Showing papers by "Holland C. Ford published in 2008"

z ~ 7-10 Galaxies in the HUDF and GOODS Fields: UV Luminosity Functions

Abstract: We use all available deep optical ACS and near-IR data over both the HUDF and the two GOODS fields to search for star-forming galaxies at -->z 7 and constrain the UV LF within the first 700 Myr. Our data set includes ~23 arcmin2 of deep NICMOS -->J + H data and ~248 arcmin2 of ground-based (ISAAC+MOIRCS) data, coincident with ACS optical data of greater or equal depths. In total, we find eight --> ~ 7.3 z-dropouts in our search fields, but no -->z ~ 9 J-dropout candidates. A careful consideration of a wide variety of different contaminants suggest an overall contamination level of just ~12% for our z-dropout selection. After performing detailed simulations to accurately estimate the selection volumes, we derive constraints on the UV LFs at -->z ~ 7 and -->z ~ 9. For a faint-end slope -->α = − 1.74, our most likely values for -->MUV* and * at -->z ~ 7 are – -->19.8 ± 0.4 mag and -->1.1+ 1.7−0.7 × 10−3 Mpc−3, respectively. Our search results for -->z ~ 9 J-dropouts set a 1 σ lower limit on -->MUV* of –19.6 mag assuming that * and α are the same as their values at slightly later times. This lower limit on -->MUV* is 1.4 mag fainter than our best-fit value at -->z ~ 4, suggesting that the UV LF has undergone substantial evolution over this time period. No evolution is ruled out at 99% confidence from -->z ~ 7 to -->z ~ 6 and at 80% confidence from -->z ~ 9 to -->z ~ 7. We find that the mass-to-light ratio of halos evolves as ~ -->(1 + z)−1 if we require that the observed brightening in -->MUV* with redshift [i.e., -->MUV* = (− 21.02 ± 0.09) + (0.36 ± 0.08) (z − 3.8) ] be consistent with the expected evolution in the halo mass function. Finally, we consider the shape of the UV LF at -->z 5 and discuss the implications of the Schechter-like form of the observed LFs, particularly the unexpected abrupt cutoff at the bright end.

z~7-10 Galaxies in the HUDF and GOODS fields, and their UV Luminosity Functions

Abstract: We use all available deep optical ACS and near-IR data over both the HUDF and the two GOODS fields to search for star-forming galaxies at z>~7 and constrain the UV LF within the first 700 Myrs. Our data set includes ~23 arcmin^2 of deep NICMOS J+H data and ~248 arcmin^2 of ground-based (ISAAC+MOIRCS) data, coincident with ACS optical data of greater or equal depths. In total, we find 8 ~7.3 z-dropouts in our search fields, but no z~9 J-dropout candidates. A careful consideration of a wide variety of different contaminants suggest an overall contamination level of just ~12% for our z-dropout selection. After performing detailed simulations to accurately estimate the selection volumes, we derive constraints on the UV LFs at z~7 and z~9. For a faint-end slope alpha=-1.74, our most likely values for M*(UV) and phi* at z~7 are -19.8+/-0.4 mag and 1.1_{-0.7}^{+1.7} x 10^{-3} Mpc^{-3}, respectively. Our search results for z~9 J-dropouts set a 1 sigma lower limit on M*(UV) of -19.6 mag assuming that phi* and alpha are the same as their values at slightly later times. This lower limit on M*(UV) is 1.4 mag fainter than our best-fit value at z~4, suggesting that the UV LF has undergone substantial evolution over this time period. No evolution is ruled out at 99% confidence from z~7 to z~6 and at 80% confidence from z~9 to z~7. The inferred brightening in M*(UV) with redshift (i.e., M*(UV) = (-21.02+/-0.09) + (0.36+/-0.08)(z - 3.8)) matches the evolution expected in the halo mass function, if the mass-to-light ratio of halos evolves as ~(1+z)**{-1}. Finally, we consider the shape of the UV LF at z>~5 and discuss the implications of the Schechter-like form of the observed LFs, particularly the unexpected abrupt cut-off at the bright end.

