1,281 research outputs found
Ecology of galaxy stellar populations from optical spectroscopic surveys
The age and chemical composition of the stars in present-day galaxies carry
important clues about their star formation processes. The latest generation of
population synthesis models have allowed to derive age and stellar metallicity
estimates for large samples of low-redshift galaxies. After reviewing the main
results about the distribution in ages and metallicities as a function of
galaxy mass, I will concentrate on recent analysis that aims at disentangling
the dependences of stellar populations properties on environment and on galaxy
stellar mass. Finally, new models that predict the response of the full
spectrum to variations in [alpha/Fe] will allow us to derive accurate estimates
of element abundance ratios and gain deeper insight into the timescales of star
formation cessation.Comment: 4 pages, 1 figure, proceedings of IAUS 262 "Stellar populations:
planning for the next decade
Stellar mass-to-light ratios from galaxy spectra: how accurate can they be?
Stellar masses play a crucial role in the exploration of galaxy properties
and the evolution of the galaxy population. In this paper, we explore the
minimum possible uncertainties in stellar mass-to-light (M/L) ratios from the
assumed star formation history (SFH) and metallicity distribution, with the
goals of providing a minimum set of requirements for observational studies. We
use a large Monte Carlo library of SFHs to study as a function of galaxy
spectral type and signal-to-noise ratio (S/N) the statistical uncertainties of
M/L values using either absorption-line data or broad band colors. The accuracy
of M/L estimates can be significantly improved by using metal-sensitive indices
in combination with age-sensitive indices, in particular for galaxies with
intermediate-age or young stellar populations. While M/L accuracy clearly
depends on the spectral S/N ratio, there is no significant gain in improving
the S/N much above 50/pix and limiting uncertainties of 0.03 dex are reached.
Assuming that dust is accurately corrected or absent and that the redshift is
known, color-based M/L estimates are only slightly more uncertain than
spectroscopic estimates (at comparable spectroscopic and photometric quality),
but are more easily affected by systematic biases. This is the case in
particular for galaxies with bursty SFHs (high Hdelta at fixed D4000), the M/L
of which cannot be constrained any better than 0.15 dex with any indicators
explored here. Finally, we explore the effects of the assumed prior
distribution in SFHs and metallicity, finding them to be higher for color-based
estimates.Comment: accepted for publication on ApJ
Charting the evolution of the ages and metallicities of massive galaxies since z=0.7
The stellar populations of intermediate-redshift galaxies can shed light onto
the growth of massive galaxies in the last 8 billion years. We perform deep,
multi-object rest-frame optical spectroscopy with IMACS/Magellan of ~70
galaxies in the E-CDFS with redshift 0.6522.7 and
stellar mass >10^{10}Msun. Following the Bayesian approach adopted for previous
low-redshift studies, we constrain the stellar mass, mean stellar age and
stellar metallicity of individual galaxies from stellar absorption features. We
characterize for the first time the dependence of stellar metallicity and age
on stellar mass at z~0.7 for all galaxies and for quiescent and star-forming
galaxies separately. These relations for the whole sample have a similar shape
as the z=0.1 SDSS analog, but are shifted by -0.28 dex in age and by -0.13 dex
in metallicity, at odds with simple passive evolution. We find that no
additional star formation and chemical enrichment are required for z=0.7
quiescent galaxies to evolve into the present-day quiescent population.
However, this must be accompanied by the quenching of a fraction of z=0.7
Mstar>10^{11}Msun star-forming galaxies with metallicities comparable to those
of quiescent galaxies, thus increasing the scatter in age without affecting the
metallicity distribution. However rapid quenching of the entire population of
massive star-forming galaxies at z=0.7 would be inconsistent with the
age/metallicity--mass relation for the population as a whole and with the
metallicity distribution of star-forming galaxies only, which are on average
0.12 dex less metal-rich than their local counterparts. This indicates chemical
enrichment until the present in at least a fraction of the z=0.7 massive
star-forming galaxies.[abridged]Comment: accepted for publication on ApJ, 26 pages, 13 figure
Halpha surface photometry of galaxies in the Virgo cluster. IV: the current star formation in nearby clusters of galaxies
Halpha+[NII] imaging observations of 369 late-type galaxies in the Virgo
cluster and in the Coma/A1367 supercluster are analyzed. They constitute an
optically selected sample (m_p<16.0) observed with 60% c.a. completeness.These
observations provide us with the current (T<10^7 yrs) star formation properties
of galaxies. The expected decrease of the star formation rate (SFR),as traced
by the Halpha E.W., with decreasing clustercentric projected distance is found
only when galaxies brighter than M_p=-19.5 are considered. We also include in
our analysis Near Infrared data, providing us with informations on the old
(T>10^9yrs) stars. Put together, the young and the old stellar indicators give
the ratio of currently formed stars over the stars formed in the past, or
"birthrate" parameter b. We also determine the "global gas content" combining
HI with CO observations. We define the "gas deficiency" parameter as the
logarithmic difference between the gas content of isolated galaxies of a given
Hubble type and the measured gas content.For the isolated objects we find that
b decreases with increasing NIR luminosity. The gas-deficient objects,
primarily members to the Virgo cluster, have their birthrate significantly
lower than the isolated objects with normal gas content and of similar NIR
luminosity. This indicates that the current star formation is regulated by the
gaseous content of spirals.Whatever mechanism (most plausibly ram-pressure
stripping) is responsible for the pattern of gas deficiency observed in spiral
galaxies members to rich clusters, it also produces the observed quenching of
the current star formation.Comment: 22 pages,14 figures,3 figures available in jpeg format.To be
published in A&
The ages and metallicities of galaxies in the local universe
We derive stellar metallicities, light-weighted ages and stellar masses for a
magnitude-limited sample of 175,128 galaxies drawn from the Sloan Digital Sky
Survey Data Release Two (SDSS DR2). We compute median-likelihood estimates of
these parameters using a large library of model spectra at medium-high
resolution, covering a comprehensive range of star formation histories. The
constraints we derive are set by the simultaneous fit of five spectral
absorption features, which are well reproduced by our population synthesis
models. By design, these constraints depend only weakly on the alpha/Fe element
abundance ratio. Our sample includes galaxies of all types spanning the full
range in star formation activity, from dormant early-type to actively
star-forming galaxies. We show that, in the mean, galaxies follow a sequence of
increasing stellar metallicity, age and stellar mass at increasing 4000AA-break
strength (D4000). For galaxies of intermediate mass, stronger Balmer absorption
at fixed D4000 is associated with higher metallicity and younger age. We
investigate how stellar metallicity and age depend on total galaxy stellar
mass. Low-mass galaxies are typically young and metal-poor, massive galaxies
old and metal-rich, with a rapid transition between these regimes over the
stellar mass range 3x10^9<M/Msun<3x10^10. Both high- and low-concentration
galaxies follow these relations, but there is a large dispersion in stellar
metallicity at fixed stellar mass, especially for low-concentration galaxies of
intermediate mass. Despite the large scatter, the relation between stellar
metallicity and stellar mass is similar to the correlation between gas-phase
oxygen abundance and stellar mass for star-forming galaxies. [abriged]Comment: 22 pages, 14 figures, accepted for publication on MNRAS, data
available at http://www.mpa-garching.mpg.de/SDSS
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