115 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 entropy of elliptical galaxies in Coma: a clue for a distance indicator
We have fitted the surface brightness of a sample of 79 elliptical galaxies
pertaining to the Coma cluster of galaxies using the Sersic profile. This model
is defined through three primary parameters: scale length (a), intensity
(\Sigma_0), and a shape parameter (\nu); physical and astrophysical quantities
may be computed from these parameters. We show that correlations are stronger
among primary parameters than the classical astrophysical ones. In particular,
the galaxies follow a high correlation in \nu and a parameters. We show that
the \nu and a correlation satisfies a constant specific entropy condition. We
propose to use this entropy relation as distance indicator for clusters.Comment: 5 pages, 3 figures, submitted to MNRAS Letter
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Atmospheric Mars Entry and Landing Investigations & Analysis (AMELIA) by ExoMars 2016 Schiaparelli Entry Descent Module
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
Gravo-thermal properties and formation of elliptical galaxies
We have analyzed a sample of galaxies belonging to three clusters: Coma, Abell 85, and Abell 496 (real galaxies) and a sample of simulated elliptical galaxies formed in a hierarchical merging scheme (virtual galaxies). We use the Sersic law to describe their light profile. The specific entropy (Boltzmann-Gibbs definition) is then calculated supposing that the galaxies behave as spherical, isotropic, one-component systems. We find that, to a good approximation (about 10%), both real and virtual galaxies have an almost unique specific entropy. Within this approximation the galaxies are distributed in a thin plane in the space defined by the three Sersic law parameters, which we call the Entropic Plane. A further analysis shows that both real and virtual galaxies are in fact located on a thin line, therefore indicating the existence of another - and yet unknown - physical property, besides the uniqueness of the specific entropy. A more careful examination of the virtual galaxies sample indicates a very small increase of their specific entropy with merging generation. In a hierarchical scenario, this implies a correlation between the specific entropy and the total mass, which is indeed seen in our data. The scatter and tilt of the Entropic Line, defined by Lima Neto et al. (1999a), are reduced when this correlation is taken into account. Although one cannot distinguish between various generations for real galaxies, the distribution of their specific entropy is similar to that in the virtual sample, suggesting that hierarchical merging processes could be an important mechanism in the building of elliptical galaxies
Dynamic Phase Transitions in Cell Spreading
We monitored isotropic spreading of mouse embryonic fibroblasts on
fibronectin-coated substrates. Cell adhesion area versus time was measured via
total internal reflection fluorescence microscopy. Spreading proceeds in
well-defined phases. We found a power-law area growth with distinct exponents
a_i in three sequential phases, which we denote basal (a_1=0.4+-0.2), continous
(a_2=1.6+-0.9) and contractile (a_3=0.3+-0.2) spreading. High resolution
differential interference contrast microscopy was used to characterize local
membrane dynamics at the spreading front. Fourier power spectra of membrane
velocity reveal the sudden development of periodic membrane retractions at the
transition from continous to contractile spreading. We propose that the
classification of cell spreading into phases with distinct functional
characteristics and protein activity patterns serves as a paradigm for a
general program of a phase classification of cellular phenotype. Biological
variability is drastically reduced when only the corresponding phases are used
for comparison across species/different cell lines.Comment: 4 pages, 5 figure
Model of Dark Matter and Dark Energy Based on Gravitational Polarization
A model of dark matter and dark energy based on the concept of gravitational
polarization is investigated. We propose an action in standard general
relativity for describing, at some effective or phenomenological level, the
dynamics of a dipolar medium, i.e. one endowed with a dipole moment vector, and
polarizable in a gravitational field. Using first-order cosmological
perturbations, we show that the dipolar fluid is undistinguishable from
standard dark energy (a cosmological constant Lambda) plus standard dark matter
(a pressureless perfect fluid), and therefore benefits from the successes of
the Lambda-CDM (Lambda-cold dark matter) scenario at cosmological scales.
Invoking an argument of "weak clusterisation" of the mass distribution of
dipole moments, we find that the dipolar dark matter reproduces the
phenomenology of the modified Newtonian dynamics (MOND) at galactic scales. The
dipolar medium action naturally contains a cosmological constant, and we show
that if the model is to come from some fundamental underlying physics, the
cosmological constant Lambda should be of the order of a0^2/c^4, where a0
denotes the MOND constant acceleration scale, in good agreement with
observations.Comment: 38 pages, 4 figures; to appear in Phys. Rev.
Energy, entropy and mass scaling relations for elliptical galaxies. Towards a physical understanding of their photometric properties
In the present paper, we show that elliptical galaxies (Es) obey a scaling
relation between potential energy and mass. Since they are relaxed systems in a
post violent-relaxation stage, they are quasi-equilibrium gravitational systems
and therefore they also have a quasi-constant specific entropy.
Assuming that light traces mass, these two laws imply that in the space
defined by the three S\'ersic law parameters (intensity Sigma_0, scale a and
shape nu), elliptical galaxies are distributed on two intersecting 2-manifolds:
the Entropic Surface and the Energy-Mass Surface.
Using a sample of 132 galaxies belonging to three nearby clusters, we have
verified that ellipticals indeed follow these laws. This also implies that they
are distributed along the intersection line (the Energy-Entropy line), thus
they constitute a one-parameter family.
These two physical laws (separately or combined), allow to find the
theoretical origin of several observed photometrical relations, such as the
correlation between absolute magnitude and effective surface brightness, and
the fact that ellipticals are located on a surface in the [log R_eff, -2.5 log
Sigma_0, log nu] space. The fact that elliptical galaxies are a one-parameter
family has important implications for cosmology and galaxy formation and
evolution models. Moreover, the Energy-Entropy line could be used as a distance
indicator.Comment: Revised version (considering non constant M/L), accepted by A&A on
10/03/0
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