144 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
Modeling and interpretation of galaxy spectra: the stellar populations of nearby galaxies
Our current understanding of structure formation in the Universe seems to be well described by a hierarchical scenario, in which small units assemble first to produce more massive systems. In recent years, much observational evidence has been accumulated, indicating that star formation proceeded instead in an antihierarchical fashion. Constraining the age and chemical composition of the stellar populations in galaxies should help shed light on this apparent dichotomy between mass assembly and star formation activity. The integrated spectra of galaxies contain valuable clues about the ages and metallicities of the stars producing the light. However, at first order, they are affected in a similar way by age and metallicity. Studies of more refined spectral diagnostics, such as individual stellar absorption features, are thus needed to provide more stringent constraints on these parameters. This method has been limited so far to small samples of elliptical galaxies, using population synthesis models with limited spectral resolution and restricted
coverage in stellar effective temperatures.
The objective of this thesis is the interpretation of the optical spectra of large samples of nearby galaxies in terms of the light-weighted metallicity, age and mass of their stellar populations. I have developed a new method to simultaneously derive median-likelihood estimates of each physical parameter and the associated confidence intervals. The method, based on a recent highresolution population synthesis code with full temperature coverage, consists
in comparing each observed spectrum with a comprehensive library of star formation histories. The constraints are set by the simultaneous fit of an optimally selected set of spectral absorption features. I have applied this method to a sample of 200,000 galaxies from the Sloan Digital Sky Survey, including galaxies with any star formation history, from quiescent early-type to actively star forming galaxies. Thanks to the unprecedented statistics, I could give an accurate description of the galaxy distribution in the full physical parameters space. The relation between stellar metallicity, age and stellar mass shows a rapid transition from low-mass, young, metal-poor to high-mass, old, metal-rich galaxies at a stellar mass of 3×10^10 solar masses, the same characteristic scale of several observed bi-modalities in galaxy properties. The stellar metallicity-mass relation is interpreted as a manifestation of galactic winds, which are more efficient in removing metals from the shallow potential well of low-mass galaxies. I then explored the implications of the above relations to re-assess the physical origin of observed scaling relations of elliptical galaxies, linking their luminous and dynamical mass to the properties of their stellar populations. The relations are driven by an increase in metallicity, age and element abundance ratios with galaxy mass. The scatter is contributed by a similar amount by both age and metallicity. The increasing spread towards younger ages at low stellar masses indicates that low-mass ellipticals either formed their stars later or have a more extended star formation history. This hints at a shift in stellar growth towards less massive galaxies in recent epochs. The large ranges in observational and physical properties covered by SDSS galaxies make it a representative sample of the local Universe. I could thus
derive the total mass density of metals and baryons locked up in stars today. I have also studied how metals and stellar mass are distributed as a function of various galaxy properties. The galaxies containing the bulk of the total stellar mass (massive, bulge-dominated galaxies with old stellar populations) are also those that contribute the largest fraction of metals, as expected from the mass-metallicity relation. These quantities set the fundamental constraints at the present epoch of the cosmic star formation and chemical enrichment histories.
The more detailed knowledge of the relations between galaxy physical parameters allows a more direct comparison with predictions from semi-analytic models of galaxy formation and evolution. Moreover, the more robust constraints represent an important calibration at redshift zero for similar studies at higher redshifts
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
Stellar Ages and Metallicities of Central and Satellite Galaxies: Implications for Galaxy Formation and Evolution
Using a large SDSS galaxy group catalogue, we study how the stellar ages and
metallicities of central and satellite galaxies depend on stellar mass and halo
mass. We find that satellites are older and metal-richer than centrals of the
same stellar mass. In addition, the slopes of the age-stellar mass and
metallicity-stellar mass relations are found to become shallower in denser
environments. This is due to the fact that the average age and metallicity of
low mass satellite galaxies increase with the mass of the halo in which they
reside. A comparison with the semi-analytical model of Wang et al. (2008) shows
that it succesfully reproduces the fact that satellites are older than centrals
of the same stellar mass and that the age difference increases with the halo
mass of the satellite. This is a consequence of strangulation, which leaves the
stellar populations of satellites to evolve passively, while the prolonged star
formation activity of centrals keeps their average ages younger. The resulting
age offset is larger in more massive environments because their satellites were
accreted earlier. The model fails, however, in reproducing the halo mass
dependence of the metallicities of low mass satellites, yields
metallicity-stellar mass and age-stellar mass relations that are too shallow,
and predicts that satellite galaxies have the same metallicities as centrals of
the same stellar mass, in disagreement with the data. We argue that these
discrepancies are likely to indicate the need to (i) modify the recipes of both
supernova feedback and AGN feedback, (ii) use a more realistic description of
strangulation, and (iii) include a proper treatment of the tidal stripping,
heating and destruction of satellite galaxies. [Abridged]Comment: 20 pages, 12 figures, submitted for publication in MNRA
Uno studio archeologico delle galassie negli ultimi 7 miliardi di anni
New observations performed with the Inamori Magellan Areal Camera and Spectrograph (IMACS) on the Magellan telescope of the Las Campanas Observatory have made it possible to determine the average age and the chemical composition of the stellar populations in the galaxies belonging to the Universe of 7 billion years ago. Through these observations it is possible to study how these physical properties changed over time and identify the ways in which the galaxies may have evolved through to the present Universe.Nuove osservazioni con lo spettrografo IMACS sul telescopio Magellan dell’Osservatorio Las Campanas hanno permesso di determinare l’età media e la composizione chimica delle popolazioni stellari in galassie appartenenti all’Universo di 7 miliardi di anni fa. Tramite queste osservazioni è possibile studiare la variazione di queste proprietà fisiche nel tempo e individuare i possibili percorsi evolutivi delle galassie fino all’Universo attuale
A merger in the dusty, galaxy A1689-zD1?
