1,195 research outputs found
The specific entropy of elliptical galaxies: an explanation for profile-shape distance indicators?
Dynamical systems in equilibrium have a stationary entropy; we suggest that
elliptical galaxies, as stellar systems in a stage of quasi-equilibrium, may
have a unique specific entropy. This uniqueness, a priori unknown, should be
reflected in correlations between the parameters describing the mass (light)
distribution in galaxies. Following recent photometrical work (Caon et al.
1993; Graham & Colless 1997; Prugniel & Simien 1997), we use the Sersic law to
describe the light profile of elliptical galaxies and an analytical
approximation to its three dimensional deprojection. The specific entropy is
calculated supposing that the galaxy behaves as a spherical, isotropic,
one-component system in hydrostatic equilibrium, obeying the ideal gas state
equations. We predict a relation between the 3 parameters of the Sersic,
defining a surface in the parameter space, an `Entropic Plane', by analogy with
the well-known Fundamental Plane. We have analysed elliptical galaxies in Coma
and ABCG 85 clusters and a group of galaxies (associated with NGC 4839). We
show that the galaxies in clusters follow closely a relation predicted by the
constant specific entropy hypothesis with a one-sigma dispersion of 9.5% around
the mean value of the specific entropy. Assuming that the specific entropy is
also the same for galaxies of different clusters, we are able to derive
relative distances between the studied clusters. If the errors are only due to
the determination of the specific entropy (about 10%), then the error in the
relative distance determination should be less than 20% for rich clusters. We
suggest that the unique specific entropy may provide a physical explanation for
the distance indicators based on the Sersic profile put forward by Young &
Currie (1994, 1995) and discussed by Binggeli & Jerjen (1998).Comment: Submitted to MNRAS (05/05/99), 15 pages, 10 figure
The cluster of galaxies Abell 376
We present a dynamical analysis of the galaxy cluster Abell 376 based on a
set of 73 velocities, most of them measured at Pic du Midi and Haute-Provence
observatories and completed with data from the literature. Data on individual
galaxies are presented and the accuracy of the determined velocities is
discussed as well as some properties of the cluster. We obtained an improved
mean redshift value z=0.0478^{+0.005}_{-0.006} and velocity dispersion
sigma=852^{+120}_{-76}km/s. Our analysis indicates that inside a radius of
900h_{70}^{-1}kpc (15 arcmin) the cluster is well relaxed without any
remarkable feature and the X-ray emission traces fairly well the galaxy
distribution. A possible substructure is seen at 20 arcmin from the centre
towards the Southwest direction, but is not confirmed by the velocity field.
This SW clump is, however, kinematically bound to the main structure of Abell
376. A dense condensation of galaxies is detected at 46 arcmin (projected
distance 2.6h_{70}^{-1}Mpc) from the centre towards the Northwest and analysis
of the apparent luminosity distribution of its galaxies suggests that this
clump is part of the large scale structure of Abell 376. X-ray spectroscopic
analysis of ASCA data resulted in a temperature kT = 4.3+/-0.4 keV and metal
abundance Z = 0.32+/-0.08 Z_solar. The velocity dispersion corresponding to
this temperature using the T_X-sigma scaling relation is in agreement with the
measured galaxies velocities.Comment: 11 pages, 10 figures, accepted for publication in A&
An XMM-Newton view of the cluster of galaxies Abell 85
We have observed the cluster of galaxies Abell 85 with XMM-Newton. These data
have allowed us to confirm in a previous paper the existence of the extended 4
Mpc filament detected by the ROSAT PSPC in the neighbourhood of this cluster,
and to determine an X-ray temperature of about about 2 keV. We now present a
thorough analysis of the properties of the X-ray gas in the cluster itself,
including temperature and metallicity maps for the entire cluster. These
results show that Abell 85 had intense merging activity in the past and is not
fully relaxed, even in the central region. We have also determined the
individual abundances for some iron-group metals and alpha-elements in various
regions; the ratios of these metallicities to the iron abundance show that both
supernova types Ia and II must be involved in the intra-cluster gas enrichment.
