196 research outputs found
Simulating Galaxy Evolution
The forwards approach to galaxy formation and evolution is extremely powerful
but leaves several questions unanswered. Foremost among these is the origin of
disks. A backwards approach is able to provide a more realistic treatment of
star formation and feedback and provides a practical guide to eventually
complement galaxy formation ab initio.Comment: 11 pages with 2 figures, to appear in "After the Dark Ages: When
Galaxies were Young", proceedings of the 9th annual October Astrophysics
Conference, ed. S. Holt and E. Smith, simulated images available at
http://astro.berkeley.edu/~bouwens/simulation.htm
Inside-Out Infall Formation of Disk Galaxies: Do Predictions Differ from Models without Size Evolution?
We develop an idealized inside-out formation model for disk galaxies to
include a realistic mix of galaxy types and luminosities that provides a fair
match to the traditional observables. The predictions of our infall models are
compared against identical models with no-size evolution by generating fully
realistic simulations of the HDF, from which we recover the angular size
distributions. We find that our infall models produce nearly identical angular
size distributions to those of our no-size evolution models in the case of a
Omega = 0 geometry but produce slightly smaller sizes in the case of a Omega =
1 geometry, a difference we associate with the fact that there is a different
amount of cosmic time in our two models for evolving to relatively low
redshifts (z \approx 1-2). Our infall models also predict a slightly smaller
(11% - 29%) number of large (disk scale lengths > 4 h_{50} ^{-1} kpc) galaxies
at z \approx 0.7 for the CFRS as well as different increases in the central
surface brightness of the disks for early-type spirals, the infall model
predicting an increase by 1.2 magnitudes out to z \approx 2 (Omega = 0), 1
(Omega = 1), while our no-size evolution models predict an increase of only 0.5
magnitude. This result suggests that infall models could be important for
explaining the 1.2-1.6 magnitude increase in surface brightness reported by
Schade et al. (1995, 1996a, 1996b).Comment: 12 pages, LaTeX (aaspp4.sty), accepted by ApJ Letter
Cloning Dropouts: Implications for Galaxy Evolution at High Redshift
The evolution of high redshift galaxies in the two Hubble Deep Fields, HDF-N
and HDF-S, is investigated using a cloning technique that replicates z~ 2-3 U
dropouts to higher redshifts, allowing a comparison with the observed B and V
dropouts at higher redshifts (z ~ 4-5). We treat each galaxy selected for
replication as a set of pixels that are k-corrected to higher redshift,
accounting for resampling, shot-noise, surface-brightness dimming, and the
cosmological model. We find evidence for size evolution (a 1.7x increase) from
z ~ 5 to z ~ 2.7 for flat geometries (Omega_M+Omega_LAMBDA=1.0). Simple scaling
laws for this cosmology predict that size evolution goes as (1+z)^{-1},
consistent with our result. The UV luminosity density shows a similar increase
(1.85x) from z ~ 5 to z ~ 2.7, with minimal evolution in the distribution of
intrinsic colors for the dropout population. In general, these results indicate
less evolution than was previously reported, and therefore a higher luminosity
density at z ~ 4-5 (~ 50% higher) than other estimates. We argue the present
technique is the preferred way to understand evolution across samples with
differing selection functions, the most relevant differences here being the
color cuts and surface brightness thresholds (e.g., due to the (1+z)^4 cosmic
surface brightness dimming effect).Comment: 56 pages, 22 figures, accepted for publication in Ap
The First 1-2 Gyrs of Galaxy Formation: Dropout Galaxies from z~3-6
The unique high-resolution wide-field imaging capabilities of HST with ACS
have allowed the characterization of galaxies at redshift 6, less than 1 Gyr
from recombination. The dropout technique, applied to deep ACS i, z images in
the RCDS 1252-2927, GOODS and UDF-Parallel fields has yielded large samples of
these objects, allowing a detailed determination of their properties (e.g.,
size, color) and meaningful comparisons against lower redshift dropout samples.
The use of cloning techniques has enabled us to control for many of the strong
selection biases that affect the study of high redshift populations. A clear
trend of size with redshift has been identified, and its impact on the
luminosity density and star formation rate can be estimated. There is a
significant, though modest, decrease in the star formation rate from redshifts
z~2.5 out through z~6. The latest data also allow for the first robust
determination of the luminosity function at z~6.Comment: 6 pages, 5 figures, to appear in the Proceedings of the ESO/USM/MPE
Workshop on "Multiwavelength Mapping of Galaxy Formation and Evolution", eds.
R. Bender and A. Renzini, left-hand axis to Figure 5a correcte
Spitzer IRAC confirmation of z_850-dropout galaxies in the Hubble Ultra Deep Field: stellar masses and ages at z~7
Using Spitzer IRAC mid-infrared imaging from the Great Observatories Origins
Deep Survey, we study z_850-dropout sources in the Hubble Ultra Deep Field.
