1,176 research outputs found
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
Constraints on the broad line region from regularized linear inversion: Velocity-delay maps for five nearby active galactic nuclei
Reverberation mapping probes the structure of the broad emission-line region
(BLR) in active galactic nuclei (AGN). The kinematics of the BLR gas can be
used to measure the mass of the central supermassive black hole. The main
uncertainty affecting black hole mass determinations is the structure of the
BLR. We present a new method for reverberation mapping based on regularized
linear inversion (RLI) that includes modelling of the AGN continuum light
curves. This enables fast calculation of velocity-resolved response maps to
constrain BLR structure. RLI allows for negative response, such as when some
areas of the BLR respond in inverse proportion to a change in ionizing
continuum luminosity. We present time delays, integrated response functions,
and velocity-delay maps for the broad emission line in five
nearby AGN, as well as for and in Arp 151,
using data from the Lick AGN Monitoring Project 2008. We find indications of
prompt response in three of the objects (Arp 151, NGC 5548 and SBS 1116+583A)
with additional prompt response in the red wing of . In SBS
1116+583A we find evidence for a multimodal broad prompt response followed by a
second narrow response at 10 days. We find no clear indications of negative
response. The results are complementary to, and consistent with, other methods
such as cross correlation, maximum entropy and dynamical modelling. Regularized
linear inversion with continuum light curve modelling provides a fast,
complementary method for velocity-resolved reverberation mapping and is
suitable for use on large datasets.Comment: 20 pages, 13 figures, accepted to MNRA
The X-shooter Lens Survey - II. Sample presentation and spatially resolved kinematics
We present the X-shooter Lens Survey (XLENS) data. The main goal of XLENS is
to disentangle the stellar and dark matter content of massive early-type
galaxies (ETGs), through combined strong gravitational lensing, dynamics and
spectroscopic stellar population studies. The sample consists of 11 lens
galaxies covering the redshift range from to and having stellar
velocity dispersions between and . All
galaxies have multi-band, high-quality HST imaging. We have obtained long-slit
spectra of the lens galaxies with X-shooter on the VLT. We are able to
disentangle the dark and luminous mass components by combining lensing and
extended kinematics data-sets, and we are also able to precisely constrain
stellar mass-to-light ratios and infer the value of the low-mass cut-off of the
IMF, by adding spectroscopic stellar population information. Our goal is to
correlate these IMF parameters with ETG masses and investigate the relation
between baryonic and non-baryonic matter during the mass assembly and structure
formation processes. In this paper we provide an overview of the survey,
highlighting its scientific motivations, main goals and techniques. We present
the current sample, briefly describing the data reduction and analysis process,
and we present the first results on spatially resolved kinematics.Comment: Accepted for publication in MNRA
HST followup observations of two bright z ~ 8 candidate galaxies from the BoRG pure-parallel survey
We present followup imaging of two bright (L > L*) galaxy candidates at z > 8
from the Brightest of Reionizing Galaxies (BoRG) survey with the F098M filter
on HST/WFC3. The F098M filter provides an additional constraint on the flux
blueward of the spectral break, and the observations are designed to
discriminate between low- and high-z photometric redshift solutions for these
galaxies. Our results confirm one galaxy, BoRG 0116+1425 747, as a highly
probable z ~ 8 source, but reveal that BoRG 0116+1425 630 - previously the
brightest known z > 8 candidate (mAB = 24.5) - is likely to be a z ~ 2
interloper. As this source was substantially brighter than any other z > 8
candidate, removing it from the sample has a significant impact on the derived
UV luminosity function in this epoch. We show that while previous BoRG results
favored a shallow power-law decline in the bright end of the luminosity
function prior to reionization, there is now no evidence for departure from a
Schechter function form and therefore no evidence for a difference in galaxy
formation processes before and after reionization.Comment: Accepted by ApJL, 7 pages, 4 figure
The Baryon Fractions and Mass-to-Light Ratios of Early-Type Galaxies
We jointly model 22 early-type gravitational lens galaxies with stellar
dynamical measurements using standard CDM halo models. The sample is
inhomogeneous in both its mass distributions and the evolution of its stellar
populations unless the true uncertainties are significantly larger than the
reported measurement errors. In general, the individual systems cannot
constrain halo models, in the sense that the data poorly constrains the stellar
mass fraction of the halo. The ensemble of systems, however, strongly
constrains the average stellar mass represented by the visible galaxies to
of the halo mass if we neglect adiabatic compression, rising to
of the halo mass if we include adiabatic compression. Both
estimates are significantly smaller than the global baryon fraction,
corresponding to a star formation efficiency for early-type galaxies of
. In the adiabatically compressed models, we find an average local
B-band stellar mass-to-light ratio of (M/L)_0 =
(7.2\pm0.5)(M_{\sun}/L_{\sun}) that evolves by
per unit redshift. Adjusting the isotropy of the stellar orbits has little
effect on the results. The adiabatically compressed models are strongly favored
if we impose either local estimates of the mass-to-light ratios of early-type
galaxies or the weak lensing measurements for the lens galaxies on 100 kpc
scales as model constraints.Comment: 9 figure
A twelve-image gravitational lens system in the z ~ 0.84 cluster Cl J0152.7-1357
Gravitational lens modeling is presented for the first discovered example of
a three-component source for which each component is quadruply imaged. The lens
is a massive galaxy member of the cluster Cl J0152.7-1357 at z ~ 0.84. Taking
advantage of this exceptional configuration and of the excellent angular
resolution of the HST/ACS, we measure the properties of the lens. Several
parametric macroscopic models were developed for the lens galaxy, starting from
pointlike to extended image models. For a lens model in terms of a singular
isothermal sphere with external shear, the Einstein radius is found to be R_{E}
= (9.54 +/- 0.15) kpc. The external shear points to the cluster's northern mass
peak. The unknown redshift of the source is determined to be higher than 1.9
and lower than 2.9. Our estimate of the lensing projected total mass inside the
Einstein radius, M_{len}(R < 9.54 kpc), depends on the source distance and lies
between 4.6 and 6.2 x 10^{11} M_{Sun}. This result turns out to be compatible
with the dynamical estimate based on an isothermal model. By considering the
constraint on the stellar mass-to-light ratio that comes from the evolution of
the Fundamental Plane, we can exclude the possibility that at more than 4 sigma
level the total mass enclosed inside the Einstein ring is only luminous matter.
