90 research outputs found
Molecular gas in low-metallicity starburst galaxies: Scaling relations and the CO-to-H conversion factor
We study the molecular content and the star formation efficiency of 21 Blue
Compact Dwarfs (BCDs). We present CO(1-0) and (2-1) observations, further
supplemented with additional CO measurements and multiwavelength ancillary data
from the literature. We find the CO luminosity to be correlated with the
stellar and HI masses, SFR tracers, the size of the starburst and its
metallicity. BCDs appear offset from the Schmidt-Kennicutt (SK) law, showing
extremely low (0.1 Gyr) H2 and H2+HI depletion timescales. The
departure from the SK law is smaller when considering H2+HI rather than H2
only, and is larger for BCDs with lower metallicity and higher specific SFR.
Thus, the molecular fraction and H2 depletion timescale of BCDs is found to be
strongly correlated with metallicity. Using this and assuming that the
empirical correlation found between the specific SFR and galaxy-averaged H2
depletion timescale of more metal-rich galaxies extends to lower masses, we
derive a metallicity-dependent CO-to-H2 conversion factor , with in qualitative agreement
with previous determinations, dust-based measurements, and recent model
predictions. Our results suggest that in vigorously star-forming dwarfs the
fraction of H2 traced by CO decreases by a factor of about 40 from to , leading to a strong underestimation of
the H2 mass in metal-poor systems when a Galactic is
considered. Adopting we find that departures from the SK law
are partially resolved. Our results suggest that starbursting dwarfs have
shorter depletion gas timescales and lower molecular fractions compared to
normal late-type disc galaxies even accounting for the molecular gas not traced
by CO emission in metal-poor environments, raising additional constraints to
model predictions (Abridged).Comment: 18 pages, 14 Figures, 4 Tables: Accepted for publication in A&
Evidence of very low metallicity and high ionization state in a strongly lensed, star-forming dwarf galaxy at z=3.417
We investigate the gas-phase metallicity and Lyman Continuum (LyC) escape
fraction of a strongly gravitationally lensed, extreme emission-line galaxy at
z=3.417, J1000+0221S, recently discovered by the CANDELS team. We derive
ionization and metallicity sensitive emission-line ratios from H+K band
LBT/LUCI medium resolution spectroscopy. J1000+0221S shows high ionization
conditions, as evidenced by its enhanced [OIII]/[OII] and [OIII]/Hbeta ratios.
Consistently, strong-line methods based on the available line ratios suggest
that J1000+0221S is an extremely metal-poor galaxy, with a metallicity of
12+log(O/H) < 7.44 (< 5% solar), placing it among the most metal-poor
star-forming galaxies at z > 3 discovered so far. In combination with its low
stellar mass (2x10^8 Msun) and high star formation rate (5 Msun/yr), the
metallicity of J1000+0221S is consistent with the extrapolation to low masses
of the mass-metallicity relation traced by Lyman-break galaxies at z > 3, but
it is 0.55 dex lower than predicted by the fundamental metallicity relation at
z < 2.5. These observations suggest the picture of a rapidly growing galaxy,
possibly fed by the massive accretion of pristine gas. Additionally, deep
LBT/LBC in the UGR bands are used to derive a limit to the LyC escape fraction,
thus allowing us to explore for the first time the regime of sub-L* galaxies at
z > 3. We find a 1sigma upper limit to the escape fraction of 23%, which adds a
new observational constraint to recent theoretical models predicting that
sub-L* galaxies at high-z have high escape fractions and thus are the
responsible for the reioization of the Universe.Comment: 5 pages, 3 figures and 1 table. Accepted for publication in ApJ
Letter
The resolved chemical composition of the starburst dwarf galaxy CGCG007-025: Direct method versus photoionization model fitting
This work focuses on the gas chemical composition of CGCG007-025. This
compact dwarf is undergoing a galaxy wide star forming burst, whose spatial
behaviour has been observed by VLT/MUSE. We present a new line measurement
library to treat almost 7800 voxels. The direct method chemical analysis is
limited to 484 voxels with good detection of the temperature
diagnostic line. The recombination fluxes are corrected for stellar absorption
via a population synthesis. Additionally, we discuss a new algorithm to fit
photoionization models via neural networks. The 8 ionic abundances analyzed
show a spatial normal distribution with a , where only half
