A new extensive catalog of optically variable AGN in the GOODS Fields and a new statistical approach to variability selection

Variability is a property shared by practically all AGN. This makes variability selection a possible technique for identifying AGN. Given that variability selection makes no prior assumption about spectral properties, it is a powerful technique for detecting both low-luminosity AGN in which the host galaxy emission is dominating and AGN with unusual spectral properties. In this paper, we will discuss and test different statistical methods for the detection of variability in sparsely sampled data that allow full control over the false positive rates. We will apply these methods to the GOODS North and South fields and present a catalog of variable sources in the z band in both GOODS fields. Out of 11931 objects checked, we find 155 variable sources at a significance level of 99.9%, corresponding to about 1.3% of all objects. After rejection of stars and supernovae, 139 variability selected AGN remain. Their magnitudes reach down as faint as 25.5 mag in z. Spectroscopic redshifts are available for 22 of the variability selected AGN, ranging from 0.046 to 3.7. The absolute magnitudes in the rest-frame z-band range from ~ -18 to -24, reaching substantially fainter than the typical luminosities probed by traditional X-ray and spectroscopic AGN selection in these fields. Therefore, this is a powerful technique for future exploration of the evolution of the faint end of the AGN luminosity function up to high redshifts.Comment: Accepted for publication in The Astrophysical Journal (version 2: minor changes to text after receiving comments

CANDELS: The Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey

The Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS) is designed to document the first third of galactic evolution, over the approximate redshift (z) range 8-1.5. It will image >250,000 distant galaxies using three separate cameras on the Hubble Space Telescope, from the mid-ultraviolet to the near-infrared, and will find and measure Type Ia supernovae at z > 1.5 to test their accuracy as standardizable candles for cosmology. Five premier multi-wavelength sky regions are selected, each with extensive ancillary data. The use of five widely separated fields mitigates cosmic variance and yields statistically robust and complete samples of galaxies down to a stellar mass of 10^9 M_☉ to z ≈ 2, reaching the knee of the ultraviolet luminosity function of galaxies to z ≈ 8. The survey covers approximately 800 arcmin^2 and is divided into two parts. The CANDELS/Deep survey (5σ point-source limit H = 27.7 mag) covers ~125 arcmin^2 within Great Observatories Origins Deep Survey (GOODS)-N and GOODS-S. The CANDELS/Wide survey includes GOODS and three additional fields (Extended Groth Strip, COSMOS, and Ultra-deep Survey) and covers the full area to a 5σ point-source limit of H ≳ 27.0 mag. Together with the Hubble Ultra Deep Fields, the strategy creates a three-tiered "wedding-cake" approach that has proven efficient for extragalactic surveys. Data from the survey are nonproprietary and are useful for a wide variety of science investigations. In this paper, we describe the basic motivations for the survey, the CANDELS team science goals and the resulting observational requirements, the field selection and geometry, and the observing design. The Hubble data processing and products are described in a companion paper

Survey design for Spectral Energy Distribution fitting: a Fisher Matrix approach

The spectral energy distribution (SED) of a galaxy contains information on the galaxy's physical properties, and multi-wavelength observations are needed in order to measure these properties via SED fitting. In planning these surveys, optimization of the resources is essential. The Fisher Matrix formalism can be used to quickly determine the best possible experimental setup to achieve the desired constraints on the SED fitting parameters. However, because it relies on the assumption of a Gaussian likelihood function, it is in general less accurate than other slower techniques that reconstruct the probability distribution function (PDF) from the direct comparison between models and data. We compare the uncertainties on SED fitting parameters predicted by the Fisher Matrix to the ones obtained using the more thorough PDF fitting techniques. We use both simulated spectra and real data, and consider a large variety of target galaxies differing in redshift, mass, age, star formation history, dust content, and wavelength coverage. We find that the uncertainties reported by the two methods agree within a factor of two in the vast majority (~ 90%) of cases. If the age determination is uncertain, the top-hat prior in age used in PDF fitting to prevent each galaxy from being older than the Universe needs to be incorporated in the Fisher Matrix, at least approximately, before the two methods can be properly compared. We conclude that the Fisher Matrix is a useful tool for astronomical survey design.Comment: Accepted by ApJ; online Fisher Matrix tool available at http://galfish.physics.rutgers.ed

