545 research outputs found
The VIMOS Ultra-Deep Survey (VUDS): fast increase in the fraction of strong Lyman-α emitters from z = 2 to z = 6
Aims. The aim of this work is to constrain the evolution of the fraction of strong Lyα emitters among UV selected star-forming galaxies at 2 <z< 6, and to measure the stellar escape fraction of Lyα photons over the same redshift range.
Methods. We exploit the ultradeep spectroscopic observations with VIMOS on the VLT collected by the VIMOS Ultra-Deep Survey (VUDS) to build an unique, complete, and unbiased sample of ~4000 spectroscopically confirmed star-forming galaxies at 2 <z< 6. Our galaxy sample includes UV luminosities brighter than M^*_(FUV) at 2 <z< 6, and luminosities down to one magnitude fainter than M^*_(FUV) at 2 <z< 3.5.
Results. We find that 80% of the star-forming galaxies in our sample have EW_0(Lyα) 25 Ă
to increase from ~5% at z ~ 2 to ~30% at z ~ 6, with the increase being stronger beyond z ~ 4. We observe no difference, for the narrow range of UV luminosities explored in this work, between the fraction of strong Lyα emitters among galaxies fainter or brighter than M^*_(FUV), although the fraction for the faint galaxies evolves faster, at 2 <z< 3.5, than for the bright ones. We do observe an anticorrelation between E(B â V) and f_(esc)(Lyα): generally galaxies with high fesc(Lyα) also have small amounts of dust (and vice versa). However, when the dust content is low (E(B â V) < 0.05) we observe a very broad range of f_(esc)(Lyα), ranging from 10-3 to 1. This implies that the dust alone is not the only regulator of the amount of escaping Lyα photons
AGN feedback at z~2 and the mutual evolution of active and inactive galaxies
The relationships between galaxies of intermediate stellar mass and moderate
luminosity active galactic nuclei (AGNs) at 1<z<3 are investigated with the
Galaxy Mass Assembly ultra-deep Spectroscopic Survey (GMASS) sample
complemented with public data in the GOODS-South field. Using X-ray data,
hidden AGNs are identified in unsuspected star-forming galaxies with no
apparent signs of non-stellar activity. In the color-mass plane, two parallel
trends emerge during the ~2 Gyr between the average redshifts z~2.2 and z~1.3:
while the red sequence becomes significantly more populated by ellipticals, the
majority of AGNs with L(2-10 keV)>10^42.3 erg s^-1 disappear from the blue
cloud/green valley where they were hosted predominantly by star-forming systems
with disk and irregular morphologies. These results are even clearer when the
rest-frame colors are corrected for dust reddening. At z~2.2, the ultraviolet
spectra of active galaxies (including two Type 1 AGNs) show possible gas
outflows with velocities up to about -500 km s^-1 that are not observed neither
in inactive systems at the same redshift, nor at lower redshifts. Such outflows
indicate the presence of gas that can move faster than the escape velocities of
active galaxies. These results suggest that feedback from moderately luminous
AGNs (logL_X~2 by contributing to
outflows capable of ejecting part of the interstellar medium and leading to a
rapid decrease in the star formation in host galaxies with stellar masses
10<logM<11 M_Sun.Comment: Astrophysical Journal Letters, in press (6 pages, 4 figures
Trapped Ar isotopes in meteorite ALH 84001 indicate Mars did not have a thick ancient atmosphere
Water is not currently stable in liquid form on the martian surface due to the present mean atmospheric pressure of âŒ7 mbar and mean global temperature of âŒ220 K. However, geomorphic features and hydrated mineral assemblages suggest that Marsâ climate was once warmer and liquid water flowed on the surface. These observations may indicate a substantially more massive atmosphere in the past, but there have been few observational constraints on paleoatmospheric pressures. Here we show how the [superscript 40]Ar/[superscript 36]Ar ratios of trapped gases within martian meteorite ALH 84001 constrain paleoatmospheric pressure on Mars during the Noachian era [âŒ4.56â3.8 billion years (Ga)]. Our model indicates that atmospheric pressures did not exceed âŒ1.5 bar during the first 400 million years (Ma) of the Noachian era, and were <400 mbar by 4.16 Ga. Such pressures of CO[subscript 2] are only sufficient to stabilize liquid water on Marsâ surface at low latitudes during seasonally warm periods. Other greenhouse gases like SO[superscript 2] and water vapor may have played an important role in intermittently stabilizing liquid water at higher latitudes following major volcanic eruptions or impact events.United States. National Aeronautics and Space Administration. Mars Fundamental Research Program (Grant MFRP05-0108)Ann and Gordon Getty Foundatio
