42 research outputs found
Star Formation at z~6: The UDF-Parallel ACS Fields
We report on the i-dropouts detected in two exceptionally deep ACS fields
(B_{435}, V_{606}, i_{775}, and z_{850} with 10 sigma limits of 28.8, 29.0,
28.5, and 27.8, respectively) taken in parallel with the UDF NICMOS
observations. Using an i-z>1.4 cut, we find 30 i-dropouts over 21 arcmin^2 down
to z_AB=28.1, or 1.4 i-dropouts arcmin^{-2}, with significant field-to-field
variation (as expected from cosmic variance). This extends i-dropout searches
some ~0.9^m further down the luminosity function than was possible in the GOODS
field, netting a ~7x increase in surface density. An estimate of the size
evolution for UV bright objects is obtained by comparing the composite radial
flux profile of the bright i-dropouts (z<27.2) with scaled versions of the
HDF-N + HDF-S U-dropouts. The best-fit is found with a (1+z)^{-1.57_{-0.53}
^{+0.50}} scaling in size (for fixed luminosity), extending lower redshift
(1<z<5) trends to z~6. Adopting this scaling and the brighter i-dropouts from
both GOODS fields, we make incompleteness estimates and construct a z~6 LF in
the rest-frame continuum UV (~1350 A) over a 3.5 magnitude baseline, finding a
shape consistent with that found at lower redshift. To evaluate the evolution
in the LF from z~3.8, we make comparisons against different scalings of a lower
redshift B-dropout sample. Though a strong degeneracy is found between
luminosity and density evolution, our best-fit model scales as (1+z)^{-2.8} in
number and (1+z)^0.1 in luminosity, suggesting a rest-frame continuum UV
luminosity density at z~6 which is just 0.38_{-0.07} ^{+0.09}x that at z~3.8.
Our inclusion of size evolution makes the present estimate lower than previous
z~6 estimates.Comment: 5 pages, 5 figures, accepted for publication in the Astrophysical
Journal Letters, labelling to the left-hand axis of Figure 4 correcte
Clusters at Half Hubble Time: Galaxy Structure and Colors in RXJ0152.7-1357 and MS1054-03
We study the photometric and structural properties of spectroscopically
confirmed members in the two massive X-ray--selected z=0.83 galaxy clusters
MS1054-03 and RXJ0152-1357 using three-band mosaic imaging with the Hubble
Space Telescope Advanced Camera for Surveys. The samples include 105 and 140
members of MS1054-03 and RXJ0152-1357, respectively, with ACS F775W magnitude <
24.0. We develop a promising new structural classification method, based on a
combination of the best-fit Sersic indices and the normalized root-mean-square
residuals from the fits; the resulting classes agree well with the visual ones,
but are less affected by galaxy orientation. We examine the color--magnitude
relations in detail and find that the color residuals correlate with the local
mass density measured from our weak lensing maps; we identify a threshold
density of , in units of the critical density, above which
the star formation appears to cease. For RXJ0152-1357, we also find a trend in
the color residuals with velocity, resulting from an offset of about 980 km/s
in the mean redshifts of the early- and late-type galaxies. Analysis of the
color--color diagrams indicates that a range of star formation time-scales are
needed to reproduce the loci of the galaxy colors. We also identify some
cluster galaxies whose colors can only be explained by large amounts, mag, of internal dust extinction. [Abstract shortened]Comment: 30 pages, emulateapj format; 23 figures, many in color. Accepted by
ApJ; scheduled for the 10 June 2006 issue. Some figures degraded; for a
higher resolution version, see: http://astro.wsu.edu/blakeslee/z1clusters
ACS Observations of a Strongly Lensed Arc in a Field Elliptical
We report the discovery of a strongly lensed arc system around a field
elliptical galaxy in Hubble Space Telescope (HST) Advanced Camera for Surveys
(ACS) images of a parallel field observed during NICMOS observations of the HST
Ultra-Deep Field. The ACS parallel data comprise deep imaging in the F435W,
F606W, F775W, and F850LP bandpasses. The main arc is at a radius of 1.6 arcsec
from the galaxy center and subtends about 120 deg. Spectroscopic follow-up at
Magellan Observatory yields a redshift z=0.6174 for the lensing galaxy, and we
photometrically estimate z_phot = 2.4\pm0.3 for the arc. We also identify a
likely counter-arc at a radius of 0.6 arcsec, which shows structure similar to
that seen in the main arc. We model this system and find a good fit to an
elliptical isothermal potential of velocity dispersion
\kms, the value expected from the fundamental plane, and some external shear.
