised absorber. Of the 16
galaxies which do not show evidence for X-ray absorption, only 2 objects are
likely to be powered by star formation, and both have 2-10 keV X-ray
luminosities of <= 10^40 cgs. The X-ray emission in the other 14 unabsorbed
NELGs and galaxies is most likely powered by AGN, which are not detected in the
optical because they are outshone by their luminous host galaxies. The Galactic
stars show multi-temperature thermal spectra which peak between 0.5 and 1 keV.
Star/AGN discrimination is possible for 4 of the 5 stars solely from their
X-ray spectra.Comment: Accepted for publication in MNRA
Hot gas flows on global and nuclear galactic scales
Since its discovery as an X-ray source with the Einstein Observatory, the hot
X-ray emitting interstellar medium of early-type galaxies has been studied
intensively, with observations of improving quality, and with extensive
modeling by means of numerical simulations. The main features of the hot gas
evolution are outlined here, focussing on the mass and energy input rates, the
relationship between the hot gas flow and the main properties characterizing
its host galaxy, the flow behavior on the nuclear and global galactic scales,
and the sensitivity of the flow to the shape of the stellar mass distribution
and the mean rotation velocity of the stars.Comment: 22 pages. Abbreviated version of chapter 2 of the book "Hot
Interstellar Matter in Elliptical Galaxies", Springer 201
The effect of non--gravitational gas heating in groups and clusters of galaxies
We present a set of gas-dynamical simulations of galaxy groups and clusters
aimed at exploring the effect of non-gravitational heating. We use GASOLINE, a
parallel Tree+SPH code, to simulate the formation of four cosmic halos with
temperature 0.5<T<8 keV. Non-gravitational heating is implemented in two
different ways: (1) by imposing a minimum entropy floor at a given redshift,
1<z<5; (2) by gradually heating gas, proportionally to the SN rate expected
from semi-analytical modeling of galaxy formation. Our main results are the
following. (a) An extra heating energy of about 1 keV per gas particle is
required to reproduce the observed Lx-T relation, independent of whether it is
provided so as to create an entropy floor of 50-100 keV cm^2, or is modulated
in redshift; our SN feedback recipe provides only 1/3 keV/part. (b) The M-T
relation is almost unaffected by non-gravitational heating and follows the M
T^{3/2} scaling, with a normalization ~40% higher than observed, independent of
the heating scheme. The inclusion of cooling in a run of a small group has the
effects of increasing T_ew by ~30%, possibly reconciling simulated and observed
M-T relations, and of decreasing Lx by ~40%. In spite of the inclusion of SN
feedback energy, almost 40% of the gas becomes cold, in excess of current
observational estimates. (abridged)Comment: 18 pages, 15 figures, to appear in MNRAS. Version with high
resolution images available at
http://www.daut.univ.trieste.it/borgani/LT/lt_1.ps.g