1,236 research outputs found
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
A spectroscopically confirmed z=1.327 galaxy-scale deflector magnifying a z~8 Lyman-Break galaxy in the Brightest of Reionizing Galaxies survey
We present a detailed analysis of an individual case of gravitational lensing
of a Lyman-Break galaxy (LBG) in a blank field, identified in Hubble
Space Telescope imaging obtained as part of the Brightest of Reionizing
Galaxies survey. To investigate the close proximity of the bright
() -dropout to a small group of foreground galaxies, we
obtained deep spectroscopy of the dropout and two foreground galaxies using
VLT/X-Shooter. We detect H-, H-, [OIII] and [OII] emission in
the brightest two foreground galaxies (unresolved at the natural seeing of
arcsec), placing the pair at . We can rule out emission lines
contributing all of the observed broadband flux in band at
, allowing us to exclude the candidate as a low redshift
interloper with broadband photometry dominated by strong emission lines. The
foreground galaxy pair lies at the peak of the luminosity, redshift and
separation distributions for deflectors of strongly lensed objects,
and we make a marginal detection of a demagnified secondary image in the
deepest () filter. We show that the configuration can be accurately
modelled by a singular isothermal ellipsoidal deflector and a S\'{e}rsic source
magnified by a factor of . The reconstructed source in the
best-fitting model is consistent with luminosities and morphologies of
LBGs in the literature. The lens model yields a group mass of
and a stellar mass-to-light ratio for the
brightest deflector galaxy of within its effective radius. The foreground galaxies'
redshifts would make this one of the few strong lensing deflectors discovered
at .Comment: Accepted for publication in MNRAS. 16 pages, 11 figures, 3 table
The Impact of Strong Gravitational Lensing on Observed Lyman-Break Galaxy Numbers at 4<z<8 in the GOODS and the XDF Blank Fields
Detection of Lyman-Break Galaxies (LBGs) at high-redshift can be affected by
gravitational lensing induced by foreground deflectors not only in galaxy
clusters, but also in blank fields. We quantify the impact of strong
magnification in the samples of , , , LBGs () observed in the XDF and GOODS/CANDELS fields, by investigating the
proximity of dropouts to foreground objects. We find that of bright
LBGs () by
foreground objects. This fraction decreases from at to
at . Since the observed fraction of strongly lensed
galaxies is a function of the shape of the luminosity function (LF), it can be
used to derive Schechter parameters, and , independently
from galaxy number counts. Our magnification bias analysis yields
Schechter-function parameters in close agreement with those determined from
galaxy counts albeit with larger uncertainties. Extrapolation of our analysis
to suggests that future surveys with JSWT, WFIRST and EUCLID
should find excess LBGs at the bright-end, even if there is an intrinsic
exponential cutoff of number counts. Finally, we highlight how the
magnification bias measurement near the detection limit can be used as probe of
the population of galaxies too faint to be detected. Preliminary results using
this novel idea suggest that the magnification bias at is not
as strong as expected if extends well below the current
detection limits in the XDF. At face value this implies a flattening of the LF
at . However, selection effects and completeness estimates
are difficult to quantify precisely. Thus, we do not rule out a steep LF
extending to .Comment: Submitted to ApJ on 18/12/201
Rest-Frame Optical Emission Lines in z~3.5 Lyman Break selected Galaxies: The Ubiquity of Unusually High [OIII]/Hbeta Ratios at 2 Gyr
We present K-band spectra of rest-frame optical emission lines for 24
star-forming galaxies at z~3.2-3.7 using MOSFIRE on the Keck 1 telescope.
Strong rest-frame optical [O III] and Hbeta emission lines were detected in 18
LBGs. The median flux ratio of [O III]5007 to Hbeta is 5.1+/-0.5, a factor of
5-10x higher than in local galaxies with similar stellar masses. The observed
Hbeta luminosities are in good agreement with expectations from the estimated
star-formation rates, and none of our sources are detected in deep X-ray
stacks, ruling out significant contamination by active galactic nuclei.
