228 research outputs found
Metallicity calibrations of low star-forming galaxies: the influence of a stochastic IMF
We present a study of the consequences of an initial mass function that is
stochastically sampled on the main emission lines used for gas-phase
metallicity estimates in extra-galactic sources. We use the stochastic stellar
population code SLUG and the photoionisation code Cloudy to show that the
stochastic sampling of the massive end of the mass function can lead to clear
variations in the relative production of energetic emission lines such as
[OIII] relative to that of Balmer lines. We use this to study the impact on the
Te, N2O2, R23 and O3N2 metallicity calibrators. We find that stochastic
sampling of the IMF leads to a systematic over-estimate of O/H in galaxies with
low star formation rates (< M/yr) when using the N2O2, R23
and O3N2 strong-line methods, and an under-estimate when using the Te method on
galaxies of sub-solar metallicity. We point out that while the
SFR(Ha)-to-SFR(UV) ratio can be used to identify systems where the initial mass
function might be insufficiently sampled, it does not provide sufficient
information to fully correct the metallicity calibrations at low star formation
rates. Care must therefore be given in the choice of metallicity indicators in
such systems, with the N2O2 indicator proving most robust of those tested by
us, with a bias of 0.08 dex for models with SFR = M/yr and
solar metallicity.Comment: 16 pages, 10 figures and 2 tables; accepted for publication on MNRA
High-z galaxies with JWST and local analogues -- it is not only star formation
I present an analysis of the JWST NIRSpec data of SMACS 0723 released as
Early Release Observations. As part of this three new redshifts are provided,
bringing the total of reliable redshifts to 14. I propose a modification to the
direct abundance determination method that reduces sensitivity to flux
calibration uncertainties by a factor of ~3 and show that the resulting
abundances are in good agreement with Bayesian photoionization models of the
rest-frame optical spectrum. I also show that 6355 is most likely a narrow-line
active galactic nucleus (AGN) with Msun at z=7.66, and argue
that 10612 might also have an AGN contribution to its flux through comparison
to photoionization models and low-redshift analogues. Under the assumption that
the lines come from star-formation I find that the galaxies have gas depletion
times of ~ years, comparable to similar galaxies locally. I also identify
a population of possibly shock-dominated galaxies at z<3 whose near-IR emission
lines plausibly come nearly all from shocks and discuss their implications. I
close with a discussion of the potential for biases in the determination of the
mass-metallicity relation using samples defined by detected [O III]4363 and
show using low-z galaxies that this can lead to biases of up to 0.5 dex with a
systematic trend with mass.Comment: Submitted to MNRA
Stars were born in significantly denser regions in the early Universe
The density of the warm ionized gas in high-redshift galaxies is known to be
higher than what is typical in local galaxies on similar scales. At the same
time, the mean global properties of the high- and low-redshift galaxies are
quite different. Here, we present a detailed differential analysis of the
ionization parameters of 14 star-forming galaxies at redshift 2.6-3.4, compiled
from the literature. For each of those high-redshift galaxies, we construct a
comparison sample of low-redshift galaxies closely matched in specific star
formation rate (sSFR) and stellar mass, thus ensuring that their global
physical conditions are similar to the high-redshift galaxy. We find that the
median log [OIII] 5007/ [OII] 3727 line ratio of the high-redshift galaxies is
0.5 dex higher than their local counterparts. We construct a new calibration
between the [OIII] 5007/ [OII] 3727 emission line ratio and ionization
parameter to estimate the difference between the ionization parameters in the
high and low-redshift samples. Using this, we show that the typical density of
the warm ionized gas in star-forming regions decreases by a median factor of
from z ~ 3.3 to z ~ 0 at fixed mass and sSFR. We show that
metallicity differences cannot explain the observed density differences.
Because the high- and low-redshift samples are comparable in size, we infer
that the relationship between star formation rate density and gas density must
have been significantly less efficient at z ~2-3 than what is observed in
nearby galaxies with similar levels of star formation activity.Comment: 16 pages, 6 figures, accepted for publication in Ap
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
Leo T Dissected with the MUSE-Faint Survey
Leo T is the lowest mass galaxy known to contain neutral gas and to show
signs of recent star formation, which makes it a valuable laboratory for
studying the nature of gas and star formation at the limits of where galaxies
are found to have rejuvenating episodes of star formation. Here we discuss a
novel study of Leo T that uses data from the MUSE integral field spectrograph
and photometric data from HST. The high sensitivity of MUSE allowed us to
increase the number of Leo T stars observed spectroscopically from 19 to 75. We
studied the age and metallicity of these stars and identified two populations,
all consistent with similar metallicity of [Fe/H] -1.5 dex, suggesting
that a large fraction of metals were ejected. Within the young population, we
discovered three emission line Be stars, supporting the conclusion that rapidly
rotating massive stars are common in metal-poor environments. We find
differences in the dynamics of young and old stars, with the young population
having a velocity dispersion consistent with the kinematics of the cold
component of the neutral gas. This finding directly links the recent star
formation in Leo T with the cold component of the neutral gas.Comment: 6 pages, 6 figures. Accepted for publication in the proceedings of
IAU Symposium 379: Dynamical Masses of Local Group Galaxies. Based on source
article arXiv:2308.1626
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
Bursty stellar populations and obscured AGN in galaxy bulges
[Abridged] We investigate trends between the recent star formation history
and black hole growth in galaxy bulges in the Sloan Digital Sky Survey (SDSS).
The galaxies lie at 0.01<z<0.07 where the fibre aperture covers only the
central 0.6-4.0kpc diameter of the galaxy. We find strong trends between black
hole growth, as measured by dust-attenuation-corrected OIII luminosity, and the
recent star formation history of the bulges. We conclude that our results
support the popular hypothesis for black hole growth occurring through gas
inflow into the central regions of galaxies, followed by a starburst and
triggering of the AGN. However, while this is a significant pathway for the
growth of black holes, it is not the dominant one in the present-day Universe.
More unspectacular processes are apparently responsible for the majority of
this growth.
In order to arrive at these conclusions we have developed a set of new high
signal-to-noise ratio (SNR) optical spectral indicators, designed to allow a
detailed study of stellar populations which have undergone recent enhanced star
formation. Working in the rest-frame wavelength range 3750-4150AA, ideally
suited to many recent and ongoing spectroscopic surveys at low and high
redshift, the first two indices are equivalent to the previously well studied
4000AA break strength and Hdelta equivalent width. The primary advantage of
this new method is a greatly improved SNR for the latter index, allowing the
present study to use spectra with SNR-per-pixel as low as 8.Comment: 27 pages, submitted to MNRAS. Due to astro-ph size restrictions 6
figures in appendix are available as separate files. Full version, with full
resolution figures available at
http://www.mpa-garching.mpg.de/~vwild/HDelta/Hd_PCAmethod.pd
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