4,498 research outputs found
Star formation rates and stellar masses of z=7-8 galaxies from IRAC observations of the WFC3/IR ERS and the HUDF field
We investigate the Spitzer/IRAC properties of 36 z~7 z-dropout galaxies and 3
z~8 Y-dropout galaxies derived from deep/wide-area WFC3/IR data of the Early
Release Science, the ultradeep HUDF09, and wide-area NICMOS data. We fit
stellar population synthesis models to the SEDs to derive mean redshifts,
stellar masses, and ages. The z~7 galaxies are best characterized by
substantial ages (>100 Myr) and M/L_V~0.2. The main trend with decreasing
luminosity is that of bluing of the far-UV slope from beta~-2.0 to beta~-3.0.
This can be explained by decreasing metallicity, except for the lowest
luminosity galaxies (0.1 L*{z=3}), where low metallicity and smooth SFHs fail
to match the blue far-UV and moderately red H-[3.6] color. Such colors may
require episodic SFHs with short periods of activity and quiescence ("on-off"
cycles) and/or a contribution from emission lines. The stellar mass of our
sample of z~7 star forming galaxies correlates with SFR according to log M* =
8.70 (+-0.09) + 1.06(+-0.10) log SFR, implying star formation may have
commenced at z>10. No galaxies are found with SFRs much higher or lower than
the past averaged SFR suggesting that the typical star formation timescales are
probably a substantial fraction of the Hubble time. We report the first IRAC
detection of Y-dropout galaxies at z~8. The average rest-frame U-V=0.3 (AB) of
the 3 galaxies are similar to faint z~7 galaxies, implying similar M/L. The
stellar mass density to M_{UV,AB}<-18 is rho*(z=8) = 1.8^{+0.7}_{-1.0} x 10^6
M_sun Mpc^-3, following log rho*(z)= 10.6(+-0.6) - 4.4(+-0.7) log(1+z) [M_sun
Mpc^-3] over 3<z< 8.Comment: 6 pages, 4 figures, 1 table, emulateapj, ApJL in press. Updated LF
corrections for mass density, evolution. Added evolution uncertainties.
Supercedes 0910.083
The Brightest Of Reionizing Galaxies Survey: Design and Preliminary Results
We present the first results on the search for very bright (M_AB -21)
galaxies at redshift z~8 from the Brightest of Reionizing Galaxies (BoRG)
survey. BoRG is a Hubble Space Telescope Wide Field Camera 3 pure-parallel
survey that is obtaining images on random lines of sight at high Galactic
latitudes in four filters (F606W, F098M, F125W, F160W), with integration times
optimized to identify galaxies at z>7.5 as F098M-dropouts. We discuss here
results from a search area of approximately 130 arcmin^2 over 23 BoRG fields,
complemented by six other pure-parallel WFC3 fields with similar filters. This
new search area is more than two times wider than previous WFC3 observations at
z~8. We identify four F098M-dropout candidates with high statistical confidence
(detected at greater than 8sigma confidence in F125W). These sources are among
the brightest candidates currently known at z~8 and approximately ten times
brighter than the z=8.56 galaxy UDFy-38135539. They thus represent ideal
targets for spectroscopic followup observations and could potentially lead to a
redshift record, as our color selection includes objects up to z~9. However,
the expected contamination rate of our sample is about 30% higher than typical
searches for dropout galaxies in legacy fields, such as the GOODS and HUDF,
where deeper data and additional optical filters are available to reject
contaminants.Comment: minor changes, ApJL accepte
The Evolution of the UV Luminosity Function from z ~ 0.75 to z ~ 2.5 using HST ERS WFC3/UVIS Observations
We present UV luminosity functions (LFs) at 1500 Angstrom derived from the
HST Early Release Science WFC3/UVIS data acquired over ~50 arcmin^2 of the
GOODS-South field. The LFs are determined over the entire redshift range
z=0.75-2.5 using two methods, similar to those used at higher redshifts for
Lyman Break Galaxies (LBGs): (1) 13-band UV+optical+NIR photometric redshifts
to study galaxies in the range z=0.5-2 in three bins of dz=0.5, and (2) dropout
samples in three redshift windows centered at z~1.5, z~1.9, and z~2.5. The
characteristic luminosity dims by ~1.5 mag from z=2.5 to z=0.75, consistent
with earlier work. However, the other Schechter function parameters, the
faint-end slope and the number density, are found to be remarkably constant
over the range z=0.75-2.5. Using these LF determinations we find the UV
luminosity density to increase by ~1.4 dex according to (1+z)^{2.58+-0.15} from
z~0 to its peak at z~2.5. Strikingly, the inferred faint-end slopes for our LFs
are all steeper than alpha=-1.5, in agreement with higher-redshift LBG studies.
