1,888 research outputs found
A simple model to interpret the ultraviolet, optical and infrared SEDs of galaxies
We present a simple, largely empirical but physically motivated model, which
is designed to interpret consistently multi-wavelength observations from large
samples of galaxies in terms of physical parameters, such as star formation
rate, stellar mass and dust content. Our model is both simple and versatile
enough to allow the derivation of statistical constraints on the star formation
histories and dust contents of large samples of galaxies using a wide range of
ultraviolet, optical and infrared observations. We illustrate this by deriving
median-likelihood estimates of a set of physical parameters describing the
stellar and dust contents of local star-forming galaxies from the Spitzer
Infrared Nearby Galaxy Sample (SINGS) and from a newly-matched sample of SDSS
galaxies observed with GALEX, 2MASS, and IRAS. The model reproduces well the
observed spectral energy distributions of these galaxies across the entire
wavelength range from the far-ultraviolet to the far-infrared. We find
important correlations between the physical parameters of galaxies which are
useful to investigate the star formation activity and dust properties of
galaxies. Our model can be straightforwardly applied to interpret observed
ultraviolet-to-infrared spectral energy distributions (SEDs) from any galaxy
sample.Comment: 4 pages, 3 figures, to appear in the Conference Proceedings of IAU
Symposium No. 262: Stellar Populations - Planning for the Next Decade, G.
Bruzual & S. Charlot ed
The K-band luminosity function at z=1: a powerful constraint on galaxy formation theory
There are two major approaches to modelling galaxy evolution. The traditional
view is that the most massive galaxies were assembled early and have evolved
with steeply declining star formation rates since a redshift of 2 or higher.
According to hierarchical theories, massive galaxies were assembled much more
recently from mergers of smaller subunits. Here we present a simple
observational test designed to differentiate between the two. The observed
K-band flux from a galaxy is a good measure of its stellar mass even at high
redshift. It is probably only weakly affected by dust extinction. We compute
the evolution of the observed K-band luminosity function for traditional, pure
luminosity evolution (PLE) models and for hierarchical models. At z=0, both
models can fit the observed local K-band luminosity function. By redshift 1,
they differ greatly in the predicted abundance of bright galaxies. We calculate
the redshift distributions of K-band selected galaxies and compare these with
available data. We show that the number of K<19 galaxies with redshifts greater
than 1 is well below the numbers predicted by the PLE models. In the Songaila
et al (1994) redshift sample of 118 galaxies with 16<K<18, 33 galaxies are
predicted to lie at z>1. Only 2 are observed. In the Cowie et al. (1996)
redshift sample of 52 galaxies with 18<K<19, 28 galaxies are predicted to lie
at at z>1. Only 5 are observed. Both these samples are more than 90% complete.
We conclude that there is already strong evidence that the abundance of massive
galaxies at z=1 is well below the local value. This is inconsistent with the
traditional model, but similar to the expectations of hierarchical models.Comment: 13 pages, Latex, 4 figures included in text, submitted to MNRAS pink
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Timing the starburst-AGN connection
The mass of super massive black holes at the centre of galaxies is tightly
correlated with the mass of the galaxy bulges which host them. This observed
correlation implies a mechanism of joint growth, but the precise physical
processes responsible are a matter of some debate. Here we report on the growth
of black holes in 400 local galactic bulges which have experienced a strong
burst of star formation in the past 600Myr. The black holes in our sample have
typical masses of 10^6.5-10^7.5 solar masses, and the active nuclei have
bolometric luminosities of order 10^42-10^44erg/s. We combine stellar continuum
indices with H-alpha luminosities to measure a decay timescale of ~300Myr for
the decline in star formation after a starburst. During the first 600Myr after
a starburst, the black holes in our sample increase their mass by on-average 5%
and the total mass of stars formed is about 1000 times the total mass accreted
onto the black hole. This ratio is similar to the ratio of stellar to black
hole mass observed in present-day bulges. We find that the average rate of
accretion of matter onto the black hole rises steeply roughly 250Myr after the
onset of the starburst. We show that our results are consistent with a simple
model in which 0.5% of the mass lost by intermediate mass stars in the bulge is
accreted by the black hole, but with a suppression in the efficiency of black
hole growth at early times plausibly caused by supernova feedback, which is
stronger at earlier times. We suggest this picture may be more generally
applicable to black hole growth, and could help explain the strong correlation
between bulge and black hole mass.Comment: 16 pages, 12 figures, accepted for publication in MNRA
Modelling the nebular emission from primeval to present-day star-forming galaxies
We present a new model of the nebular emission from star-forming galaxies in
a wide range of chemical compositions, appropriate to interpret observations of
galaxies at all cosmic epochs. The model relies on the combination of
state-of-the-art stellar population synthesis and photoionization codes to
describe the ensemble of HII regions and the diffuse gas ionized by young stars
in a galaxy. A main feature of this model is the self-consistent yet versatile
treatment of element abundances and depletion onto dust grains, which allows
one to relate the observed nebular emission from a galaxy to both gas-phase and
dust-phase metal enrichment. We show that this model can account for the
rest-frame ultraviolet and optical emission-line properties of galaxies at
different redshifts and find that ultraviolet emission lines are more sensitive
than optical ones to parameters such as C/O abundance ratio, hydrogen gas
density, dust-to-metal mass ratio and upper cutoff of the stellar initial mass
function. We also find that, for gas-phase metallicities around solar to