ECO and RESOLVE: Galaxy Disk Growth in Environmental Context

We study the relationships between galaxy environments and galaxy properties related to disk (re)growth, considering two highly complete samples that are approximately baryonic mass limited into the high-mass dwarf galaxy regime, the Environmental COntext (ECO) catalog (data release herein) and the B-semester region of the REsolved Spectroscopy Of a Local VolumE (RESOLVE) survey. We quantify galaxy environments using both group identification and smoothed galaxy density field methods. We use by-eye and quantitative morphological classifications plus atomic gas content measurements and estimates. We find that blue early-type (E/S0) galaxies, gas-dominated galaxies, and UV-bright disk host galaxies all become distinctly more common below group halo mass ~10^11.5 Msun, implying that this low group halo mass regime may be a preferred regime for significant disk growth activity. We also find that blue early-type and blue late-type galaxies inhabit environments of similar group halo mass at fixed baryonic mass, consistent with a scenario in which blue early types can regrow late-type disks. In fact, we find that the only significant difference in the typical group halo mass inhabited by different galaxy classes is for satellite galaxies with different colors, where at fixed baryonic mass red early and late types have higher typical group halo masses than blue early and late types. More generally, we argue that the traditional morphology-environment relation (i.e., that denser environments tend to have more early types) can be largely attributed to the morphology-galaxy mass relation for centrals and the color-environment relation for satellites.Comment: 26 pages and 28 figures; v2 contains minor figure and text updates to match final published version in ApJ; ECO data table release now available at http://resolve.astro.unc.edu/pages/data.ph

HST Imaging in the Chandra Deep Field South: II. WFPC2 Observations of an X-Ray Flux-Limited Sample from the 1 Msec Chandra Catalog

We present HST/WFPC2 observations of a well-defined sample of 40 X-ray sources with X-ray fluxes above the detection threshold of the full 1 Msec Chandra Deep Field South (CDFS). The sensitivity and spatial resolution of our HST observations are sufficient to detect the optical counterparts of 37 of the X-ray sources, yielding information on their morphologies and environments. In this paper we extend the results obtained in our previous study on the 300 ks CDFS X-ray data (Schreier et al. 2001, Paper I). Specifically, we show that the optical counterparts to the X-ray sources are divided into two distinct populations: 1) an optically faint group with relatively blue colors, similar to the faint blue field galaxy population, and 2) an optically brighter group, including resolved galaxies with average colors significantly redder than the corresponding bright field galaxy population. The brighter objects comprise a wide range of types, including early and late type galaxies, starbursts, and AGN. By contrast, we show that the faint blue X-ray population are most consistent with being predominantly Type 2 AGN of low to moderate luminosity, located at higher redshifts (z ~ 1 - 2). This conclusion is supported by luminosity function models of the various classes of objects. Hence, the combination of deep X-ray data with the high spatial resolution of HST are for the first time allowing us to probe the faint end of the AGN luminosity function at cosmologically interesting redshifts.Comment: AASTEX-Latex, 25 pages, 4 postscript figures, 9 jpg figures. Accepted by the Astrophysical Journal. Full-size postscript images and figures, included in the preprint, are available from: http://www.stsci.edu/~koekemoe/papers/cdfs-hst

Hubble Space Telescope Imaging in the Chandra Deep Field South: III. Quantitative Morphology of the 1Ms Chandra Counterparts and Comparison with the Field Population

We present quantitative morphological analyses of 37 HST/WFPC2 counterparts of X-ray sources in the 1 Ms Chandra Deep Field-South (CDFS). We investigate: 1) 1-D surface brightness profiles via isophotal ellipse fitting; 2) 2-D, PSF- convolved, bulge+disk+nucleus profile-fitting; 3) asymmetry and concentration indices compared with all ~3000 sources in our three WFPC2 fields; and 4) near- neighbor analyses comparing local environments of X-ray sources versus the field control sample. Significant nuclear point-source optical components appear in roughly half of the resolved HST/WFPC2 counterparts, showing a narrow range of F_X/F_{opt,nuc} consistent with the several HST-unresolved X-ray sources (putative type-1 AGN) in our fields. We infer roughly half of the HST/WFPC2 counterparts host unobscured AGN, which suggests no steep decline in the type-1/type-2 ratio out to the redshifts z~0.5-1 typical of our sources. The concentration indices of the CDFS counterparts are clearly larger on average than those of the field distribution, at 5-sigma, suggesting that the strong correlation between central black hole mass and host galaxy properties (including concentration index) observed in nearby galaxies is already evident by z~0.5-1. By contrast, the asymmetry index distribution of the 21 resolved CDFS sources at I<23 is indistinguishable from the I<23 field. Moreover, the frequency of I<23 near neighbors around the CDFS counterparts is not significantly different from the field sample. These results, combined with previous similar findings for local samples, suggest that recent merger/ interaction history is not a good indicator of AGN activity over a substantial range of look-back time.Comment: 30 pages, incl. 8 figures; accepted for publication in the Astrophysical Journa