Several other galaxies in the field have colors similar to the lensing galaxy
and likely make up a small group.Comment: Accepted for publication in ApJ Letters. 10 pages, 3 figures. Figures
have been degraded to meet size limit; a higher resolution version and
addtional pictures available at http://acs.pha.jhu.edu/~jpb/UDFparc
Advanced Camera for Surveys Observations of Young Star Clusters in the Interacting Galaxy UGC 10214
We present the first Advanced Camera for Surveys (ACS) observations of young
star clusters in the colliding/merging galaxy UGC 10214. The observations were
made as part of the Early Release Observation (ERO) program for the newly
installed ACS during service mission SM3B for the Hubble Space Telescope (HST).
Many young star clusters can be identified in the tails of UGC 10214, with ages
ranging from ~3 Myr to 10 Myr. The extreme blue V-I (F606W-F814W) colors of the
star clusters found in the tail of UGC 10214 can only be explained if strong
emission lines are included with a young stellar population. This has been
confirmed by our Keck spectroscopy of some of these bright blue stellar knots.
The most luminous and largest of these blue knots has an absolute magnitude of
M_V = -14.45, with a half-light radius of 161 pc, and if it is a single star
cluster, would qualify as a super star cluster (SSC). Alternatively, it could
be a superposition of multiple scaled OB associations or clusters. With an
estimated age of ~ 4-5 Myr, its derived mass is < 1.3 x 10^6 solar masses. Thus
the young stellar knot is unbound and will not evolve into a normal globular
cluster. The bright blue clusters and associations are much younger than the
dynamical age of the tail, providing strong evidence that star formation occurs
in the tail long after it was ejected. UGC 10214 provides a nearby example of
processes that contributed to the formation of halos and intra-cluster media in
the distant and younger Universe.Comment: 6 pages with embedded figures, ApJ in pres
Star Formation at z~6: i-dropouts in the ACS GTO fields
Using an i-z dropout criterion, we determine the space density of z~6
galaxies from two deep ACS GTO fields with deep optical-IR imaging. A total of
23 objects are found over 46 arcmin^2, or ~0.5 objects/arcmin^2 down to z~27.3
(6 sigma; all AB mag) (including one probable z~6 AGN). Combining deep ISAAC
data for our RDCS1252-2927 field (J~25.7 and Ks~25.0 (5 sigma)) and NICMOS data
for the HDF North (JH~27.3 (5 sigma)), we verify that these dropouts have flat
spectral slopes. i-dropouts in our sample range in luminosity from ~1.5 L*
(z~25.6) to ~0.3 L* (z~27.3) with the exception of one very bright candidate at
z~24.2. The half-light radii vary from 0.09" to 0.29", or 0.5 kpc to 1.7 kpc.
We derive the z~6 rest-frame UV luminosity density using three different
procedures, each utilizing simulations based on a CDF South V dropout sample.
First, we compare our findings with a no-evolution projection of this V-dropout
sample. We find 23+/-25% more i-dropouts than we predict. Adopting previous
results to z~5, this works out to a 20+/-29% drop in the luminosity density
from z~3 to z~6. Second, we use these same V-dropout simulations to derive a
selection function for our i-dropout sample and compute the UV-luminosity
density (7.2+/-2.5 x 10^25 ergs/s/Hz/Mpc^3 down to z~27). We find a 39+/-21%
drop over the same redshift range. This is our preferred value and suggests a
star formation rate of 0.0090+/-0.0031 M_sol/yr/Mpc^3 to z~27, or ~0.036+/-
0.012 M_sol/yr/Mpc^3 extrapolating the LF to the faint limit. Third, we follow
a very similar procedure, but assume no incompleteness, finding a luminosity
density which is ~2-3X lower. This final estimate constitutes a lower limit.
