20 research outputs found
Physical and morphological properties of z~3 LBGs: dependence on Lyalpha line emission
We investigate the physical and morphological properties of LBGs at z ~2.5 to
~3.5, to determine if and how they depend on the nature and strength of the
Lyalpha emission. We selected U-dropout galaxies from the z-detected GOODS
MUSIC catalog, by adapting the classical Lyman Break criteria on the GOODS
filter set. We kept only those galaxies with spectroscopic confirmation, mainly
from VIMOS and FORS public observations. Using the full multi-wavelength
14-bands photometry, we determined the physical properties of the galaxies,
through a standard spectral energy distribution fitting with the updated
Charlot & Bruzual (2009) templates. We also added other relevant observations,
i.e. the 24mu m observations from Spitzer/MIPS and the 2 MSec Chandra X-ray
observations. Finally, using non parametric diagnostics (Gini, Concentration,
Asymmetry, M_20 and ellipticity), we characterized the rest-frame UV morphology
of the galaxies. We then analyzed how these physical and morphological
properties correlate with the presence of the Lyalpha line in the optical
spectra. We find that, unlike at higher redshift, the dependence of physical
properties on the Lyalpha line is milder: galaxies without Lyalpha in emission
tend to be more massive and dustier than the rest of the sample, but all other
parameters, ages, SFRs, X-ray emission as well as UV morphology do not depend
strongly on the presence of the line emission. A simple scenario where all LBGs
have intrinsically high Lyalpha emission, but where dust and neutral hydrogen
content (which shape the final appearance of the Lyalpha) depend on the mass of
the galaxies, is able to reproduce the majority of the observed properties at
z~3. Some modification might be needed to account for the observed evolution of
these properties with cosmic epoch, which is also discussed.Comment: Accepted for publication on A&
The stellar masses and specific star-formation rates of submillimetre galaxies
Establishing the stellar masses (M*), and hence specific star-formation rates
(sSFRs) of submillimetre galaxies (SMGs) is crucial for determining their role
in the cosmic galaxy/star formation. However, there is as yet no consensus over
the typical M* of SMGs. Specifically, even for the same set of SMGs, the
reported average M* have ranged over an order of magnitude, from ~5x10^10 Mo to
~5x10^11 Mo. Here we study how different methods of analysis can lead to such
widely varying results. We find that, contrary to recent claims in the
literature, potential contamination of IRAC 3-8 um photometry from hot dust
associated with an active nucleus is not the origin of the published
discrepancies in derived M*. Instead, we expose in detail how inferred M*
depends on assumptions made in the photometric fitting, and quantify the
individual and cumulative effects of different choices of initial mass
function, different brands of evolutionary synthesis models, and different
forms of assumed star-formation history. We review current observational
evidence for and against these alternatives as well as clues from the
hydrodynamical simulations, and conclude that, for the most justifiable choices
of these model inputs, the average M* of SMGs is ~2x10^11 Mo. We also confirm
that this number is perfectly reasonable in the light of the latest
measurements of their dynamical masses, and the evolving M* function of the
overall galaxy population. M* of this order imply that the average sSFR of SMGs
is comparable to that of other star-forming galaxies at z>2, at 2-3 Gyr^-1.
This supports the view that, while rare outliers may be found at any M*, most
SMGs simply form the top end of the main-sequence of star-forming galaxies at
these redshifts. Conversely, this argues strongly against the viewpoint that
SMGs are extreme pathological objects, of little relevance in the cosmic
history of star-formation.Comment: Accepted to A&A. 13 pages, 5 figures, 3 tables. Main changes: 1)
investigation that the main-sequence does not change the location as much as
SMGs when changing SFHs; 2) a new table added with all stellar mass estimates
for individual SMGs (machine-readable version in the source file). V3:
missing references adde
Stellar Population Trends in S0 Galaxies
We present stellar population age and metallicity trends for a sample of 59
S0 galaxies based on optical SDSS and NIR J & H photometry. When combined with
optical g and r passband imaging data from the SDSS archive and stellar
population models, we obtain radial age and metallicity trends out to at least
5 effective radii for most of the galaxies in our sample. The sample covers a
range in stellar mass and light concentration. We find an average central
light-weighted age of ~ 4 Gyr and central metallicity [Z/H] ~ 0.2 dex. Almost
all galaxies show a negative metallicity gradient from the center out, with an
average value of Delta[Z/H]/Delta(log(r/Re)) = -0.6. An age increase, decrease,
and minimal change with radius is observed for 58%, 19%, and 23%, respectively,
for a mean age gradient of Delta(age)/Delta(log(r/Re)) = 2.3 Gyr dex^{-1}. For
14 out of 59 galaxies, the light-weighted age of the outer region is greater
than 10 Gyr. We find that galaxies with both lower mass and lower concentration
have younger light-weighted ages and lower light-weighted metallicities. This
mass-metallicity relation extends into the outer regions of our S0 galaxies.