Since the faint-end slope in the local universe is found to be much flatter
with alpha~=-1.2, this poses the question as to when and how the expected
flattening occurs. Despite relatively large uncertainties, our data suggest
alpha~=-1.7 at least down to z~1. These new results from such a shallow early
dataset demonstrate very clearly the remarkable potential of WFC3/UVIS for the
thorough characterization of galaxy evolution over the full redshift range
z~0.5 to z~3.Comment: 6 pages, 4 figures, ApJL, updated to match printed versio
CLASH: Discovery of a Bright z~6.2 Dwarf Galaxy Quadruply Lensed by MACS J0329.6-0211
We report the discovery of a z_{phot}=6.18^{+0.05}_{-0.07} (95% confidence
level) dwarf galaxy, lensed into four images by the galaxy cluster MACS
J0329.6-0211 (z_{l}=0.45). The galaxy is observed as a high-redshift dropout in
HST/ACS/WFC3 CLASH and Spitzer/IRAC imaging. Its redshift is securely
determined due to a clear detection of the Lyman-break in the 18-band
photometry, making this galaxy one of the highest-redshift multiply-lensed
objects known to date with an observed magnitude of F125W=24.00\pm0.04 AB mag
for its highest-magnified image. We also present the first strong-lensing
analysis of this cluster uncovering 15 additional multiply-imaged candidates of
five lower-redshift sources spanning the range z_{s}~2-4. The mass model
independently supports the high photometric redshift and reveals magnifications
of 11.6^{+8.9}_{-4.1}, 17.6^{+6.2}_{-3.9}, 3.9^{+3.0}_{-1.7}, and
3.7^{+1.3}_{-0.2}, respectively, for the four images of the high-redshift
galaxy. With this we construct a source image with a physical resolution of
~200 pc when the universe was ~0.9 Gyr old, where the z~6.2 galaxy occupies a
source-plane area of approximately 2.2 kpc^{2}. Modeling the observed spectral
energy distribution using population synthesis models, we find a demagnified
stellar mass of ~10^{9} {M}_{sun}, subsolar metallicity (Z/Z_{sun}~0.5), low
dust content (A_{V}~0.1 mag), a demagnified star formation rate (SFR) of ~3.2
{M}_{sun} yr^{-1}, and a specific SFR of ~3.4 Gyr^{-1}, all consistent with the
properties of local dwarf galaxies.Comment: 6 pages, 1 table, 4 figures, accepted for publication in ApJ Letters;
v3: minor changes including shortened text and figure
Expanded Search for z~10 Galaxies from HUDF09, ERS, and CANDELS Data: Evidence for Accelerated Evolution at z>8?
We search for z~10 galaxies over ~160 arcmin^2 of WFC3/IR data in the Chandra
Deep Field South, using the public HUDF09, ERS, and CANDELS surveys, that reach
to 5sigma depths ranging from 26.9 to 29.4 in H_160 AB mag. z>~9.5 galaxy
candidates are identified via J_125-H_160>1.2 colors and non-detections in any
band blueward of J_125. Spitzer IRAC photometry is key for separating the
genuine high-z candidates from intermediate redshift (z~2-4) galaxies with
evolved or heavily dust obscured stellar populations. After removing 16 sources
of intermediate brightness (H_160~24-26 mag) with strong IRAC detections, we
only find one plausible z~10 galaxy candidate in the whole data set, previously
reported in Bouwens et al. (2011). The newer data cover a 3x larger area and
provide much stronger constraints on the evolution of the UV luminosity
function (LF). If the evolution of the z~4-8 LFs is extrapolated to z~10, six
z~10 galaxies are expected in our data. The detection of only one source
suggests that the UV LF evolves at an accelerated rate before z~8. The
luminosity density is found to increase by more than an order of magnitude in
only 170 Myr from z~10 to z~8. This increase is >=4x larger than expected from
the lower redshift extrapolation of the UV LF. We are thus likely witnessing
the first rapid build-up of galaxies in the heart of cosmic reionization.
Future deep HST WFC3/IR data, reaching to well beyond 29 mag, can enable a more
robust quantification of the accelerated evolution around z~10.Comment: 13 pages, 11 figures, ApJ resubmitted after referee repor
A Glimpse of the First Galaxies
The recently refurbished Hubble Space Telescope reveals a galaxy from a time
when the Universe was just 500 million years old, providing insights into the
first throes of galaxy formation and the reionization of the Universe.Comment: Invited Nature "News and Views" Commentary on Bouwens et al. 2011,
Nature, 469, 504-507; 5 pages, 1 figur
z~7 galaxy candidates from NICMOS observations over the HDF South and the CDF-S and HDF-N GOODS fields
We use ~88 arcmin**2 of deep (>~26.5 mag at 5 sigma) NICMOS data over the two
GOODS fields and the HDF South to conduct a search for bright z>~7 galaxy
candidates. This search takes advantage of an efficient preselection over 58
arcmin**2 of NICMOS H-band data where only plausible z>~7 candidates are
followed up with NICMOS J-band observations. ~248 arcmin**2 of deep
ground-based near-infrared data (>~25.5 mag, 5 sigma) is also considered in the
search. In total, we report 15 z-dropout candidates over this area -- 7 of
which are new to these search fields. Two possible z~9 J-dropout candidates are
also found, but seem unlikely to correspond to z~9 galaxies. The present z~9
search is used to set upper limits on the prevalence of such sources. Rigorous
testing is undertaken to establish the level of contamination of our selections
by photometric scatter, low mass stars, supernovae (SNe), and spurious sources.
