95 research outputs found
The VANDELS survey: A strong correlation between Ly equivalent width and stellar metallicity at
We present the results of a new study investigating the relationship between
observed Ly equivalent width ((Ly)) and the
metallicity of the ionizing stellar population () for a sample of
star-forming galaxies at drawn from the VANDELS survey.
Dividing our sample into quartiles of rest-frame (Ly)
across the range -58 \unicode{xC5} \lesssim (Ly)
\lesssim 110 \unicode{xC5} we determine from full spectral
fitting of composite far-ultraviolet (FUV) spectra and find a clear
anti-correlation between (Ly) and . Our results
indicate that decreases by a factor between the lowest
(Ly) quartile
((Ly)\rangle=-18\unicode{xC5}) and the highest
(Ly) quartile
((Ly)\rangle=24\unicode{xC5}). Similarly,
galaxies typically defined as Lyman Alpha Emitters (LAEs;
(Ly) >20\unicode{xC5}) are, on average, metal poor with
respect to the non-LAE galaxy population ((Ly)
\leq20\unicode{xC5}) with
. Finally, based on the best-fitting stellar models, we
estimate that the increasing strength of the stellar ionizing spectrum towards
lower is responsible for of the observed variation
in (Ly) across our sample, with the remaining contribution
() being due to a decrease in the HI/dust covering fractions in
low galaxies.Comment: 10 pages, 6 figures, MNRAS accepte
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The VIMOS Ultra-Deep Survey: A major merger origin for the high fraction of galaxies at 2 < z < 6 with two bright clumps
(Abridged) The properties of stellar clumps in star forming galaxies and
their evolution over the redshift range are presented
and discussed in the context of the build-up of massive galaxies at early
cosmic times. We use HST/ACS images of galaxies with spectroscopic redshifts
from the VIMOS Ultra Deep Survey (VUDS) to identify clumps within a 20 kpc
radius. We find that the population of galaxies with more than one clump is
dominated by galaxies with two clumps, representing \% of the
population, while the fraction of galaxies with 3, or 4 and more, clumps is
8-11 and 7-9\%, respectively. The fraction of clumpy galaxies is in the range
over , increasing at higher redshifts, indicating that the
fraction of irregular galaxies remains high up to the highest redshifts. The
large and bright clumps (M up to M) are
found to reside predominantly in galaxies with two clumps. Smaller and lower
luminosity clumps () are
found in galaxies with three clumps or more. We interpret these results as
evidence for two different modes of clump formation working in parallel. The
small low luminosity clumps are likely the result of disc fragmentation, with
violent disc instabilities (VDI) forming several long-lived clumps in-situ, as
suggested from simulations. A fraction of these clumps is also likely coming
from minor mergers. The clumps in the dominating population of galaxies with
two clumps are significantly more massive and have properties akin to those in
merging pairs observed at similar redshifts; they appear as more massive than
the most massive clumps observed in VDI numerical simulations
Extreme emission-line galaxies out to z1 in zCOSMOS. I. Sample and characterization of global properties
We present a thorough characterization of a large sample of 183 extreme
emission-line galaxies (EELGs) at redshift 0.11 < z < 0.93 selected from the
20k zCOSMOS Bright Survey because of their unusually large emission line
equivalent widths. We use multiwavelength COSMOS photometry, HST-ACS I-band
imaging and optical zCOSMOS spectroscopy to derive the main global properties
of EELGs, such as sizes, masses, SFRs, reliable metallicities from both
"direct" and "strong-line" methods. The EELGs are compact (R_50 ~ 1.3 kpc),
low-mass (log(M*/Msol)~7-10) galaxies forming stars at unusually high specific
SFR (log(sSFR/yr) up to ~ -7) compared to main sequence SFGs of the same
stellar mass and redshift. At UV wavelengths, the EELGs are luminous and show
high surface brightness and include strong Ly emitters, as revealed by
GALEX spectroscopy. We show that zCOSMOS EELGs are high-ionization,
low-metallicity systems, with median 12+log(O/H)=8.16, including a handful of
extremely metal-deficient galaxies (<10% solar). While ~80% of the EELGs show
non-axisymmetric morphologies, including clumpy and tadpole galaxies, we find
that ~29% of them show additional low surface-brightness features, which
strongly suggest recent or ongoing interactions. As star-forming dwarfs in the
local Universe, EELGs are most often found in relative isolation. While only
very few EELGs belong to compact groups, almost one third of them are found in
spectroscopically confirmed loose pairs or triplets. We conclude that EELGs are
galaxies caught in a transient and probably early period of their evolution,
where they are efficiently building-up a significant fraction of their
present-day stellar mass in an ongoing galaxy-wide starburst. Therefore, the
EELGs constitute an ideal benchmark for comparison studies between low- and
high-redshift low-mass star-forming galaxies.Comment: Accepted in A&A. Final replacement to match the version in press. It
includes a minor change in the title and a new figur
The VIMOS Ultra Deep Survey: Ly Emission and Stellar Populations of Star-Forming Galaxies at 2<z<2.5
The aim of this paper is to investigate spectral and photometric properties
of 854 faint (<~25 mag) star-forming galaxies (SFGs) at 2<z<2.5 using
