418 research outputs found
On the shape of the light profiles of early-type galaxies
We have obtained the best fit to the light profiles of a luminosity limited
sample of elliptical and S0 galaxies with a power law \rn, letting the exponent
remain free rather than keeping it fixed at as in the well known \GV
formula. The introduction of a free parameter in the fitting formula (ranging
from for kpc to for kpc) is justified by
the existence of a good correlation between and the global galaxian
parameters, such as total luminosity and scale-radius. This result seems to be
in line with the segregation of properties between the `ordinary' and `bright'
families of early-type galaxies, and has consequence for the claimed
independence of the shape of galaxy profiles with respect to the Fundamental
Plane parameters.Comment: 10 pages, postscript file including figures, PADOVA (archived file
truncated during email transfer
The Tilt of the Fundamental Plane: Three-quarters Structural Nonhomology, One-quarter Stellar Population
The variation of the mass-to-light ratios M/L of early type galaxies as
function of their luminosities L is investigated. It is shown that the tilt
beta=0.27 (in the B--band) of the fundamental plane relation M/L ~ L^{beta} can
be understood as a combination of two effects: about one-quarter (i.e. dbeta
=0.07) is a result of systematic variations of the stellar population
properties with increasing luminosity. The remaining three-quarters (i.e. dbeta
=0.2) can be completely attributed to nonhomology effects that lead to a
systematic change of the surface brightness profiles with increasing
luminosity. Consequently, the observed tilt in the K-band (beta=0.17) where
stellar population effects are negligible, is explained by nonhomology effects
alone. After correcting for nonhomology, the mean value of the mass-to-light
ratio of elliptical galaxies (M/L_B) is 7.1+-2.8 (1 sigma scatter).Comment: 8 pages, 3 figures, ApJL, 600, 39, minor changes made to match the
published versio
An X-ray Survey of Galaxies in Pairs
Results are reported from the first survey of X-ray emission from galaxies in
pairs. The sample consists of fifty-two pairs of galaxies from the Catalog of
Paired Galaxies Karachentsev (1972) whose coordinates overlap ROSAT Position
Sensitive Proportional Counter pointed observations. The mean observed log l_x
for early-type pairs is 41.35 +/-0.21 while the mean log l_x predicted using
the l_x-l_b relationship for isolated early-type galaxies is 42.10 +/-0.19.
With 95% confidence, the galaxies in pairs are underluminous in the X-ray,
compared to isolated galaxies, for the same l_b. A significant fraction of the
mixed pair sample also appear similarly underluminous. A spatial analysis shows
that the X-ray emission from pairs of both types typically has an extent of ~10
- 50 kpc, much smaller than group intergalactic medium and thus likely
originates from the galaxies. CPG 564, the most X-ray luminous early-type pair,
4.7x10^42 ergs/sec, is an exception. The extent of it's X-ray emission, >169
kpc, and HWHM, ~80 kpc, is comparable to that expected from an intergalactic
medium. The sample shows only a weak correlation, ~81% confidence, between l_x
and l_b, presumably due to variations in gas content within the galaxies. No
correlation between l_x and the pair velocity difference, separation, or
far-infrared luminosity is found though the detection rate is low, 22%.Comment: 40 pages, 6 jpg figures, ApJ (in press
A New Empirical Model for the Structural Analysis of Early-type Galaxies and a Critical Review of the Nuker Model
The Nuker law was designed to match the inner few (~3-10) arcseconds of
predominantly nearby (< 30 Mpc) early-type galaxy light-profiles; it was never
intended to describe an entire profile. The Sersic model, on the other hand,
was developed to fit the entire profile; however, due to the presence of
partially depleted galaxy cores, the Sersic model cannot always describe the
very inner region. We have therefore developed a new empirical model consisting
of an inner power-law, a transition region, and an outer Sersic model to
connect the inner and outer structure of elliptical galaxies. Moreover, the
stability of the Nuker model parameters are investigated. Surprisingly, none
are found to be stable quantities; all are shown to vary systematically with a
profile's fitted radial extent, and often by more than 100%. Considering
elliptical galaxies spanning a range of 7.5 magnitudes, the central stellar
density of the underlying host galaxy is observed to increase with galaxy
luminosity until the onset of core formation, detected only in the brightest
elliptical galaxies. We suggest that the so-called ``power-law'' galaxies may
actually be described by the Sersic model over their entire radial range
Halo Cores and Phase Space Densities: Observational Constraints on Dark Matter Physics and Structure Formation
We explore observed dynamical trends in a wide range of dark matter dominated
systems (about seven orders of magnitude in mass) to constrain hypothetical
dark matter candidates and scenarios of structure formation. First, we argue
that neither generic warm dark matter (collisionless or collisional) nor
self-interacting dark matter can be responsible for the observed cores on all
scales. Both scenarios predict smaller cores for higher mass systems, in
conflict with observations; some cores must instead have a dynamical origin.