Discovery of a Very Bright Strongly Lensed Galaxy Candidate at z ≈ 7.6*

Abstract: Using Hubble Space Telescope (HST) and Spitzer IRAC imaging, we report the discovery of a very bright strongly lensed Lyman break galaxy (LBG) candidate at -->z ~ 7.6 in the field of the massive galaxy cluster Abell 1689 ( -->z = 0.18). The galaxy candidate, which we refer to as A1689-zD1, shows a strong -->z850 − J110 break of at least 2.2 mag and is completely undetected ( J110 − H160 and -->H160 − [ 4.5 μ m ] colors, are exactly the properties of an -->z ~ 7.6 LBG, and can only be reasonably fit by a star-forming galaxy at -->z = 7.6 ± 0.4 ( -->χ2ν = 1.1). Attempts to reproduce these properties with a model galaxy at -->z χ2ν ≥ 25). A1689-zD1 has an observed (lensed) magnitude of 24.7 AB (8 σ) in the NICMOS H160 band and is ~1.3 mag brighter than the brightest known z850-dropout galaxy. When corrected for the cluster magnification of ~9.3 at -->z ~ 7.6, the candidate has an intrinsic magnitude of -->H160 = 27.1 AB, or about an L* galaxy at -->z ~ 7.6. The source-plane deprojection shows that the star formation is occurring in compact knots of size 300 pc. The best-fit stellar population synthesis models yield a median redshift of 7.6, stellar masses -->(1.6–3.9) × 109 M☉, stellar ages 45-320 Myr, star formation rates 7.6 M☉ yr−1, and low reddening with -->AV ≤ 0.3. These properties are generally similar to those of LBGs found at -->z ~ 5–6. The inferred stellar ages suggest a formation redshift of -->z ~ 8–10 ( -->t 0.63 Gyr). A1689-zD1 is the brightest observed, highly reliable -->z > 7.0 galaxy candidate found to date.

Lyman break galaxies, Lyα emitters, and a radio galaxy in a protocluster at z = 4.1

Abstract: We present deep HST ACS observations in g_(475)r_(625)i_(775)z_(850) toward the z = 4.1 radio galaxy TN J1338–1942 and its overdensity of >30 spectroscopically confirmed Lyα emitters (LAEs). We select 66 g_(475) band dropouts to z_(850),(5σ) = 27, 6 of which are also LAEs. Although our color-color selection results in a relatively broad redshift range centered on z = 4.1, the field of TN J1338–1942 is richer than the average field at the > 5 σ significance, based on a comparison with GOODS. The angular distribution is filamentary with about half of the objects clustered near the radio galaxy, and a small, excess signal (2 σ) in the projected pair counts at separations of θ = 0.13") galaxies, and we derive a mean stellar mass of ~10^8-10^9 M⊙ based on a stacked Ks band image. We determine star formation rates, sizes, morphologies, and color-magnitude relations of the g_(475)-dropouts and find no evidence for a difference between galaxies near TN J1338–1942 and in the field. We conclude that environmental trends as observed in clusters at much lower redshift are either not yet present or washed out by the relatively broad selection in redshift. The large galaxy overdensity, its corresponding mass overdensity, and the subclustering at the approximate redshift of TN J1338–1942 suggest the assemblage of a >10^14 M⊙ structure, confirming that it is possible to find and study cluster progenitors in the linear regime at z ≳ 4.

Formation epochs, star formation histories and sizes of massive early-type galaxies in cluster and field environments at z=1.2: insights from the rest-frame UV

Abstract: We derive stellar masses, ages and star formation histories of massive early-type galaxies in the z=1.237 RDCS1252.9-2927 cluster and compare them with those measured in a similarly mass-selected sample of field contemporaries drawn from the GOODS South Field. Robust estimates of these parameters are obtained by comparing a large grid of composite stellar population models with 8-9 band photometry in the rest-frame NUV, optical and IR, thus sampling the entire relevant domain of emission of the different stellar populations. Additionally, we present new, deep $U$-band photometry of both fields, giving access to the critical FUV rest-frame, in order to constrain empirically the dependence on the environment of the most recent star formation processes. We find that early-type galaxies, both in the cluster and in the field, show analogous optical morphologies, follow comparable mass vs. size relation, have congruent average surface stellar mass densities and lie on the same Kormendy relation. We also that a fraction of early-type galaxies in the field employ longer timescales, $\tau$, to assemble their mass than their cluster contemporaries. Hence we conclude that, while the formation epoch of early-type only depends on their mass, the environment does regulate the timescales of their star formation histories. Our deep $U$-band imaging strongly supports this conclusions. It shows that cluster galaxies are at least 0.5 mag fainter than their field contemporaries of similar mass and optical-to-infrared colors, implying that the last episode of star formation must have happened more recently in the field than in the cluster.

