234 research outputs found
Modelling and interpreting spectral energy distributions of galaxies with BEAGLE
We present a new-generation tool to model and interpret spectral energy
distributions (SEDs) of galaxies, which incorporates in a consistent way the
production of radiation and its transfer through the interstellar and
intergalactic media. This flexible tool, named BEAGLE (for BayEsian Analysis of
GaLaxy sEds), allows one to build mock galaxy catalogues as well as to
interpret any combination of photometric and spectroscopic galaxy observations
in terms of physical parameters. The current version of the tool includes
versatile modeling of the emission from stars and photoionized gas, attenuation
by dust and accounting for different instrumental effects, such as
spectroscopic flux calibration and line spread function. We show a first
application of the BEAGLE tool to the interpretation of broadband SEDs of a
published sample of galaxies at redshifts . We find that the constraints derived on photometric redshifts
using this multi-purpose tool are comparable to those obtained using public,
dedicated photometric-redshift codes and quantify this result in a rigorous
statistical way. We also show how the post-processing of BEAGLE output data
with the Python extension PYP-BEAGLE allows the characterization of systematic
deviations between models and observations, in particular through posterior
predictive checks. The modular design of the BEAGLE tool allows easy extensions
to incorporate, for example, the absorption by neutral galactic and
circumgalactic gas, and the emission from an active galactic nucleus, dust and
shock-ionized gas. Information about public releases of the BEAGLE tool will be
maintained on http://www.jacopochevallard.org/beagle.Comment: added missing term in equation 4.1 (Erratum submitted to MNRAS
The [OIII]H Equivalent Width Distribution at z7: Implications for the Contribution of Galaxies to Reionization
We quantify the distribution of [OIII]+H line strengths at z7
using a sample of 20 bright (M 21) galaxies. We
select these systems over wide-area fields (2.3 deg total) using a new
colour-selection which precisely selects galaxies at z6.636.83, a
redshift range where blue Spitzer/IRAC [3.6][4.5] colours unambiguously
indicate strong [OIII]H emission. These 20 galaxies suggest a
log-normal [OIII]H EW distribution with median EW =
759 and standard deviation =
0.26 dex. We find no evidence for strong variation in this EW
distribution with UV luminosity. The typical [OIII]+H EW at z7
implied by our sample is considerably larger than that in massive star forming
galaxies at z2, consistent with a shift toward larger average sSFR (4.4
Gyr) and lower metallicities (0.16 Z). We also find evidence for
the emergence of a population with yet more extreme nebular emission
([OIII]+H EW1200 ) that is rarely seen at
lower redshifts. These objects have extremely large sSFR (30 Gyr), as
would be expected for systems undergoing a burst or upturn in star formation.
While this may be a short-lived phase, our results suggest that 20% of the
z7 population has such extreme nebular emission, implying that galaxies
likely undergo intense star formation episodes regularly at z6. We argue
that this population may be among the most effective ionizing agents in the
reionization era, both in terms of photon production efficiency and escape
fraction. We furthermore suggest that galaxies passing through this large sSFR
phase are likely to be very efficient in forming bound star clusters.Comment: 20 pages, 11 figures. Accepted in MNRAS with minor revision
Resolved maps of stellar mass and SED of galaxies from optical/NIR imaging and SPS models
We report on the method developed by Zibetti, Charlot and Rix (2009) to
construct resolved stellar mass maps of galaxies from optical and NIR imaging.