this value can be explained by the uncertainty in the measurements. The oxygen
abundance distribution is . The and
are also normally distributed. However, in the central and brightest
region, the is almost thrice the mean galaxy value. This is also
reflected in the extinction measurements. The ionization parameter has a
distribution of . The parameter spatial behaviour
agrees with the map. Finally, the discrepancies between the
direct method and the photoionization model fitting are discussed. In the
latter technique, we find that mixing lines with uneven uncertainty magnitudes
can impact the accuracy of the results. In these fittings, we recommend
overestimating the minimum flux uncertainty one order below the maximum line
flux uncertainty. This provides a better match with the direct method.Comment: Minor revision of your manuscript is requested before it is
reconsidered for publication in MNRA
Spitzer UltRa Faint SUrvey Program (SURFS UP). II. IRAC-Detected Lyman-Break Galaxies at 6 < z < 10 Behind Strong-Lensing Clusters
We study the stellar population properties of the IRAC-detected galaxy candidates from the Spitzer UltRa Faint SUrvey Program
(SURFS UP). Using the Lyman Break selection technique, we find a total of 16
new galaxy candidates at with in at
least one of the IRAC m and m bands. According to the best mass
models available for the surveyed galaxy clusters, these IRAC-detected galaxy
candidates are magnified by factors of --. We find that the
IRAC-detected sample is likely not a homogeneous
galaxy population: some are relatively massive (stellar mass as high as ) and evolved (age Myr) galaxies, while
others are less massive () and very
young ( Myr) galaxies with strong nebular emission lines that boost
their rest-frame optical fluxes. We identify two Ly emitters in our
sample from the Keck DEIMOS spectra, one at (in
RXJ1347) and one at (in MACS0454). We show that IRAC
color, when combined with photometric redshift, can be used to
identify galaxies likely with strong nebular emission lines within certain
redshift windows.Comment: ApJ in pres
The Bursty Star Formation Histories of Low-mass Galaxies at Revealed by Star Formation Rates Measured from H and FUV
We investigate the burstiness of star formation histories (SFHs) of galaxies
at by using the ratio of star formation rates (SFRs) measured from
H and FUV (1500 \AA) (H--to--FUV ratio). Our sample contains 164
galaxies down to stellar mass (M*) of in the CANDELS GOODS-N
region, where Team Keck Redshift Survey DEIMOS spectroscopy and HST/WFC3 F275W
images from CANDELS and Hubble Deep UV Legacy Survey are available. When the
{\it ratio} of H- and FUV-derived SFRs is measured, dust extinction
correction is negligible (except for very dusty galaxies) with the Calzetti
attenuation curve. The H--to--FUV ratio of our sample increases with M*
and SFR. The median ratio is 0.7 at M* (or SFR) and increases to 1 at M* (or SFR
). At M*, our median H--to--FUV
ratio is lower than that of local galaxies at the same M*, implying a redshift
evolution. Bursty SFH on a timescale of a few tens of megayears on galactic
scales provides a plausible explanation of our results, and the importance of
the burstiness increases as M* decreases. Due to sample selection effects, our
H--to--FUV ratio may be an upper limit of the true value of a complete
sample, which strengthens our conclusions. Other models, e.g., non-universal
initial mass function or stochastic star formation on star cluster scales, are
unable to plausibly explain our results.Comment: 13 pages, 8 figures. ApJ accepted. The main conclusions are not
changed. Major modifications include: (1) to be consistent with the
literature, now reporting H\beta--to--FUV ratio (rather than FUV--to--H\beta\
in the first version); (2) detailed discussions on dust extinction
correction; (3) new SF bustiness calculation; and (4) enriched discussions in
Introductio
Stellar Mass--Gas-phase Metallicity Relation at : A Power Law with Increasing Scatter toward the Low-mass Regime
We present the stellar mass ()--gas-phase metallicity relation (MZR)
and its scatter at intermediate redshifts () for 1381 field
galaxies collected from deep spectroscopic surveys. The star formation rate
(SFR) and color at a given of this magnitude-limited ( AB)
sample are representative of normal star-forming galaxies. For masses below
, our sample of 237 galaxies is 10 times larger than those
in previous studies beyond the local universe. This huge gain in sample size
enables superior constraints on the MZR and its scatter in the low-mass regime.
