1,661 research outputs found
The Ages of Elliptical Galaxies in a Merger Model
The tightness of the observed colour-magnitude and Mg- velocity
dispersion relations for elliptical galaxies has often been cited as an
argument against a picture in which ellipticals form by the merging of spiral
disks. A common view is that merging would mix together stars of disparate ages
and produce a large scatter in these relations. Here I use semi-analytic models
of galaxy formation to derive the distribution of the mean ages, colours and
metallicities of the stars in elliptical galaxies formed by mergers in a flat
CDM universe. It is seen that most of the stars in ellipticals form at
relatively high redshift (z > 1.9) and that the predicted scatter in the
colour-magnitude and Mg_2 - sigma relations falls within observational bounds.
I conclude that the apparent homogeneity in the properties of the stellar
populations of ellipticals is not inconsistent with a merger scenario for the
origin of these systems.Comment: latex file, figures available upon reques
The Properties of Satellite Galaxies in External Systems. I. Morphology and Structural Parameters
We present the first results of an ongoing project to study the
morphological, kinematical, dynamical, and chemical properties of satellite
galaxies of external giant spiral galaxies. The sample of objects has been
selected from the catalogue by Zaritsky et al. (1997). The paper analyzes the
morphology and structural parameters of a subsample of 60 such objects. The
satellites span a great variety of morphologies and surface brightness
profiles. About two thirds of the sample are spirals and irregulars, the
remaining third being early-types. Some cases showing interaction between pairs
of satellites are presented and briefly discussed.Comment: Accepted for publication in Astrophys. Journal Supp. Se
Does the Number Density of Elliptical Galaxies Change at z<1?
We have performed a detailed V/Vmax test for a sample of the Canada-France
Redshift Survey (CFRS) for the purpose of examining whether the comoving number
density of field galaxies changes significantly at redshifts of z<1. Taking
into account the luminosity evolution of galaxies which depends on their
morphological type through different history of star formation, we obtain
\sim 0.5 in the range of 0.3<z<0.8, where reliable redshifts were
secured by spectroscopy of either absorption or emission lines for the CFRS
sample. This indicates that a picture of mild evolution of field galaxies
without significant mergers is consistent with the CFRS data. Early-type
galaxies, selected by their (V-I)_{AB} color, become unnaturally deficient in
number at z>0.8 due to the selection bias, thereby causing a fictitious
decrease of . We therefore conclude that a reasonable choice of upper
bound of redshift z \sim 0.8 in the V/Vmax test saves the picture of passive
evolution for field ellipticals in the CFRS sample, which was rejected by
Kauffman, Charlot, & White (1996) without confining the redshift range.
However, about 10% of the CFRS sample consists of galaxies having colors much
bluer than predicted for irregular galaxies, and their \avmax is significantly
larger than 0.5. We discuss this population of extremely blue galaxies in terms
of starburst that has just turned on at their observed redshifts.Comment: 11 pages including 3 figures, to appear in ApJ Letter
The Evolution of Early-Type Galaxies in Distant Clusters
We present results from an optical-IR photometric study of early-type
galaxies in 19 galaxy clusters out to z=0.9. The galaxy sample is selected on
the basis of morphologies determined from HST WFPC2 images, and is
photometrically defined in the K-band to minimize redshift-dependent selection
biases. The optical-IR colors of the early-type cluster galaxies become bluer
with increasing redshift in a manner consistent with the passive evolution of
an old stellar population formed at an early cosmic epoch. The degree of color
evolution is similar for clusters at similar redshift, and does not depend
strongly on the optical richness or X-ray luminosity of the cluster, suggesting
that the history of early-type galaxies is relatively insensitive to
environment. The slope of the color-magnitude relationship shows no significant
change out to z=0.9, providing evidence that it arises from a correlation
between galaxy mass and metallicity, not age. Finally, the intrinsic scatter in
the optical-IR colors is small and nearly constant with redshift, indicating
that the majority of giant, early-type galaxies in clusters share a common star
formation history, with little perturbation due to uncorrelated episodes of
later star formation. Taken together, our results are consistent with models in
which most early-type galaxies in rich clusters are old, formed the majority of
their stars at high redshift in a well-synchronized fashion, and evolved
quiescently thereafter.Comment: 55 pages, 24 figures, uses AASTeX. Accepted for publication in The