the VIMOS Ultra-Deep Survey (VUDS) spectroscopic data and deep multi-wavelength
photometric data in three extensively studied extragalactic fields (ECDFS,
VVDS, COSMOS). These SFGs were targeted for spectroscopy based on their
photometric redshifts. The VUDS spectra are used to measure the UV spectral
slopes () as well as Ly equivalent widths (EW). On average, the
spectroscopically measured (-1.360.02), is comparable to the
photometrically measured (-1.320.02), and has smaller measurement
uncertainties. The positive correlation of with the Spectral Energy
Distribution (SED)-based measurement of dust extinction, E(B-V),
emphasizes the importance of as an alternative dust indicator at high
redshifts. To make a proper comparison, we divide these SFGs into three
subgroups based on their rest-frame Ly EW: SFGs with no Ly
emission (SFG; EW0\AA), SFGs with Ly emission (SFG; EW0\AA), and Ly emitters (LAEs; EW20\AA). The fraction of
LAEs at these redshifts is 10%, which is consistent with previous
observations. We compared best-fit SED-estimated stellar parameters of the
SFG, SFG and LAE samples. For the luminosities probed here
(L), we find that galaxies with and without Ly in emission
have small but significant differences in their SED-based properties. We find
that LAEs have less dust, and lower star-formation rates (SFR) compared to
non-LAEs. We also find that LAEs are less massive compared to non-LAEs, though
the difference is smaller and less significant compared to the SFR and E(B-V). [abridged]Comment: Accepted for publication in A&A, 19 pages, 10 figures, 1 tabl
The stellar host in blue compact dwarf galaxies: the need for a two-dimensional fit
The structural properties of the low surface brightness stellar host in blue
compact dwarf galaxies are often studied by fitting r^{1/n} models to the outer
regions of their radial profiles. The limitations imposed by the presence of a
large starburst emission overlapping the underlying component makes this kind
of analysis a difficult task. We propose a two-dimensional fitting methodology
in order to improve the extraction of the structural parameters of the LSB
host. We discuss its advantages and weaknesses by using a set of simulated
galaxies and compare the results for a sample of eight objects with those
already obtained using a one-dimensional technique. We fit a PSF convolved
Sersic model to synthetic galaxies, and to real galaxy images in the B, V, R
filters. We restrict the fit to the stellar host by masking out the starburst
region and take special care to minimize the sky-subtraction uncertainties. In
order to test the robustness and flexibility of the method, we carry out a set
of fits with synthetic galaxies. Furthermore consistency checks are performed
to assess the reliability and accuracy of the derived structural parameters.
The more accurate isolation of the starburst emission is the most important
advantage and strength of the method. Thus, we fit the host galaxy in a range
of surface brightness and in a portion of area larger than in previous
published 1D fits with the same dataset. We obtain robust fits for all the
sample galaxies, all of which, except one, show Sersic indices n very close to
1, with good agreement in the three bands. These findings suggest that the
stellar hosts in BCDs have near-exponential profiles, a result that will help
us to understand the mechanisms that form and shape BCD galaxies, and how they
relate to the other dwarf galaxy classes.Comment: 22 pages, 15 figures (low resolution), accepted for publication in
A&A. A higher resolution version of the figures can be provided upon reques
The VIMOS Ultra Deep Survey: The Reversal of the Star Formation Rate -- Density Relation at
Utilizing spectroscopic observations taken for the VIMOS Ultra-Deep Survey
(VUDS), new observations from Keck/DEIMOS, and publicly available observations
of large samples of star-forming galaxies, we report here on the relationship
between the star formation rate (SFR) and the local environment
() of galaxies in the early universe (). Unlike what is
observed at lower redshifts (), we observe a definite, nearly monotonic
increase in the average SFR with increasing galaxy overdensity over more than
an order of magnitude in . The robustness of this trend is
quantified by accounting for both uncertainties in our measurements and galaxy
populations that are either underrepresented or not present in our sample
finding that the trend remains significant under all circumstances. This trend
appears to be primarily driven by the fractional increase of galaxies in high
density environments that are more massive in their stellar content and are
forming stars at a higher rate than their less massive counterparts. We find
that, even after stellar mass effects are accounted for, there remains a weak
but significant SFR- trend in our sample implying that additional
environmentally-related processes are helping to drive this trend. We also find
clear evidence that the average SFR of galaxies in the densest environments
increases with increasing redshift. These results lend themselves to a picture
in which massive gas-rich galaxies coalesce into proto-cluster environments at
, interact with other galaxies or with a forming large-scale medium,
subsequently using or losing most of their gas in the process, and begin to
seed the nascent red sequence that is present in clusters at slightly lower
