53 research outputs found
Optical Variability of Infrared Power Law-Selected Galaxies & X-ray Sources in the GOODS-South Field
We investigate the use of optical variability to identify and study Active
Galactic Nuclei (AGN) in the GOODS-South field. A sample of 22 mid-infrared
power law sources and 102 X-ray sources with optical counterparts in the HST
ACS images were selected. Each object is classified with a variability
significance value related to the standard deviation of its magnitude in five
epochs separated by 45-day intervals. The variability significance is compared
to the optical, mid-IR, and X-ray properties of the sources. We find that 26%
of all AGN candidates (either X-ray- or mid-IR-selected) are optical variables.
The fraction of optical variables increases to 51% when considering sources
with soft X-ray band ratios. For the mid-IR AGN candidates which have
multiwavelength SEDs, we find optical variability for 64% of those classified
with SEDs like Broad Line AGNs. While mostly unobscured AGN appear to have the
most significant optical variability, some of the more obscured AGNs are also
observed as variables. In particular, we find two mid-IR power law-selected AGN
candidates without X-ray emission that display optical variability, confirming
their AGN nature.Comment: 11 pages, 4 figures, 1 table, accepted for publication in Ap
The SEDs, Host Galaxies and Environments of Variability Selected AGN in GOODS-S
Variability selection has been proposed as a powerful tool for identifying
both low-luminosity AGN and those with unusual SEDs. However, a systematic
study of sources selected in such a way has been lacking. In this paper, we
present the multi-wavelength properties of the variability selected AGN in
GOODS South. We demonstrate that variability selection indeed reliably
identifies AGN, predominantly of low luminosity. We find contamination from
stars as well as a very small sample of sources that show no sign of AGN
activity, their number is consistent with the expected false positive rate. We
also study the host galaxies and environments of the AGN in the sample.
Disturbed host morphologies are relatively common. The host galaxies span a
wide range in the level of ongoing star-formation. However, massive star-bursts
are only present in the hosts of the most luminous AGN in the sample. There is
no clear environmental preference for the AGN sample in general but we find
that the most luminous AGN on average avoid dense regions while some
low-luminosity AGN hosted by late-type galaxies are found near the centres of
groups. AGN in our sample have closer nearest neighbours than the general
galaxy population. We find no indications that major mergers are a dominant
triggering process for the moderate to low luminosity AGN in this sample. The
environments and host galaxy properties instead suggest secular processes, in
particular tidal processes at first passage and minor mergers, as likely
triggers for the objects studied. This study demonstrates the strength of
variability selection for AGN and gives first hints at possibly triggering
mechanisms for high-redshift low luminosity AGN.Comment: Accepted for publication in MNRAS (missing references added in
version 2
Requirements for Investigating the Connection Between Lyman Alpha Absorption Clouds and the Large-Scale Distribution of Galaxies
We model the requirements on observational data that would allow an accurate
determination of the degree of association between Lyman alpha absorbers and
peaks in the redshift distribution of galaxies (large-scale structures like
clusters of galaxies). We compare simulated distributions of low-redshift Lyman
alpha absorption systems, constrained to be consistent with the distribution
observed with HST, with the large-scale distribution of galaxies determined
from pencil-beam redshift surveys. We estimate the amount of observational data
required from catalogues of Lyman alpha absorbers and galaxies to allow a
statistically significant test of the association of absorbers with large-scale
structures of galaxies.