Astrophysical Journa
The Tidal Evolution of Local Group Dwarf Spheroidals
(Abridged) We use N-body simulations to study the evolution of dwarf
spheroidal galaxies (dSphs) driven by galactic tides. We adopt a
cosmologically-motivated model where dSphs are approximated by a King model
embedded within an NFW halo. We find that these NFW-embedded King models are
extraordinarily resilient to tides; the stellar density profile still resembles
a King model even after losing more than 99% of the stars. As tides strip the
galaxy, the stellar luminosity, velocity dispersion, central surface
brightness, and core radius decrease monotonically. Remarkably, we find that
the evolution of these parameters is solely controlled by the total amount of
mass lost from within the luminous radius. Of all parameters, the core radius
is the least affected: after losing 99% of the stars, R_c decreases by just a
factor of ~2. Interestingly, tides tend to make dSphs more dark-matter
dominated because the tightly bound central dark matter ``cusp'' is more
resilient to disruption than the ``cored'' King profile. We examine whether the
extremely large M/L ratios of the newly-discovered ultra-faint dSphs might have
been caused by tidal stripping of once brighter systems. Although dSph tidal
evolutionary tracks parallel the observed scaling relations in the
luminosity-radius plane, they predict too steep a change in velocity dispersion
compared with the observational estimates hitherto reported in the literature.
The ultra-faint dwarfs are thus unlikely to be the tidal remnants of systems
like Fornax, Draco, or Sagittarius. Despite spanning four decades in
luminosity, dSphs appear to inhabit halos of comparable peak circular velocity,
lending support to scenarios that envision dwarf spheroidals as able to form
only in halos above a certain mass threshold.Comment: 17 pages, 12 figs., accepted by Ap
Quantitative Morphology of Galaxies in the Hubble Deep Field
We measure quantitative structural parameters of galaxies in the Hubble Deep
Field (HDF) on the drizzled F814W images. Our structural parameters are based
on a two-component surface brightness made up of a S\'ersic profile and an
exponential profile. We compare our results to the visual classification of van
den Bergh et al. (1996) and the classification of Abraham et al. (1996a).
Our morphological analysis of the galaxies in the HDF indicates that the
spheroidal galaxies, defined here as galaxies with a dominant bulge profile,
make up for only a small fraction, namely 8% of the galaxy population down to
m = 26.0. We show that the larger fraction of early-type systems
in the van den Bergh sample is primarily due to the difference in
classification of 40% of small round galaxies with half-light radii <
0\arcsecpoint 31. Although these objects are visually classified as elliptical
galaxies, we find that they are disk-dominated with bulge fractions < 0.5.
Given the existing large dataset of HDF galaxies with measured spectroscopic
redshifts, we are able to determine that the majority of distant galaxies
() from this sample are disk-dominated. Our analysis reveals a subset of
HDF galaxies which have profiles flatter than a pure exponential profile.Comment: 35 pages, LaTeX, 18 Postscript Figures, Tables available at
http://astro.berkeley.edu/~marleau/. Accepted for Publication in The
Astrophysical Journa
Evidence for Evolving Spheroidals in the Hubble Deep Fields North and South
We investigate the dispersion in the internal colours of faint spheroidals in
the HDFs North and South. We find that a remarkably large fraction ~30% of the
morphologically classified spheroidals with I<24 mag show strong variations in
internal colour, which we take as evidence for recent episodes of
star-formation. In most cases these colour variations manifest themselves via
the presence of blue cores, an effect of opposite sign to that expected from
metallicity gradients. Examining similarly-selected ellipticals in five rich
clusters with 0.37<z<0.83 we find a significant lower dispersion in their
internal colours. This suggests that the colour inhomogeneities have a strong
environmental dependence being weakest in dense environments where spheroidal
formation was presumably accelerated at early times. We use the trends defined
by the cluster sample to define an empirical model based on a high-redshift of
formation and estimate that at z~1 about half the field spheroidals must be
undergoing recent episodes of star-formation. Using spectral synthesis models,
we construct the time dependence of the density of star-formation. Although the
samples are currently small, we find evidence for an increase in
between z=0 to z=1. We discuss the implications of this rise in the context of
that observed in the similar rise in the abundance of galaxies with irregular
morphology. Regardless of whether there is a connection our results provide