The Cosmic Evolution Survey (COSMOS): a large-scale structure at z=0.73 and the relation of galaxy morphologies to local environment
We have identified a large-scale structure at z~0.73 in the COSMOS field,
coherently described by the distribution of galaxy photometric redshifts, an
ACS weak-lensing convergence map and the distribution of extended X-ray sources
in a mosaic of XMM observations. The main peak seen in these maps corresponds
to a rich cluster with Tx= 3.51+0.60/-0.46 keV and Lx=(1.56+/-0.04) x 10^{44}
erg/s ([0.1-2.4] keV band). We estimate an X-ray mass within
corresponding to M500~1.6 x 10^{14} Msun and a total lensing mass (extrapolated
by fitting a NFW profile) M(NFW)=(6+/-3) x 10^15 Msun. We use an automated
morphological classification of all galaxies brighter than I_AB=24 over the
structure area to measure the fraction of early-type objects as a function of
local projected density Sigma_10, based on photometric redshifts derived from
ground-based deep multi-band photometry. We recover a robust morphology-density
relation at this redshift, indicating, for comparable local densities, a
smaller fraction of early-type galaxies than today. Interestingly, this
difference is less strong at the highest densities and becomes more severe in
intermediate environments. We also find, however, local "inversions'' of the
observed global relation, possibly driven by the large-scale environment. In
particular, we find direct correspondence of a large concentration of disk
galaxies to (the colder side of) a possible shock region detected in the X-ray
temperature map and surface brightness distribution of the dominant cluster. We
interpret this as potential evidence of shock-induced star formation in
existing galaxy disks, during the ongoing merger between two sub-clusters.Comment: 15 pages (emulateapj style), 16 figs (low res.); to appear in the ApJ
Supplement COSMOS Special Issue. Low-resolution figures; full resolution
version available at:
http://www.astro.caltech.edu/~cosmos/publications/files/guzzo_0701482.pd
A Coherent Study of Emission Lines from Broad-Band Photometry: Specific Star-Formation Rates and [OIII]/H{\beta} Ratio at 3 < z < 6
We measure the H{\alpha} and [OIII] emission line properties as well as
specific star-formation rates (sSFR) of spectroscopically confirmed 3<z<6
galaxies in COSMOS from their observed colors vs. redshift evolution. Our model
describes consistently the ensemble of galaxies including intrinsic properties
(age, metallicity, star-formation history), dust-attenuation, and optical
emission lines. We forward-model the measured H{\alpha} equivalent-widths (EW)
to obtain the sSFR out to z~6 without stellar mass fitting. We find a strongly
increasing rest-frame H{\alpha} EW that is flattening off above z~2.5 with
average EWs of 300-600A at z~6. The sSFR is increasing proportional to
(1+z)^2.4 at z<2.2 and (1+z)^1.5 at higher redshifts, indicative of a fast mass
build-up in high-z galaxies within e-folding times of 100-200Myr at z~6. The
redshift evolution at z>3 cannot be fully explained in a picture of cold
accretion driven growth. We find a progressively increasing
[OIII]{\lambda}5007/H{\beta} ratio out to z~6, consistent with the ratios in
local galaxies selected by increasing H{\alpha} EW (i.e., sSFR). This
demonstrates the potential of using "local high-z analogs" to investigate the
spectroscopic properties and relations of galaxies in the re-ionization epoch.Comment: 18 pages, 11 figures, 3 table
The extended epoch of galaxy formation: age dating of ~3600 galaxies with 2<z<6.5 in the VIMOS Ultra-Deep Survey
We aim at improving constraints on the epoch of galaxy formation by measuring
the ages of 3597 galaxies with spectroscopic redshifts 2<z<6.5 in the VIMOS
Ultra Deep Survey (VUDS). We derive ages and other physical parameters from the
simultaneous fitting with the GOSSIP+ software of observed UV rest-frame
spectra and photometric data from the u-band up to 4.5 microns using composite
stellar population models. We conclude from extensive simulations that at z>2
the joint analysis of spectroscopy and photometry combined with restricted age
possibilities when taking into account the age of the Universe substantially
reduces systematic uncertainties and degeneracies in the age derivation. We
find galaxy ages ranging from very young with a few tens of million years to
substantially evolved with ages up to ~1.5-2 Gyr. The formation redshifts z_f
derived from the measured ages indicate that galaxies may have started forming
stars as early as z_f~15. We produce the formation redshift function (FzF), the
number of galaxies per unit volume formed at a redshift z_f, and compare the
FzF in increasing redshift bins finding a remarkably constant 'universal' FzF.