redshifts.Comment: 22 pages, nine figures, submitted to A&
The impact of the Star Formation Histories on the SFR-M relation at z2
In this paper we investigate the impact of different star formation histories
(SFHs) on the relation between stellar mass M and star formation rate
(SFR) using a sample of galaxies with reliable spectroscopic redshift zspec>2
drawn from the VIMOS Ultra-Deep Survey (VUDS). We produce an extensive database
of dusty model galaxies, calculated starting from the new library of single
stellar population (SSPs) models presented in Cassara' et al. 2013 and weighted
by a set of 28 different SFHs based on the Schmidt function, and characterized
by different ratios of the gas infall time scale to the star
formation efficiency . The treatment of dust extinction and re-emission
has been carried out by means of the radiative transfer calculation. The
spectral energy distribution (SED) fitting technique is performed by using
GOSSIP+, a tool able to combine both photometric and spectroscopic information
to extract the best value of the physical quantities of interest, and to
consider the Intergalactic Medium (IGM) attenuation as a free parameter. We
find that the main contribution to the scatter observed in the
plane is the possibility of choosing between different families of SFHs in the
SED fitting procedure, while the redshift range plays a minor role. The
majority of the galaxies, at all cosmic times, are best-fit by models with SFHs
characterized by a high ratio. We discuss the
reliability of the presence of a small percentage of dusty and highly star
forming galaxies, in the light of their detection in the FIR.Comment: 14 pages, 13 figures, accepted for pubblication in A&
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
Metal content of the circumgalactic medium around star-forming galaxies at z 2.6 as revealed by the VIMOS Ultra-Deep Survey
The circumgalactic medium (CGM) is the location where the interplay between
large-scale outflows and accretion onto galaxies occurs. Metals in different
ionization states flowing between the circumgalactic and intergalactic mediums
are affected by large galactic outflows and low-ionization state inflowing gas.
Observational studies on their spatial distribution and their relation with
galaxy properties may provide important constraints on models of galaxy
formation and evolution. To provide new insights into the spatial distribution
of the circumgalactic of star-forming galaxies, we select a sample of 238 close
pairs at (2.6) from the VIMOS Ultra Deep
Survey. We then generate composite spectra by co-adding spectra of
galaxies that provide different sight-lines across the CGM to examine the
spatial distribution of the gas located around these galaxies and investigate
possible correlations between the strength of the low- and high-ionization
absorption features with different galaxy properties. We detect C II, Si II, Si
IV and C IV) up to separations 172 kpc and 146 kpc. Our
radial profiles suggest a potential redshift evolution for the CGM gas
content producing these absorptions. We find a correlation between C II and C
IV with star formation rate, stellar mass and trends with galaxy size estimated
by the effective radius and azimuthal angle. Galaxies with high star formation
rate show stronger C IV absorptions compared with star-forming galaxies with
low SFR and low stellar mass. These results could be explained by stronger
outflows, softer radiation fields unable to ionize high-ionization state lines
or by the galactic fountain scenario where metal-rich gas ejected from previous
star-formation episodes fall back to the galaxy.Comment: Accepted for publication in A&
Gas Accretion and Star Formation Rates
Cosmological numerical simulations of galaxy evolution show that accretion of
metal-poor gas from the cosmic web drives the star formation in galaxy disks.
Unfortunately, the observational support for this theoretical prediction is
still indirect, and modeling and analysis are required to identify hints as
actual signs of star-formation feeding from metal-poor gas accretion. Thus, a
meticulous interpretation of the observations is crucial, and this
observational review begins with a simple theoretical description of the
physical process and the key ingredients it involves, including the properties
of the accreted gas and of the star-formation that it induces. A number of
observations pointing out the connection between metal-poor gas accretion and
star-formation are analyzed, specifically, the short gas consumption time-scale
compared to the age of the stellar populations, the fundamental metallicity
relationship, the relationship between disk morphology and gas metallicity, the
existence of metallicity drops in starbursts of star-forming galaxies, the
so-called G dwarf problem, the existence of a minimum metallicity for the
star-forming gas in the local universe, the origin of the alpha-enhanced gas
forming stars in the local universe, the metallicity of the quiescent BCDs, and
the direct measurements of gas accretion onto galaxies. A final section
discusses intrinsic difficulties to obtain direct observational evidence, and
points out alternative observational pathways to further consolidate the
current ideas.Comment: Invited review to appear in Gas Accretion onto Galaxies, Astrophysics
and Space Science Library, eds. A. J. Fox & R. Dav\'e, to be published by
Springe
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