1,337 research outputs found
The Structure & Dynamics of Massive Early-type Galaxies: On Homology, Isothermality and Isotropy inside one Effective Radius
Based on 58 SLACS strong-lens early-type galaxies with direct total-mass and
stellar-velocity dispersion measurements, we find that inside one effective
radius massive elliptical galaxies with M_eff >= 3x10^10 M_sun are
well-approximated by a power-law ellipsoid with an average logaritmic density
slope of = -dlog(rho_tot)/dlog(r)=2.085^{+0.025}_{-0.018} (random
error on mean) for isotropic orbits with beta_r=0, +-0.1 (syst.) and
sigma_gamma' <= 0.20^{+0.04}_{-0.02} intrinsic scatter (all errors indicate the
68 percent CL). We find no correlation of gamma'_LD with galaxy mass (M_eff),
rescaled radius (i.e. R_einst/R_eff) or redshift, despite intrinsic differences
in density-slope between galaxies. Based on scaling relations, the average
logarithmic density slope can be derived in an alternative manner, fully
independent from dynamics, yielding =1.959 +- 0.077. Agreement
between the two values is reached for =0.45 +- 0.25, consistent with
mild radial anisotropy. This agreement supports the robustness of our results,
despite the increase in mass-to-light ratio with total galaxy mass: M_eff ~
L_{V,eff}^(1.363+-0.056). We conclude that massive early-type galaxies are
structurally close-to homologous with close-to isothermal total density
profiles (<=10 percent intrinsic scatter) and have at most some mild radial
anisotropy. Our results provide new observational limits on galaxy formation
and evolution scenarios, covering four Gyr look-back time.Comment: Accepted for publication by ApJL; 4 pages, 2 figure
The Sloan Lens ACS Survey. IX. Colors, Lensing and Stellar Masses of Early-type Galaxies
We present the current photometric dataset for the Sloan Lens ACS (SLACS)
Survey, including HST photometry from ACS, WFPC2, and NICMOS. These data have
enabled the confirmation of an additional 15 grade `A' (certain) lens systems,
bringing the number of SLACS grade `A' lenses to 85; including 13 grade `B'
(likely) systems, SLACS has identified nearly 100 lenses and lens candidates.
Approximately 80% of the grade `A' systems have elliptical morphologies while
~10% show spiral structure; the remaining lenses have lenticular morphologies.
Spectroscopic redshifts for the lens and source are available for every system,
making SLACS the largest homogeneous dataset of galaxy-scale lenses to date. We
have developed a novel Bayesian stellar population analysis code to determine
robust stellar masses with accurate error estimates. We apply this code to
deep, high-resolution HST imaging and determine stellar masses with typical
statistical errors of 0.1 dex; we find that these stellar masses are unbiased
compared to estimates obtained using SDSS photometry, provided that informative
priors are used. The stellar masses range from 10^10.5 to 10^11.8 M and
the typical stellar mass fraction within the Einstein radius is 0.4, assuming a
Chabrier IMF. The ensemble properties of the SLACS lens galaxies, e.g. stellar
masses and projected ellipticities, appear to be indistinguishable from other
SDSS galaxies with similar stellar velocity dispersions. This further supports
that SLACS lenses are representative of the overall population of massive
early-type galaxies with M* >~ 10^11 M, and are therefore an ideal
dataset to investigate the kpc-scale distribution of luminous and dark matter
in galaxies out to z ~ 0.5.Comment: 20 pages, 18 figures, 5 tables, published in Ap
The Cosmic Evolution of Faint Satellite Galaxies as a Test of Galaxy Formation and the Nature of Dark Matter
The standard cosmological model based on cold dark matter (CDM) predicts a
large number of subhalos for each galaxy-size halo. It is well known that
matching the subhalos to the observed properties of luminous satellites of
galaxies in the local universe poses a significant challenge to our
understanding of the astrophysics of galaxy formation. We show that the cosmic
evolution and host mass dependence of the luminosity function of satellites
provides a powerful new diagnostic to disentangle astrophysical effects from
variations in the underlying dark matter mass function. We illustrate this by
comparing the results of recent observations of satellites out to based
on Hubble Space Telescope images with the predictions of three different sets
of state-of-the art semi-analytic models with underlying CDM power spectra and
one semi-analytic model with an underlying Warm Dark Matter (WDM) power
spectrum. We find that even though CDM models provide a reasonable fit to the
local luminosity function of satellites around galaxies comparable or slightly
larger than the Milky Way, they do not reproduce the data as well for different
redshift and host galaxy stellar mass. This tension indicates that further
improvements are likely to be needed in the description of star formation if
the models are to be reconciled with the data. The WDM model matches the
observed mass dependence and redshift evolution of satellite galaxies more
closely than any of the CDM models, indicating that a modification of the
underlying power spectrum may offer an alternative solution to this tension. We
conclude by presenting predictions for the color magnitude relation of
satellite galaxies to demonstrate how future observations will be able to
further distinguish between these models and help constrain baryonic and
non-baryonic physics.Comment: Accepted for publication in ApJ, revised to incorporate referee
comment
Can dry merging explain the size evolution of early-type galaxies?
