211 research outputs found
Discovery of a possibly old galaxy at , multiply imaged by the massive cluster Abell 383
We report the discovery of a unique galaxy, multiply imaged by the
cluster Abell 383 and detected in new Hubble Space Telescope ACS and WFC3
imaging, as well as in Warm Spitzer observations. This galaxy was selected as a
pair of i-dropouts; its suspected high redshift was confirmed by the
measurement of a strong Lyman-alpha line in both images using Keck/DEIMOS.
Combining Hubble and Spitzer photometry after correcting for contamination by
line emission (estimated to be a small effect), we identify a strong Balmer
break of 1.5 magnitudes. Taking into account the magnification factor of
11.4+/-1.9 (2.65+/-0.17 mag) for the brightest image, the unlensed AB magnitude
for the source is 27.2+/-0.05 in the H band, corresponding to a 0.4 L* galaxy,
and 25.7+/-0.08 at 3.6 um. The UV slope is consistent with beta~2.0, and from
the rest-frame UV continuum we measure a current star formation rate of
2.4+/-1.1 Msol/yr. The unlensed half-light radius is measured to be 300 pc,
from which we deduce a star-forming surface density of ~10 Msol/yr/kpc2. The
Lyman-alpha emission is found to be extended over ~3" along the slit,
corresponding to ~5 kpc in the source plane. This can be explained by the
presence of a much larger envelope of neutral hydrogen around the star-forming
region. Finally, fitting the spectral energy distribution using 7 photometric
data points with simple SED models, we derive the following properties: very
little reddening, an inferred stellar mass of M*=6e9 Msol, and an inferred age
of ~800 Myrs (corresponding to a redshift of formation of ~18). The
star-formation rate of this object was likely much stronger in the past than at
the time of observation, suggesting that we may be missing a fraction of
galaxies at z~6 which have already faded in rest-frame UV wavelengths.Comment: 6 pages, 3 figures, MNRAS in press, replaced with accepted version
including minor comment
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
The Rest Frame UV to Optical Colors and SEDs of z~4-7 Galaxies
We use the ultra-deep HUDF09 and the deep ERS data from the HST WFC3/IR
camera, along with the wide area Spitzer/IRAC data from GOODS-S to derive SEDs
of star-forming galaxies from the rest-frame UV to the optical over a wide
luminosity range (M_1500 ~ -21 to M_1500 ~ -18) from z ~ 7 to z ~ 4. The sample
contains ~ 400 z ~ 4, ~ 120 z ~ 5, ~ 60 z ~ 6, and 36 prior z ~ 7 galaxies.
Median stacking enables the first comprehensive study of very faint high-z
galaxies at multiple redshifts (e.g., [3.6] = 27.4 +/- 0.1 AB mag for the
M_1500 ~ -18 sources at z ~ 4). At z ~ 4 our faint median-stacked SEDs reach to
~ 0.06 L*(z=4) and are combined with recently published results at high
luminosity L > L* that extend to M_1500 ~ -23. We use the observed SEDs and
template fits to derive rest frame UV-to-optical colors (U - V) at all
redshifts and luminosities. We find that this color does not vary significantly
with redshift at a fixed luminosity. The UV-to-optical color does show a weak
trend with luminosity, becoming redder at higher luminosities. This is most
likely due to dust. At z >~ 5 we find blue colors [3.6]-[4.5] ~ -0.3 mag that
are most likely due to rest-frame optical emission lines contributing to the
flux in the IRAC filter bandpasses. The scatter across our derived SEDs remains
substantial, but the results are most consistent with a lack of any evolution
in the SEDs with redshift at a given luminosity. The similarity of the SEDs
suggests a self-similar mode of evolution over a timespan from 0.7 Gyr to 1.5
Gyr that encompasses very substantial growth in the stellar mass density in the
universe (from ~ 4x10^6 to ~ 2x10^7 Msun Mpc^-3).Comment: 15 pages, 11 figures, 3 tables, submitted to Ap
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
Reionization and feedback in overdense regions at high redshift
Observations of galaxy luminosity function at high redshifts typically focus
on fields of view of limited sizes preferentially containing bright sources.
These regions possibly are overdense and hence biased with respect to the
globally averaged regions. Using a semi-analytic model based on Choudhury &
Ferrara (2006) which is calibrated to match a wide range of observations, we
study the reionization and thermal history of the universe in overdense
regions. The main results of our calculation are: (i) Reionization and thermal
histories in the biased regions are markedly different from the average ones
because of enhanced number of sources and higher radiative feedback. (ii) The
galaxy luminosity function for biased regions is markedly different from those
corresponding to average ones. In particular, the effect of radiative feedback
arising from cosmic reionization is visible at much brighter luminosities.
(iii) Because of the enhanced radiative feedback within overdense locations,
the luminosity function in such regions is more sensitive to reionization
history than in average regions. The effect of feedback is visible for absolute
AB magnitude at , almost within the reach of present
day observations and surely to be probed by JWST. This could possibly serve as
an additional probe of radiative feedback and hence reionization at high
redshifts.Comment: 10 pages, 5 figures. Added Section 2.5, Figure 5, and an Appendix.