LensPerfect: Gravitational Lens Mass Map Reconstructions Yielding Exact Reproduction of All Multiple Images

Abstract: We present a new approach to gravitational lens mass map reconstruction. Our mass map solutions perfectly reproduce the positions, fluxes, and shears of all multiple images, and each mass map accurately recovers the underlying mass distribution to a resolutionlimitedby the number of multiple images detected.We demonstrate ourtechnique given a mock galaxy cluster similar to Abell 1689, which gravitationally lenses 19 mock background galaxies to produce 93 multiple images. We also explore cases in which as few as four multiple images are observed. Mass map solutions are never unique, and our method makes it possible to explore an extremely flexible range of physical (and unphysical) solutions, all of which perfectly reproduce the data given. Each reconfiguration of the source galaxies produces a new mass map solution. An optimization routine is provided to find those source positions (and redshifts, within uncertainties) that produce the ‘‘most physical’’ mass map solution, according to a new figure of merit developed here. Our method imposes no assumptions about the slope of the radial profile or mass following light. However, unlike ‘‘nonparametric’’ grid-based methods, the number of free parameters that we solve for is only as many as the number of observable constraints (or slightly greater if fluxes are constrained). For each set of source positions and redshifts, mass map solutions are obtained ‘‘instantly’’ via direct matrix inversion by smoothly interpolating thedeflectionfieldusingarecently developedmathematical technique. Our LensPerfect software isstraightforward and easy to use, and is publicly available on our Web site.

Discovery of a Very Bright Strongly-Lensed Galaxy Candidate at z ~ 7.6

Abstract: Using HST and Spitzer IRAC imaging, we report the discovery of a very bright strongly lensed Lyman break galaxy (LBG) candidate at z~7.6 in the field of the massive galaxy cluster Abell 1689. The galaxy candidate, which we refer to as A1689-zD1, shows a strong z-J break of at least 2.2 mag and is completely undetected ( 7.0 galaxy candidate found to date.

The fundamental plane of cluster ellipticals at z = 1.25 1,2

Abstract: Using deep HST ACS imaging and VLT FORS2 spectra, we determined the velocity dispersions, effective radii and surface brightnesses for four early-type galaxies in the z = 1.237 cluster RDCS 1252.92927. All four galaxies are massive, > 10 11 M⊙. These four galaxies, combined with three from RDCS 0848+4453 at z = 1.276, establish the Fundamental Plane of massive early-type cluster galaxies at ¯ z = 1.25. The offset of the Fundamental Plane shows that the luminosity evolution in rest-frame �

Evolution of the Color-Magnitude Relation in Galaxy Clusters at z ~1 from the ACS Intermediate Redshift Cluster Survey

Abstract: We apply detailed observations of the Color-Magnitude Relation (CMR) with the ACS/HST to study galaxy evolution in eight clusters at z~1. The early-type red sequence is well defined and elliptical and lenticular galaxies lie on similar CMRs. We analyze CMR parameters as a function of redshift, galaxy properties and cluster mass. For bright galaxies (M_B < -21mag), the CMR scatter of the elliptical population in cluster cores is smaller than that of the S0 population, although the two become similar at faint magnitudes. While the bright S0 population consistently shows larger scatter than the ellipticals, the scatter of the latter increases in the peripheral cluster regions. If we interpret these results as due to age differences, bright elliptical galaxies in cluster cores are on average older than S0 galaxies and peripheral elliptical galaxies (by about 0.5Gyr). CMR zero point, slope, and scatter in the (U-B)_z=0 rest-frame show no significant evolution out to redshift z~1.3 nor significant dependence on cluster mass. Two of our clusters display CMR zero points that are redder (by ~2sigma) than the average (U-B)_z=0 of our sample. We also analyze the fraction of morphological early-type and late-type galaxies on the red sequence. We find that, while in the majority of the clusters most (80% to 90%) of the CMR population is composed of early-type galaxies, in the highest redshift, low mass cluster of our sample, the CMR late-type/early-type fractions are similar (~50%), with most of the late-type population composed of galaxies classified as S0/a. This trend is not correlated with the cluster's X-ray luminosity, nor with its velocity dispersion, and could be a real evolution with redshift.