Moreover, the photometric-stellar mass measurement within the Einstein radius
gives a minimum value of 50% (1 sigma) for the dark-to-total matter fraction.
The lensing analysis has allowed us to investigate the distribution of mass of
the deflector, also providing some interesting indications on scales that are
larger and smaller than the Einstein radius of the lens galaxy. The combination
of different diagnostics has proved to be essential in determining quantities
that otherwise would have not been directly measurable with the current data.Comment: 10 pages, 9 figures, accepted by Astronomy & Astrophysic
The SWELLS Survey. I. A large spectroscopically selected sample of edge-on late-type lens galaxies
The relative contribution of baryons and dark matter to the inner regions of
spiral galaxies provides critical clues to their formation and evolution, but
it is generally difficult to determine. For spiral galaxies that are strong
gravitational lenses, however, the combination of lensing and kinematic
observations can be used to break the disk-halo degeneracy. In turn, such data
constrain fundamental parameters such as i) the mass density profile slope and
axis ratio of the dark matter halo, and by comparison with dark matter-only
numerical simulations the modifications imposed by baryons; ii) the mass in
stars and therefore the overall star formation efficiency, and the amount of
feedback; iii) by comparison with stellar population synthesis models, the
normalization of the stellar initial mass function. In this first paper of a
series, we present a sample of 16 secure, 1 probable, and 6 possible strong
lensing spiral galaxies, for which multi-band high-resolution images and
rotation curves were obtained using the Hubble Space Telescope and Keck-II
Telescope as part of the Sloan WFC Edge-on Late-type Lens Survey (SWELLS). The
sample includes 8 newly discovered secure systems. [abridged] We find that the
SWELLS sample of secure lenses spans a broad range of morphologies (from
lenticular to late-type spiral), spectral types (quantified by Halpha
emission), and bulge to total stellar mass ratio (0.22-0.85), while being
limited to M_*>10^{10.5} M_sun. The SWELLS sample is thus well-suited for
exploring the relationship between dark and luminous matter in a broad range of
galaxies. We find that the deflector galaxies obey the same size-mass relation
as that of a comparison sample of elongated non-lens galaxies selected from the
SDSS survey. We conclude that the SWELLS sample is consistent with being
representative of the overall population of high-mass high-inclination disky
galaxies.Comment: 21 pages, 6 figures, MNRAS, in pres
Luminous Satellites II: Spatial Distribution, Luminosity Function and Cosmic Evolution
We infer the normalization and the radial and angular distributions of the
number density of satellites of massive galaxies
() between redshifts 0.1 and 0.8 as a function
of host stellar mass, redshift, morphology and satellite luminosity. Exploiting
the depth and resolution of the COSMOS HST images, we detect satellites up to
eight magnitudes fainter than the host galaxies and as close as 0.3 (1.4)
arcseconds (kpc). Describing the number density profile of satellite galaxies
to be a projected power law such that P(R)\propto R^{\rpower}, we find
\rpower=-1.1\pm 0.3. We find no dependency of \rpower on host stellar mass,
redshift, morphology or satellite luminosity. Satellites of early-type hosts
have angular distributions that are more flattened than the host light profile
and are aligned with its major axis. No significant average alignment is
detected for satellites of late-type hosts. The number of satellites within a
fixed magnitude contrast from a host galaxy is dependent on its stellar mass,
with more massive galaxies hosting significantly more satellites. Furthermore,
high-mass late-type hosts have significantly fewer satellites than early-type
galaxies of the same stellar mass, likely a result of environmental
differences. No significant evolution in the number of satellites per host is
detected. The cumulative luminosity function of satellites is qualitatively in
good agreement with that predicted using subhalo abundance matching techniques.
However, there are significant residual discrepancies in the absolute
normalization, suggesting that properties other than the host galaxy luminosity
or stellar mass determine the number of satellites.Comment: 23 pages, 12 figures, Accepted for publication in the Astrophysical
Journa
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