5,561 research outputs found
Nonisotropic 3-level Quantum Systems: Complete Solutions for Minimum Time and Minimum Energy
We apply techniques of subriemannian geometry on Lie groups and of optimal
synthesis on 2-D manifolds to the population transfer problem in a three-level
quantum system driven by two laser pulses, of arbitrary shape and frequency. In
the rotating wave approximation, we consider a nonisotropic model i.e. a model
in which the two coupling constants of the lasers are different. The aim is to
induce transitions from the first to the third level, minimizing 1) the time of
the transition (with bounded laser amplitudes),
2) the energy of lasers (with fixed final time). After reducing the problem
to real variables, for the purpose 1) we develop a theory of time optimal
syntheses for distributional problem on 2-D-manifolds, while for the purpose 2)
we use techniques of subriemannian geometry on 3-D Lie groups. The complete
optimal syntheses are computed.Comment: 29 pages, 6 figure
Extragalactic Globular Clusters in the Near Infrared: IV. Quantifying the Age Structure using Monte-Carlo Simulations
In previous papers of the series we used a combination of optical and
near-infrared colours to derive constraints on the relative age structure in
globular cluster systems. Here, we present the details, strength and
limitations of our method based on Monte-Carlo simulations of colour-colour
diagrams and cumulative age distributions. The simulations are based on general
informations about the globular cluster systems (e.g. colour-ranges, the number
ratios between sub-populations) and the different single stellar population
models (SSP's) which are used to derive relative ages. For both the modeled
systems and the observed globular cluster systems we derive the cumulative age
distribution and introduce two parameters to define it, the so-called 50 % age
and the result of the reduced test of the comparison between models
and observations. The method was tested successfully on several systems and
allowed to reveal significant intermediate age populations in two of them.Comment: 14 pages, 20 figures, accepted for publication in A&
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
Near Infrared spectroscopy of post-starburst galaxies: a limited impact of TP-AGB stars on galaxy SEDs
We present VLT-ISAAC NIR spectro-photometric observations of 16
post-starburst (PSB) galaxies aimed at constraining the debated influence of
TP-AGB stars on the SED of galaxies with stellar ages of 0.5-2 Gyr, critical
for high-redshift studies. PSB galaxies have negligible on-going star formation
and a SED dominated by the stars formed in a recent (<2 Gyr) burst. By
spectroscopically selecting PSB galaxies with mean luminosity-weighted ages
between 0.5 and 1.5 Gyr and a broad range of metallicities, we explore the
parameter space around the peak of the relative energy output of TP-AGB stars.
Crucially, we target galaxies at z~0.2, so that two main spectral features of
TP-AGB stars (C-molecule band-head drops at 1.41 and 1.77mum), which are
heavily absorbed by our atmosphere for targets at z~0, move inside the H and K
atmospheric windows and can be constrained for the first time to high accuracy.
Our findings provide key constraints to stellar population synthesis models: i)
The NIR regions around 1.41 and 1.77mum (rest-frame) are featureless for all
galaxies in our sample over the whole range of relevant ages and metallicities
at variance with the Maraston (2005) TP-AGB heavy models, which exhibit marked
drops there; ii) No flux boosting is observed in the NIR. The optical-NIR SEDs
of most of our PSB galaxies can be consistently reproduced with Bruzual &
Charlot (2003) models, using either simple stellar populations of corresponding
light-weighted ages and metallicities, or a more realistic burst plus an old
population containing <~60% of the total stellar mass. In contrast, all
combinations of this kind based on the Maraston (2005) models are unable to
simultaneously reproduce the smoothness of the NIR spectra and the relatively
blue optical-NIR colours in the observations. The data collected in this study
appear to disfavour TP-AGB heavy models with respect to TP-AGB light ones.Comment: MNRAS accepted. 21 pages, 11 figures. New version improves mainly on
clarity of figures and discussion of burst fraction/SF
The role of minor mergers in the recent star formation history of early-type galaxies
We demonstrate that the large scatter in the ultra-violet (UV) colours of
intermediate-mass early-type galaxies in the local Universe and the inferred
low-level recent star formation in these objects can be reproduced by minor
mergers in the standard LCDM cosmology. Numerical simulations of mergers with
mass ratios less than or equal to 1:4, with reasonable assumptions for the
ages, metallicities and dust properties of the merger progenitors, produce good
agreement to the observed UV colours of the early-type population, if the
infalling satellites are assumed to have (cold) gas fractions greater than 20%.
