283 research outputs found
Surface Brightness Profiles of Composite Images of Compact Galaxies at z~4-6 in the HUDF
The Hubble Ultra Deep Field (HUDF) contains a significant number of B, V and
i'-band dropout objects, many of which were recently confirmed to be young
star-forming galaxies at z~4-6. These galaxies are too faint individually to
accurately measure their radial surface brightness profiles. Their average
light profiles are potentially of great interest, since they may contain clues
to the time since the onset of significant galaxy assembly. We separately
co-add V, i' and z'-band HUDF images of sets of z~4,5 and 6 objects,
pre-selected to have nearly identical compact sizes and the roundest shapes.
From these stacked images, we are able to study the averaged radial structure
of these objects at much higher signal-to-noise ratio than possible for an
individual faint object. Here we explore the reliability and usefulness of a
stacking technique of compact objects at z~4-6 in the HUDF. Our results are:
(1) image stacking provides reliable and reproducible average surface
brightness profiles; (2) the shape of the average surface brightness profiles
show that even the faintest z~4-6 objects are resolved; and (3) if late-type
galaxies dominate the population of galaxies at z~4-6, as previous HST studies
have shown, then limits to dynamical age estimates for these galaxies from
their profile shapes are comparable with the SED ages obtained from the
broadband colors. We also present accurate measurements of the sky-background
in the HUDF and its associated 1-sigma uncertainties.Comment: 10 pages, 9 figures, 2 tables, emulateapj; Accepted for publication
in The Astronomical Journa
Ultra-deep Large Binocular Camera U-band Imaging of the GOODS-North Field: Depth vs. Resolution
We present a study of the trade-off between depth and resolution using a
large number of U-band imaging observations in the GOODS-North field
(Giavalisco et al. 2004) from the Large Binocular Camera (LBC) on the Large
Binocular Telescope (LBT). Having acquired over 30 hours of data (315 images
with 5-6 mins exposures), we generated multiple image mosaics, starting with
the best atmospheric seeing images (FWHM 0.8"), which constitute
10% of the total data set. For subsequent mosaics, we added in data with
larger seeing values until the final, deepest mosaic included all images with
FWHM 1.8" (94% of the total data set). From the mosaics, we
made object catalogs to compare the optimal-resolution, yet shallower image to
the lower-resolution but deeper image. We show that the number counts for both
images are 90% complete to . Fainter than
27, the object counts from the optimal-resolution image start to
drop-off dramatically (90% between = 27 and 28 mag), while the deepest
image with better surface-brightness sensitivity ( 32
mag arcsec) show a more gradual drop (10% between 27
and 28 mag). For the brightest galaxies within the GOODS-N field, structure and
clumpy features within the galaxies are more prominent in the
optimal-resolution image compared to the deeper mosaics. Finally, we find - for
220 brighter galaxies with 24 mag - only marginal
differences in total flux between the optimal-resolution and lower-resolution
light-profiles to 32 mag arcsec. In only 10% of
the cases are the total-flux differences larger than 0.5 mag. This helps
constrain how much flux can be missed from galaxy outskirts, which is important
for studies of the Extragalactic Background Light.Comment: 24 pages, 14 figures, submitted to PASP, comments welcom
A distortion of very--high--redshift galaxy number counts by gravitational lensing
The observed number counts of high-redshift galaxy candidates have been used
to build up a statistical description of star-forming activity at redshift z >~
7, when galaxies reionized the Universe. Standard models predict that a high
incidence of gravitational lensing will probably distort measurements of flux
and number of these earliest galaxies. The raw probability of this happening
has been estimated to be ~ 0.5 percent, but can be larger owing to
observational biases. Here we report that gravitational lensing is likely to
dominate the observed properties of galaxies with redshifts of z >~ 12, when
the instrumental limiting magnitude is expected to be brighter than the
characteristic magnitude of the galaxy sample. The number counts could be
modified by an order of magnitude, with most galaxies being part of multiply
imaged systems, located less than 1 arcsec from brighter foreground galaxies at
z ~ 2. This lens-induced association of high-redshift and foreground galaxies
has perhaps already been observed among a sample of galaxy candidates
identified at z ~ 10.6. Future surveys will need to be designed to account for
a significant gravitational lensing bias in high-redshift galaxy samples.Comment: Nature, Jan. 13, 2011 issue (in press
Constraining stellar assembly and AGN feedback at the peak epoch of star formation
We study stellar assembly and feedback from active galactic nuclei (AGN)
around the epoch of peak star formation (1<z<2), by comparing hydrodynamic
simulations to rest-frame UV-optical galaxy colours from the Wide Field Camera
3 (WFC3) Early-Release Science (ERS) Programme. Our Adaptive Mesh Refinement
simulations include metal-dependent radiative cooling, star formation, kinetic
outflows due to supernova explosions, and feedback from supermassive black
holes. Our model assumes that when gas accretes onto black holes, a fraction of
the energy is used to form either thermal winds or sub-relativistic
momentum-imparting collimated jets, depending on the accretion rate. We find
that the predicted rest-frame UV-optical colours of galaxies in the model that
includes AGN feedback is in broad agreement with the observed colours of the
WFC3 ERS sample at 1<z<2. The predicted number of massive galaxies also matches
well with observations in this redshift range. However, the massive galaxies
are predicted to show higher levels of residual star formation activity than
the observational estimates, suggesting the need for further suppression of
star formation without significantly altering the stellar mass function. We
discuss possible improvements, involving faster stellar assembly through
enhanced star formation during galaxy mergers while star formation at the peak
epoch is still modulated by the AGN feedback.Comment: 6 pages, 4 figures, accepted for publication in MNRAS Letter
UV-dropout Galaxies in the GOODS-South Field from WFC3 Early Release Science Observations
We combine new high sensitivity ultraviolet (UV) imaging from the Wide Field
Camera 3 (WFC3) on the Hubble Space Telescope (HST) with existing deep
HST/Advanced Camera for Surveys (ACS) optical images from the Great
Observatories Origins Deep Survey (GOODS) program to identify UV-dropouts,
which are Lyman break galaxy (LBG) candidates at z~1-3. These new HST/WFC3
observations were taken over 50 sq.arcmin in the GOODS-South field as a part of
the Early Release Science program. The uniqueness of these new UV data is that
they are observed in 3 UV/optical (WFC3 UVIS) channel filters (F225W, F275W and
F336W), which allows us to identify three different sets of UV-dropout samples.