strong evidence for the continued formation of field spheroidals over 0<z<1.Comment: 13 pages, 11 figures. To appear in MNRAS in response to referee's
Report. Figures and paper also available at
http://www.ast.cam.ac.uk/~fmenante/HDFs
Concentrations of Dark Halos from their Assembly Histories
(abridged) We study the relation between the density profiles of dark matter
halos and their mass assembly histories, using a statistical sample of halos in
a high-resolution N-body simulation of the LCDM cosmology. For each halo at
z=0, we identify its merger-history tree, and determine concentration
parameters c_vir for all progenitors, thus providing a structural merger tree
for each halo. We fit the mass accretion histories by a universal function with
one parameter, the formation epoch a_c, defined when the log mass accretion
rate dlogM/dloga falls below a critical value S. We find that late forming
galaxies tend to be less concentrated, such that c_vir ``observed'' at any
epoch a_o is strongly correlated with a_c via c_vir=c_1*a_o/a_c. Scatter about
this relation is mostly due to measurement errors in c_v and a_c, implying that
the actual spread in c_vir for halos of a given mass can be mostly attributed
to scatter in a_c. We demonstrate that this relation can also be used to
predict the mass and redshift dependence of c_v, and the scatter about the
median c_vir(M,z), using accretion histories derived from the Extended
Press-Schechter (EPS) formalism, after adjusting for a constant offset between
the formation times as predicted by EPS and as measured in the simulations;this
new ingredient can thus be easily incorporated into semi-analytic models of
galaxy formation. The correlation found between halo concentration and mass
accretion rate suggests a physical interpretation: for high mass infall rates
the central density is related to the background density; when the mass infall
rate slows, the central density stays approximately constant and the halo
concentration just grows as R_vir. The tight correlation demonstrated here
provides an essential new ingredient for galaxy formation modeling.Comment: 19 pages, 18 figures, uses emulateapj5.tex. ApJ, in press; revised to
match accepted versio
Using the filaments in the LCRS to test the LambdaCDM model
It has recently been established that the filaments seen in the Las Campanas
Redshift Survey (LCRS) are statistically significant at scales as large as 70
to 80 Mpc/h in the slice, and 50 to 70 Mpc/h in the five other
LCRS slices. The ability to produce such filamentary features is an important
test of any model for structure formation. We have tested the LCDM model with a
featureless, scale invariant primordial power spectrum by quantitatively
comparing the filamentarity in simulated LCRS slices with the actual data. The
filamentarity in an unbiased LCDM model, we find, is less than the LCRS.
Introducing a bias b=1.15, the model is in rough consistency with the data,
though in two of the slices the filamentarity falls below the data at a low
level of statistical significance. The filamentarity is very sensitive to the
bias parameter and a high value b=1.5, which enhances filamentarity at small
scales and suppresses it at large scales, is ruled out. A bump in the power
spectrum at k~0.05 Mpc/h is found to have no noticeable effect on the
filamentarity.Comment: 16 pages, 3 figures; Minor Changes, Accepted to Ap
The Evolution of the Galaxy Sizes in the NTT Deep Field: a Comparison with CDM Models
The sizes of the field galaxies with I<25 have been measured in the NTT Deep
Field. Intrinsic sizes have been obtained after deconvolution of the PSF with a
multigaussian method. The reliability of the method has been tested using both
simulated data and HST observations of the same field. The distribution of the
half light radii is peaked at r_{hl} 0.3 arcsec, in good agreement with that
derived from HST images at the same magnitude. An approximate morphological
classification has been obtained using the asymmetry and concentration
parameters. The intrinsic sizes of the galaxies are shown as a function of
their redshifts and absolute magnitudes using photometric redshifts derived
from the multicolor catalog. While the brighter galaxies with morphological
parameters typical of the normal spirals show a flat distribution in the range
r_{d}=1-6 kpc, the fainter population at 0.4<z<0.8 dominates at small sizes. To
explore the significance of this behaviour, an analytical rendition of the
standard CDM model for the disc size evolution has been computed. The model
showing the best fit to the local luminosity function and the Tully-Fisher
relation is able to reproduce at intermediate redshifts a size distribution in
general agreement with the observations, although it tends to underestimate the
number of galaxies fainter than M_B~ -19 with disk sizes r_d~ 1-2 kpc.Comment: 16 pages, 11 figures, ApJ in press, Dec 199
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