54 research outputs found
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
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CLASSY III: The Properties of Starburst-Driven Warm Ionized Outflows
We report the results of analyses of galactic outflows in a sample of 45
low-redshift starburst galaxies in the COS Legacy Archive Spectroscopic SurveY
(CLASSY), augmented by five additional similar starbursts with COS data. The
outflows are traced by blueshifted absorption-lines of metals spanning a wide
range of ionization potential. The high quality and broad spectral coverage of
CLASSY data enable us to disentangle the absorption due to the static ISM from
that due to outflows. We further use different line multiplets and doublets to
determine the covering fraction, column density, and ionization state as a
function of velocity for each outflow. We measure the outflow's mean velocity
and velocity width, and find that both correlate in a highly significant way
with the star-formation rate, galaxy mass, and circular velocity over ranges of
four orders-of-magnitude for the first two properties. We also estimate outflow
rates of metals, mass, momentum, and kinetic energy. We find that, at most,
only about 20% of silicon created and ejected by supernovae in the starburst is
carried in the warm phase we observe. The outflows' mass-loading factor
increases steeply and inversely with both circular and outflow velocity
(log-log slope -1.6), and reaches for dwarf galaxies. We find
that the outflows typically carry about 10 to 100% of the momentum injected by
massive stars and about 1 to 20% of the kinetic energy. We show that these
results place interesting constraints on, and new insights into, models and
simulations of galactic winds.Comment: 34 pages, 16 figures, 6 tables, submitted to Ap
The Low-redshift Lyman Continuum Survey: Radio continuum properties of low- Lyman continuum emitters
The sources that leak Lyman-continuum (LyC) photons and lead to the
reionisation of the universe are intensely studied using multiple observing
facilities. Recently, the Low-redshift LyC Survey (LzLCS) has found the first
large sample of LyC emitting galaxies at low redshift () with the
Hubble Space Telescope/Cosmic Origins Spectrograph. The LzLCS sample contains a
robust estimate of the LyC escape fraction () for
66 galaxies spanning a wide range of . Here we,
for the first time, aim to study the radio continuum (RC) properties of LzLCS
sources and their dependence on . We present Karl
G. Jansky Very Large Array RC observations at C (4-8 GHz), S (2-4 GHz) and L
(1-2 GHz) bands for a sub-sample of the LzLCS sources. The radio spectral index
() spans a wide range from being flat (
) to very steep (). We find that the strongest leakers in
our sample show flat , weak leakers have
close to normal star-forming galaxies,
and non-leakers are characterized by steep
. We argue that a combination of young
ages, free-free absorption, and a flat cosmic-ray energy spectrum can
altogether lead to a flat for strong
leakers. Non-leakers are characterized by steep spectra which can arise due to
break/cutoff at high frequencies. Such a cutoff in the spectrum can arise in a
single injection model of CRs characteristic of galaxies which have recently
stopped star formation. Such a relation between
and hints
at the interesting role of supernovae, CRs, and magnetic fields in facilitating
the escape (and/or the lack) of LyC photons.Comment: 25 pages, 14 figures, 3 tables, Submitted to Astronomy & Astrophysic
CLASSY VII Ly\alpha\ Profiles: The Structure and Kinematics of Neutral Gas and Implications for LyC Escape in Reionization-Era Analogs
Lyman-alpha line profiles are a powerful probe of ISM structure, outflow
speed, and Lyman continuum escape fraction. In this paper, we present the
Ly line profiles of the COS Legacy Archive Spectroscopic SurveY, a
sample rich in spectroscopic analogs of reionization-era galaxies. A large
fraction of the spectra show a complex profile, consisting of a double-peaked
Ly emission profile in the bottom of a damped, Ly absorption
trough. Such profiles reveal an inhomogeneous interstellar medium (ISM). We
successfully fit the damped Ly absorption (DLA) and the Ly
emission profiles separately, but with complementary covering factors, a
surprising result because this approach requires no Ly exchange between
high- and low- paths. The combined distribution
of column densities is qualitatively similar to the bimodal distributions
observed in numerical simulations. We find an inverse relation between
Ly peak separation and the [O III]/[O II] flux ratio, confirming that
the covering fraction of Lyman-continuum-thin sightlines increases as the
Ly peak separation decreases. We combine measurements of Ly
peak separation and Ly red peak asymmetry in a diagnostic diagram which
identifies six Lyman continuum leakers in the CLASSY sample. We find a strong
correlation between the Ly trough velocity and the outflow velocity
measured from interstellar absorption lines. We argue that greater vignetting
of the blueshifted Ly peak, relative to the redshifted peak, is the
source of the well-known discrepancy between shell-model parameters and
directly measured outflow properties. The CLASSY sample illustrates how
scattering of Ly photons outside the spectroscopic aperture reshapes
Ly profiles as the distances to these compact starbursts span a large
range.Comment: 40 pages, 19 figures, 5 tables, submitted to ApJ, comments welcom
Cosmic Evolution Early Release Science (CEERS) survey: The colour evolution of galaxies in the distant Universe
The wavelength-coverage and sensitivity of JWST now enables us to probe the
rest-frame UV - optical spectral energy distributions (SEDs) of galaxies at
high-redshift (). From these SEDs it is, in principle, through SED fitting
possible to infer key physical properties, including stellar masses, star
formation rates, and dust attenuation. These in turn can be compared with the
predictions of galaxy formation simulations allowing us to validate and refine
the incorporated physics. However, the inference of physical properties,
particularly from photometry alone, can lead to large uncertainties and
potential biases. Instead, it is now possible, and common, for simulations to
be \emph{forward-modelled} to yield synthetic observations that can be compared
directly to real observations. In this work, we measure the JWST broadband
fluxes and colours of a robust sample of galaxies using the Cosmic
Evolution Early Release Science (CEERS) Survey. We then analyse predictions
from a variety of models using the same methodology and compare the
NIRCam/F277W magnitude distribution and NIRCam colours with observations. We
find that the predicted and observed magnitude distributions are similar, at
least at the distributions differ somewhat, though our
observed sample size is small and thus susceptible to statistical fluctuations.