The characteristic size of early-type galaxies (ETGs) of given stellar mass
is observed to increase significantly with cosmic time, from redshift z>2 to
the present. A popular explanation for this size evolution is that ETGs grow
through dissipationless ("dry") mergers, thus becoming less compact. Combining
N-body simulations with up-to-date scaling relations of local ETGs, we show
that such an explanation is problematic, because dry mergers do not decrease
the galaxy stellar-mass surface-density enough to explain the observed size
evolution, and also introduce substantial scatter in the scaling relations.
Based on our set of simulations, we estimate that major and minor dry mergers
increase half-light radius and projected velocity dispersion with stellar mass
(M) as M^(1.09+/-0.29) and M^(0.07+/-0.11), respectively. This implies that: 1)
if the high-z ETGs are indeed as dense as estimated, they cannot evolve into
present-day ETGs via dry mergers; 2) present-day ETGs cannot have assembled
more than ~45% of their stellar mass via dry mergers. Alternatively, dry
mergers could be reconciled with the observations if there was extreme fine
tuning between merger history and galaxy properties, at variance with our
assumptions. Full cosmological simulations will be needed to evaluate whether
this fine-tuned solution is acceptable.Comment: 5 pages, 2 figures. Accepted for publication in ApJ Letter
The Missing Satellite Problem in 3D
It is widely believed that the large discrepancy between the observed number
of satellite galaxies and the predicted number of dark subhalos can be resolved
via a variety of baryonic effects which suppress star formation in low mass
halos.Supporting this hypothesis, numerous high resolution simulations with
star formation, and associated feedback have been shown to reproduce the
satellite luminosity function around Milky Way-mass simulated galaxies at
redshift zero. However, a more stringent test of these models is their ability
to simultaneously match the satellite luminosity functions of a range of host
halo masses and redshifts. In this work we measure the luminosity function of
faint (sub-Small Magellanic Cloud luminosity) satellites around hosts with
stellar masses 10.5M/M to an unprecedented
redshift of 1.5. This new measurement of the satellite luminosity function
provides powerful new constraining power; we compare these results with
predictions from four different simulations and show that although the models
perform similarly over-all, no one model reproduces the satellite luminosity
function reliably at all redshifts and host stellar masses. This result
highlights the continued need for improvement in understanding the fundamental
physics that governs satellite galaxy evolution.Comment: 9 pages, 2 figures, submitted to MNRA
Measuring the mass of high-z galaxies with NGST
We discuss dynamical mass measurements of high-z galaxies with the Next
Generation Space Telescope (NGST). In particular, we review some of the
observational limits with the current instrument/telescope generation, we
discuss the redshift limits and caveats for absorption and emission lines
studies with NGST, and the existence of suitable targets at high redshift. We
also briefly summarize strengths and weaknesses of proposed NGST instruments
for dynamical studies.Comment: to appear in "The Mass of Galaxies at Low and High Redshift" R.
Bender and A. Renzini ed
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