arXiv subjects

Garth Illingworth

Publications and source records attributed to Garth Illingworth.

47 records · Page 3Linked to original sources

arXiv2005

The NICMOS Ultra Deep Field: Observations, Data Reduction, and Galaxy Photometry

This paper describes the observations and data reduction techniques for the version 2.0 images and catalog of the NICMOS Ultra Deep Field Treasury program. All sources discussed in this paper are based on detections in the combined NICMOS F110W and F160W bands only. The NICMOS images are drizzled to 0.09 arc second pixels and aligned to the ACS UDF F850LP image which was rebinned to the same pixel scale. These form the NICMOS version 2.0 UDF images. The catalog sources are chosen with a conservative detection limit to avoid the inclusion of numerous spurious sources. The catalog contains 1293 objects in the 144 x 144 arc sececonds NICMOS subfield of the UDF. The 5 sigma signal to noise level is an average 0.6 arc second diameter aperture AB magnitude of ~27.7 at 1.1 and 1.6 microns. The catalog sources, listed in order of right ascension, satisfy a minimum signal to noise criterion of 1.4 sigma in at least 7 contiguous pixels of the combined F110W and F160W image

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arXiv2004

Stellar Populations and Kinematics of Red Galaxies at z>2: Implications for the Formation of Massive Galaxies

We recently identified a substantial population of galaxies at z>2 with red rest-frame optical colors. These distant red galaxies (DRGs) are efficiently selected by the simple observed color criterion J-K>2.3. In this paper we present NIR spectroscopy with Keck/NIRSPEC of six DRGs at 2.4<z<3.2. We detect continuum emission and emission lines of all observed galaxies. Equivalent widths of H alpha are 20-30 Ang, smaller than measured for LBGs and nearby LIRGs, and comparable to normal nearby galaxies. The modest equivalent widths imply that the galaxies either have a decreasing star formation rate, or that they are very dusty. Fitting both the photometry and the H alpha lines, we find continuum extinction A_V=1-2 mag, ages 1-2.5 Gyr, star formation rates 200-400 solar masses/yr, and stellar masses 1-5x10^11 solar masses for models with constant star formation rates. From [NII]/H alpha ratios we infer that the metallicities are high, 1-1.5 x Solar. For four galaxies we can determine line widths from the optical emission lines. The widths are high, ranging from 130-240 km/s, and by combining data for LBGs and DRGs we find significant correlations between linewidth and restframe U-V color, and between linewidth and stellar mass. The latter correlation has a similar slope and offset as the ``baryonic Tully-Fisher relation'' for nearby galaxies. The median dynamical mass is ~2x10^11 solar masses, supporting the high stellar masses inferred from the photometry. We find that the median M/L_V ~ 0.8, a factor of ~5 higher than measured for LBGs. We infer from our small sample that DRGs are dustier, more metal rich, more massive, and have higher ages than z=3 LBGs of the same rest-frame V-band luminosity. Their high M/L ratios imply that they contribute significantly to the stellar mass density at z~2.5. [ABRIDGED]

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arXiv2004

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.

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arXiv2003

The Nature of E+A Galaxies in Intermediate Redshift Clusters

[Abridged] Combining HST/WFPC2 mosaics with extensive ground-based spectroscopy, we study the nature of E+A galaxies in three intermediate redshift clusters (z=0.33, 0.58, & 0.83). From a sample of ~500 confirmed cluster members, we isolate 46 E+A candidates to determine the E+A fraction and study their physical properties. We find E+A's comprise a non-negligible component (~7-13%) of the cluster population at these redshifts, and their diverse nature indicates a heterogeneous parent population. Cluster E+A's span the range in Hubble type and cover a wide range in luminosity, internal velocity dispersion, and half-light radius. From their velocity dispersions and half-light radii, we infer that the descendants of E+A's in our highest redshift cluster are massive early-type galaxies. We find a decrease in the characteristic E+A mass similar to the decrease in luminosity of rapidly star-forming field galaxies since z~1, i.e. galaxy ``down-sizing.'' In addition, we argue our statistics imply that >30% of the E-S0 members have undergone an E+A phase; the true fraction could be 100% if the effects of E+A down-sizing, an increasing E+A fraction with redshift, and the conversion of spirals into early-types are also considered. Thus, the E+A phase may indeed be an important stage in the transformation of star-forming galaxies into early-type cluster members.

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arXiv2003

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).

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arXiv2003

CL 1358+62: Characterizing the Physical Properties of Cluster Galaxies at z=0.33

[Abridged] We examine the physical properties of 173 cluster members in CL1358+62 (z=0.33) from HST WFPC2 imaging taken in the F606W & F814W filters (rest-frame B & V) over a 2.2x2.2 Mpc^2 field. Structural parameters are measured for each member by fitting a PSF-convolved, two component model to their 2D surface brightness distribution. We examine bulge+disk models using three different bulge profiles (de Vaucouleurs, Sersic, and exponential), and rigorously test the robustness of our results by analyzing several thousand artificial galaxies in the same manner as the cluster data. Combining structural parameters with ground-based spectroscopy, we test for correlations between morphological characteristics, current star formation, total galaxy colors, and cluster substructure. We find that: (1) Using a de Vaucouleurs bulge, bulge-to-total ratios and Hubble type (-5<T<8) are strongly correlated (99% CL), but the scatter is large and early-type spirals are not reliably distinguished from E's and S0's based on (B/T). (2) The low luminosity ellipticals in our sample are likely to be face-on S0 galaxies. (3) High galaxy asymmetry and strong [OII] emission are strongly correlated for disk-dominated members. (4) There exists a small population (~5%) of bulge-dominated members whose significant [OII] emission (<-5A) suggest they harbor AGN. (5) At these redshifts, determining the correct Sersic index can be highly unreliable.