Our results are consistent with the formation of S0 galaxies through the
transformation of spiral galaxy disks. Determining the structural component
that makes up the outer region of galaxies with old outksirts is a necessary
step to understand the formation history of S0 galaxies.Comment: accepted to MNRA
Comparing six evolutionary population synthesis models through spectral synthesis on galaxies
We compare six popularly used evolutionary population synthesis (EPS) models
(BC03, CB07, Ma05, GALEV, GRASIL, Vazdekis/Miles) through fitting the full
optical spectra of six representative types of galaxies (star-forming and
composite galaxies, Seyfert 2s, LINERs, E+A and early-type galaxies), which are
taken from the Sloan Digital Sky Survey (SDSS). Throughout our paper, we use
the simple stellar populations (SSPs) from each EPS model and the software
STARLIGHT to do our fits. Our main results are: Using different EPS models the
resulted numerical values of contributed light fractions change obviously, even
though the dominant populations are consistent. The stellar population
synthesis does depend on the selection of age and metallicity, while it does
not depend on the stellar evolution track much. The importance of young
populations decreases from star-forming, composite, Seyfert 2, LINER to
early-type galaxies, and E+A galaxies lie between composite galaxies and
Seyfert 2s in most cases. We conclude that different EPS models do derive
different stellar populations, so that it is not reasonable to directly compare
stellar populations estimated from different EPS models. To get reliable
results, we should use the same EPS model for the compared samples.Comment: 13 pages, 4 figures, accepted for publication in A&
Physical properties of galaxies and their evolution in the VIMOS VLT Deep Survey. I. The evolution of the mass-metallicity relation up to z~0.9
We derive the mass-metallicity relation of star-forming galaxies up to
, using data from the VIMOS VLT Deep Survey. Automatic measurement of
emission-line fluxes and equivalent widths have been performed on the full
spectroscopic sample. This sample is divided into two sub-samples depending on
the apparent magnitude selection: wide () and deep
). These two samples span two different ranges of stellar
masses. Emission-line galaxies have been separated into star-forming galaxies
and active galactic nuclei using emission line ratios. For the star-forming
galaxies the emission line ratios have also been used to estimate gas-phase
oxygen abundance, using empirical calibrations renormalized in order to give
consistent results at low and high redshifts. The stellar masses have been
estimated by fitting the whole spectral energy distributions with a set of
stellar population synthesis models. We assume at first order that the shape of
the mass-metallicity relation remains constant with redshift. Then we find a
stronger metallicity evolution in the wide sample as compared to the deep
sample. We thus conclude that the mass-metallicity relation is flatter at
higher redshift. The observed flattening of the mass-metallicity relation at
high redshift is analyzed as an evidence in favor of the open-closed model.Comment: 21 pages, revised version submitted to A&
The Formation and Evolution of Virgo Cluster Galaxies - II. Stellar Populations
We use a combination of deep optical and near-infrared light profiles for a
morphologically diverse sample of Virgo cluster galaxies to study the
radially-resolved stellar populations of cluster galaxies over a wide range of
galaxy structure. We find that, in the median, the age gradients of Virgo
galaxies are either flat (lenticulars and Sa-Sb spirals) or positive
(ellipticals, Sbc+Sc spirals, gas-rich dwarfs, and irregulars), while all
galaxy types have a negative median metallicity gradient. Comparison of the
galaxy stellar population diagnostics (age, metallicity, and gradients thereof)
against structural and environmental parameters also reveals that the ages of
gas-rich systems depend mainly on their atomic gas deficiencies. Conversely,
the metallicities of Virgo gas-poor galaxies depend on their concentrations,
luminosities, and surface brightnesses. The stellar population gradients of all
Virgo galaxies exhibit no dependence on either their structure or environment.
