32 research outputs found
A statistically-selected Chandra sample of 20 galaxy clusters -- I. Temperature and cooling time profiles
We present an analysis of 20 galaxy clusters observed with the Chandra X-ray
satellite, focussing on the temperature structure of the intracluster medium
and the cooling time of the gas. Our sample is drawn from a flux-limited
catalogue but excludes the Fornax, Coma and Centaurus clusters, owing to their
large angular size compared to the Chandra field-of-view. We describe a
quantitative measure of the impact of central cooling, and find that the sample
comprises 9 clusters possessing cool cores and 11 without. The properties of
these two types differ markedly, but there is a high degree of uniformity
amongst the cool core clusters, which obey a nearly universal radial scaling in
temperature of the form T \propto r^~0.4, within the core. This uniformity
persists in the gas cooling time, which varies more strongly with radius in
cool core clusters (t_cool \propto r^~1.3), reaching t_cool <1Gyr in all cases,
although surprisingly low central cooling times (<5Gyr) are found in many of
the non-cool core systems. The scatter between the cooling time profiles of all
the clusters is found to be remarkably small, implying a universal form for the
cooling time of gas at a given physical radius in virialized systems, in
agreement with recent previous work. Our results favour cluster merging as the
primary factor in preventing the formation of cool cores.Comment: 14 pages, 9 figures; accepted for publication in MNRA
X-ray Scaling Properties of Early-type Galaxies
We present an analysis of 39 X-ray luminous early-type galaxies observed with
the ROSAT PSPC. Using multi-component spectral and spatial fits to these data
we have measured halo abundance, temperature, luminosity and surface brightness
profile. We compare these measurements to similar results from galaxy groups
and clusters, fitting a number of relations commonly used in the study of these
larger objects. In particular, we find that the sigma:Tx relation for our
sample is similar to that reported for clusters, consistent with beta_{spec}=1,
and that the Lx:Tx relation has a steep slope (gradient 4.8+-0.7) comparable
with that found for galaxy groups. Assuming isothermality, we construct
3-dimensional models of our galaxies, allowing us to measure gas entropy. We
find no correlation between gas entropy and system mass, but do find a trend
for low temperature systems to have reduced gas fractions. We conclude that the
galaxies in our sample are likely to have developed their halos through galaxy
winds, influenced by their surrounding environment.Comment: Accepted for publication in MNRAS, 26 pages incl.14 postscript
figure
X-ray luminosities of galaxies in groups
We have derived the X-ray luminosities of a sample of galaxies in groups,
making careful allowance for contaminating intragroup emission. The L_X:L_B and
L_X:L_{FIR} relations of spiral galaxies in groups appear to be
indistinguishable from those in other environments, however the elliptical
galaxies fall into two distinct classes. The first class is central-dominant
group galaxies which are very X-ray luminous, and may be the focus of group
cooling flows. All other early-type galaxies in groups belong to the second
class, which populates an almost constant band of L_X/L_B over the range 9.8 <
log L_B < 11.3. The X-ray emission from these galaxies can be explained by a
superposition of discrete galactic X-ray sources together with a contribution
from hot gas lost by stars, which varies a great deal from galaxy to galaxy. In
the region where the optical luminosity of the non-central group galaxies
overlaps with the dominant galaxies, the dominant galaxies are over an order of
magnitude more luminous in X-rays.
We also compared these group galaxies with a sample of isolated early-type
galaxies, and used previously published work to derive L_X:L_B relations as a
function of environment. The non-dominant group galaxies have mean L_X/L_B
ratios very similar to that of isolated galaxies, and we see no significant
correlation between L_X/L_B and environment. We suggest that previous findings
of a steep L_X:L_B relation for early-type galaxies result largely from the
inclusion of group-dominant galaxies in samples.Comment: 18 pages, 8 figures. Accepted for publication in MNRA
The Complete Local Volume Groups Sample - I. Sample Selection and X-ray Properties of the High-Richness Subsample
We present the Complete Local-Volume Groups Sample (CLoGS), a statistically
complete optically-selected sample of 53 groups within 80 Mpc. Our goal is to
combine X-ray, radio and optical data to investigate the relationship between
member galaxies, their active nuclei, and the hot intra-group medium (IGM). We
describe sample selection, define a 26-group high-richness subsample of groups
containing at least 4 optically bright (log L_B>=10.2 LBsol) galaxies, and
report the results of XMM-Newton and Chandra observations of these systems. We
find that 14 of the 26 groups are X-ray bright, possessing a group-scale IGM
extending at least 65kpc and with luminosity >10^41 erg/s, while a further 3
groups host smaller galaxy-scale gas halos. The X-ray bright groups have masses
in the range M_500=0.5-5x10^13 Msol, based on system temperatures of 0.4-1.4
keV, and X-ray luminosities in the range 2-200x10^41 erg/s. We find that
~53-65% of the X-ray bright groups have cool cores, a somewhat lower fraction
than found by previous archival surveys. Approximately 30% of the X-ray bright
groups show evidence of recent dynamical interactions (mergers or sloshing),
and ~35% of their dominant early-type galaxies host AGN with radio jets. We
find no groups with unusually high central entropies, as predicted by some
simulations, and confirm that CLoGS is in principle capable of detecting such
systems. We identify three previously unrecognized groups, and find that they
are either faint (L_X,R500<10^42 erg/s) with no concentrated cool core, or
highly disturbed. This leads us to suggest that ~20% of X-ray bright groups in
the local universe may still be unidentified.Comment: Accepted for publication by MNRAS, 25 Manuscript pages with 6 tables
and 10 figures, plus 30 pages of appendices. v2 corrects minor typographical
errors identified at proof stag
The dark haloes of early-type galaxies in low-density environments: XMM-Newton and Chandra observations of NGC 57, NGC 7796 and IC 1531
We present analysis of Chandra and XMM-Newton observations of three
early-type galaxies, NGC 57, NGC 7796 and IC 1531. All three are found in very
low density environments, and appear to have no neighbours of comparable size.