We find a power-law MZR at :
. Our MZR
shows good agreement with others measured at similar redshifts in the
literature in the intermediate and massive regimes, but is shallower than the
extrapolation of the MZRs of others to masses below . The SFR
dependence of the MZR in our sample is weaker than that found for local
galaxies (known as the Fundamental Metallicity Relation). Compared to a variety
of theoretical models, the slope of our MZR for low-mass galaxies agrees well
with predictions incorporating supernova energy-driven winds. Being robust
against currently uncertain metallicity calibrations, the scatter of the MZR
serves as a powerful diagnostic of the stochastic history of gas accretion, gas
recycling, and star formation of low-mass galaxies. Our major result is that
the scatter of our MZR increases as decreases. Our result implies that
either the scatter of the baryonic accretion rate or the scatter of the
-- relation increases as decreases. Moreover, our
measures of scatter at appears consistent with that found for local
galaxies.Comment: 18 pages, 10 figures. Accepted by ApJ. Typos correcte
Discovering extremely compact and metal-poor, star-forming dwarf galaxies out to z ~ 0.9 in the VIMOS Ultra-Deep Survey
We report the discovery of 31 low-luminosity (-14.5 > M_{AB}(B) > -18.8),
extreme emission line galaxies (EELGs) at 0.2 < z < 0.9 identified by their
unusually high rest-frame equivalent widths (100 < EW[OIII] < 1700 A) as part
of the VIMOS Ultra Deep Survey (VUDS). VIMOS optical spectra of unprecedented
sensitivity ( ~ 25 mag) along with multiwavelength photometry and HST
imaging are used to investigate spectrophotometric properties of this unique
sample and explore, for the first time, the very low stellar mass end (M* <
10^8 M) of the luminosity-metallicity (LZR) and mass-metallicity
(MZR) relations at z < 1. Characterized by their extreme compactness (R50 < 1
kpc), low stellar mass and enhanced specific star formation rates (SFR/M* ~
10^{-9} - 10^{-7} yr^{-1}), the VUDS EELGs are blue dwarf galaxies likely
experiencing the first stages of a vigorous galaxy-wide starburst. Using
T_e-sensitive direct and strong-line methods, we find that VUDS EELGs are
low-metallicity (7.5 < 12+log(O/H) < 8.3) galaxies with high ionization
conditions, including at least three EELGs showing HeII 4686A emission and four
EELGs of extremely metal-poor (<10% solar) galaxies. The LZR and MZR followed
by EELGs show relatively large scatter, being broadly consistent with the
extrapolation toward low luminosity and mass from previous studies at similar
redshift. However, we find evidences that galaxies with younger and more
vigorous star formation -- as characterized by their larger EWs, ionization and
sSFR -- tend to be more metal-poor at a given stellar mass.Comment: Letter in A&A 568, L8 (2014). This replacement matches the published
versio
Hubble Frontier Field Photometric Catalogues of Abell 370 and RXC J2248.7-4431: Multiwavelength photometry, photometric redshifts, and stellar properties
This paper presents multiwavelength photometric catalogues of the last two
Hubble Frontier Fields (HFF), the massive galaxy clusters Abell 370 and RXC
J2248.7-4431. The photometry ranges from imaging performed on the Hubble Space
Telescope (HST) to ground based Very Large Telescope (VLT) and Spitzer/IRAC, in
collaboration with the ASTRODEEP team, and using the ASTRODEEP pipeline. While
the main purpose of this paper is to release the catalogues, we also perform,
as a proof of concept, a brief analysis of z > 6 objects selected using
drop-out method, as well as spectroscopically confirmed sources and multiple
images in both clusters. While dropout methods yield a sample of high-z
galaxies, the addition of longer wavelength data reveals that as expected the
samples have substantial contamination at the ~30-45% level by dusty galaxies
at lower redshifts. Furthermore, we show that spectroscopic redshifts are still
required to unambiguously determine redshifts of multiply imaged systems.
Finally, the now publicly available ASTRODEEP catalogues were combined for all
HFFs and used to explore stellar properties of a large sample of 20,000
galaxies across a large photometric redshift range. The powerful magnification
provided by the HFF clusters allows us an exploration of the properties of
galaxies with intrinsic stellar masses as low as
and intrinsic star formation rates \mbox{SFRs}\sim 0.1\mbox{-}1M_\odot/\mbox
yr at z > 6.Comment: Replaced to match the accepted versio
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