We interpret these stellar population data for Virgo galaxies in the context of
popular formation and evolution scenarios, and suggest that gas-poor giants
grew hierarchically (through dissipative starbursts), gas-poor dwarfs have
descended from at least two different production channels (e.g., environmental
transformation and merging), while spirals formed inside-out, but with star
formation in the outskirts of a significant fraction of the population having
been quenched due to ram pressure stripping. (Abridged)Comment: 54 pages, 16 figures, 3 tables, re-submitted to MNRAS (edited to
reflect the referee's suggestions
Central mass-to-light ratios and dark matter fractions in early-type galaxies
Dynamical studies of local ETGs and the Fundamental Plane point to a strong
dependence of M/L ratio on luminosity (and stellar mass) with a relation of the
form . The "tilt" may be caused by various
factors, including stellar population properties, IMF, rotational support,
luminosity profile non-homology and dark matter (DM) fraction. We evaluate the
impact of all these factors using a large uniform dataset of local ETGs from
Prugniel & Simien (1997). We take particular care in estimating the stellar
masses, using a general star formation history, and comparing different
population synthesis models. We find that the stellar M/L contributes little to
the tilt. We estimate the total M/L using simple Jeans dynamical models, and
find that adopting accurate luminosity profiles is important but does not
remove the need for an additional tilt component, which we ascribe to DM. We
survey trends of the DM fraction within one effective radius, finding it to be
roughly constant for galaxies fainter than , and increasing
with luminosity for the brighter galaxies; we detect no significant differences
among S0s and fast- and slow-rotating ellipticals. We construct simplified
cosmological mass models and find general consistency, where the DM transition
point is caused by a change in the relation between luminosity and effective
radius. A more refined model with varying galaxy star formation efficiency
suggests a transition from total mass profiles (including DM) of faint galaxies
distributed similarly to the light, to near-isothermal profiles for the bright
galaxies. These conclusions are sensitive to various systematic uncertainties
which we investigate in detail, but are consistent with the results of dynamics
studies at larger radii.Comment: 21 Pages, 19 figures, accepted for publication on MNRAS. Version
including revisions after the referee's report and an updated list of
reference
Physical interpretation of the near-infrared colours of low-redshift galaxies
International audienceWe use empirical techniques to interpret the near-infrared (near-IR) colours of a sample of 5800 galaxies drawn from Sloan Digital Sky Survey (SDSS) main spectroscopic sample with YJHK photometry from the United Kingdom Infrared Deep Sky Survey (UKIDSS) data release 1. Our study focuses on the inner 3 arcsec regions of the galaxies sampled by the SDSS fibre spectra. We study correlations between near-IR colours measured within this aperture and physical parameters derived from the spectra. These parameters include specific star formation rate (SFR), stellar age, metallicity and dust attenuation. All correlations are analysed for samples of galaxies that are closely matched in redshift, in stellar mass and in concentration index. Whereas more strongly star-forming galaxies have bluer optical colours, the opposite is true at near-IR wavelengths - galaxies with higher specific SFR have redder near-IR colours. This result agrees qualitatively with the predictions of models in which thermally pulsing asymptotic giant branch (TP-AGB) stars dominate the H- and K-band light of a galaxy following a burst of star formation. We also find a surprisingly strong correlation between the near-IR colours of star-forming galaxies and their dust attenuation as measured from the Balmer decrement. Unlike optical colours, however, near-IR colours exhibit very little dependence on galaxy inclination. This suggests that the correlation of near-IR colours with dust attenuation arises because TP-AGB stars are the main source of dust in the galaxy. Finally, we compare the near-IR colours of the galaxies in our sample to the predictions of three different stellar population models: the Bruzual & Charlot model, a preliminary version of a new model under development which includes a new prescription for AGB star evolution, and the Maraston model