1,420 research outputs found
Evidence for a fast evolution of the UV luminosity function beyond redshift 6 from a deep HAWK-I survey of the GOODS-S field
We perform a deep search for galaxies in the redshift range 6.5<z<7.5, to
measure the evolution of the number density of luminous galaxies in this
redshift range and derive useful constraints on the evolution of their
Luminosity Function. We present here the first results of an ESO Large Program,
that exploits the unique combination of area and sensitivity provided in the
near-IR by the camera Hawk-I at the VLT. We have obtained two Hawk-I pointings
on the GOODS South field for a total of 32 observing hours, covering ~90
arcmin2. The images reach Y=26.7 mags for the two fields. We have used public
ACS images in the z band to select z-dropout galaxies with the colour criteria
Z-Y>1, Y-J<1.5 and Y-K<2. The other public data in the UBVRIJHK bands are used
to reject possible low redshift interlopers. The output has been compared with
extensive Monte Carlo simulations to quantify the observational effects of our
selection criteria as well as the effects of photometric errors. We detect 7
high quality candidates in the magnitude range Y=25.5-26.7. This interval
samples the critical range for M* at z>6 (M_1500 ~- 19.5 to -21.5). After
accounting for the expected incompleteness, we rule out at a 99% confidence
level a Luminosity Function constant from z=6 to z=7, even including the
effects of cosmic variance. For galaxies brighter than M_1500=-19.0 we derive a
luminosity density rho_UV=1.5^{+2.0}_{-0.9} 10^25 erg/s/Hz/Mpc3, implying a
decrease by a factor 3.5 from z=6 to z~6.8. On the basis of our findings, we
make predictions for the surface densities expected in future surveys surveys,
based on ULTRA-VISTA, HST-WFC3 or JWST-NIRCam, evaluating the best
observational strategy to maximise their impact.Comment: Accepted for publication in Astronomy & Astrophysic
Estimating Luminosity Function Constraints from High-Redshift Galaxy Surveys
The installation of the Wide Field Camera 3 (WFC3) on the Hubble Space
Telescope (HST) will revolutionize the study of high-redshift galaxy
populations. Initial observations of the HST Ultra Deep Field (UDF) have
yielded multiple z>~7 dropout candidates. Supplemented by the Great Observatory
Origins Deep Survey (GOODS) Early Release Science (ERS) and further UDF
pointings, these data will provide crucial information about the most distant
known galaxies. However, achieving tight constraints on the z~7 galaxy
luminosity function (LF) will require even more ambitious photometric surveys.
Using a Fisher matrix approach to fully account for Poisson and cosmic sample
variance, as well as covariances in the data, we estimate the uncertainties on
LF parameters achieved by surveys of a given area and depth. Applying this
method to WFC3 z~7 dropout galaxy samples, we forecast the LF parameter
uncertainties for a variety of model surveys. We demonstrate that performing a
wide area (~1 deg^2) survey to H_AB~27 depth or increasing the UDF depth to
H_AB~30 provides excellent constraints on the high-z LF when combined with the
existing UDF GO and GOODS ERS data. We also show that the shape of the matter
power spectrum may limit the possible gain of splitting wide area (>~0.5 deg^2)
high-redshift surveys into multiple fields to probe statistically independent
regions; the increased root-mean-squared density fluctuations in smaller
volumes mostly offset the improved variance gained from independent samples.Comment: Version accepted by ApJ
A candidate redshift z ~ 10 galaxy and rapid changes in that population at an age of 500 Myr
Searches for very-high-redshift galaxies over the past decade have yielded a
large sample of more than 6,000 galaxies existing just 900-2,000 million years
(Myr) after the Big Bang (redshifts 6 > z > 3; ref. 1). The Hubble Ultra Deep
Field (HUDF09) data have yielded the first reliable detections of z ~ 8
galaxies that, together with reports of a gamma-ray burst at z ~ 8.2 (refs 10,
