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Chandra Observations of A 1.9 Kpc Separation Double X-Ray Source in A Candidate Dual Active Galactic Nucleus Galaxy At Z=0.16
We report Chandra observations of a double X-ray source in the z = 0.1569 galaxy SDSS J171544.05+600835.7. The galaxy was initially identified as a dual active galactic nucleus (AGN) candidate based on the double-peaked [O III] lambda 5007 emission lines, with a line-of-sight velocity separation of 350 km s(-1), in its Sloan Digital Sky Survey spectrum. We used the Kast Spectrograph at Lick Observatory to obtain two long-slit spectra of the galaxy at two different position angles, which reveal that the two Type 2 AGN emission components have not only a velocity offset, but also a projected spatial offset of 1.9 h(70)(-1) kpc on the sky. Chandra/ACIS observations of two X-ray sources with the same spatial offset and orientation as the optical emission suggest that the galaxy most likely contains Compton-thick dual AGNs, although the observations could also be explained by AGN jets. Deeper X-ray observations that reveal Fe K lines, if present, would distinguish between the two scenarios. The observations of a double X-ray source in SDSS J171544.05+600835.7 are a proof of concept for a new, systematic detection method that selects promising dual AGN candidates from ground-based spectroscopy that exhibits both velocity and spatial offsets in the AGN emission features.W.J. McDonald Postdoctoral FellowshipCollege of Natural SciencesDepartment of Astronomy at the University of Texas at AustinMcDonald ObservatoryU.S. Department of Energy DE-AC02-76SF00515Astronom
Measuring Galaxy Environments with Deep Redshift Surveys
We study the applicability of several galaxy environment measures
(n^th-nearest-neighbor distance, counts in an aperture, and Voronoi volume)
within deep redshift surveys. Mock galaxy catalogs are employed to mimic
representative photometric and spectroscopic surveys at high redshift (z ~ 1).
We investigate the effects of survey edges, redshift precision, redshift-space
distortions, and target selection upon each environment measure. We find that
even optimistic photometric redshift errors (\sigma_z = 0.02) smear out the
line-of-sight galaxy distribution irretrievably on small scales; this
significantly limits the application of photometric redshift surveys to
environment studies. Edges and holes in a survey field dramatically affect the
estimation of environment, with the impact of edge effects depending upon the
adopted environment measure. These edge effects considerably limit the
usefulness of smaller survey fields (e.g. the GOODS fields) for studies of
galaxy environment. In even the poorest groups and clusters, redshift-space
distortions limit the effectiveness of each environment statistic; measuring
density in projection (e.g. using counts in a cylindrical aperture or a
projected n^th-nearest-neighbor distance measure) significantly improves the
accuracy of measures in such over-dense environments. For the DEEP2 Galaxy
Redshift Survey, we conclude that among the environment estimators tested the
projected n^th-nearest-neighbor distance measure provides the most accurate
estimate of local galaxy density over a continuous and broad range of scales.Comment: 17 pages including 16 figures, accepted to Ap
Kiloparsec-scale Spatial Offsets in Double-peaked Narrow-line Active Galactic Nuclei. I. Markers for Selection of Compelling Dual Active Galactic Nucleus Candidates
Merger-remnant galaxies with kpc-scale separation dual active galactic nuclei
(AGNs) should be widespread as a consequence of galaxy mergers and triggered
gas accretion onto supermassive black holes, yet very few dual AGNs have been
observed. Galaxies with double-peaked narrow AGN emission lines in the Sloan
Digital Sky Survey are plausible dual AGN candidates, but their double-peaked
profiles could also be the result of gas kinematics or AGN-driven outflows and
jets on small or large scales. To help distinguish between these scenarios, we
have obtained spatial profiles of the AGN emission via follow-up long-slit
spectroscopy of 81 double-peaked narrow-line AGNs in SDSS at 0.03 < z < 0.36
using Lick, Palomar, and MMT Observatories. We find that all 81 systems exhibit
double AGN emission components with ~kpc projected spatial separations on the
sky, which suggests that they are produced by kpc-scale dual AGNs or kpc-scale
outflows, jets, or rotating gaseous disks. In addition, we find that the
subsample (58%) of the objects with spatially compact emission components may
be preferentially produced by dual AGNs, while the subsample (42%) with
spatially extended emission components may be preferentially produced by AGN
outflows. We also find that for 32% of the sample the two AGN emission
components are preferentially aligned with the host galaxy major axis, as
expected for dual AGNs orbiting in the host galaxy potential. Our results both
narrow the list of possible physical mechanisms producing the double AGN
components, and suggest several observational criteria for selecting the most
promising dual AGN candidates from the full sample of double-peaked narrow-line
AGNs. Using these criteria, we determine the 17 most compelling dual AGN
candidates in our sample.Comment: 12 pages, 8 figures, published in ApJ. Modified from original version
to reflect referee's comment
Dependence of Galaxy Quenching on Halo Mass and Distance from its Centre
We study the dependence of star-formation quenching on galaxy mass and
environment, in the SDSS (z~0.1) and the AEGIS (z~1). It is crucial that we
define quenching by low star-formation rate rather than by red colour, given
that one third of the red galaxies are star forming. We address stellar mass
M*, halo mass Mh, density over the nearest N neighbours deltaN, and distance to
the halo centre D. The fraction of quenched galaxies appears more strongly
correlated with Mh at fixed M* than with M* at fixed Mh, while for satellites
quenching also depends on D. We present the M*-Mh relation for centrals at z~1.