NGC 57 has a halo of kT~0.9 keV, solar metallicity gas, while NGC 7796 and IC
1531 both have ~0.55 keV, 0.5-0.6 Zsol haloes. IC 1531 has a relatively compact
halo, and we consider it likely that gas has been removed from the system by
the effects of AGN heating. For NGC 57 and NGC 7796 we estimate mass, entropy
and cooling time profiles and find that NGC 57 has a fairly massive dark halo
with a mass-to-light ratio of 44.7 (4.0,-8.5) Msol/Lsol (1 sigma uncertainties)
at 4.75 Re. This is very similar to the mass-to-light ratio found for NGC 4555
and confirms that isolated ellipticals can possess sizable dark matter haloes.
We find a significantly lower mass-to-light ratio for NGC 7796, 10.6
(+2.5,-2.3) Msol/Lsol at 5 Re, and discuss the possibility that NGC 7796 hosts
a galactic wind, causing us to underestimate its mass.Comment: 14 pages, 9 figures, accepted for publication in MNRA
AGN Driven Weather and Multiphase Gas in the Core of the NGC 5044 Galaxy Group
A deep Chandra observation of the X-ray bright group, NGC 5044, shows that
the central region of this group has been strongly perturbed by repeated AGN
outbursts. These recent AGN outbursts have produced many small X-ray cavities,
cool filaments and cold fronts. We find a correlation between the coolest X-ray
emitting gas and the morphology of the Ha filaments. The Ha filaments are
oriented in the direction of the X-ray cavities, suggesting that the warm gas
responsible for the Halpha emission originated near the center of NGC 5044 and
was dredged up behind the buoyant, AGN-inflated X-ray cavities. A detailed
spectroscopic analysis shows that the central region of NGC 5044 contains
spatially varying amounts of multiphase gas. The regions with the most
inhomogeneous gas temperature distribution tend to correlate with the extended
235 MHz and 610 MHz radio emission detected by the GMRT. This may result from
gas entrainment within the radio emitting plasma or mixing of different
temperature gas in the regions surrounding the radio emitting plasma by AGN
induced turbulence. Accounting for the effects of multiphase gas, we find that
the abundance of heavy elements is fairly uniform within the central 100 kpc,
with abundances of 60-80% solar for all elements except oxygen, which has a
significantly sub-solar abundance. In the absence of continued AGN outbursts,
the gas in the center of NGC 5044 should attain a more homogeneous distribution
of gas temperature through the dissipation of turbulent kinetic energy and heat
conduction in approximately 10e8 yr. The presence of multiphase gas in NGC 5044
indicates that the time between recent AGN outbursts has been less than
approximately 10e8 yr
A galaxy lacking dark matter
Studies of galaxy surveys in the context of the cold dark matter paradigm
have shown that the mass of the dark matter halo and the total stellar mass are
coupled through a function that varies smoothly with mass. Their average ratio
M_{halo}/M_{stars} has a minimum of about 30 for galaxies with stellar masses
near that of the Milky Way (approximately 5x10^{10} solar masses) and increases
both towards lower masses and towards higher masses. The scatter in this
relation is not well known; it is generally thought to be less than a factor of
two for massive galaxies but much larger for dwarf galaxies. Here we report the
radial velocities of ten luminous globular-cluster-like objects in the
ultra-diffuse galaxy NGC1052-DF2, which has a stellar mass of approximately
2x10^8 solar masses. We infer that its velocity dispersion is less than 10.5
kilometers per second with 90 per cent confidence, and we determine from this
that its total mass within a radius of 7.6 kiloparsecs is less than 3.4x10^8
solar masses. This implies that the ratio M_{halo}/M_{stars} is of order unity
(and consistent with zero), a factor of at least 400 lower than expected.
NGC1052-DF2 demonstrates that dark matter is not always coupled with baryonic
matter on galactic scales.Comment: To appear in the 29 March issue of Natur