11), constitute the earliest objects reliably reported to date. Observations of
z ~ 7-8 galaxies suggest substantial star formation at z > 9-10. Here we use
the full two-year HUDF09 data to conduct an ultra-deep search for z ~ 10
galaxies in the heart of the reionization epoch, only 500 Myr after the Big
Bang. Not only do we find one possible z ~ 10 galaxy candidate, but we show
that, regardless of source detections, the star formation rate density is much
smaller (~10%) at this time than it is just ~200 Myr later at z ~ 8. This
demonstrates how rapid galaxy build-up was at z ~ 10, as galaxies increased in
both luminosity density and volume density from z ~ 8 to z ~ 10. The 100-200
Myr before z ~ 10 is clearly a crucial phase in the assembly of the earliest
galaxies.Comment: 41 pages, 14 figures, 2 tables, Nature, in pres
A critical analysis of the UV Luminosity Function at redshift~7 from deep WFC3 data
The study of the Luminosity Function (LF) of Lyman Break Galaxies (LBGs) at
z=7 is important for ascertaining their role in the reionization of the
Universe. We perform a detailed and critical analysis of the statistical and
systematic errors in the z~7 LF determination: we have assembled a large sample
of candidate LBGs at z~7 from different surveys, spanning a large variety of
areas and depths. In particular, we have combined data from the deep (J<27.4)
and ultradeep (J<29.2) surveys recently acquired with the new WFC3 NIR camera
on HST, over the GOODS-ERS and the HUDF fields, with ground based surveys in
wide and shallow areas from VLT and Subaru. We have used public ACS images in
the z-band to select z-dropout galaxies, and other public data both in the blue
(BVI) and in the red bands to reject possible low-redshift interlopers. We have
compared our results with extensive simulations to quantify the observational
effects of our selection criteria as well as the effects of photometric
scatter, color selections or the morphology of the candidates. We have found
that the number density of faint LBGs at z~7 is only marginally sensitive to
the color selection adopted, but it is strongly dependent from the assumption
made on the half light distributions of the simulated galaxies, used to correct
the observed sample for incompleteness. The slope of the faint end of the LBGs
LF has thus a rather large uncertainty, due to the unknown distribution of
physical sizes of the z~7 LBGs. We conclude that galaxies at z~7 are unable to
reionize the Universe unless there is a significant evolution in the clumpiness
of the IGM or in the escape fraction of ionising photons or, alternatively,
there is a large population of z~7 LBGs with large physical dimensions but
still not detected by the present observations.Comment: 15 pages, 11 figures, accepted for publication in A&
The bright end of the z ~ 7 UV Luminosity Function from a wide and deep HAWK-I survey
(Abridged) We present here the second half of an ESO Large Programme, which
exploits the unique combination of area and sensitivity provided in the near-IR
by the camera Hawk-I at the VLT. We have obtained - 30 observing hours with
Hawk-I in the Y-band of two high galactic latitude fields. We combined the
Y-band data with deep J and K Hawk-I observations, and with FORS1/FORS2 U, B,
V, R, I, and Z observations to select z-drop galaxies having Z - Y > 1, no
optical detection and flat Y - J and Y - K colour terms. We detect 8
high-quality candidates in the magnitude range Y = 25.5 - 26.5 that we add to
the z-drop candidates selected in two Hawk-I pointings over the GOODS-South
field. We use this full sample of 15 objects found in -161 arcmin^2 of our
survey to constrain the average physical properties and the evolution of the
number density of z ~ 7 LBGs. A stacking analysis yields a best-fit SED with
photometric redshift z= 6.85 +0.20 -0.15 and an E(B-V)=0.05 +0.15 -0.05. We
compute a binned estimate of the z ~ 7 LF and explore the effects of
photometric scatter and model uncertainties on the statistical constraints.
