2,013 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
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
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
Immune relevant molecules identified in the skin mucus of fish using omics technologies
Author's accepted version (postprint)
Charting the evolution of the ages and metallicities of massive galaxies since z=0.7
The stellar populations of intermediate-redshift galaxies can shed light onto
the growth of massive galaxies in the last 8 billion years. We perform deep,
multi-object rest-frame optical spectroscopy with IMACS/Magellan of ~70
galaxies in the E-CDFS with redshift 0.6522.7 and
stellar mass >10^{10}Msun. Following the Bayesian approach adopted for previous
low-redshift studies, we constrain the stellar mass, mean stellar age and
stellar metallicity of individual galaxies from stellar absorption features. We
characterize for the first time the dependence of stellar metallicity and age
on stellar mass at z~0.7 for all galaxies and for quiescent and star-forming
galaxies separately. These relations for the whole sample have a similar shape
as the z=0.1 SDSS analog, but are shifted by -0.28 dex in age and by -0.13 dex
in metallicity, at odds with simple passive evolution. We find that no
additional star formation and chemical enrichment are required for z=0.7
quiescent galaxies to evolve into the present-day quiescent population.
However, this must be accompanied by the quenching of a fraction of z=0.7
Mstar>10^{11}Msun star-forming galaxies with metallicities comparable to those
of quiescent galaxies, thus increasing the scatter in age without affecting the
metallicity distribution. However rapid quenching of the entire population of
massive star-forming galaxies at z=0.7 would be inconsistent with the
age/metallicity--mass relation for the population as a whole and with the
metallicity distribution of star-forming galaxies only, which are on average
0.12 dex less metal-rich than their local counterparts. This indicates chemical
enrichment until the present in at least a fraction of the z=0.7 massive
star-forming galaxies.[abridged]Comment: accepted for publication on ApJ, 26 pages, 13 figure
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
Challenges in Stellar Population Studies
The stellar populations of galaxies contain a wealth of detailed information.
From the youngest, most massive stars, to almost invisible remnants, the
history of star formation is encoded in the stars that make up a galaxy.
Extracting some, or all, of this informationhas long been a goal of stellar
population studies. This was achieved in the last couple of decades and it is
now a routine task, which forms a crucial ingredient in much of observational
galaxy evolution, from our Galaxy out to the most distant systems found. In
many of these domains we are now limited not by sample size, but by systematic
uncertainties and this will increasingly be the case in the future.
The aim of this review is to outline the challenges faced by stellar
population studies in the coming decade within the context of upcoming
observational facilities. I will highlight the need to better understand the
near-IR spectral range and outline the difficulties presented by less well
understood phases of stellar evolution such as thermally pulsing AGB stars,
horizontal branch stars and the very first stars. The influence of rotation and
binarity on stellar population modeling is also briefly discussed.Comment: Plenary review talk at IAU GA in Rio de Janeiro to be published in
the proceedings of IAU Symposium 262. Movies and talk slides available at
http://www.strw.leidenuniv.nl/~jarle/IAU0
Alien Registration- Brinchmann, Christian Aron Juel N. (Portland, Cumberland County)
https://digitalmaine.com/alien_docs/21343/thumbnail.jp
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