After carefully removing contaminating flux from foreground sources, we clearly
detect two z_850-dropouts at 3.6 micron and 4.5 micron, while two others are
marginally detected. The mid-infrared fluxes strongly support their
interpretation as galaxies at z~7, seen when the Universe was only 750 Myr old.
The IRAC observations allow us for the first time to constrain the rest-frame
optical colors, stellar masses, and ages of the highest redshift galaxies.
Fitting stellar population models to the spectral energy distributions, we find
photometric redshifts in the range 6.7-7.4, rest-frame colors U-V=0.2-0.4,
V-band luminosities L_V=0.6-3 x 10^10 L_sun, stellar masses 1-10 x 10^9 M_sun,
stellar ages 50-200 Myr, star formation rates up to ~25 M_sun/yr, and low
reddening A_V<0.4. Overall, the z=7 galaxies appear substantially less massive
and evolved than Lyman break galaxies or Distant Red Galaxies at z=2-3, but
fairly similar to recently identified systems at z=5-6. The stellar mass
density inferred from our z=7 sample is rho* = 1.6^{+1.6}_{-0.8} x 10^6 M_sun
Mpc^-3 (to 0.3 L*(z=3)), in apparent agreement with recent cosmological
hydrodynamic simulations, but we note that incompleteness and sample variance
may introduce larger uncertainties. The ages of the two most massive galaxies
suggest they formed at z>8, during the era of cosmic reionization, but the star
formation rate density derived from their stellar masses and ages is not nearly
sufficient to reionize the universe. The simplest explanation for this
deficiency is that lower-mass galaxies beyond our detection limit reionized the
universe.Comment: 4 pages, 3 figures, emulateapj, Accepted for publication in ApJ
Letter
The Sizes of Candidate Galaxies: confirmation of the bright CANDELS sample and relation with luminosity and mass
Recently, a small sample of six candidates was discovered in
CANDELS that are more luminous than any of the previous
galaxies identified over the HUDF/XDF and CLASH fields. We measure
the sizes of these candidates to map out the size evolution of galaxies from
the earliest observable times. Their sizes are also used to provide a valuable
constraint on whether these unusual galaxy candidates are at high redshift.
Using galfit to derive sizes from the CANDELS F160W images of these candidates,
we find a mean size of 0.130.02" (or 0.50.1 kpc at ). This
handsomely matches the 0.6 kpc size expected extrapolating lower redshift
measurements to , while being much smaller than the 0.59" mean size
for lower-redshift interlopers to photometric selections lacking
the blue IRAC color criterion. This suggests that source size may be an
effective constraint on contaminants from selections lacking IRAC
data. Assuming on the basis of the strong photometric evidence that the Oesch
et al. 2014 sample is entirely at , we can use this sample to extend
current constraints on the size-luminosity, size-mass relation, and size
evolution of galaxies to . We find that the candidate
galaxies have broadly similar sizes and luminosities as -8 counterparts
with star-formation-rate surface densities in the range of . The stellar mass-size
relation is uncertain, but shallower than those inferred for lower-redshift
galaxies. In combination with previous size measurements at z=4-7, we find a
size evolution of with for galaxies,
consistent with the evolution previously derived from galaxies.Comment: 9 figures, 5 tables, accepted by Ap
Characterization and modeling of contamination for Lyman break galaxy samples at high redshift
The selection of high redshift sources from broad-band photometry using the
Lyman-break galaxy (LBG) technique is a well established methodology, but the
characterization of its contamination for the faintest sources is still
incomplete. We use the optical and near-IR data from four (ultra)deep Hubble
Space Telescope legacy fields to investigate the contamination fraction of LBG
samples at z~5-8 selected using a colour-colour method. Our approach is based
on characterizing the number count distribution of interloper sources, that is
galaxies with colors similar to those of LBGs, but showing detection at
wavelengths shorter than the spectral break. Without sufficient sensitivity at
bluer wavelengths, a subset of interlopers may not be properly classified, and
contaminate the LBG selection. The surface density of interlopers in the sky
gets steeper with increasing redshift of LBG selections. Since the intrinsic
number of dropouts decreases significantly with increasing redshift, this
implies increasing contamination from misclassified interlopers with increasing
redshift, primarily by intermediate redshift sources with unremarkable
properties (intermediate ages, lack of ongoing star formation and low/moderate
dust content). Using Monte Carlo simulations, we estimate that the CANDELS deep
data have contamination induced by photometric scatter increasing from ~2% at
z~5 to ~6% at z~8 for a typical dropout color >1 mag, with contamination
naturally decreasing for a more stringent dropout selection. Contaminants are
expected to be located preferentially near the detection limit of surveys,
ranging from 0.1 to 0.4 contaminants per arcmin2 at J=30, depending on the
field considered. This analysis suggests that the impact of contamination in
future studies of z>10 galaxies needs to be carefully considered.Comment: 17 pages, 13 figures, ApJ in pres
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