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arXiv2001

Cluster Galaxy Populations from z=0.33 - 0.83: Characterizing Evolution with CL1358+62

We combine HST WFPC mosaics of clusters at multiple redshifts with Keck LRIS spectroscopy to characterize different galaxy populations and how they evolve. By combining quantitative measurements of structural parameters with velocity dispersions, line indices, and morphological types, we attempt to establish links between ellipticals, S0's, E+A's, and later type galaxies. Here we focus on galaxies in CL1358+62, a massive cluster at z=0.33.

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arXiv2000

Formation and Evolution of E and S0 Galaxies from HST and Keck Studies of z~0.3-1 Clusters

We have witnessed a dramatic increase over the last five years in results on distant galaxies, in large part because of the high resolution imaging capability of HST, and the multiobject spectroscopic capability of the Keck telescopes. Our program to obtain wide-field, multi-color WFPC2 mosaics with HST of intermediate redshift clusters, and spectroscopic membership and high S/N spectroscopy with LRIS on Keck, has provided new insights into the nature of elliptical and S0 galaxies in the cluster environment over a wide range of densities. In particular, most ellipticals, and a significant fraction of the S0 population, have large luminosity-weighted ages, suggesting that their stellar populations were formed at redshifts beyond z~2, though the existence of substantial numbers of major mergers in MS 1054-03 at z=0.83 suggests that final assembly of such galaxies may not have occurred until much later for a significant fraction of early-type galaxies.

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arXiv2000

Galaxies at High Redshift

Within just the last few years, we have advanced from knowing only a few galaxies at z>2 to having redshifts for nearly 1000 z~2-5 objects. The majority of this sample has been detected through the photometric ``drop-out'' technique used so successfully by Steidel and his collaborators. In addition, a handful of objects have already been detected at z>5, and we may already have a few objects at z>6! These data, plus that at z<2 have resulted in a characterization of the star formation history of the universe, commonly known as the ``Madau plot'', which gives SFR per comoving volume vs redshift from the present day at z=0 to z~5. Recently, we have realized that dust has significantly affected our SFR estimates for the objects that have been detected in the optical, and have developed approaches to correct for that extinction. We now also have increasingly good evidence that a substantial fraction (>50%?) of the high redshift star formation occurs in dust-enshrouded starbursts that were detected at 850 micron with a submm bolometer SCUBA on the JCMT telescope. These objects will typically be too faint to detect and measure redshifts in the optical, though those that can have already provided valuable constraints. The highest redshift objects at z~5 and beyond are intriguing sources, but test even the limits of Keck and of HST. The detailed study of such sources may well only be practical for gravitationally-lensed objects, where the high magnification allows for a much more detailed study of the proto-galaxy's structure, until even larger space-based (NGST) and ground-based (30-m plus) telescopes are developed.

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arXiv1999

The Hubble Constant from the HST Key Project on the Extragalactic Distance Scale

The final efforts of the HST Key Project on the Extragalactic Distance Scale are presented. Four distance indicators, the Surface Brightness Fluctuation method, the Fundamental Plane for early-type galaxies, the Tully-Fisher relation and the Type Ia Supernovae, are calibrated using Cepheid distances to galaxies within 25 Mpc. The calibration is then applied to distant samples reaching cz~10000 km/s and (in the case of SNIa) beyond. By combining the constraints imposed on the Hubble constant by the four distance indicators, we obtain H0 = 71+/-6 km/s/Mpc.

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arXiv1998

The HST Key Project on the Extragalactic Distance Scale. XV. A Cepheid Distance to the Fornax Cluster and Its Implications

Using the Hubble Space Telescope (HST) 37 long-period Cepheid variables have been discovered in the Fornax Cluster spiral galaxy NGC 1365. The resulting V and I period-luminosity relations yield a true distance modulus of 31.35 +/- 0.07 mag, which corresponds to a distance of 18.6 +/- 0.6 Mpc. This measurement provides several routes for estimating the Hubble Constant. (1) Assuming this distance for the Fornax Cluster as a whole yields a local Hubble Constant of 70 +/-18_{random} [+/-7]_{systematic} km/s/Mpc. (2) Nine Cepheid-based distances to groups of galaxies out to and including the Fornax and Virgo clusters yield Ho = 73 (+/-16)_r [+/-7]_s km/s/Mpc. (3) Recalibrating the I-band Tully-Fisher relation using NGC 1365 and six nearby spiral galaxies, and applying it to 15 galaxy clusters out to 100 Mpc gives Ho = 76 (+/-3)_r [+/-8]_s km/s/Mpc. (4) Using a broad-based set of differential cluster distance moduli ranging from Fornax to Abell 2147 gives Ho = 72 (+/-)_r [+/-6]_s km/s/Mpc. And finally, (5) Assuming the NGC 1365 distance for the two additional Type Ia supernovae in Fornax and adding them to the SnIa calibration (correcting for light curve shape) gives Ho = 67 (+/-6)_r [+/-7]_s km/s/Mpc out to a distance in excess of 500 Mpc. All five of these Ho determinations agree to within their statistical errors. The resulting estimate of the Hubble Constant combining all these determinations is Ho = 72 (+/-5)_r [+/-12]_s km/s/Mpc.

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