At z~1, the dependence of quenching on M* at fixed Mh is somewhat more
pronounced than at z~0, but the quenched fraction is low (10%) and the haloes
are less massive. For satellites, M*-dependent quenching is noticeable at high
D, suggesting a quenching dependence on sub-halo mass for recently captured
satellites. At small D, where satellites likely fell in more than a few Gyr
ago, quenching strongly depends on Mh, and not on M*. The Mh-dependence of
quenching is consistent with theoretical wisdom where virial shock heating in
massive haloes shuts down accretion and triggers ram-pressure stripping,
causing quenching. The interpretation of deltaN is complicated by the fact that
it depends on the number of observed group members compared to N, motivating
the use of D as a better measure of local environment.Comment: 23 pages, 13 figures, accepted by MNRA
AEGIS: Chandra Observation of DEEP2 Galaxy Groups and Clusters
We present a 200 ksec Chandra observation of seven spectroscopically
selected, high redshift (0.75 < z < 1.03) galaxy groups and clusters discovered
by the DEEP2 Galaxy Redshift Survey in the Extended Groth Strip (EGS). X-ray
emission at the locations of these systems is consistent with background. The
3-sigma upper limits on the bolometric X-ray luminosities (L_X) of these
systems put a strong constraint on the relation between L_X and the velocity
dispersion of member galaxies sigma_gal at z~1; the DEEP2 systems have lower
luminosity than would be predicted by the local relation. Our result is
consistent with recent findings that at high redshift, optically selected
clusters tend to be X-ray underluminous. A comparison with mock catalogs
indicates that it is unlikely that this effect is entirely caused by a
measurement bias between sigma_gal and the dark matter velocity dispersion.
Physically, the DEEP2 systems may still be in the process of forming and hence
not fully virialized, or they may be deficient in hot gas compared to local
systems. We find only one possibly extended source in this Chandra field, which
happens to lie outside the DEEP2 coverage.Comment: 5 pages, 3 figures. Accepted for publication in AEGIS ApJ Letters
special editio
The DEEP2 Galaxy Redshift Survey: Clustering of Groups and Group Galaxies at z~1
We study the clustering properties of groups and of galaxies in groups in the
DEEP2 Galaxy Redshift Survey dataset at z~1. Four clustering measures are
presented: 1) the group correlation function for 460 groups with estimated
velocity dispersions of sigma>200 km/s, 2) the galaxy correlation for the full
galaxy sample, using a flux-limited sample of 9800 objects between 0.7<z<1.0,
3) the galaxy correlation for galaxies in groups, and 4) the group-galaxy
cross-correlation function. Using the observed number density and clustering
amplitude of the groups, the estimated minimum group dark matter halo mass is
M_min~6 10^12 h^-1 M_Sun for a flat LCDM cosmology. Groups are more clustered
than galaxies, with a relative bias of b=1.7 +/-0.04 on scales r_p=0.5-15
Mpc/h. Galaxies in groups are also more clustered than the full galaxy sample,
with a scale-dependent relative bias which falls from b~2.5 +/-0.3 at r_p=0.1
Mpc/h to b~1 +/-0.5 at r_p=10 Mpc/h. The correlation functions for all galaxies
and galaxies in groups can be fit by a power-law on scales r_p=0.05-20 Mpc/h.
We empirically measure the contribution to the projected correlation function
for galaxies in groups from a `one-halo' term and a `two-halo' term by counting
pairs of galaxies in the same or in different groups. The projected
cross-correlation between shows that red galaxies are more centrally
concentrated in groups than blue galaxies at z~1. DEEP2 galaxies in groups
appear to have a shallower radial distribution than that of mock galaxy
catalogs made from N-body simulations, which assume a central galaxy surrounded
by satellite galaxies with an NFW profile. We show that the clustering of
galaxies in groups can be used to place tighter constraints on the halo model
than can be gained from using the usual galaxy correlation function alone.Comment: 22 pages, 12 figures, in emulateapj format, accepted to ApJ, minor
changes made to match published versio
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