Accurate pixel-by-pixel colour information (specifically g-i and i-H) is
converted into stellar mass-to-light ratios with typical accuracy of 30%, based
on median likelihoods derived from a Monte Carlo library of 50,000 stellar
population synthesis models that include dust and updated TP-AGB phase
prescriptions. Hence, surface mass densities are computed. In a pilot study, we
analyze 9 galaxies spanning a broad range of morphologies. Among the main
results, we find that: i) galaxies appear much smoother in stellar mass maps
than at any optical or NIR wavelength; ii) total stellar mass estimates based
on unresolved photometry are biased low with respect to the integral of
resolved stellar mass maps, by up to 40%, due to dust obscured regions being
under-represented in global colours; iii) within a galaxy, on local scales
colours correlate with surface stellar mass density; iv) the slope and
tightness of this correlation reflect/depend on the morphology of the galaxy.Comment: 4 pages, 2 figures, to appear in the proceedings of the IAU Symposium
262 "Stellar Populations: Planning for the Next Decade", Charlot & Bruzual
ed
Strong Gravitational Lensing and the Stellar IMF of Early-type Galaxies
Systematic variations of the IMF in early-type galaxies, and their connection
with possible drivers such as velocity dispersion or metallicity, have been
much debated in recent years. Strong lensing over galaxy scales combined with
photometric and spectroscopic data provides a powerful method to constrain the
stellar mass-to-light ratio and hence the functional form of the IMF. We
combine photometric and spectroscopic constraints from the latest set of
population synthesis models of Charlot & Bruzual, including a varying IMF, with
a non-parametric analysis of the lens masses of 18 ETGs from the SLACS survey,
with velocity dispersions in the range 200-300 km/s. We find that very
bottom-heavy IMFs are excluded. However, the upper limit to the bimodal IMF
slope (, accounting for a dark matter fraction of 20-30%,
where corresponds to a Kroupa-like IMF) is compatible at the
level with constraints imposed by gravity-sensitive line strengths. A
two-segment power law parameterisation of the IMF (Salpeter-like for high
masses) is more constrained (, where is the power
index at low masses) but requires a dark matter contribution of
to reconcile the results with a Salpeter IMF. For a standard Milky Way-like IMF
to be applicable, a significant dark matter contribution is required within
. Our results reveal a large range of allowed IMF slopes, which, when
interpreted as intrinsic scatter in the IMF properties of ETGs, could explain
the recent results of Smith et al., who find Milky Way-like IMF normalisations
in a few massive lensing ETGs.Comment: Accepted for publication in MNRAS, 18 pages, 12 figures, 4 table
Angular Sizes of Faint Field Disk Galaxies: Intrinsic Luminosity Evolution
In order to explain the small scale-lengths detected in the recent deep field
observations performed from large ground-based telescopes and from the Hubble
Space Telescope, we investigate the predictions at high redshifts for disk
galaxies that formed by infall. Changes with redshift in the observed
properties of field galaxies are directly related to the evolution of the disks
and of the stellar populations. We see that changes in the rest frame
luminosity of a galaxy induce smaller values of half-light radii than are
predicted assuming no evolution. Comparisons are presented with two observed
samples from Mutz et al. (1994) and Smail et al. (1995).Comment: plain tex file + 3 postscript figures. To be published in ApJ
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
Integrated Stellar Populations: Confronting Photometry with Spectroscopy
We investigate the ability of spectroscopic techniques to yield realistic
star formation histories (SFHs) for the bulges of spiral galaxies based on a
comparison with their observed broadband colors. Full spectrum fitting to
optical spectra indicates that recent (within ~1 Gyr) star formation activity
can contribute significantly to the V-band flux, whilst accounting for only a
minor fraction of the stellar mass budget which is made up primarily of old
stars. Furthermore, recent implementations of stellar population (SP) models
reveal that the inclusion of a more complete treatment of the thermally
pulsating asymptotic giant branch (TP-AGB) phase to SP models greatly increases
the NIR flux for SPs of ages 0.2-2 Gyr. Comparing the optical--NIR colors
predicted from population synthesis fitting, using models which do not include
all stages of the TP-AGB phase, to the observed colors reveals that observed
optical--NIR colors are too red compared to the model predictions. However,
when a 1 Gyr SP from models including a full treatment the TP-AGB phase is
used, the observed and predicted colors are in good agreement. This has strong
implications for the interpretation of stellar populations, dust content, and
SFHs derived from colors alone.Comment: 6 pages, 6 figures, accepted for publication in the Astrophysical
Journa
Ultraviolet spectra of extreme nearby star-forming regions --- approaching a local reference sample for JWST
Nearby dwarf galaxies provide a unique laboratory in which to test stellar