After accounting for the expected incompleteness through MonteCarlo
simulations, we strengthen our previous finding that a Schechter luminosity
function constant from z=6 to z=7 is ruled out at a >99% confidence level, even
including the effects of cosmic variance. For galaxies brighter than M_1500=
-19.0, we derive a luminosity density rho_UV = 1.5^{+2.1}{-0.8} x 10^25
erg/s/Hz/Mpc^3, implying a decrease by a factor 3.5 from z=6 to z=6.8. We find
that, under standard assumptions, the emission rate of ionizing photons coming
from UV bright galaxies is lower by at least a factor of two than the value
required for reionization. Finally, we exploit deep Hawk-I J and K band
observations to derive an upper limit on the number density of M1500<~ -22.0
LBGs at z-8 (Y-dropouts).Comment: 12 pages, 8 figures. Accepted for publication in Astronomy &
Astrophysic
Connecting the Gamma Ray Burst Rate and the Cosmic Star Formation History: Implications for Reionization and Galaxy Evolution
(Abridged) The contemporary discoveries of galaxies and gamma ray bursts
(GRBs) at high redshift have supplied the first direct information on star
formation when the universe was only a few hundred million years old. The
probable origin of long duration GRBs in the deaths of massive stars would link
the universal GRB rate to the redshift-dependent star formation rate density,
although exactly how is currently unknown. As the most distant GRBs and
star-forming galaxies probe the reionization epoch, the potential rewards of
understanding the redshift-dependent ratio Psi(z) of the GRB rate to star
formation rate are significant and include addressing fundamental questions
such as incompleteness in rest-frame UV surveys for determining the star
formation rate at high redshift and time variations in the stellar initial mass
function. Using an extensive sample of 112 GRBs above a fixed luminosity limit
drawn from the Second Swift Burst Alert Telescope catalog, we compare the
cumulative redshift distribution N(< z) of GRBs with the star formation density
rho_sfr(z) measured from UV-selected galaxies over 0 < z < 4. Strong evolution
(e.g., Psi(z) \propto (1+z)^{1.5}) is disfavored, while more modest evolution
(e.g., Psi(z) \propto (1+z)^{0.5}) is consistent with the data. If such trends
continue beyond z ~ 4, we find the discovery rate of distant GRBs implies a
star formation rate density much higher than that inferred from UV-selected
galaxies. We show that such a star formation history would over-predict the
observed stellar mass density at z > 4 measured from rest-frame optical
surveys. The resolution of this important disagreement is currently unclear,
and the GRB production rate at early times is likely more complex than a simple
function of star formation rate and progenitor metallicity.Comment: Version accepted by Ap
The Great Observatories Origins Deep Survey: Constraints on the Lyman Continuum Escape Fraction Distribution of Lyman--Break Galaxies at 3.4<z<4.5
We use ultra-deep ultraviolet VLT/VIMOS intermediate-band and VLT/FORS1
narrow-band imaging in the GOODS Southern field to derive limits on the
distribution of the escape fraction (f_esc) of ionizing radiation for L >~
L*(z=3) Lyman Break Galaxies (LBGs) at redshift 3.4--4.5. Only one LBG, at
redshift z=3.795, is detected in its Lyman continuum (LyC; S/N~5.5), the
highest redshift galaxy currently known with a direct detection. Its
ultraviolet morphology is quite compact (R_eff=0.8, kpc physical). Three out of
seven AGN are also detected in their LyC, including one at redshift z=3.951 and
z850 = 26.1. From stacked data (LBGs) we set an upper limit to the average
f_esc in the range 5%--20%, depending on the how the data are selected (e.g.,
by magnitude and/or redshift). We undertake extensive Monte Carlo simulations
that take into account intergalactic attenuation, stellar population synthesis
models, dust extinction and photometric noise in order to explore the moments
of the distribution of the escaping radiation. Various distributions
(exponential, log-normal and Gaussian) are explored. We find that the median
f_esc is lower than ~6% with an 84% percentile limit not larger than 20%. If
this result remains valid for fainter LBGs down to current observational
limits, then the LBG population might be not sufficient to account for the
entire photoionization budget at the redshifts considered here, with the exact
details dependent upon the assumed ionizing background and QSO contribution
thereto. It is possible that f_esc depends on the UV luminosity of the
galaxies, with fainter galaxies having higher f_esc, and estimates of f_esc
from a sample of faint LBG from the HUDF (i775<28.5) are in broad quantitative
agreement with such a scenario.Comment: 58 pages, 23 figures; submitted to ApJ, revised version in response
to referee's comment
Dust Obscuration in Lyman Break Galaxies at z~4
Measuring star formation rates (SFRs) in high-z galaxies with their
rest-frame ultraviolet (UV) continuum can be uncertain because of dust
obscuration. Prior studies had used the submillimeter emission at 850 um to