Early-types that satisfy (NUV-r) < 3.8 are likely to have experienced mergers
with mass ratios between 1:4 and 1:6 within the last ~1.5 Gyrs, while those
that satisfy 3.8<(NUV-r)<5.5 are consistent with either recent mergers with
mass ratios < 1:6 or mergers with higher mass ratios that occurred more than
~1.5 Gyrs in the past. We demonstrate that the early-type colour-magnitude
relations and colour distributions in both the UV and optical spectral ranges
are consistent with the expected frequency of minor merging activity in the
standard LCDM cosmology at low redshift. We present a strong plausibility
argument for minor mergers to be the principal mechanism behind the large UV
scatter and associated low-level recent star formation observed in early-type
galaxies in the nearby Universe.Comment: MNRAS in pres
The star formation histories of galaxies in the Sloan Digital Sky Survey
We present the results of a MOPED analysis of ~3 x 10^5 galaxy spectra from
the Sloan Digital Sky Survey Data Release Three (SDSS DR3), with a number of
improvements in data, modelling and analysis compared with our previous
analysis of DR1. The improvements include: modelling the galaxies with
theoretical models at a higher spectral resolution of 3\AA; better calibrated
data; an extended list of excluded emission lines, and a wider range of dust
models. We present new estimates of the cosmic star formation rate, the
evolution of stellar mass density and the stellar mass function from the fossil
record. In contrast to our earlier work the results show no conclusive peak in
the star formation rate out to a redshift around 2 but continue to show
conclusive evidence for `downsizing' in the SDSS fossil record. The star
formation history is now in good agreement with more traditional instantaneous
measures. The galaxy stellar mass function is determined over five decades of
mass, and an updated estimate of the current stellar mass density is presented.
We also investigate the systematic effects of changes in the stellar population
modelling, the spectral resolution, dust modelling, sky lines, spectral
resolution and the change of data set. We find that the main changes in the
results are due to the improvements in the calibration of the SDSS data,
changes in the initial mass function and the theoretical models used.Comment: replaced to match accepted version in MNRA
Differential stellar population models: how to reliably measure [Fe/H] and [alpha/Fe] in galaxies
We present differential stellar population models, which allow improved
determinations of the ages, iron and alpha-element abundances of old stellar
populations from spectral fitting. These new models are calibrated at solar
abundances using the predictions from classical, semi-empirical stellar
population models. We then use the predictive power of fully synthetic models
to compute predictions for different [Fe/H] and [alpha/Fe]. We show that these
new differential models provide remarkably accurate fits to the integrated
optical spectra of the bulge globular clusters NGC6528 and NGC6553, and that
the inferred [Fe/H] and [alpha/Fe] agree with values derived elsewhere from
stellar photometry and spectroscopy. The analysis of a small sample of SDSS
early-type galaxies further confirms that our alpha-enhanced models provide a
better fit to the spectra of massive ellipticals than the solar-scaled ones.
