304 research outputs found
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
Imaging Spectropolarimetry -- A New Observing Mode on the Hubble Space Telescope's Advanced Camera for Surveys
Imaging spectropolarimetry is a new observing mode on the Advanced Camera for
Surveys (ACS) aboard the Hubble Space Telescope (HST) that was commissioned in
Cycle 30 and is available to HST observers starting in Cycle 31 (i.e., from
2023). It is a technique that is accessible from ground-based observatories,
but the superb spatial resolution afforded by HST/ACS combined with the
slitless nature of HST/ACS grism spectroscopy opens up the possibility of
studying polarized extended emission in a way that is not currently possible
even with Adaptive Optics facilities on the ground. This mode could help to
study interesting targets including (but not limited to) QSOs, AGN and Radio
Galaxies, ISM Dust Properties, Pre-Planetary Nebulae, Proto-Planetary and
Debris Disks, Supernovae/Supernova Remnants, and Solar System objects. This
research note presents the preliminary results from the calibration programs
used to calibrate imaging spectropolarimetry on HST/ACS.Comment: To be published in Research Notes of the AAS (RNAAS), 6 pages, 1
figur
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
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