Detecting Transits in Sparsely Sampled Surveys

Abstract: The small sizes of low mass stars in principle provide an opportunity to find Earth‐like planets and “super Earths” in habitable zones via transits. Large area synoptic surveys like Pan‐STARRS and LSST will observe large numbers of low mass stars, albeit with widely spaced (sparse) time sampling relative to the planets’ periods and transit durations. We present simple analytical equations that can be used to estimate the feasibility of a survey by setting upper limits to the number of transiting planets that will be detected. We use Monte Carlo simulations to find upper limits for the number of transiting planets that may be discovered in the Pan‐STARRS Medium Deep and 3π surveys. Our search for transiting planets and M‐dwarf eclipsing binaries in the SDSS‐II supernova data is used to illustrate the problems (and successes) in using sparsely sampled surveys.

The Fourier-Kelvin Stellar Interferometer: A Progress Report and Preliminary Results from our Laboratory Testbed

Abstract: The Fourier-Kelvin Stellar Interferometer (FKSI) is a passively cooled, space-borne interferometer that has been developed for high angular resolution astrophysics in the near-to-mid IR wavelength range (3-8 microns). The scientific emphasis of the mission is on the direct detection of extrasolar giant planets, characterization of their atmospheres, and observation of secular changes in their atmospheric spectra. FKSI will also facilitate the search for brown dwarfs and Jupiter and sub-Jupiter mass planets and the study of the evolution of protostellar systems from the collapse of the precursor molecular cloud core through the formation of the disk surrounding the protostar, the formation of planets in the disk, and subsequent dispersal of the disk material. FKSI could also play a powerful role in the investigation of the structure of active galactic nuclei and extra-galactic star formation. While FKSI is a high angular resolution system complementary to JWST, it is also an important scientific and technological pathfinder for the TPF Interferometer and Darwin missions as well as NASA Vision missions SPIRIT, SPECS, and SAFIR.

SPECTROSCOPY OF Z ∼ 6 I-DROPOUT GALAXIES: FREQUENCY OF LY α EMISSION AND THE SIZES

Abstract: We report on deep spectroscopy using LRIS on Keck I and FORS2 on the VLT of a sample of 22 candidate z ∼ 6 Lyman Break galaxies (LBGs) selected by the i775 − z850 > 1.3 dropout criterion. Redshifts could be measured for eight objects. These redshifts are all in the range z = 5.5 - 6.1, confirming the efficiency of thei775 − z850 color selection technique. Six of the confirmed galaxies show Lyα emission. Assuming that the 14 objects without redshifts are z ∼ 6 LBGs, but lack detectable Lyα emission lines, we infer that the fraction of Lyα emitting LBGs with Lyα equivalent widths greater than 20 u among z ∼ 6 LBGs is ≈30%, similar to that found at z ∼ 3. Every Lyα emitting object in our sample is compact with rhl ≤ 0. ′′ 14. Furthermore, all the Lyα emitting objects in our sample are more compact than average relative to the observed size-magnitude relation of a large i-dropout sample (332 candidate z ∼ 6 objects). We can reject the hypothesis that the Lyα emitting population is a subset of the rest of the z ∼ 6 LBG population at >97% confidence. We speculate the small sizes of Lyα emitting LBGs are due to these objects being less massive than other LBGs at z ∼ 6. Subject headings: galaxies: evolution — galaxies: formation — galaxies: high-redshift — galaxies: starburst — early universe