The FzF is parametrized with (1+z)^\zeta, with \zeta~0.58+/-0.06, indicating a
smooth 2 dex increase from z~15 to z~2. Remarkably this observed increase is of
the same order as the observed rise in the star formation rate density (SFRD).
The ratio of the SFRD with the FzF gives an average SFR per galaxy of
~7-17Msun/yr at z~4-6, in agreement with the measured SFR for galaxies at these
redshifts. From the smooth rise in the FzF we infer that the period of galaxy
formation extends from the highest possible redshifts that we can probe at z~15
down to redshifts z~2. This indicates that galaxy formation is a continuous
process over cosmic time, with a higher number of galaxies forming at the peak
in SFRD at z~2 than at earlier epochs. (Abridged)Comment: Submitted to A&A, 24 page
HeII emitters in the VIMOS VLT Deep Survey: PopIII star formation or peculiar stellar populations in galaxies at 2<z<4.6?
The aim of this work is to identify HeII emitters at 2<z<4.6 and to constrain
the source of the hard ionizing continuum that powers the HeII emission. We
have assembled a sample of 277 galaxies with a high quality spectroscopic
redshift at 2<z<4.6 from the VVDS survey, and we have identified 39 HeII1640A
emitters. We study their spectral properties, measuring the fluxes, equivalent
widths (EW) and FWHM for most relevant lines. About 10% of galaxies at z~3 show
HeII in emission, with rest frame equivalent widths EW0~1-7A, equally
distributed between galaxies with Lya in emission or in absorption. We find 11
high-quality HeII emitters with unresolved HeII line (FWHM_0<1200km/s), 13
high-quality emitters with broad He II emission (FWHM_0>1200km/s), 3 AGN, and
an additional 12 possible HeII emitters. The properties of the individual broad
emitters are in agreement with expectations from a W-R model. On the contrary,
the properties of the narrow emitters are not compatible with such model,
neither with predictions of gravitational cooling radiation produced by gas
accretion. Rather, we find that the EW of the narrow HeII line emitters are in
agreement with expectations for a PopIII star formation, if the episode of star
formation is continuous, and we calculate that a PopIII SFR of 0.1-10 Mo yr-1
only is enough to sustain the observed HeII flux. We conclude that narrow HeII
emitters are either powered by the ionizing flux from a stellar population rare
at z~0 but much more common at z~3, or by PopIII star formation. As proposed by
Tornatore et al. (2007), incomplete ISM mixing may leave some small pockets of
pristine gas at the periphery of galaxies from which PopIII may form, even down
to z~2 or lower. If this interpretation is correct, we measure at z~3 a SFRD in
PopIII stars of 10^6Mo yr^-1 Mpc^-3 qualitatively comparable to the value
predicted by Tornatore et al. (2007).Comment: accepted for publication in A&
Identifying Ly{\alpha} emitter candidates with Random Forest: learning from galaxies in CANDELS survey
The physical processes which make a galaxy a Lyman Alpha Emitter have been
extensively studied for the past 25 years. However, the correlations between
physical and morphological properties of galaxies and the strength of the
Ly emission line are still highly debated. Therefore, we investigate
the correlations between the rest-frame Ly equivalent width and stellar
mass, star formation rate, dust reddening, metallicity, age, half-light
semi-major axis, S\'ersic index and projected axis ratio in a sample of 1578
galaxies in the redshift range from the GOODS-S, UDS and
COSMOS fields. From the large sample of Ly emitters (LAEs) in the
dataset we find that LAEs are typically common main sequence star forming
galaxies which show stellar mass , star formation
rate , and
half-light semi-major axis . Building on these findings we
develop a new method based on Random Forest (i.e. a Machine Learning
classifier) in order to select galaxies which have the highest probability of
being Ly emitters. When applied to a population in the redshift range
, our classifier holds a accuracy and precision. At higher redshifts (), we obtain a
accuracy and a precision. These results highlight it is possible to
overcome the current limitations in assembling large samples of LAEs by making
informed predictions that can be used for planning future large scale
spectroscopic surveys.Comment: Accepted for publication in the 4. Extragalactic astronomy section of
Astronomy & Astrophysics, 13 pages, 13 figure
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