Likewise, the predicted and observed colour evolution show broad agreement, at
least at . There is however some disagreement between the observed and
modelled strength of the strong line contribution. In particular all the models
fails to reproduce the F410M-F444W colour at , though, again, the sample
size is small here.Comment: 11 pages, 10 figures, submitted to MNRA
The Low-Redshift Lyman Continuum Survey. Unveiling the ISM properties of low- Lyman continuum emitters
Combining 66 ultraviolet (UV) spectra and ancillary data from the
Low-Redshift Lyman Continuum Survey (LzLCS) and 23 LyC observations by earlier
studies, we form a statistical sample of star-forming galaxies at
to study the role of the cold interstellar medium (ISM) gas in the leakage of
ionizing radiation. We first constrain the massive star content (ages and
metallicities) and UV attenuation, by fitting the stellar continuum with a
combination of simple stellar population models. The models, together with
accurate LyC flux measurements, allow to determine the absolute LyC photon
escape fraction for each galaxy (). We measure the
equivalent widths and residual fluxes of multiple HI and low-ionization state
(LIS) lines, and the geometrical covering fraction adopting the picket-fence
model. The spans a wide range, with a median (0.16,
0.84 quantiles) of 0.04 (0.02, 0.20), and 50 out of the 89 galaxies detected in
the LyC. The HI and LIS line equivalent widths scale with the UV luminosity and
attenuation, and inversely with the residual flux of the lines. The HI and LIS
residual fluxes are correlated, indicating that the neutral gas is spatially
traced by the LIS transitions. We find the observed trends of the absorption
lines and the UV attenuation are primarily driven by the covering fraction. The
non-uniform gas coverage demonstrates that LyC photons escape through
low-column density channels in the ISM. The equivalent widths and residual
fluxes of the UV lines strongly correlate with : strong
LyC leakers show weak absorption lines, low UV attenuation, and large
Ly equivalent widths. We finally show that simultaneous UV absorption
line and dust attenuation measurements can predict, on average, the escape
fraction of galaxies and the method can be applied to galaxies across a wide
redshift range.Comment: 30 pages, 16 figures, 3 tables; accepted for publication in Astronomy
and Astrophysics on December 16, 2021. Tables A1 to A4 are part of the LzLCS
science products and will be publicly available in a dedicated websit
CLASSY VIII: Exploring the Source of Ionization with UV ISM diagnostics in local High- Analogs
In the current JWST era, rest-frame UV spectra play a crucial role in
enhancing our understanding of the interstellar medium (ISM) and stellar
properties of the first galaxies in the epoch of reionization (EoR, ).
Here, we compare well-known and reliable optical diagrams sensitive to the main
ionization source (i.e., star formation, SF; active galactic nuclei, AGN;
shocks) to UV counterparts proposed in the literature - the so-called ``UV-BPT
diagrams'' - using the HST COS Legacy Archive Spectroscopic SurveY (CLASSY),
the largest high-quality, high-resolution and broad-wavelength range atlas of
far-UV spectra for 45 local star-forming galaxies. In particular, we explore
where CLASSY UV line ratios are located in the different UV diagnostic plots,
taking into account state-of-the-art photoionization and shock models and, for
the first time, the measured ISM and stellar properties (e.g., gas-phase
metallicity, ionization parameter, carbon abundance, stellar age). We find that
the combination of C III] 1907,9 He II and O III]
1666 can be a powerful tool to separate between SF, shocks and AGN at
sub-solar metallicities. We also confirm that alternative diagrams without O
III] 1666 still allow us to define a SF-locus with some caveats.
Diagrams including C IV 1548,51 should be taken with caution
given the complexity of this doublet profile. Finally, we present a discussion
detailing the ISM conditions required to detect UV emission lines, visible only
in low gas-phase metallicity (12+log(O/H) ) and high ionization
parameter (log() ) environments. Overall, CLASSY and our UV
toolkit will be crucial in interpreting the spectra of the earliest galaxies
that JWST is currently revealing.Comment: 31 pages, submitted to ApJ, comments welcom
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