All three estimates are within the canonical range of luminosity densities
necessary for reionization of the universe at this epoch. (abridged)Comment: 36 pages, 13 figures, 2 tables, accepted for publication in ApJ,
postscript version with high-resolution figures can be downloaded at
http://www.ucolick.org/~bouwens/idropout.p
UV Luminosity Functions from 132 z~7 and z~8 Lyman-Break Galaxies in the ultra-deep HUDF09 and wide-area ERS WFC3/IR Observations
We identify 73 z~7 and 59 z~8 candidate galaxies in the reionization epoch,
and use this large 26-29.4 AB mag sample of galaxies to derive very deep
luminosity functions to <-18 AB mag and the star formation rate density at z~7
and z~8. The galaxy sample is derived using a sophisticated Lyman-Break
technique on the full two-year WFC3/IR and ACS data available over the HUDF09
(~29.4 AB mag, 5 sigma), two nearby HUDF09 fields (~29 AB mag, 14 arcmin) and
the wider area ERS (~27.5 AB mag) ~40 arcmin**2). The application of strict
optical non-detection criteria ensures the contamination fraction is kept low
(just ~7% in the HUDF). This very low value includes a full assessment of the
contamination from lower redshift sources, photometric scatter, AGN, spurious
sources, low mass stars, and transients (e.g., SNe). From careful modelling of
the selection volumes for each of our search fields we derive luminosity
functions for galaxies at z~7 and z~8 to <-18 AB mag. The faint-end slopes
alpha at z~7 and z~8 are uncertain but very steep at alpha = -2.01+/-0.21 and
alpha=-1.91+/-0.32, respectively. Such steep slopes contrast to the local
alpha<~-1.4 and may even be steeper than that at z~4 where alpha=-1.73+/-0.05.
With such steep slopes (alpha<~-1.7) lower luminosity galaxies dominate the
galaxy luminosity density during the epoch of reionization. The star formation
rate densities derived from these new z~7 and z~8 luminosity functions are
consistent with the trends found at later times (lower redshifts). We find
reasonable consistency, with the SFR densities implied from reported stellar
mass densities, being only ~40% higher at z<7. This suggests that (1) the
stellar mass densities inferred from the Spitzer IRAC photometry are reasonably
accurate and (2) that the IMF at very high redshift may not be very different
from that at later times.Comment: 38 pages, 21 figures, 20 tables, ApJ, accepted for publicatio
Internal Color Properties of Resolved Spheroids in the Deep HST/ACS field of UGC 10214
(Abridged) We study the internal color properties of a morphologically
selected sample of spheroidal galaxies taken from HST/ACS ERO program of UGC
10214 (``The Tadpole''). By taking advantage of the unprecedented high
resolution of the ACS in this very deep dataset we are able to characterize
spheroids at sub-arcseconds scales. Using the V_606W and I_814W bands, we
construct V-I color maps and extract color gradients for a sample of spheroids
at I_814W < 24 mag. We investigate the existence of a population of
morphologically classified spheroids which show extreme variation in their
internal color properties similar to the ones reported in the HDFs. These are
displayed as blue cores and inverse color gradients with respect to those
accounted from metallicity variations. Following the same analysis we find a
similar fraction of early-type systems (~30%-40%) that show non-homologous
internal colors, suggestive of recent star formation activity. We present two
statistics to quantify the internal color variation in galaxies and for tracing
blue cores, from which we estimate the fraction of non-homogeneous to
homogeneous internal colors as a function of redshift up to z<1.2. We find that
it can be described as about constant as a function of redshift, with a small
increase with redshift for the fraction of spheroids that present strong color
dispersions. The implications of a constant fraction at all redshifts suggests
the existence of a relatively permanent population of evolving spheroids up to
z~1. We discuss the implications of this in the context of spheroidal
formation.Comment: Fixed URL for high resolution version. 13 Pages, 10 Figures. Accepted
for Publication in ApJ. Sep 1st issue. Higher resolution version and complete
table3B at http://acs.pha.jhu.edu/~felipe/e-prints/Tadpol
An Observational Test for the Anthropic Origin of the Cosmological Constant
The existence of multiple regions of space beyond the observable Universe
(within the so-called "multiverse") where the vacuum energy density takes
different values, has been postulated as an explanation for the low non-zero
value observed for it in our Universe. It is often argued that our existence
pre-selects regions where the cosmological constant is sufficiently small to
allow galaxies like the Milky Way to form and intelligent life to emerge. Here
we propose a simple empirical test for this anthropic argument within the
boundaries of the observable Universe. We make use of the fact that dwarf
galaxies formed in our Universe at redshifts as high as z~10 when the mean
matter density was larger by a factor of ~10^3 than today. Existing technology
enables to check whether planets form in nearby dwarf galaxies and globular
clusters by searching for microlensing or transit events of background stars.
