10 research outputs found
X-ray properties of BzK-selected galaxies in the deepest X-ray fields
We investigate the X-ray properties of BzK-selected galaxies at z 2
using deep X-ray data in the Chandra Deep Field South and North (CDFS and
CDFN). Of these we directly detect in X-rays 49 sBzKs in CDFS and 32 sBzKs in
CDFN. Stacking the undetected sources also reveals a significant X-ray signal.
Investigating the X-ray detection rate and stacked flux versus the IR excess
parameter (i.e. SFRtotal/SFRUV,corr), we find no strong evidence for an
increased X-ray detection rate, or a harder X-ray spectrum in IR Excess sBzKs.
This is particularly the case when one accounts for the strong correlation
between the IR excess parameter and the bolometric IR luminosity (LIR), e.g.
when controlling for LIR, the IR Non-Excess sBzKs show a detection rate at
least as high. While both direct detections and stacking suggest that the AGN
fraction in sBzK galaxies is high, there is no clear evidence for widespread
Compton thick activity in either the sBzK population generally, or the IR
Excess sBzK subsample. The very hard X-ray signal obtained for the latter in
earlier work was most likely contaminated by a few hard X-ray sources now
directly detected in deeper X-ray data. The X-ray detection fraction of passive
BzK galaxies in our sample is if anything higher than that of sBZKs, so there
is no evidence for coeval black hole growth and star formation from X-ray
analysis of the BzK populations. Because increased AGN activity in the IR
excess population is not indicated by our X-ray analysis, it appears that the
bulk of the IR Excess sBzK population are luminous star-forming galaxies whose
SFRs are either overestimated at 24 microns, underestimated in the UV, or both.
This conclusion reinforces recent results from Herschel which show similar
effects.Comment: 17 pages, 8 Figures, 7 Table
Obscuration-dependent evolution of Active Galactic Nuclei
We aim to constrain the evolution of AGN as a function of obscuration using
an X-ray selected sample of AGN from a multi-tiered survey including
the CDFS, AEGIS-XD, COSMOS and XMM-XXL fields. The spectra of individual X-ray
sources are analysed using a Bayesian methodology with a physically realistic
model to infer the posterior distribution of the hydrogen column density and
intrinsic X-ray luminosity. We develop a novel non-parametric method which
allows us to robustly infer the distribution of the AGN population in X-ray
luminosity, redshift and obscuring column density, relying only on minimal
smoothness assumptions. Our analysis properly incorporates uncertainties from
low count spectra, photometric redshift measurements, association
incompleteness and the limited sample size. We find that obscured AGN with
account for of the number
density and luminosity density of the accretion SMBH population with , averaged over cosmic time. Compton-thick AGN account
for approximately half the number and luminosity density of the obscured
population, and of the total. We also find evidence that the
evolution is obscuration-dependent, with the strongest evolution around
. We highlight this by measuring the
obscured fraction in Compton-thin AGN, which increases towards , where
it is higher than the local value. In contrast the fraction of
Compton-thick AGN is consistent with being constant at ,
independent of redshift and accretion luminosity. We discuss our findings in
the context of existing models and conclude that the observed evolution is to
first order a side-effect of anti-hierarchical growth.Comment: Published in Ap
CANDELS/GOODS-S, CDFS, ECDFS: Photometric Redshifts For Normal and for X-Ray-Detected Galaxies
We present photometric redshifts and associated probability distributions for
all detected sources in the Extended Chandra Deep Field South (ECDFS). The work
makes use of the most up-to-date data from the Cosmic Assembly Near-IR Deep
Legacy Survey (CANDELS) and the Taiwan ECDFS Near-Infrared Survey (TENIS) in
addition to other data. We also revisit multi-wavelength counterparts for
published X-ray sources from the 4Ms-CDFS and 250ks-ECDFS surveys, finding
reliable counterparts for 1207 out of 1259 sources (). Data used for
photometric redshifts include intermediate-band photometry deblended using the
TFIT method, which is used for the first time in this work. Photometric
redshifts for X-ray source counterparts are based on a new library of
AGN/galaxy hybrid templates appropriate for the faint X-ray population in the
CDFS. Photometric redshift accuracy for normal galaxies is 0.010 and for X-ray
sources is 0.014, and outlier fractions are and respectively. The
results within the CANDELS coverage area are even better as demonstrated both
by spectroscopic comparison and by galaxy-pair statistics. Intermediate-band
photometry, even if shallow, is valuable when combined with deep broad-band
photometry. For best accuracy, templates must include emission lines.Comment: The paper has been accepted by ApJ. The materials we provide are
available under [Surveys] > [CDFS] through the portal
http://www.mpe.mpg.de/XraySurvey
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
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 relationship between obscured AGN and their host galaxies
This thesis presents an investigation into obscured Active Galactic Nuclei (AGN) activity at high redshift and the interactions they share with their host galaxies. Using a combination of X-ray spectral fitting and X-ray stacking analyses, three studies are undertaken in this work. The first study is a reinvestigation of a specific group of X-ray undetected mid-IR excess galaxies at z ~ 2 that have previously been identified as Compton thick AGN candidates through X-ray stacking analysis. The parent sample of optically identified galaxies is found to possess above average obscured AGN activity. The galaxies exhibiting mid-IR excess, however, do not exhibit elevated levels of obscured AGN activity relative to the parent sample. Key to this result is the increased depth of X-ray observations, which resolves hard X-ray sources that had biased earlier stacking analyses.
The second study concerns the nature of AGN residing in massive galaxies at z ~ 2. The highlight of this research is the identification of two accretion modes which are dependent upon host galaxy compactness: a “transformative mode for compact galaxies and a “maintenance mode for extended galaxies. AGN in the transformative mode are heavily obscured and X-ray luminous and are thought to rapidly quench star formation in their host galaxies through violent feedback. The AGN in “maintenance mode have lower luminosities and tend to be unobscured, but appear to suppress further star formation in their host galaxies through a gentler feedback process.
The third study is a comparison of colour-excitation (CEx) and mass-excitation (MEx) classification techniques designed to identify Type 2 AGN out to z ~ 0.8. The CEx technique is found to identify obscured AGN with a high degree of accuracy, with X-ray stacking revealing many as yet X-ray undetected sources. The MEx technique is adept at identifying X-ray detected AGN but is less accurate at isolating obscured sources.Open Acces
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CANDELS: Constraining the AGN-Merger Connection with Host Morphologies at z~2
Using HST/WFC3 imaging taken as part of the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS), we examine the role that major galaxy mergers play in triggering active galactic nuclei (AGN) activity at z~2. Our sample consists of 72 moderate-luminosity (Lx ~ 1E42-1E44 erg/s) AGN at 1.