determine the intrinsic SFRs of rest-frame UV selected galaxies, but the
results suffered from the low sensitivity and poor resolution (~15''). Here, we
use ultradeep Very Large Array 1.4 GHz images with ~1''-2'' resolutions to
measure the intrinsic SFRs. We perform stacking analyses in the radio images
centered on ~3500 Lyman break galaxies (LBGs) at z~4 in the Great Observatories
Origins Deep Survey-North and South fields selected with Hubble Space
Telescope/Advanced Camera for Surveys data. The stacked radio flux is very low,
0.08+/-0.15 uJy, implying a mean SFR of 6+/-11 M/yr. This is comparable to the
uncorrected mean UV SFRs of 5 M/yr, implying that the z~4 LBGs have little dust
extinction. The low SFR and dust extinction support the previous results that
z~4 LBGs are in general not submillimeter galaxies. We further show that there
is no statistically significant excess of dust-hidden star-forming components
within ~22 kpc from the LBGs.Comment: 6 pages, 3 figures, and 2 tables. Accepted to the Astrophysical
Journal (ApJ
Optical dropout galaxies lensed by the cluster A2667
We investigate the nature and the physical properties of z, Y and J-dropout
galaxies selected behind the lensing cluster A2667. This field is part of our
project aimed at identifying z~7-10 candidates accessible to spectroscopic
studies, based on deep photometry with ESO/VLT HAWK-I and FORS2 (zYJH and
Ks-band images, AB(3 sigma)~26-27) on a sample of lensing clusters extracted
from our multi-wavelength combined surveys with SPITZER, HST, and Herschel. In
this paper we focus on the complete Y and J-dropout sample, as well as the
bright z-dropouts fulfilling the selection criteria by Capak et al. (2011). 10
candidates are selected within the common field of ~33 arcmin2 (effective area
once corrected for contamination and lensing dilution). All of them are
detected in H and Ks bands in addition to J and/or IRAC 3.6/4.5, with
H(AB)~23.4 to 25.2, and have modest magnification factors. Although best-fit
photometric redshifts place all these candidates at high-z, the contamination
by low-z interlopers is estimated at 50-75% level based on previous studies,
and the comparison with the blank-field WIRCAM Ultra-Deep Survey (WUDS). The
same result is obtained when photometric redshifts include a luminosity prior,
allowing us to remove half of the original sample as likely z~1.7-3 interlopers
with young stellar pulations and strong extinction. Two additional sources
among the remaining sample could be identified at low-z based on a detection at
24 microns and on the HST z_850 band. These low-z interlopers are not well
described by current templates given the large break, and cannot be easily
identified based solely on optical and near-IR photometry. Given the estimated
dust extinction and high SFRs, some of them could be also detected in the IR or
sub-mm bands. After correction for likely contaminants, the observed counts at
z>7.5 seem to be in agreement with an evolving LF. (abridged)Comment: 18 pages, 11 figures. Accepted for publication in A&
Overdensities of Y-dropout Galaxies from the Brightest-of-Reionizing Galaxies Survey: A Candidate Protocluster at Redshift z~8
Theoretical and numerical modeling of dark-matter halo assembly predicts that
the most luminous galaxies at high redshift are surrounded by overdensities of
fainter companions. We test this prediction with HST observations acquired by
our Brightest of Reionizing Galaxies (BoRG) survey, which identified four very
bright z~8 candidates as Y-dropout sources in four of the 23 non-contiguous
WFC3 fields observed. We extend here the search for Y-dropouts to fainter
luminosities (M_* galaxies with M_AB\sim-20), with detections at >5sigma
confidence (compared to >8sigma confidence adopted earlier) identifying 17 new
candidates. We demonstrate that there is a correlation between number counts of
faint and bright Y-dropouts at >99.84% confidence. Field BoRG58, which contains
the best bright z\sim8 candidate (M_AB=-21.3), has the most significant
overdensity of faint Y-dropouts. Four new sources are located within 70arcsec
(corresponding to 3.1 comoving Mpc at z=8) from the previously known brighter
z\sim8 candidate. The overdensity of Y-dropouts in this field has a physical
origin to high confidence (p>99.975%), independent of completeness and
contamination rate of the Y-dropout selection. We modeled the overdensity by
means of cosmological simulations and estimate that the principal dark matter
halo has mass M_h\sim(4-7)x10^11Msun (\sim5sigma density peak) and is
surrounded by several M_h\sim10^11Msun halos which could host the fainter
dropouts. In this scenario, we predict that all halos will eventually merge
into a M_h>2x10^14Msun galaxy cluster by z=0. Follow-up observations with
ground and space based telescopes are required to secure the z\sim8 nature of
the overdensity, discover new members, and measure their precise redshift.Comment: Minor revision: ApJ accepted [17 pages (emulateapj style), 7 figures,
2 tables
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