This version accepted for publication in MNRA
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
Expanding the search for galaxies at z ~7-10 with new NICMOS Parallel Fields
We have carried out a search for galaxies at z ~ 7-10 in ~14.4 sq. arcmin of
new NICMOS parallel imaging taken in the Great Observatories Origins Deep
Survey (GOODS, 5.9 sq. arcmin), the Cosmic Origins Survey (COSMOS, 7.2 sq.
arcmin), and SSA22 (1.3 sq. arcmin). These images reach 5 sigma sensitivities
of J110 = 26.0-27.5 (AB), and combined they increase the amount of deep
near-infrared data by more than 60% in fields where the investment in deep
optical data has already been made. We find no z>7 candidates in our survey
area, consistent with the Bouwens et al. (2008) measurements at z~7 and 9 (over
23 sq. arcmin), which predict 0.7 galaxies at z~7 and <0.03 galaxies at z~9. We
estimate that 10-20% of z>7 galaxies are missed by this survey, due to
incompleteness from foreground contamination by faint sources. For the case of
luminosity evolution, assuming a Schecter parameterization with a typical phi*
= 10^-3 Mpc^-3, we find M* > -20.0 for z~7 and M* > -20.7 for z~9 (68%
confidence). This suggests that the downward luminosity evolution of LBGs
continues to z~7, although our result is marginally consistent with the z~6 LF
of Bouwens et al.(2006, 2007). In addition we present newly-acquired deep
MMT/Megacam imaging of the z~9 candidate JD2325+1433, first presented in Henry
et al. (2008). The resulting weak but significant detection at i' indicates
that this galaxy is most likely an interloper at z~2.7.Comment: Accepted to ApJ. Replacement includes updated discussion of
incompleteness from foreground contaminatio
The Taiwan ECDFS Near-Infrared Survey: Very Bright End of the Luminosity Function at z>7
The primary goal of the Taiwan ECDFS Near-Infrared Survey (TENIS) is to find
well screened galaxy candidates at z>7 (z' dropout) in the Extended Chandra
Deep Field-South (ECDFS). To this end, TENIS provides relatively deep J and Ks
data (~25.3 ABmag, 5-sigma) for an area of 0.5*0.5 degree. Leveraged with
existing data at mid-infrared to optical wavelengths, this allows us to screen
for the most luminous high-z objects, which are rare and thus require a survey
over a large field to be found. We introduce new color selection criteria to
select a z>7 sample with minimal contaminations from low-z galaxies and
Galactic cool stars; to reduce confusion in the relatively low angular
resolution IRAC images, we introduce a novel deconvolution method to measure
the IRAC fluxes of individual sources. Illustrating perhaps the effectiveness
at which we screen out interlopers, we find only one z>7 candidate, TENIS-ZD1.
The candidate has a weighted z_phot of 7.8, and its colors and luminosity
indicate a young (45M years old) starburst galaxy with a stellar mass of
3.2*10^10 M_sun. The result matches with the observational luminosity function
analysis and the semi-analytic simulation result based on the Millennium
Simulations, which may over predict the volume density for high-z massive
galaxies. The existence of TENIS-ZD1, if confirmed spectroscopically to be at
z>7, therefore poses a challenge to current theoretical models for how so much
mass can accumulate in a galaxy at such a high redshift.Comment: 14 pages, 11 figures, ApJ accepte
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
An exponential decline at the bright end of the z=6 galaxy luminosity function
We present the results of a search for the most luminous star-forming
galaxies at redshifts z~6 based on CFHT Legacy Survey data. We identify a
sample of 40 Lyman break galaxies brighter than magnitude z'=25.3 across an
area of almost 4 square degrees. Sensitive spectroscopic observations of seven
galaxies provide redshifts for four, of which only two have moderate to strong
Lyman alpha emission lines. All four have clear continuum breaks in their
spectra. Approximately half of the Lyman break galaxies are spatially resolved
in 0.7 arcsec seeing images, indicating larger sizes than lower luminosity
galaxies discovered with the Hubble Space Telescope, possibly due to on-going
mergers. The stacked optical and infrared photometry is consistent with a
galaxy model with stellar mass ~ 10^{10} solar masses. There is strong evidence
for substantial dust reddening with a best-fit A_V=0.7 and A_V>0.48 at 2 sigma
confidence, in contrast to the typical dust-free galaxies of lower luminosity
at this epoch. The spatial extent and spectral energy distribution suggest that
the most luminous z~6 galaxies are undergoing merger-induced starbursts. The
luminosity function of z=5.9 star-forming galaxies is derived. This agrees well
with previous work and shows strong evidence for an exponential decline at the
bright end, indicating that the feedback processes which govern the shape of
the bright end are occurring effectively at this epoch.Comment: 14 pages, 11 figures, AJ in press, revised to address referee
comment
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