Combining our sample with a variety of LBGs from the literature, including 49
galaxies selected in a very similar manner, we find a high median ratio of
[OIII]/Hbeta = 4.8+0.8-1.7. This high ratio seems to be an ubiquitous feature
of z~3-4 LBGs, very different from typical local star-forming galaxies at
similar stellar masses. The only comparable systems at z~0 are those with
similarly high specific star-formation rates, though ~5x lower stellar masses.
High specific star-formation rates either result in a much higher ionization
parameter or other unusual conditions for the interstellar medium, which result
in a much higher [OIII]/Hbeta line ratio. This implies a strong relation
between a global property of a galaxy, the specific star-formation rate, and
the local conditions of ISM in star-forming regions.Comment: 14 pages, 8 figures, 5 color, published in ApJ, updated to reflect
published versio
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&
High-precision Photometric Redshifts from Spitzer/IRAC: Extreme [3.6]-[4.5] Colors Identify Galaxies in the Redshift Range z~6.6-6.9
One of the most challenging aspects of studying galaxies in the z>~7 universe
is the infrequent confirmation of their redshifts through spectroscopy, a
phenomenon thought to occur from the increasing opacity of the intergalactic
medium to Lya photons at z>6.5. The resulting redshift uncertainties inhibit
the efficient search for [C II] in z~7 galaxies with sub-mm instruments such as
ALMA, given their limited scan speed for faint lines. One means by which to
improve the precision of the inferred redshifts is to exploit the potential
impact of strong nebular emission lines on the colors of z~4-8 galaxies as
observed by Spitzer/IRAC. At z~6.8, galaxies exhibit IRAC colors as blue as
[3.6]-[4.5] ~-1, likely due to the contribution of [O III]+Hb to the 3.6 mum
flux combined with the absence of line contamination in the 4.5 mum band. In
this paper we explore the use of extremely blue [3.6]-[4.5] colors to identify
galaxies in the narrow redshift window z~6.6-6.9. When combined with an
I-dropout criterion, we demonstrate that we can plausibly select a relatively
clean sample of z~6.8 galaxies. Through a systematic application of this
selection technique to our catalogs from all five CANDELS fields, we identify
20 probable z~6.6-6.9 galaxies. We estimate that our criteria select the ~50%
strongest line emitters at z~6.8 and from the IRAC colors we estimate a typical
[O III]+Hb rest-frame equivalent width of 1085A for this sample. The small
redshift uncertainties on our sample make it particularly well suited for
follow-up studies with facilities such as ALMA.Comment: In submission to the Astrophysical Journal, updated in response to
the referee report, 13 pages, 11 figures, 1 tabl
UV Luminosity Functions at redshifts z~4 to z~10: 10000 Galaxies from HST Legacy Fields
The remarkable HST datasets from the CANDELS, HUDF09, HUDF12, ERS, and
BoRG/HIPPIES programs have allowed us to map out the evolution of the UV LF
from z~10 to z~4. We have identified 5859, 3001, 857, 481, 217, and 6 galaxy
candidates at z~4, z~5, z~6, z~7, z~8, and z~10, respectively from the ~1000
arcmin**2 area probed. The selection of z~4-8 galaxies over the five CANDELS
fields allows us to assess the cosmic variance; the largest variations are
apparent at z>=7. Our new LF determinations at z~4 and z~5 span a 6-mag
baseline (-22.5 to -16 AB mag). These determinations agree well with previous
estimates, but the larger samples and volumes probed here result in a more
reliable sampling of >L* galaxies and allow us to reassess the form of the UV
LFs. Our new LF results strengthen our earlier findings to 3.4 sigma
significance for a steeper faint-end slope to the UV LF at z>4, with alpha
evolving from alpha=-1.64+/-0.04 at z~4 to alpha=-2.06+/-0.13 at z~7 (and alpha
= -2.02+/-0.23 at z~8), consistent with that expected from the evolution of the
halo mass function. With our improved constraints at the bright end, we find
less evolution in the characteristic luminosity M* over the redshift range z~4
to z~7; the observed evolution in the LF is now largely represented by changes
in phi*. No evidence for a non-Schechter-like form to the z~4-8 LFs is found. A
simple conditional LF model based on halo growth and evolution in the M/L ratio
of halos ((1+z)**-1.5) provides a good representation of the observed
evolution.Comment: 53 pages, 28 figures, 11 tables, accepted for publication in the
Astrophysical Journa
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