3,449 research outputs found
Applications of Stellar Population Synthesis in the Distant Universe
Comparison with artificial galaxy models is essential for translating the
incomplete and low signal-to-noise data we can obtain on astrophysical stellar
populations to physical interpretations which describe their composition,
physical properties, histories and internal conditions. In particular, this is
true for distant galaxies, whose unresolved light embeds clues to their
formation and evolution as well as their impact on their wider environs.
Stellar population synthesis models are now used as the foundation of analysis
at all redshifts, but are not without their problems. Here we review the use of
stellar population synthesis models, with a focus on applications in the
distant Universe.Comment: 32 page review, published in Galaxies special issue, "Star Formation
in the UV", ed. Jorick Vin
Empirical modeling of the stellar spectrum of galaxies
An empirical method of modeling the stellar spectrum of galaxies is proposed,
based on two successive applications of Principal Component Analysis (PCA). PCA
is first applied to the newly available stellar library STELIB, supplemented by
the J, H and K magnitudes taken mainly from the 2 Micron All Sky Survey
(2MASS). Next the resultant eigen-spectra are used to fit the observed spectra
of a sample of 1016 galaxies selected from the Sloan Digital Sky Survey Data
Release One (SDSS DR1). PCA is again applied, to the fitted spectra to
construct the eigen-spectra of galaxies with zero velocity dispersion. The
first 9 galactic eigen-spectra so obtained are then used to model the stellar
spectrum of the galaxies in SDSS DR1, and synchronously to estimate the stellar
velocity dispersion, the spectral type, the near-infrared SED, and the average
reddening. Extensive tests show that the spectra of different type galaxies can
be modeled quite accurately using these eigen-spectra. The method can yield
stellar velocity dispersion with accuracies better than 10%, for the spectra of
typical S/N ratios in SDSS DR1.Comment: 34 pages with 18 figures, submitted to A
Fitting the integrated Spectral Energy Distributions of Galaxies
Fitting the spectral energy distributions (SEDs) of galaxies is an almost
universally used technique that has matured significantly in the last decade.
Model predictions and fitting procedures have improved significantly over this
time, attempting to keep up with the vastly increased volume and quality of
available data. We review here the field of SED fitting, describing the
modelling of ultraviolet to infrared galaxy SEDs, the creation of
multiwavelength data sets, and the methods used to fit model SEDs to observed
galaxy data sets. We touch upon the achievements and challenges in the major
ingredients of SED fitting, with a special emphasis on describing the interplay
between the quality of the available data, the quality of the available models,
and the best fitting technique to use in order to obtain a realistic
measurement as well as realistic uncertainties. We conclude that SED fitting
can be used effectively to derive a range of physical properties of galaxies,
such as redshift, stellar masses, star formation rates, dust masses, and
metallicities, with care taken not to over-interpret the available data. Yet
there still exist many issues such as estimating the age of the oldest stars in
a galaxy, finer details ofdust properties and dust-star geometry, and the
influences of poorly understood, luminous stellar types and phases. The
challenge for the coming years will be to improve both the models and the
observational data sets to resolve these uncertainties. The present review will
be made available on an interactive, moderated web page (sedfitting.org), where
the community can access and change the text. The intention is to expand the
text and keep it up to date over the coming years.Comment: 54 pages, 26 figures, Accepted for publication in Astrophysics &
Space Scienc
Evidence for TP-AGB stars in high redshift galaxies, and their effect on deriving stellar population parameters
We explore the effects of stellar population models on estimating star
formation histories, ages and masses of high redshift galaxies. The focus is on
the Thermally-Pulsing Asymptotic Giant Branch (TP-AGB) phase of stellar
evolution, whose treatment is a source of major discrepancy among different
evolutionary population synthesis. In particular, besides the models usually
adopted in the literature, we use models (by Maraston 2005), in which the
contribution of the TP-AGB phase is observationally calibrated and is the
dominant source of near-IR energy for stellar populations in the age range 0.2
to 2 Gyr. We use a sample of high-z galaxies in the HUDF, with spectroscopic
redshifts, and Spitzer IRAC and MIPS photometry from GOODS. We find that the
TP-AGB phase plays a key role in the interpretation of Spitzer data for high-z
galaxies, when the rest-frame near-IR is sampled. When fitting without dust
reddening, the models with the empirically-calibrated TP-AGB phase always
reproduce better the observed spectral energy distributions (SEDs). Allowing
for dust reddening improves the fits with literature models in some cases. In
both cases, the results from Maraston models imply younger ages by factors up
to 6 and lower stellar masses (by ~60 % on average). The observed strengths of
the MgUV spectral feature compare better to the predicted ones in the case of
the Maraston models, implying a better overall consistency of SED fitting.
Finally, we find that photometric redshifts improve significantly using these
models on the SEDs extending over the IRAC bands. This work provides the first
direct evidence of TP-AGB stars in the primeval Universe.Comment: 14 pages, 10 figures, 3 tables, submitted to the Astrophysical
Journa
Stellar Metallicities and SNIa Rates in the Early-type Galaxy NGC5846 from ROSAT and ASCA Observations
In this paper we analyze the diffuse X-ray coronae surrounding the elliptical
galaxy NGC5846, combining measurements from two observatories, ROSAT and ASCA.
