99 research outputs found
An extreme [OIII] emitter at : a low metallicity Lyman continuum source
[Abridged] We investigate the physical properties of a Lyman continuum
emitter candidate at with photometric coverage from to MIPS
24m band and VIMOS/VLT and MOSFIRE/Keck spectroscopy. Investigation of the
UV spectrum confirms a direct spectroscopic detection of the Lyman continuum
emission with . Non-zero Ly flux at the systemic redshift and
high Lyman- escape fraction suggest a low HI column density. The weak C
and Si low-ionization absorption lines are also consistent with a low covering
fraction along the line of sight. The
[OIII] equivalent width is one of the
largest reported for a galaxy at
(, rest-frame) and the NIR spectrum shows that this is mainly due to an
extremely strong [OIII] emission. The large observed [OIII]/[OII] ratio ()
and high ionization parameter are consistent with prediction from
photoionization models in case of a density-bounded nebula scenario.
Furthermore, the
is
comparable to recent measurements reported at , in the reionization
epoch. We also investigate the possibility of an AGN contribution to explain
the ionizing emission but most of the AGN identification diagnostics suggest
that stellar emission dominates instead. This source is currently the first
high- example of a Lyman continuum emitter exhibiting indirect and direct
evidences of a Lyman continuum leakage and having physical properties
consistent with theoretical expectation from Lyman continuum emission from a
density-bounded nebula.Comment: 14 pages, 11 figures, accepted for publication in A&A. Minor
modifications, Figure 2 updated, Figure 9 adde
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 Metallicities of Low Stellar Mass Galaxies and the Scatter in the Mass-Metallicity Relation
In this investigation we quantify the metallicities of low mass galaxies by
constructing the most comprehensive census to date. We use galaxies from the
SDSS and DEEP2 survey and estimate metallicities from their optical emission
lines. We also use two smaller samples from the literature which have
metallicities determined by the direct method using the temperature sensitive
[OIII]4363 line. We examine the scatter in the local mass-metallicity (MZ)
relation determined from ~20,000 star-forming galaxies in the SDSS and show
that it is larger at lower stellar masses, consistent with the theoretical
scatter in the MZ relation determined from hydrodynamical simulations. We
determine a lower limit for the scatter in metallicities of galaxies down to
stellar masses of ~10^7 M_solar that is only slightly smaller than the expected
scatter inferred from the SDSS MZ relation and significantly larger than what
is previously established in the literature. The average metallicity of
star-forming galaxies increases with stellar mass. By examining the scatter in
the SDSS MZ relation, we show that this is mostly due to the lowest metallicity
galaxies. The population of low mass, metal-rich galaxies have properties which
are consistent with previously identified galaxies that may be transitional
objects between gas-rich dwarf irregulars and gas-poor dwarf spheroidals and
ellipticals.Comment: Accepted to ApJ. 17 pages, 17 figure
Extreme Emission Line Galaxies in CANDELS: Broad-Band Selected, Star-Bursting Dwarf Galaxies at z>1
We identify an abundant population of extreme emission line galaxies (EELGs)
at redshift z~1.7 in the Cosmic Assembly Near-IR Deep Extragalactic Legacy
Survey (CANDELS) imaging from Hubble Space Telescope/Wide Field Camera 3
(HST/WFC3). 69 EELG candidates are selected by the large contribution of
exceptionally bright emission lines to their near-infrared broad-band
magnitudes. Supported by spectroscopic confirmation of strong [OIII] emission
lines -- with rest-frame equivalent widths ~1000\AA -- in the four candidates
that have HST/WFC3 grism observations, we conclude that these objects are
galaxies with 10^8 Msol in stellar mass, undergoing an enormous starburst phase
with M_*/(dM_*/dt) of only ~15 Myr. These bursts may cause outflows that are
strong enough to produce cored dark matter profiles in low-mass galaxies. The
individual star formation rates and the co-moving number density (3.7x10^-4
Mpc^-3) can produce in ~4 Gyr much of the stellar mass density that is
presently contained in 10^8-10^9 Msol dwarf galaxies. Therefore, our
observations provide a strong indication that many or even most of the stars in
present-day dwarf galaxies formed in strong, short-lived bursts, mostly at z>1.Comment: accepted for publication in ApJ; 10 pages; 6 figures; 1 tabl
Near-infrared Emission Lines in Starburst Galaxies at 0.5 < z < 0.9: Discovery of a Merger Sequence of Extreme Obscurations
We obtained optical/near-IR rest-frame Magellan FIRE spectra (including
Pa and Pa) of 25 starburst galaxies at 0.5<z<0.9, with average
star formation rates (SFR) x7 above the Main Sequence (MS). We find that
Paschen-to-Balmer line ratios saturate around a constant value corresponding to
2-3 mag, while line to IR luminosity ratios suggest a large
range of more extreme obscurations and appear to be uncorrelated to the former.