population models below . Such tests are particularly important for
interpreting the surprising high-ionization UV line emission detected at
in recent years. We present HST/COS ultraviolet spectra of ten nearby
metal-poor star-forming galaxies selected to show He II emission in SDSS
optical spectra. The targets span nearly a dex in gas-phase oxygen abundance
() and present uniformly large specific star
formation rates (sSFR ). The UV spectra confirm
that metal-poor stellar populations can power extreme nebular emission in
high-ionization UV lines, reaching C III] equivalent widths comparable to those
seen in systems at . Our data reveal a marked transition in UV
spectral properties with decreasing metallicity, with systems below
() presenting minimal
stellar wind features and prominent nebular emission in He II and C IV. This is
consistent with nearly an order of magnitude increase in ionizing photon
production beyond the -ionizing edge relative to H-ionizing flux
as metallicity decreases below a fifth solar, well in excess of standard
stellar population synthesis predictions. Our results suggest that often
neglected sources of energetic radiation such as stripped binary products and
very massive O-stars produce a sharper change in the ionizing spectrum with
decreasing metallicity than expected. Consequently, nebular emission in C IV
and He II powered by these stars may provide useful metallicity constraints in
the reionization era.Comment: 27 pages, 13 figures, 11 tables, accepted for publication in MNRA
On the ages of bright galaxies Myr after the Big Bang: insights into star formation activity at with JWST
With JWST, new opportunities to study the formation and evolution of galaxies
in the early Universe are now emerging. Spitzer constraints on rest-optical
properties of galaxies demonstrated the power of using stellar
masses and star formation histories (SFHs) of galaxies to indirectly infer the
star formation history of the Universe. However, only the brightest individual
objects at could be detected with Spitzer, making it difficult to
robustly constrain past activity at . Here, we leverage the
greatly improved rest-optical sensitivity of JWST at to constrain
the ages and SFHs of eleven UV-bright () galaxies
selected to lie at , then investigate implications for star
formation activity at . We infer the properties of individual
objects in our sample with two spectral energy distribution modelling codes,
then infer a distribution of ages for bright galaxies. We
find a median age of Myr, younger than that inferred at
with a similar analysis, which is consistent with an evolution towards larger
specific star formation rates at early times. The age distribution suggests
that only percent of bright galaxies would be
similarly luminous at , implying that the number density of
bright galaxies declines by approximately an order of magnitude between and . This evolution is challenging to reconcile with some
early JWST results suggesting that the abundance of bright galaxies does not
significantly decrease towards very early times, but we suggest this tension
may be eased if young stellar populations form on top of older stellar
components, or if bright galaxies at are observed during a burst of
star formation.Comment: 13 pages, 9 figure
Searching for Extremely Blue UV Continuum Slopes at in JWST/NIRCam Imaging: Implications for Stellar Metallicity and Ionizing Photon Escape in Early Galaxies
The ultraviolet (UV) continuum slope ( where f) of galaxies is sensitive to a variety of properties, from the
metallicity and age of the stellar population to the attenuation from dust
through the galaxy. Considerable attention has focused on identifying
reionization-era galaxies with very blue UV slopes (). Not only do
such systems provide a signpost of low metallicity stars, but they also
identify galaxies that likely have ionizing photons leaking from their HII
regions as such blue UV slopes can only be seen if the reddening effect of
nebular continuum has been diminished. In this paper we present a search for
reionization-era galaxies with very blue UV colors in recent JWST/NIRCam
imaging of the EGS field. We characterize UV slopes for a large sample of
galaxies, finding a median value of . Three of the
lower luminosity (M) and lower stellar mass
(5-6M) systems exhibit both extremely blue UV slopes
( to ) and rest-optical photometry indicating weak nebular
line emission. Each system is very compact (r260 pc) with very high star
formation rate surface densities. We model the SEDs with a suite of BEAGLE
models with varying levels of ionizing photon escape. The SEDs cannot be
reproduced with our fiducial (f=0) or alpha enhanced
(Z) models. The combined blue UV slopes and weak nebular
emission are best-fit by models with significant ionizing photon escape from
HII regions (f=0.6-0.8) and extremely low metallicity massive
stars (Z=0.01-0.06 Z). The discovery of these galaxies highlights
the potential for JWST to identify large numbers of candidate Lyman Continuum
leaking galaxies in the reionization era and suggests low metallicity stellar
populations may be veryComment: 11 pages, 8 figures, 2 tables; Submitted to Ap
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