The estimated contamination rate of our z~7 selection is ~24%. Through careful
simulations, the effective volume available to our z>~7 selections is estimated
and used to establish constraints on the volume density of luminous (L*(z=3),
or -21 mag) galaxies from these searches. We find that the volume density of
luminous star-forming galaxies at z~7 is 13_{-5}^{+8}x lower than at z~4 and
>25x lower (1 sigma) at z~9 than at z~4. This is the most stringent constraint
yet available on the volume density of >~L* galaxies at z~9. The present
wide-area, multi-field search limits cosmic variance to <20%. The evolution we
find at the bright end of the UV LF is similar to that found from recent Subaru
Suprime-Cam, HAWK-I or ERS WFC3/IR searches. The present paper also includes a
complete summary of our final z~7 z-dropout sample (18 candidates) identified
from all NICMOS observations to date (over the two GOODS fields, the HUDF,
galaxy clusters).Comment: 13 pages, 6 figures, 6 tables, accepted for publication in the
Astrophysical Journal, replaced to match accepted version, see
http://firstgalaxies.org/astronomers-area/ for a link to a complete reduction
of the NICMOS observations over the two GOODS field
Very blue UV-continuum slopes of low luminosity z~7 galaxies from WFC3/IR: Evidence for extremely low metallicities?
We use the ultra-deep WFC3/IR data over the HUDF and the Early Release
Science WFC3/IR data over the CDF-South GOODS field to quantify the broadband
spectral properties of candidate star-forming galaxies at z~7. We determine the
UV-continuum slope beta in these galaxies, and compare the slopes with galaxies
at later times to measure the evolution in beta. For luminous L*(z=3) galaxies,
we measure a mean UV-continuum slope beta of -2.0+/-0.2, which is comparable to
the beta~-2 derived at similar luminosities at z~5-6. However, for the lower
luminosity 0.1L*(z=3) galaxies, we measure a mean beta of -3.0+/-0.2. This is
substantially bluer than is found for similar luminosity galaxies at z~4, just
800 Myr later, and even at z~5-6. In principle, the observed beta of -3.0 can
be matched by a very young, dust-free stellar population, but when nebular
emission is included the expected beta becomes >~-2.7. To produce these very
blue beta's (i.e., beta~-3), extremely low metallicities and mechanisms to
reduce the red nebular emission are likely required. For example, a large
escape fraction (i.e., f_{esc}>~0.3) could minimize the contribution from this
red nebular emission. If this is correct and the escape fraction in faint z~7
galaxies is >~0.3, it may help to explain how galaxies reionize the universe.Comment: 5 pages, 5 figures, accepted for publication in Astrophysical Journal
Letter
Lower-Luminosity Galaxies could reionize the Universe: Very Steep Faint-End Slopes to the UV Luminosity Functions at z>=5-8 from the HUDF09 WFC3/IR Observations
The HUDF09 data are the deepest near-IR observations ever, reaching to 29.5
mag. Luminosity functions (LF) from these new HUDF09 data for 132 z\sim7 and
z\sim8 galaxies are combined with new LFs for z\sim5-6 galaxies and the earlier
z\sim4 LF to reach to very faint limits (<0.05 L*(z=3)). The faint-end slopes
alpha are steep: -1.79+/-0.12 (z\sim5), -1.73+/-0.20 (z\sim6), -2.01+/-0.21
(z\sim7), and -1.91+/-0.32 (z\sim8). Slopes alpha\lesssim-2 lead to formally
divergent UV fluxes, though galaxies are not expected to form below \sim-10 AB
mag. These results have important implications for reionization. The weighted
mean slope at z\sim6-8 is -1.87+/-0.13. For such steep slopes, and a faint-end
limit of -10 AB mag, galaxies provide a very large UV ionizing photon flux.
While current results show that galaxies can reionize the universe by z\sim6,
matching the Thomson optical depths is more challenging. Extrapolating the
current LF evolution to z>8, taking alpha to be -1.87+/-0.13 (the mean value at
z\sim6-8), and adopting typical parameters, we derive Thomson optical depths of
0.061_{-0.006}^{+0.009}. However, this result will change if the faint-end
slope alpha is not constant with redshift. We test this hypothesis and find a
weak, though uncertain, trend to steeper slopes at earlier times
(dalpha/dz\sim-0.05+/-0.04), that would increase the Thomson optical depths to
0.079_{-0.017}^{+0.063}, consistent with recent WMAP estimates
(tau=0.088+/-0.015). It may thus not be necessary to resort to extreme
assumptions about the escape fraction or clumping factor. Nevertheless, the
uncertainties remain large. Deeper WFC3/IR+ACS observations can further
constrain the ionizing flux from galaxies.Comment: 6 pages, 4 figures, 1 table, accepted for publication in
Astrophysical Journal Letters, updated to match the version in pres
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