74 research outputs found
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
On the X-ray properties of submm-selected galaxies
We present an analysis of the X-ray properties of 35 submm galaxies(SMGs) in
the CDF-N. Using a sample of robust 850micron-selected galaxies, with accurate
positions from Spitzer and/or radio counterparts, we find 16 objects (45+/-8%)
with significant X-ray detections in the 2Ms Chandra data. 6 of these SMGs
(~17+/-6%) have measured X-ray luminosities or upper limits consistent with
those expected based on the far-infrared (FIR) or radio-derived star formation
rate (SFR), and hence with the X-rays coming solely from star formation. In
another 7 sources (20+/-7%) a dominant AGN contribution to the X-ray emission
is required, while in 3 more it is unclear whether stellar process or accretion
are responsible. Stacking of the X-ray undetected SMGs reveals a highly
significant detection. If due to star formation, this corresponds to an average
X-ray derived SFR of ~150 Msun/yr. We deduce that the AGN fraction in SMGs
based on X-ray observations is 20-29 (+/-7) %, which is towards the lower limit
of previous estimates. Spectral analysis shows that in general the SMGs are not
heavily obscured in the X-ray but most of the SMGs classfied as AGN show
absorption with N_H in excess of 10^22 cm^-2. Of the secure AGN, the bolometric
luminosity appears to be dominated by the AGN in only 3 cases. In ~85% of the
SMGs, the X-ray spectrum effectively rules out an AGN contribution that
dominates the bolometric emission, even if the AGN is Compton thick. The
evidence therefore suggests that intense star formation accounts for both the
FIR and X-ray emission in most SMGs. We argue that, rather than having an
especially high AGN fraction or duty cycle, SMGs have a high X-ray detection
rate at very faint fluxes partly because of their high star formation rates
and, in rarer cases, because the submm emission is from an AGN.(abridged)Comment: 12 pages, 5 figures, accepted for publication in MNRA
AEGIS: New Evidence Linking Active Galactic Nuclei to the Quenching of Star Formation
Utilizing Chandra X-ray observations in the All-wavelength Extended Groth
Strip International Survey (AEGIS) we identify 241 X-ray selected Active
Galactic Nuclei (AGNs, L > 10^{42} ergs/s) and study the properties of their
host galaxies in the range 0.4 < z < 1.4. By making use of infrared photometry
from Palomar Observatory and BRI imaging from the Canada-France-Hawaii
Telescope, we estimate AGN host galaxy stellar masses and show that both
stellar mass and photometric redshift estimates (where necessary) are robust to
the possible contamination from AGNs in our X-ray selected sample. Accounting
for the photometric and X-ray sensitivity limits of the survey, we construct
the stellar mass function of X-ray selected AGN host galaxies and find that
their abundance decreases by a factor of ~2 since z~1, but remains roughly flat
as a function of stellar mass. We compare the abundance of AGN hosts to the
rate of star formation quenching observed in the total galaxy population. If
the timescale for X-ray detectable AGN activity is roughly 0.5-1 Gyr--as
suggested by black hole demographics and recent simulations--then we deduce
that the inferred AGN "trigger" rate matches the star formation quenching rate,
suggesting a link between these phenomena. However, given the large range of
nuclear accretion rates we infer for the most massive and red hosts, X-ray
selected AGNs may not be directly responsible for quenching star formation.Comment: 12 pages. Submitted to ApJ. Comments welcom
Groups of Galaxies in AEGIS: The 200 ksec Chandra Extended X-ray Source catalogue
We present the discovery of seven X-ray emitting groups of galaxies selected
as extended X-ray sources in the 200 ksec Chandra coverage of the
All-wavelength Extended Groth Strip International Survey (AEGIS). In addition,
we report on AGN activity associated to these systems. Using the DEEP2 Galaxy
Redshift Survey coverage, we identify optical counterparts and determine
velocity dispersions. In particular, we find three massive high-redshift groups
at z>0.7, one of which is at z=1.13, the first X-ray detections of
spectroscopically selected DEEP2 groups. We also present a first look at the
the L_X-T, L_X-sigma, and sigma-T scaling relations for high-redshift massive
groups. We find that the properties of these X-ray selected systems agree well
with the scaling relations of similar systems at low redshift, although there
are X-ray undetected groups in the DEEP2 catalogue with similar velocity
dispersions. The other three X-ray groups with identified redshifts are
associated with lower mass groups at z~0.07 and together form part of a large
structure or "supergroup" in the southern portion of the AEGIS field. All of
the low-redshift systems are centred on massive elliptical galaxies, and all of
the high-redshift groups have likely central galaxies or galaxy pairs. All of
the central group galaxies host X-ray point sources, radio sources, and/or show
optical AGN emission. Particularly interesting examples of central AGN activity
include a bent-double radio source plus X-ray point source at the center of a
group at z=0.74, extended radio and double X-ray point sources associated to
the central galaxy in the lowest-redshift group at z=0.066, and a bright green
valley galaxy (part of a pair) in the z=1.13 group which shows optical AGN
emission lines.Comment: accepted to MNRAS, 15 pages, 11 figures, for version with full
resolution figures see http://www.ucolick.org/~tesla/aegis_groups.ps.g
A CANDELS WFC3 Grism Study of Emission-Line Galaxies at z~2: A Mix of Nuclear Activity and Low-Metallicity Star Formation
We present Hubble Space Telescope Wide Field Camera 3 slitless grism
spectroscopy of 28 emission-line galaxies at z~2, in the GOODS-S region of the
Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS). The
high sensitivity of these grism observations, with 1-sigma detections of
emission lines to f > 2.5x10^{-18} erg/s/cm^2, means that the galaxies in the
sample are typically ~7 times less massive (median M_* = 10^{9.5} M_sun) than
previously studied z~2 emission-line galaxies. Despite their lower mass, the
galaxies have OIII/Hb ratios which are very similar to previously studied z~2
galaxies and much higher than the typical emission-line ratios of local
galaxies. The WFC3 grism allows for unique studies of spatial gradients in
emission lines, and we stack the two-dimensional spectra of the galaxies for
this purpose. In the stacked data the OIII emission line is more spatially
concentrated than the Hb emission line with 98.1 confidence. We additionally
stack the X-ray data (all sources are individually undetected), and find that
the average L(OIII)/L(0.5-10 keV) ratio is intermediate between typical z~0
obscured active galaxies and star-forming galaxies. Together the compactness of
the stacked OIII spatial profile and the stacked X-ray data suggest that at
least some of these low-mass, low-metallicity galaxies harbor weak active
galactic nuclei.Comment: ApJ accepted. 8 pages, 6 figure
AEGIS: Demographics of X-ray and Optically Selected AGNs
We develop a new diagnostic method to classify galaxies into AGN hosts,
star-forming galaxies, and absorption-dominated galaxies by combining the [O
III]/Hbeta ratio with rest-frame U-B color. This can be used to robustly select
AGNs in galaxy samples at intermediate redshifts (z<1). We compare the result
of this optical AGN selection with X-ray selection using a sample of 3150
galaxies with 0.3<z<0.8 and I_AB<22, selected from the DEEP2 Galaxy Redshift
Survey and the All-wavelength Extended Groth Strip International Survey
(AEGIS). Among the 146 X-ray sources in this sample, 58% are classified
optically as emission-line AGNs, the rest as star-forming galaxies or
absorption-dominated galaxies. The latter are also known as "X-ray bright,
optically normal galaxies" (XBONGs). Analysis of the relationship between
optical emission lines and X-ray properties shows that the completeness of
optical AGN selection suffers from dependence on the star formation rate and
the quality of observed spectra. It also shows that XBONGs do not appear to be
a physically distinct population from other X-ray detected, emission-line AGNs.
On the other hand, X-ray AGN selection also has strong bias. About 2/3 of all
emission-line AGNs at L_bol>10^44 erg/s in our sample are not detected in our
200 ks Chandra images, most likely due to moderate or heavy absorption by gas
near the AGN. The 2--7 keV detection rate of Seyfert 2s at z~0.6 suggests that
their column density distribution and Compton-thick fraction are similar to
that of local Seyferts. Multiple sample selection techniques are needed to
obtain as complete a sample as possible.Comment: 24 pages, 14 figures, submitted to ApJ. Version 2 matches the ApJ
accepted version. Sec 3 was reorganized and partly rewritten with one
additional figure (Fig.3
A CANDELS WFC3 Grism Study of Emission-Line Galaxies at Z approximates 2: A mix of Nuclear Activity and Low-Metallicity Star Formation
We present Hubble Space Telescope Wide Field Camera 3 slitless grism spectroscopy of 28 emission-line galaxies at z approximates 2, in the GOODS-S region of the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS). The high sensitivity of these grism observations, with > 5-sigma detections of emission lines to f > 2.5 X 10(exp -18( erg/s/ square cm, means that the galaxies in the sample are typically approximately 7 times less massive (median M(star). = 10(exp 9.5)M(solar)) than previously studied z approximates 2 emission-line galaxies. Despite their lower mass, the galaxies have [O-III]/H-Beta ratios which are very similar to previously studied z approximates 2 galaxies and much higher than the typical emission-line ratios of local galaxies. The WFC3 grism allows for unique studies of spatial gradients in emission lines, and we stack the two-dimensional spectra of the galaxies for this purpose. In the stacked data the [O-III] emission line is more spatially concentrated than the H-Beta emission line with 98.1% confidence. We additionally stack the X-ray data (all sources are individually undetected), and find that the average L(sub [O-III])/L(sub 0.5.10keV) ratio is intermediate between typical z approximates 0 obscured active galaxies and star-forming galaxies. Together the compactness of the stacked [O-III] spatial profile and the stacked X-ray data suggest that at least some of these low-mass, low-metallicity galaxies harbor weak active galactic nuclei
The DEEP Groth Strip Survey VI. Spectroscopic, Variability, and X-ray Detection of AGN
We identify active galactic nuclei (AGN) in the Groth-Westphal Survey Strip
(GSS) using the independent and complementary selection techniques of optical
spectroscopy and photometric variability. We discuss the X-ray properties of
these AGN using Chandra/XMM data for this region. From a sample of 576 galaxies
with high quality spectra we identify 31 galaxies with AGN signatures. Seven of
these have broad emission lines (Type 1 AGNs). We also identify 26 galaxies
displaying nuclear variability in HST WFPC2 images of the GSS separated by ~7
years. The primary overlap of the two selected AGN samples is the set of
broad-line AGNs, of which 80% appear as variable. Only a few narrow-line AGNs
approach the variability threshold. The broad-line AGNs have an average
redshift of z~1.1 while the other spectroscopic AGNs have redshifts closer to
the mean of the general galaxy population (z~0.7). Eighty percent of the
identified broad-line AGNs are detected in X-rays and these are among the most
luminous X-ray sources in the GSS. Only one narrow-line AGN is X-ray detected.