The gravitationally-lensed galaxy A1689-zD1 is one of the most distant
spectroscopically confirmed sources (). It is the earliest known galaxy
where the interstellar medium (ISM) has been detected; dust emission was
detected with the Atacama Large Millimetre Array (ALMA). A1689-zD1 is also
unusual among high-redshift dust emitters as it is a sub-L* galaxy and is
therefore a good prospect for the detection of gaseous ISM in a more typical
galaxy at this redshift. We observed A1689-zD1 with ALMA in bands 6 and 7 and
with the Green Bank Telescope (GBT) in band . To study the structure of
A1689-zD1, we map the mm thermal dust emission and find two spatial components
with sizes about \,kpc (lensing-corrected). The rough spatial
morphology is similar to what is observed in the near-infrared with {\it HST}
and points to a perturbed dynamical state, perhaps indicative of a major merger
or a disc in early formation. The ALMA photometry is used to constrain the
far-infrared spectral energy distribution, yielding a dust temperature (--\,K for ). We do not detect the CO(3-2) line
in the GBT data with a 95\% upper limit of 0.3\,mJy observed. We find a slight
excess emission in ALMA band~6 at 220.9\,GHz. If this excess is real, it is
likely due to emission from the [CII] 158.8\,m line at . The stringent upper limits on the [CII]/ luminosity ratio
suggest a [CII] deficit similar to several bright quasars and massive
starbursts.Comment: 9 pages, accepted to MNRAS, in pres
Stellar mass as the "glocal" driver of galaxies' stellar population properties
The properties of the stellar populations in a galaxy are known to correlate
with the amount and the distribution of stellar mass. We take advantage of the
maps of light-weighted mean stellar age Agewr and metallicity Z*wr for a sample
of 362 galaxies from the integral-field spectroscopic survey CALIFA (summing up
to >600,000 individual regions of approximately 1 kpc linear size), produced in
our previous works, to investigate how these local properties react to the
local stellar-mass surface density mu* and to the global total stellar mass M*
and mean stellar-mass surface density e. We establish the existence of i) a
dual mu*-Agewr relation, resulting in a young sequence and an old ridge, and
ii) a mu*-Z*wr relation, overall independent of the age of the regions. The
global mass parameters (M* and, possibly secondarily, e) determine the
distribution of mu* in a galaxy and set the maximum attainable mu*, which
increases with M*. M* affects the shape and normalization of the local
relations up to a threshold mass of MSun, above which they
remain unchanged. We conclude that stellar mass is a "glocal" (i.e.
simultaneously global and local) driver of the stellar population properties.
We consider how the local and global mass-age and mass-metallicity relations
are connected, and in particular discuss how it is possible, from a single
local relation, to produce two different global mass-metallicity relations for
quiescent and star-forming galaxies respectively, as reported in the
literature. Structural differences in these two classes of galaxies are key to
explain the duality in global scaling relations and appear as essential in
modelling the baryonic cycle of galaxies
A dynamical view on stellar metallicity gradient diversity across the Hubble sequence with CALIFA
We analyze radial stellar metallicity and kinematic profiles out to 1Re in
244 CALIFA galaxies ranging from morphological type E to Sd, to study the
evolutionary mechanisms of stellar population gradients. We find that linear
metallicity gradients exhibit a clear correlation with galaxy morphological
type - with early type galaxies showing the steepest gradients. We show that
the metallicity gradients simply reflect the local mass-metallicity relation
within a galaxy. This suggests that the radial stellar population distribution
within a galaxys effective radius is primarily a result of the \emph{in-situ}
local star formation history. In this simple picture, the dynamically derived
stellar surface mass density gradient directly predicts the metallicity
gradient of a galaxy. We show that this correlation and its scatter can be
reproduced entirely by using independent empirical galaxy structural and
chemical scaling relations. Using Schwarzschild dynamical models, we also
explore the link between a galaxys local stellar populations and their orbital
structures. We find that galaxies angular momentum and metallicity gradients
show no obvious causal link. This suggests that metallicity gradients in the
inner disk are not strongly shaped by radial migration, which is confirmed by
the lack of correlation between the metallicity gradients and observable probes
of radial migration in the galaxies, such as bars and spiral arms. Finally, we
find that galaxies with positive metallicity gradients become increasingly
common towards low mass and late morphological types - consistent with stellar
feedback more efficiently modifying the baryon cycle in the central regions of
these galaxies.Comment: 20 pages, 13 figure
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