The oldest of these nearby systems may have formed at z~10. If planets are as
common per stellar mass in these descendents as they are in the Milky Way
galaxy, then the anthropic argument would be weakened considerably since
planets could have formed in our Universe even if the cosmological constant was
three orders of magnitude larger than observed. For a flat probability
distribution, this would imply that the probability for us to reside in a
region where the cosmological constant obtains its observed value is lower than
\~10^{-3}. A precise version of the anthropic argument could then be ruled-out
at a confidence level of ~99.9%, which constitutes a satisfactory measure of a
good experimental test.Comment: JCAP, in pres
The Luminosity Function of Early-Type Galaxies at z~0.75
We measure the luminosity function of morphologically selected E/S0 galaxies
from to using deep high resolution Advanced Camera for Surveys
imaging data. Our analysis covers an area of 48\Box\arcmin (8 the
area of the HDF-N) and extends 2 magnitudes deeper ( mag) than was
possible in the Deep Groth Strip Survey (DGSS). At , we find
and , and at
, we find . These luminosity
functions are similar in both shape and number density to the luminosity
function using morphological selection (e.g., DGSS), but are much steeper than
the luminosity functions of samples selected using morphological proxies like
the color or spectral energy distribution (e.g., CFRS, CADIS, or COMBO-17). The
difference is due to the `blue', , E/S0 galaxies, which make up to
of the sample at all magnitudes and an increasing proportion of faint
galaxies. We thereby demonstrate the need for {\it both morphological and
structural information} to constrain the evolution of galaxies.
We find that the `blue' E/S0 galaxies have the same average sizes and Sersic
parameters as the `red', , E/S0 galaxies at brighter luminosities
(), but are increasingly different at fainter magnitudes where
`blue' galaxies are both smaller and have lower Sersic parameters. Fits of the
colors to stellar population models suggest that most E/S0 galaxies have short
star-formation time scales ( Gyr), and that galaxies have formed at an
increasing rate from until after which there has been a
gradual decline.Comment: 39 pages, 21 figures, accepted in A
The Morphology - Density Relation in z ~ 1 Clusters
We measure the morphology--density relation (MDR) and morphology-radius
relation (MRR) for galaxies in seven z ~ 1 clusters that have been observed
with the Advanced Camera for Surveys on board the Hubble Space Telescope.
Simulations and independent comparisons of ourvisually derived morphologies
indicate that ACS allows one to distinguish between E, S0, and spiral
morphologies down to zmag = 24, corresponding to L/L* = 0.21 and 0.30 at z =
0.83 and z = 1.24, respectively. We adopt density and radius estimation methods
that match those used at lower redshift in order to study the evolution of the
MDR and MRR. We detect a change in the MDR between 0.8 < z < 1.2 and that
observed at z ~ 0, consistent with recent work -- specifically, the growth in
the bulge-dominated galaxy fraction, f_E+SO, with increasing density proceeds
less rapidly at z ~ 1 than it does at z ~ 0. At z ~ 1 and density <= 500
galaxies/Mpc^2, we find = 0.72 +/- 0.10. At z ~ 0, an E+S0 population
fraction of this magnitude occurs at densities about 5 times smaller. The
evolution in the MDR is confined to densities >= 40 galaxies/Mpc^2 and appears
to be primarily due to a deficit of S0 galaxies and an excess of Spiral+Irr
galaxies relative to the local galaxy population. The Elliptical fraction -
density relation exhibits no significant evolution between z = 1 and z = 0. We
find mild evidence to suggest that the MDR is dependent on the bolometric X-ray
luminosity of the intracluster medium. Implications for the evolution of the
disk galaxy population in dense regions are discussed in the context of these
observations.Comment: 30 pages, 18 figures. Accepted for publication in ApJ. Full
resolution versions of figs 2,3,6,8 are available at
http://www.stsci.edu/~postman/mdr_figure