We find that for each line-of-sight observed for Lyman alpha absorption lines
(assuming that the entire redshift range out to z=0.4 is observable), redshifts
must be obtained for at least 18 galaxies brighter than Mb=-18 and having
redshifts between 0.2 and 0.4. Based on the redshift surveys used in this
study, a search radius of 10' from the quasar line-of-sight is required. This
will ensure that all peaks in the galaxy redshift distribution are represented
by at least one galaxy in the observed sample. If Lyman alpha absorbers are
intrinsically uncorrelated with galaxies, we find that 8 lines-of-sight must be
observed to show that the distributions are different at the 95% confidence
level. However, if a fraction of the Lyman alpha absorbers are distributed with
the peaks in the galaxy distribution, 38 lines-of-sight must be mapped for the
distribution of both Lyman alpha absorbers and galaxies in order to determine
the fraction of absorbers distributed with the peaks of the galaxy distribution
to an accuracy of 10%.Comment: 17 pages LaTeX file, plus 6 uuencoded compressed .eps figures to be
printed seperately, to appear in Feb. 1, 1996 Ap
Selection of AGN candidates in the GOODS-South Field through SPITZER/MIPS 24 m variability
We present a study of galaxies showing mid-infrared variability in data taken
in the deepest Spitzer/MIPS 24 m surveys in the GOODS-South field. We
divide the dataset in epochs and subepochs to study the long-term
(months-years) and the short-term (days) variability. We use a
-statistics method to select AGN candidates with a probability
1% that the observed variability is due to statistical errors alone. We find 39
(1.7% of the parent sample) sources that show long-term variability and 55
(2.2% of the parent sample) showing short-term variability. That is, 0.03
sources arcmin for both, long-term and short-term variable
sources. After removing the expected number of false positives inherent to the
method, the estimated percentages are 1.0% and 1.4% of the parent sample for
the long-term and short-term respectively. We compare our candidates with AGN
selected in the X-ray and radio bands, and AGN candidates selected by their IR
emission. Approximately, 50% of the MIPS 24 m variable sources would be
identified as AGN with these other methods. Therefore, MIPS 24 m
variability is a new method to identify AGN candidates, possibly dust obscured
and low luminosity AGN, that might be missed by other methods. However, the
contribution of the MIPS 24 m variable identified AGN to the general AGN
population is small ( 13%) in GOODS-South.Comment: Accepted for publication in MNRA
Compact Nuclei in Galaxies at Moderate Redshift: I. Imaging and Spectroscopy
This study explores the space density and properties of active galaxies to
z=0.8. We have investigated the frequency and nature of unresolved nuclei in
galaxies at moderate redshift as indicators of nuclear activity such as Active
Galactic Nuclei (AGN) or starbursts. Candidates are selected by fitting imaged
galaxies with multi-component models using maximum likelihood estimate
techniques to determine the best model fit. We select those galaxies requiring
an unresolved, point source component in the galaxy nucleus, in addition to a
disk and/or bulge component, to adequately model the galaxy light. We have
searched 70 WFPC2 images primarily from the Medium Deep Survey for galaxies
containing compact nuclei. In our survey of 1033 galaxies, the fraction
containing an unresolved nuclear component greater than 3% of the total galaxy
light is 16+/-3% corrected for incompleteness and 9+/-1% for nuclei greater
than 5% of the galaxy light. Spectroscopic redshifts have been obtained for 35
of our AGN/starburst candidates and photometric redshifts are estimated to an
accuracy of sigma_z=0.1 for the remaining sample. In this paper, the first of
two in this series, we present the selected HST imaged galaxies having
unresolved nuclei and discuss the selection procedure. We also present the
ground-based spectroscopy for these galaxies as well as the photometric
redshifts estimated for those galaxies without spectra.Comment: 56 pages, 22 figures, to appear in ApJ Supplement Series, April 199
Compact Nuclei in Moderately Redshifted Galaxies
The Hubble Space Telescope WFPC2 is being used to obtain high-resolution
images in the V and I bands for several thousand distant galaxies as part of
the Medium Deep Survey (MDS). An important scientific aim of the MDS is to
identify possible AGN candidates from these images in order to measure the
faint end of the AGN luminosity function as well as to study the host galaxies
of AGNs and nuclear starburst systems. We are able to identify candidate
objects based on morphology. Candidates are selected by fitting bulge+disk
models and bulge+disk+point source nuclei models to HST imaged galaxies and
determining the best model fit to the galaxy light profile. We present results
from a sample of MDS galaxies with I less than 21.5 mag that have been searched
for AGN/starburst nuclei in this manner. We identify 84 candidates with
unresolved nuclei in a sample of 825 galaxies. For the expected range of galaxy
redshifts, all normal bulges are resolved. Most of the candidates are found in
galaxies displaying exponential disks with some containing an additional bulge
component. 5% of the hosts are dominated by an r^-1/4 bulge. The V-I color
distribution of the nuclei is consistent with a dominant population of
Seyfert-type nuclei combined with an additional population of starbursts. Our
results suggest that 10% +/- 1% of field galaxies at z less than 0.6 may
contain AGN/starburst nuclei that are 1 to 5 magnitudes fainter than the host
galaxies.Comment: 12 pages AASTeX manuscript, 3 separate Postscript figures, to be
published in ApJ Letter
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