This behavior is not consistent with standard attenuation laws derived for
local and distant galaxies, while being remarkably consistent with observations
of starburst cores in which young stars and dust are homogeneously mixed. This
model implies 2-30 mag attenuation to the center of starburst
cores, with a median of ~9 mag (a factor of 4000). X-ray hardness ratios for 6
AGNs in our sample and column densities derived from observed dust masses and
radio sizes independently confirm this level of attenuation. In these
conditions observed optical/near-IR emission comes from surface regions, while
inner starburst cores are invisible. We thus attribute the high [NII]/H
ratios to widespread shocks from accretion, turbulence and dynamic disturbances
rather than to AGNs. The large range of optical depths demonstrates that
substantial diversity is present within the starburst population, possibly
connected to different merger phases or progenitor properties. The majority of
our targets are, in fact, morphologically classified as mergers. We argue that
the extreme obscuration provides in itself smoking gun evidence of their merger
origin, and a powerful tool for identifying mergers at even higher redshifts
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
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&
The ALMA-ALPINE [CII] survey: Kennicutt-Schmidt relation in four massive main-sequence galaxies at <i>z</i> ⌠4.5
Aims. The Kennicutt-Schmidt (KS) relation between the gas and the star formation rate (SFR) surface density (ÎŁâ
ââ
ÎŁ) is essential to understand star formation processes in galaxies. To date, it has been measured up to z ⌠2.5 in main-sequence galaxies. In this Letter our aim is to put constraints at zââŒâ4.5 using a sample of four massive main-sequence galaxies observed by ALMA at high resolution.
Methods. We obtained âŒ0.3âł-resolution [CII] and continuum maps of our objects, which we then converted into gas and obscured SFR surface density maps. In addition, we produced unobscured SFR surface density maps by convolving Hubble ancillary data in the rest-frame UV. We then derived the average ÎŁ in various ÎŁ bins, and estimated the uncertainties using a Monte Carlo sampling.
Results. Our galaxy sample follows the KS relation measured in main-sequence galaxies at lower redshift, and is slightly lower than the predictions from simulations. Our data points probe the high end both in terms of ÎŁ and ÎŁ, and gas depletion timescales (285â843 Myr) remain similar to z ⌠2 objects. However, three of our objects are clearly morphologically disturbed, and we could have expected shorter gas depletion timescales (âČ100 Myr) similar to merger-driven starbursts at lower redshifts. This suggests that the mechanisms triggering starbursts at high redshift may be different than in the low- and intermediate-z Universe
Dust and gas in star forming galaxies at z~3 - extending galaxy uniformity to 11.5 billion years
We present millimetre dust emission measurements of two Lyman Break Galaxies at z~3 and construct for the first time fully sampled infrared spectral energy distributions (SEDs), from mid-IR to the Rayleigh-Jeans tail, of individually detected, unlensed, UV-selected, main sequence (MS) galaxies at z=3. The SED modelling of the two sources confirms previous findings, based on stacked ensembles, of an increasing mean radiation field with redshift, consistent with a rapidly decreasing gas metallicity in z > 2 galaxies. Complementing our study with CO[3-2] emission line observations, we measure the molecular gas mass (M_H2) reservoir of the systems using three independent approaches: 1) CO line observations, 2) the dust to gas mass ratio vs metallicity relation and 3) a single band, dust emission flux on the Rayleigh-Jeans side of the SED. All techniques return consistent M_H2 estimates within a factor of ~2 or less, yielding gas depletion time-scales (tau_dep ~ 0.35 Gyrs) and gas-to-stellar mass ratios (M_H2/M* ~ 0.5-1) for our z~3 massive MS galaxies. The overall properties of our galaxies are consistent with trends and relations established at lower redshifts, extending the apparent uniformity of star-forming galaxies over the last 11.5 billion years
Spectroscopic verification of very luminous galaxy candidates in the early universe
During the first 500 million years of cosmic history, the first stars and
galaxies formed and seeded the cosmos with heavy elements. These early galaxies
illuminated the transition from the cosmic "dark ages" to the reionization of
the intergalactic medium. This transitional period has been largely
inaccessible to direct observation until the recent commissioning of JWST,
which has extended our observational reach into that epoch. Excitingly, the
first JWST science observations uncovered a surprisingly high abundance of
early star-forming galaxies. However, the distances (redshifts) of these
galaxies were, by necessity, estimated from multi-band photometry. Photometric
redshifts, while generally robust, can suffer from uncertainties and/or
degeneracies. Spectroscopic measurements of the precise redshifts are required
to validate these sources and to reliably quantify their space densities,
stellar masses, and star formation rates, which provide powerful constraints on
galaxy formation models and cosmology. Here we present the results of JWST
follow-up spectroscopy of a small sample of galaxies suspected to be amongst
the most distant yet observed. We confirm redshifts z > 10 for two galaxies,
including one of the first bright JWST-discovered candidates with z = 11.4, and
show that another galaxy with suggested z ~ 16 instead has z = 4.9, with strong
emission lines that mimic the expected colors of more distant objects. These
results reinforce the evidence for the rapid production of luminous galaxies in
the very young Universe, while also highlighting the necessity of spectroscopic
verification for remarkable candidates.Comment: Submitted to Natur
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