Spectral analysis of the central region suggests a different redshift of the
X-ray emitting gas compared to the mean cluster velocity derived from galaxy
member redshifts. We discuss the implications of the difference between the cD
galaxy redshift, the mean galaxy redshift and the hot gas redshift, as well as
the possibility of several groups being accreted on to Abell 85. Finally, we
obtain the dynamical mass profile and baryon fraction taking into account the
new determined temperature profile. The dynamical mass in Abell 85 has a steep
density profile, similar to the ones found in N-body simulations.Comment: Accepted for publication in Astronomy & Astrophysic
The rich cluster of galaxies ABCG 85. II. X-ray analysis using the ROSAT HRI
We present a new X-ray analysis mainly based on ROSAT HRI data. The HRI
spatial resolution combined with an improved wavelet analysis method and with
complementary radio and optical data provides new results compared to a
previous paper based on ROSAT PSPC data (Pislar et al. 1997). We use also
redshift data in order to identify galaxies dynamically belonging to the main
body of the cluster and/or to superimposed substructures. Various kinds of
emission are superimposed on a mean thermal X-ray emission due to the
intra-cluster gas: a) an X-ray flux excess in the centre; b) a south blob,
partially generated by individual galaxies. The mean velocity and velocity
dispersion of the galaxies located in this region are the same as those of the
cluster as a whole: it therefore does not seem to be a bound subgroup; c) West
emission due to a foreground group with self-emission from a Seyfert galaxy
located at the north-west; d) emission in the south-west due to inverse Compton
emission associated to a very steep radio source (the remnant of an active
galactic nucleus). We have examined the possibility for the central peak to be
an "unusual" galaxy, as assumed for the central galaxy of J2310-43 (Tananbaum
et al. 1997). We conclude on the existence of a cooling flow region, in which
the presence of at least three small features certainly related to cooler blobs
is revealed by the wavelet analysis. We have performed a pixel-to-pixel
modelling of the double X-ray emission. The large scale emission component is
comparable to those derived from by the PSPC data and the small scale one is
interpreted as a cooling-flow. A multiphase gas model analysis leads to a mass
deposit of 50-150 M_\odot/yr.Comment: 11 pages, 6 figures, 3 tables, LaTeX Accepted for publication in
Astronomy & Astrophysics main journa
The rich cluster of galaxies ABCG 85.I. X-ray analysis
We present an X-ray analysis of the rich cluster ABCG 85 based on ROSAT PSPC
data. By applying an improved wavelet analysis, we show that our view of this
cluster is notably changed from what was previously believed (a main region and
a south blob). The main emission comes from the central part of the main body
of the cluster on which is superimposed that of a foreground group of galaxies.
The foreground group and the main cluster are separated (if redshifts are
cosmological) by 46 1/h_50 Mpc. The southern blob is clearly not a group: it is
resolved into X-ray emitting galaxies (in particular the second more luminous
galaxy of the main cluster). Several X-ray features are identified with bright
galaxies. We performed a spectral analysis and derived the temperature (T),
metallicity (Z) and hydrogen column density NH. The global quantities are:
T=4keV (in agreement with the velocity dispersion of 760km/s) and
. We cannot derive accurate gradients for these quantities with
our data, but there is strong evidence that the temperature is lower () and the metallicity much higher (Z ) in the very centre
(within about 50 1/h_50 kpc). We present a pixel by pixel method to model the
physical properties of the X-ray gas and derive its density distribution. We
apply classical methods to estimate the dynamical, gas and stellar masses, as
well as the cooling time and cooling flow characteristics. At the limiting
radius of the image (1.4 1/h_50 Mpc), we find _{\odot}M_{gas}/M_{Dyn}\sim 0.18 h_{50}^{-1.5}6.7\ 10^{12}M_{\odot}M/L_{V}\sim 300$.
The cooling time is estimated for different models, leading to a cooling radius
of 30-80 kpc depending on theComment: 14 pages incl 16 postscript figures available, 4 tables, corrected
stellar mass. Accepted for publication in Astronomy & Astrophysic
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