We map the gas temperature distribution and find a central cool region within
an approximately isothermal gas halo extending to a radius of about 50 kpc, and
evidence for a temperature decrease at larger radii. With a radially falling
temperature profile, the total mass converges to 9.6+/-1.0 10^12 Msun at ~230
kpc radius. Using the spectroscopic measurements, we also derive radial
distributions for the heavy elements silicon and iron and find that the
abundances of both decrease with galaxy radius. The mass ratio of Si to Fe lies
between the theoretical predictions for element production in SN Ia and SN II,
suggesting an important role for SN Ia, as well as SN II, for gas enrichment in
ellipticals. Using the SN Ia yield of Si, we set an upper limit of 0.012 SNU
for the SN Ia rate at radii >50 kpc, which is independent of possible
uncertainties in the iron L-shell modeling. We compare our observations with
the theoretical predictions for the chemical evolution of ellipticals, taken
from Matteucci & Gibson (1995). We conclude that the metal content in stars, if
explained by the star formation duration, requires a significant decline in the
duration of star formation with galaxy radius, ranging from ~1 Gyr at the
center to ~0.01 Gyr at 100 kpc radius. Alternatively, the decline in
metallicity with galaxy radius may be caused by a similar drop with radius in
the efficiency of star formation. Based on the Si and Fe measurements presented
in this paper, we conclude that the latter scenario is preferred, unless a
dependence of the SN Ia rate on stellar metallicity is invoked. (Abridged).Comment: 11 pages, figures&tables included, emulapj.sty, accepted for Ap
ArborZ: Photometric Redshifts Using Boosted Decision Trees
Precision photometric redshifts will be essential for extracting cosmological
parameters from the next generation of wide-area imaging surveys. In this paper
we introduce a photometric redshift algorithm, ArborZ, based on the
machine-learning technique of Boosted Decision Trees. We study the algorithm
using galaxies from the Sloan Digital Sky Survey and from mock catalogs
intended to simulate both the SDSS and the upcoming Dark Energy Survey. We show
that it improves upon the performance of existing algorithms. Moreover, the
method naturally leads to the reconstruction of a full probability density
function (PDF) for the photometric redshift of each galaxy, not merely a single
"best estimate" and error, and also provides a photo-z quality figure-of-merit
for each galaxy that can be used to reject outliers. We show that the stacked
PDFs yield a more accurate reconstruction of the redshift distribution N(z). We
discuss limitations of the current algorithm and ideas for future work.Comment: 10 pages, 13 figures, submitted to Ap
The GalMer database: Galaxy Mergers in the Virtual Observatory
We present the GalMer database, a library of galaxy merger simulations, made
available to users through tools compatible with the Virtual Observatory (VO)
standards adapted specially for this theoretical database. To investigate the
physics of galaxy formation through hierarchical merging, it is necessary to
simulate galaxy interactions varying a large number of parameters:
morphological types, mass ratios, orbital configurations, etc. On one side,
these simulations have to be run in a cosmological context, able to provide a
large number of galaxy pairs, with boundary conditions given by the large-scale
simulations, on the other side the resolution has to be high enough at galaxy
scales, to provide realistic physics. The GalMer database is a library of
thousands simulations of galaxy mergers at moderate spatial resolution and it
is a compromise between the diversity of initial conditions and the details of
underlying physics. We provide all coordinates and data of simulated particles
in FITS binary tables. The main advantages of the database are VO access
interfaces and value-added services which allow users to compare the results of
the simulations directly to observations: stellar population modelling, dust
extinction, spectra, images, visualisation using dedicated VO tools. The GalMer
value-added services can be used as virtual telescope producing broadband
images, 1D spectra, 3D spectral datacubes, thus making our database oriented
towards the usage by observers. We present several examples of the GalMer
database scientific usage obtained from the analysis of simulations and
modelling their stellar population properties, including: (1) studies of the
star formation efficiency in interactions; (2) creation of old counter-rotating
components; (3) reshaping metallicity profiles in elliptical galaxies; (4)
orbital to internal angular momentum transfer; (5) reproducing observed colour
bimodality of galaxies.Comment: 15 pages, 11 figures, 10 tables accepted to A&A. Visualisation of
GalMer simulations, access to snapshot files and value-added tools described
in the paper are available at http://galmer.obspm.fr
Stellar Content from high resolution galactic spectra via Maximum A Posteriori
This paper describes STECMAP (STEllar Content via Maximum A Posteriori), a
flexible, non-parametric inversion method for the interpretation of the
integrated light spectra of galaxies, based on synthetic spectra of single
stellar populations (SSPs). We focus on the recovery of a galaxy's star
formation history and stellar age-metallicity relation. We use the high
resolution SSPs produced by PEGASE-HR to quantify the informational content of
the wavelength range 4000 - 6800 Angstroms.
A detailed investigation of the properties of the corresponding simplified
linear problem is performed using singular value decomposition. It turns out to
be a powerful tool for explaining and predicting the behaviour of the
inversion. We provide means of quantifying the fundamental limitations of the
problem considering the intrinsic properties of the SSPs in the spectral range
of interest, as well as the noise in these models and in the data.
We performed a systematic simulation campaign and found that, when the time
elapsed between two bursts of star formation is larger than 0.8 dex, the
properties of each episode can be constrained with a precision of 0.04 dex in
age and 0.02 dex in metallicity from high quality data (R=10 000,
signal-to-noise ratio SNR=100 per pixel), not taking model errors into account.
The described methods and error estimates will be useful in the design and in
the analysis of extragalactic spectroscopic surveys.Comment: 31 pages, 23 figures, accepted for publication in MNRA
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