We apply Lyman break dropout selection criteria to identify F225W-, F275W- and
F336W-dropouts, which are z~1.7, 2.1 and 2.7 LBG candidates, respectively. Our
results are as follows: (1) these WFC3 UVIS filters are very reliable in
selecting LBGs with z~2.0, which helps to reduce the gap between the well
studied z~>3 and z~0 regimes, (2) the combined number counts agrees very well
with the observed change in the surface densities as a function of redshift
when compared with the higher redshift LBG samples; and (3) the best-fit
Schechter function parameters from the rest-frame UV luminosity functions at
three different redshifts fit very well with the evolutionary trend of the
characteristic absolute magnitude, and the faint-end slope, as a function of
redshift. This is the first study to illustrate the usefulness of the WFC3 UVIS
channel observations to select z<3 LBGs. The addition of the new WFC3 on the
HST has made it possible to uniformly select LBGs from z~1 to z~9, and
significantly enhance our understanding of these galaxies using HST sensitivity
and resolution.Comment: Accepted for publication in ApJ (24 pages, 7 figures
Hubble Space Telescope Imaging of Lyman Alpha Emission at z=4.4
We present the highest redshift detections of resolved Lyman alpha emission,
using Hubble Space Telescope/ACS F658N narrowband-imaging data taken in
parallel with the Wide Field Camera 3 Early Release Science program in the
GOODS CDF-S. We detect Lyman alpha emission from three spectroscopically
confirmed z = 4.4 Lyman alpha emitting galaxies (LAEs), more than doubling the
sample of LAEs with resolved Lyman alpha emission. Comparing the light
distribution between the rest-frame ultraviolet continuum and narrowband
images, we investigate the escape of Lyman alpha photons at high redshift.
While our data do not support a positional offset between the Lyman alpha and
rest-frame ultraviolet (UV) continuum emission, the half-light radii in two out
of the three galaxies are significantly larger in Lyman alpha than in the
rest-frame UV continuum. This result is confirmed when comparing object sizes
in a stack of all objects in both bands. Additionally, the narrowband flux
detected with HST is significantly less than observed in similar filters from
the ground. These results together imply that the Lyman alpha emission is not
strictly confined to its indigenous star-forming regions. Rather, the Lyman
alpha emission is more extended, with the missing HST flux likely existing in a
diffuse outer halo. This suggests that the radiative transfer of Lyman alpha
photons in high-redshift LAEs is complicated, with the interstellar-medium
geometry and/or outflows playing a significant role in galaxies at these
redshifts.Comment: Submitted to the Astrophysical Journal. 11 pages, 10 figure
Spectrophotometrically Identified stars in the PEARS-N and PEARS-S fields
Deep ACS slitless grism observations and identification of stellar sources
are presented within the Great Observatories Origins Deep Survey (GOODS) North
and South fields which were obtained in the Probing Evolution And Reionization
Spectroscopically (PEARS) program. It is demonstrated that even low resolution
spectra can be a very powerful means to identify stars in the field, especially
low mass stars with stellar types M0 and later. The PEARS fields lay within the
larger GOODS fields, and we used new, deeper images to further refine the
selection of stars in the PEARS field, down to a magnitude of mz = 25 using a
newly developed stellarity parameter. The total number of stars with reliable
spectroscopic and morphological identification was 95 and 108 in the north and
south fields respectively. The sample of spectroscopically identified stars
allows constraints to be set on the thickness of the Galactic thin disk as well
as contributions from a thick disk and a halo component. We derive a thin disk
scale height, as traced by the population of M4 to M9 dwarfs along two
independent lines of sight, of h_thin = 370 +60/-65 pc. When including the more
massive M0 to M4 dwarf population, we derive h_thin = 300 +/- 70pc. In both
cases, we observe that we must include a combination of thick and halo
components in our models in order to account for the observed numbers of faint
dwarfs. The required thick disk scale height is typically h_thick=1000 pc and
the acceptable relative stellar densities of the thin disk to thick disk and
the thin disk to halo components are in the range of 0.00025<f_halo<0.0005 and
0.05<f_thick<0.08 and are somewhat dependent on whether the more massive M0 to
M4 dwarfs are included in our sample
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