Ages and Metallicities of Early-Type Void Galaxies from Line Strength Measurements

We present spectroscopic observations of 26 galaxies of type E and S0, based on their blue morphologies, located in voids by the study of Grogin & Geller (1999). Measurements of redshift, velocity dispersion, and four Lick line indices, Mgb, Fe5270, Fe5335, and Hbeta with their errors are given for all of these galaxies, along with Hbeta, [OIII], Halpha, and [NII] emission line strengths for a subset of these objects. These sources are brighter than M star for low density regions and tend to be bluer than their counterpart early-type objects in high density regions. Using the models of Thomas et al. (2003) gives metal abundances and ages with a median alpha enhancement, [alpha/Fe] = +0.13, and median metals abundance, [Z/H] = +0.22, values comparable to those found for E and S0 galaxies in clusters, but with a wider spread in [Z/H] towards low values. If the emission line subsample is interpreted as younger, the proportion of young objects is higher than for early-types in higher density regions. There is a significant incidence of sources in the sample with emission lines in their spectra (46% with Hbeta and [OIII] and 69% with Halpha or [NII]) as well as shells and rings in their morphologies (19%). The diagnostic log[NII/Halpha], log[OIII/Hbeta] diagram places 10 of 12 emission line galaxies in or near the star forming and liner region and two among the Seyferts. The Halpha fluxes indicate star formation rates of 0.2 to 1.0 Msun per yr. The percentage of these early-type void galaxies undergoing star formation appears to be higher compared to their cluster counterparts and the range of ages wider.Comment: 45 pages, 13 figures, accepted for publication in the Astronomical Journa

CANDELS: The Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey

The Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS) is designed to document the first third of galactic evolution, over the approximate redshift (z) range 8-1.5. It will image >250,000 distant galaxies using three separate cameras on the Hubble Space Telescope, from the mid-ultraviolet to the near-infrared, and will find and measure Type Ia supernovae at z > 1.5 to test their accuracy as standardizable candles for cosmology. Five premier multi-wavelength sky regions are selected, each with extensive ancillary data. The use of five widely separated fields mitigates cosmic variance and yields statistically robust and complete samples of galaxies down to a stellar mass of 10^9 M_☉ to z ≈ 2, reaching the knee of the ultraviolet luminosity function of galaxies to z ≈ 8. The survey covers approximately 800 arcmin^2 and is divided into two parts. The CANDELS/Deep survey (5σ point-source limit H = 27.7 mag) covers ~125 arcmin^2 within Great Observatories Origins Deep Survey (GOODS)-N and GOODS-S. The CANDELS/Wide survey includes GOODS and three additional fields (Extended Groth Strip, COSMOS, and Ultra-deep Survey) and covers the full area to a 5σ point-source limit of H ≳ 27.0 mag. Together with the Hubble Ultra Deep Fields, the strategy creates a three-tiered "wedding-cake" approach that has proven efficient for extragalactic surveys. Data from the survey are nonproprietary and are useful for a wide variety of science investigations. In this paper, we describe the basic motivations for the survey, the CANDELS team science goals and the resulting observational requirements, the field selection and geometry, and the observing design. The Hubble data processing and products are described in a companion paper

Lyman-Alpha Absorption Systems and the Nearby Galaxy Distribution

We study the galaxy number density (smoothed on a 5h^{-1} Mpc scale) around 18 low-redshift Lyman-alpha absorbers previously observed with HST. The absorbers lie in the foregrounds of Mrk 335, Mrk 421, Mrk 501, I Zw 1, and 3C 273, all within regions where there are now complete redshift surveys to m_{Zw}=15.5. We construct a smoothed galaxy number density field from the redshift survey data and determine the distribution of densities at the Lyman-alpha absorber locations. We also find the distribution of galaxy number density for a variety of test samples: all galaxy locations within the Center for Astrophysics Redshift Survey (CfA2), CfA2 galaxy locations along randomly selected lines of sight (LOS), and randomly chosen redshifts along random LOS. The Lyman-alpha absorbers are present in dense regions of the survey, but occur far more frequently in underdense regions than do typical luminous galaxies. The distribution of smoothed galaxy density around the Lyman-alpha absorbers is inconsistent at the 4-sigma level with the density distribution around survey galaxies. It is highly consistent with a density distribution at randomly chosen redshifts along random LOS. This supports earlier evidence that the nearby, low column density (log N_{HI} < 14) Lyman-alpha forest systems are spatially distributed at random; they are not well correlated with the local large-scale structure.Comment: Accepted for publication in ApJ, 38 pages including 10 figure