Second, we show that the core phase space densities of dwarf spheroidals,
rotating dwarf and low surface brightness galaxies, and clusters of galaxies
decrease with increasing velocity dispersion like Q ~ sigma^-3 ~ M^-1, as
predicted by a simple scaling argument based on merging equilibrium systems,
over a range of about eight orders of magnitude in Q. We discuss the processes
which set the overall normalization of the observed phase density hierarchy. As
an aside, we note that the observed phase-space scaling behavior and density
profiles of dark matter halos both resemble stellar components in elliptical
galaxies, likely reflecting a similar collisionless, hierarchical origin. Thus,
dark matter halos may suffer from the same systematic departures from homology
as seen in ellipticals, possibly explaining the shallower density profiles
observed in low mass halos. Finally, we use the maximum observed phase space
density in dwarf spheroidal galaxies to fix a minimum mass for relativistically
decoupled warm dark matter candidates of roughly 700 eV for thermal fermions,
and 300 eV for degenerate fermions.Comment: Submitted to the Astrophysical Journal, LaTeX, 26 pages including 4
pages of figure
Major axis kinematics of 15 early-type galaxies in the Fornax cluster
Major axis rotation curves and velocity dispersions profiles, extending out
to about one effective radius, are presented for 15 ellipticals and S0's out of
the photometric sample of Fornax cluster galaxies studied by Caon \etal\
(1994). A brief description of the spectroscopic and photometric
characteristics of each galaxy is provided, together with a comparison with
previous studies. Six of the nine E's are possibly misclassified S0's or
ellipticals harboring a disk-like component. Two galaxies (NGC~1399 and 1404)
show a hint of counter-rotation. Evidence is given that the \bright\ and the
\ordinary\ families of early-type galaxies, first introduced by Capaccioli
\etal\ (1992), look distinct also in term of the anisotropy parameter
(\Vm/\sigma)^*.Comment: 33 pages with figures, uuencoded compressed postscrip
The stellar host in blue compact dwarf galaxies: the need for a two-dimensional fit
The structural properties of the low surface brightness stellar host in blue
compact dwarf galaxies are often studied by fitting r^{1/n} models to the outer
regions of their radial profiles. The limitations imposed by the presence of a
large starburst emission overlapping the underlying component makes this kind
of analysis a difficult task. We propose a two-dimensional fitting methodology
in order to improve the extraction of the structural parameters of the LSB
host. We discuss its advantages and weaknesses by using a set of simulated
galaxies and compare the results for a sample of eight objects with those
already obtained using a one-dimensional technique. We fit a PSF convolved
Sersic model to synthetic galaxies, and to real galaxy images in the B, V, R
filters. We restrict the fit to the stellar host by masking out the starburst
region and take special care to minimize the sky-subtraction uncertainties. In
order to test the robustness and flexibility of the method, we carry out a set
of fits with synthetic galaxies. Furthermore consistency checks are performed
to assess the reliability and accuracy of the derived structural parameters.
The more accurate isolation of the starburst emission is the most important
advantage and strength of the method. Thus, we fit the host galaxy in a range
of surface brightness and in a portion of area larger than in previous
published 1D fits with the same dataset. We obtain robust fits for all the
sample galaxies, all of which, except one, show Sersic indices n very close to
1, with good agreement in the three bands. These findings suggest that the
stellar hosts in BCDs have near-exponential profiles, a result that will help
us to understand the mechanisms that form and shape BCD galaxies, and how they
relate to the other dwarf galaxy classes.Comment: 22 pages, 15 figures (low resolution), accepted for publication in
A&A. A higher resolution version of the figures can be provided upon reques
A Disk Galaxy of Old Stars at z ~ 2.5
We describe observations of a galaxy in the field of the radio
galaxy 4C 23.56, photometrically selected to have a spectral-energy
distribution consistent with an old stellar population at the redshift of the
radio galaxy. Exploration of redshift--stellar-population-reddening constraints
from the photometry indicates that the galaxy is indeed at a redshift close to
that of 4C23.56, that the age of the most recent significant star formation is
roughly >~2 Gyr, and that reddening is fairly modest, with more reddening
required for the younger end of stellar age range. From analysis of a deep
adaptive-optics image of the galaxy, we find that an r^1/4-law profile, common
for local spheroidal galaxies, can be excluded quite strongly. On the other
hand, a pure exponential profile fits remarkably well, while the best fit is
given by a Sersic profile with index n=1.49. Reconstruction of the
two-dimensional form of the galaxy from the best-fit model is consistent with a
disk galaxy with neither a significant bulge component nor gross azimuthal
structure. The assembly of roughly 2L* of old stars into such a configuration
this early in the history of the universe is not easily explainable by any of
the currently popular scenarios for galaxy formation. A galaxy with these
properties would seem to require smooth but rapid infall of the large mass of
gas involved, followed by a burst of extremely vigorous and efficient star
formation in the resulting disk.Comment: 8 pages, 6 figures, emulateapj.sty, accepted for publication in The
Astrophysical Journa
A New Approach to the Study of Stellar Populations in Early-Type Galaxies: K-band Spectral Indices and an Application to the Fornax Cluster
New measurements of K-band spectral features are presented for eleven
early-type galaxies in the nearby Fornax galaxy cluster. Based on these
measurements, the following conclusions have been reached: (1) in galaxies with
no signatures of a young stellar component, the K-band Na I index is highly
correlated with both the optical metallicity indicator [MgFe]' and central
velocity dispersion; (2) in the same galaxies, the K-band Fe features saturate
in galaxies with sigma > 150 km/s while Na I (and [MgFe]') continues to
increase; (3) [Si/Fe] (and possibly [Na/Fe]) is larger in all observed Fornax
galaxies than in Galactic open clusters with near-solar metallicity; (4) in
various near-IR diagnostic diagrams, galaxies with signatures of a young
stellar component (strong Hbeta, weak [MgFe]') are clearly separated from
galaxies with purely old stellar populations; furthermore, this separation is
consistent with the presence of an increased number of M-giant stars (most
likely to be thermally pulsating AGB stars); (5) the near-IR diagrams discussed
here seem as efficient for detecting putatively young stellar components in
early-type galaxies as the more commonly used age/metallicity diagnostic plots
using optical indices (e.g Hbeta vs. [MgFe]').Comment: 47 pages, 16 figures, ApJ accepte
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