slightly sub-solar, widely used formulae to constrain oxygen ionic fractions
and the C/O ratio from ultraviolet and optical emission-line luminosities are
reasonable faithful. However, the recipes break down at non-solar
metallicities, making them inappropriate to study chemically young galaxies. In
such cases, a fully self-consistent model of the kind presented in this paper
is required to interpret the observed nebular emission.Comment: 20 pages, 15 figures, Accepted for publication in MNRA
Theoretical Predictions for Surface Brightness Fluctuations and Implications for Stellar Populations of Elliptical Galaxies
(Abridged) We present new theoretical predictions for surface brightness
fluctuations (SBFs) using models optimized for this purpose. Our predictions
agree well with SBF data for globular clusters and elliptical galaxies. We
provide refined theoretical calibrations and k-corrections needed to use SBFs
as standard candles. We suggest that SBF distance measurements can be improved
by using a filter around 1 micron and calibrating I-band SBFs with the
integrated V-K galaxy color. We also show that current SBF data provide useful
constraints on population synthesis models, and we suggest SBF-based tests for
future models. The data favor specific choices of evolutionary tracks and
spectra in the models among the several choices allowed by comparisons based on
only integrated light. In addition, the tightness of the empirical I-band SBF
calibration suggests that model uncertainties in post-main sequence lifetimes
are less than +/-50% and that the IMF in ellipticals is not much steeper than
that in the solar neighborhood. Finally, we analyze the potential of SBFs for
probing unresolved stellar populations. We find that optical/near-IR SBFs are
much more sensitive to metallicity than to age. Therefore, SBF magnitudes and
colors are a valuable tool to break the age/metallicity degeneracy. Our initial
results suggest that the most luminous stellar populations of bright cluster
galaxies have roughly solar metallicities and about a factor of three spread in
age.Comment: Astrophysical Journal, in press (uses Apr 20, 2000 version of
emulateapj5.sty). Reposted version has a minor cosmetic change to Table
Relative merits of different types of rest-frame optical observations to constrain galaxy physical parameters
We present a new approach to constrain galaxy physical parameters from the
combined interpretation of stellar and nebular emission in wide ranges of
observations. This approach relies on the Bayesian analysis of any type of
galaxy spectral energy distribution using a comprehensive library of synthetic
spectra assembled using state-of-the-art models of star formation and chemical
enrichment histories, stellar population synthesis, nebular emission and
attenuation by dust. We focus on the constraints set by 5-band photometry and
low- and medium-resolution spectroscopy at optical rest wavelengths on a set of
physical parameters characterizing the stars and the interstellar medium. Since
these parameters cannot be known a priori for any galaxy sample, we assess the
accuracy to which they can be retrieved by simulating `pseudo-observations'
using models with known parameters. Assuming that these models are good
approximations of true galaxies, we find that the combined analysis of stellar
and nebular emission in low-resolution galaxy spectra provides valuable
constraints on all physical parameters. At higher resolution, the analysis of
the combined stellar and nebular emission in 12,660 SDSS star-forming galaxies
using our approach yields likelihood distributions of stellar mass, gas-phase
oxygen abundance, optical depth of the dust and specific star formation rate
similar to those obtained in previous separate analyses of the stellar and
nebular emission at the original (twice higher) SDSS spectral resolution. We
show that the constraints derived on galaxy physical parameters from these
different types of observations depend sensitively on signal-to-noise ratio.
Our approach can be extended to the analysis of any type of observation across
the wavelength range covered by spectral evolution models. [abridged]Comment: 24 pages, 19 figures, accepted for publication in MNRAS.
Full-resolution version available from
ftp://ftp.iap.fr/pub/from_users/pacifici/paper_pacifici_hr.pd
Extracting Star Formation Histories from Medium-resolution Galaxy Spectra
We adapt an existing data compression algorithm, MOPED, to the extraction of
median-likelihood star formation (SF) histories from medium-resolution galaxy
spectra. By focusing on the high-pass components of galaxy spectra, we minimize
potential uncertainties arising from the spectro-photometric calibration and
intrinsic attenuation by dust. We validate our approach using model high-pass
spectra of galaxies with different SF histories covering the wavelength range
3650-8500 A at a resolving power of about 2000. We show that the method can
recover the full SF histories of these models, without prior knowledge of the
metallicity, to within an accuracy that depends sensitively on signal-to-noise
ratio. The investigation of the sensitivity of the flux at each wavelength to
the mass fraction of stars of different ages allows us to identify new
age-sensitive features in galaxy spectra. We also highlight a fundamental
limitation in the recovery of the SF histories of galaxies for which the
optical signatures of intermediate-age stars are masked by those of younger and
older stars. We apply this method to derive average SF histories from the
highest-quality spectra of morphologically identified early- and late-type
galaxies in the SDSS EDR [...]. We also investigate the constraints set by the
high-pass signal in the stacked spectra of a magnitude-limited sample of
SDSS-EDR galaxies on the global SF history of the Universe. We confirm that the
stellar populations in the most massive galaxies today appear to have formed on
average earlier than those in the least massive ones. Our results do not
support the recent suggestion of a statistically significant peak in the SF
activity of the Universe at redshifts below unity, although such a peak is not
ruled out [abridged].Comment: 18 pages, 14 figures, to appear in MNRAS; version with full
resolution figures available at http://www.mpa-garching.mpg.de/~charlot/SFH
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