Of the variable nuclei galaxies within the X-ray survey, 27% are X-ray
detected. We find that 1.9+/-0.6% of GSS galaxies to V=24 are broad-line AGNs,
1.4+/-0.5% are narrow-line AGNs, and 4.5+/-1.4% contain variable nuclei. The
fraction of spectroscopically identified BLAGNs and NLAGNs at z~1 reveals a
marginally significant increase of 1.3+/-0.9% when compared to the local
population.Comment: 29 pages, 8 figures, accepted for publication in ApJ
AEGIS: The Clustering of X-ray AGN Relative to Galaxies at z~1
We measure the clustering of non-quasar X-ray AGN at z=0.7-1.4 in the AEGIS
field. Using the cross-correlation of 113 Chandra-selected AGN, with a median
log L_X=42.8 erg s^-1, with ~5,000 DEEP2 galaxies, we find that the X-ray AGN
are fit by a power law with a clustering scale length of r_0=5.95 +/-0.90 h^-1
Mpc and slope gamma=1.66 +/-0.22. X-ray AGN have a similar clustering amplitude
as red, quiescent and `green' transition galaxies at z~1 and are significantly
more clustered than blue, star-forming galaxies. The X-ray AGN clustering
strength is primarily determined by the host galaxy color; AGN in red host
galaxies are significantly more clustered than AGN in blue host galaxies, with
a relative bias that is similar to that of red to blue DEEP2 galaxies. We
detect no dependence of clustering on optical brightness, X-ray luminosity, or
hardness ratio within the ranges probed here. We find evidence for galaxies
hosting X-ray AGN to be more clustered than a sample of galaxies with matching
joint optical color and magnitude distributions. This implies that galaxies
hosting X-ray AGN are more likely to reside in groups and more massive dark
matter halos than galaxies of the same color and luminosity without an X-ray
AGN. In comparison to optically-selected quasars in the DEEP2 fields, we find
that X-ray AGN at z~1 are more clustered than optically-selected quasars (with
a 2.6-sigma significance) and therefore likely reside in more massive dark
matter halos. Our results are consistent with galaxies undergoing a quasar
phase while in the blue cloud before settling on the red sequence with a
lower-luminosity X-ray AGN, if they are similar objects at different
evolutionary stages.Comment: 18 pages, accepted to ApJ, minor changes made to match accepted
version, Figure 1 adde
Variability and Multiwavelength Detected AGN in the GOODS Fields
We identify 85 variable galaxies in the GOODS North and South fields using 5
epochs of HST ACS V-band (F606W) images spanning 6 months. The variables are
identified through significant flux changes in the galaxy's nucleus and
represent ~2% of the survey galaxies. With the aim of studying the active
galaxy population in the GOODS fields, we compare the variability-selected
sample with X-ray and mid-IR AGN candidates. Forty-nine percent of the
variables are associated with X-ray sources identified in the 2Ms Chandra
surveys. Twenty-four percent of X-ray sources likely to be AGN are optical
variables and this percentage increases with decreasing hardness ratio of the
X-ray emission. Stacking of the non-X-ray detected variables reveals marginally
significant soft X-ray emission. Forty-eight percent of mid-IR power-law
sources are optical variables, all but one of which are also X-ray detected.
Thus, about half of the optical variables are associated with either X-ray or
mid-IR power-law emission. The slope of the power-law fit through the Spitzer
IRAC bands indicates that two-thirds of the variables have BLAGN-like SEDs.
Among those galaxies spectroscopically identified as AGN, we observe
variability in 74% of broad-line AGNs and 15% of NLAGNs. The variables are
found in galaxies extending to z~3.6. We compare the variable galaxy colors and
magnitudes to the X-ray and mid-IR sample and find that the non-X-ray detected
variable hosts extend to bluer colors and fainter intrinsic magnitudes. The
variable AGN candidates have Eddington ratios similar to those of X-ray
selected AGN.Comment: 36 pages, 12 figures, ApJ accepte
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