Our approach opens new opportunities for precision measurements of abundance
ratios in galaxies.Comment: 5 pages, 5 figures, MNRAS in pres
The propagation of uncertainties in stellar population synthesis modeling I: The relevance of uncertain aspects of stellar evolution and the IMF to the derived physical properties of galaxies
The stellar masses, mean ages, metallicities, and star formation histories of
galaxies are now commonly estimated via stellar population synthesis (SPS)
techniques. SPS relies on stellar evolution calculations from the main sequence
to stellar death, stellar spectral libraries, phenomenological dust models, and
stellar initial mass functions (IMFs). The present work is the first in a
series that explores the impact of uncertainties in key phases of stellar
evolution and the IMF on the derived physical properties of galaxies and the
expected luminosity evolution for a passively evolving set of stars. A
Monte-Carlo Markov-Chain approach is taken to fit near-UV through near-IR
photometry of a representative sample of low- and high-redshift galaxies with
this new SPS model. Significant results include the following: 1) including
uncertainties in stellar evolution, stellar masses at z~0 carry errors of ~0.3
dex at 95% CL with little dependence on luminosity or color, while at z~2, the
masses of bright red galaxies are uncertain at the ~0.6 dex level; 2) either
current stellar evolution models, current observational stellar libraries, or
both, do not adequately characterize the metallicity-dependence of the
thermally-pulsating asymptotic giant branch phase; 3) conservative estimates on
the uncertainty of the slope of the IMF in the solar neighborhood imply that
luminosity evolution per unit redshift is uncertain at the ~0.4 mag level in
the K-band, which is a substantial source of uncertainty for interpreting the
evolution of galaxy populations across time; 4) The more plausible assumption
of a distribution of stellar metallicities, rather than a fixed value as is
usually assumed, can have significant effects on the interpretation of colors
blueward of the V-band. (ABRIDGED)Comment: 21 pages, 17 figures, ApJ in pres
Probing recent star formation with absorption-line strengths in hierarchical models and observations
Stellar population parameters derived from spectral line-strengths provide a
powerful probe of galaxy properties and formation histories. We implement the
machinery for extracting single-stellar-population-equivalent stellar
population parameters from synthetic spectra generated by a hierarchical galaxy
formation model. We find that the SSP-equivalent age is related to the
light-weighted age in a complicated fashion that reflects the influence of
recently-formed stars and is poorly correlated with the mass-weighted age. The
tendency for SSP-equivalent ages to be biased young means that archaeological
downsizing overstates the mass-weighted downsizing in age with mass. We find
that the SSP-equivalent metallicity closely tracks the mass- and light-weighted
metallicities, so that observed mass--metallicity relations for old galaxies
closely reflect the underlying trends. We construct mock catalogues of
early-type galaxies in a Coma cluster-sized halo and compare them directly to
observations of early-type galaxies in the Coma cluster. The similarity of the
SSP-equivalent ages in the observational samples and the mock catalogues gives
us confidence that the star-formation quenching implemented in the hierarchical
galaxy formation model produces roughly the correct amount of recent star
formation. The SSP-equivalent metallicities are however too low and have the
wrong slope as a function of velocity dispersion, and the SSP-equivalent ages
of the model galaxies may have an incorrect slope as a function of velocity
dispersion. (Abridged)Comment: Accepted for publication in Monthly Notices of the Royal Astronomical
Society. 17 page
The stellar evolution of Luminous Red Galaxies, and its dependence on colour, redshift, luminosity and modelling
We present a series of colour evolution models for Luminous Red Galaxies
(LRGs) in the 7th spectroscopic data release of the Sloan Digital Sky Survey
(SDSS), computed using the full-spectrum fitting code VESPA on high
signal-to-noise stacked spectra. The colour-evolution models are computed as a
function of colour, luminosity and redshift, and we do not a-priori assume that
LRGs constitute a uniform population of galaxies in terms of stellar evolution.
By computing star-formation histories from the fossil record, the measured
stellar evolution of the galaxies is decoupled from the survey's selection
function, which also evolves with redshift. We present these evolutionary
models computed using three different sets of Stellar Population Synthesis
(SPS) codes. We show that the traditional fiducial model of purely passive
stellar evolution of LRGs is broadly correct, but it is not sufficient to
explain the full spectral signature. We also find that higher-order corrections
to this model are dependent on the SPS used, particularly when calculating the
amount of recent star formation. The amount of young stars can be
non-negligible in some cases, and has important implications for the
interpretation of the number density of LRGs within the selection box as a
function of redshift. Dust extinction, however, is more robust to the SPS
modelling: extinction increases with decreasing luminosity, increasing
redshift, and increasing r-i colour. We are making the colour evolution tracks
publicly available at http://www.icg.port.ac.uk/~tojeiror/lrg_evolution/.Comment: 29 pages, 34 figures, re-submitted to MNRAS after addressing the
referee's repor
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