1,194 research outputs found
The X-ray luminosity function of AGN at z~3
We combine Lyman-break colour selection with ultradeep (> 200 ks) Chandra
X-ray imaging over a survey area of ~0.35 deg^2 to select high redshift AGN.
Applying careful corrections for both the optical and X-ray selection
functions, the data allow us to make the most accurate determination to date of
the faint end of the X-ray luminosity function (XLF) at z~3. Our methodology
recovers a number density of X-ray sources at this redshift which is at least
as high as previous surveys, demonstrating that it is an effective way of
selecting high z AGN. Comparing to results at z=1, we find no evidence that the
faint slope of the XLF flattens at high z, but we do find significant (factor
~3.6) negative evolution of the space density of low luminosity AGN. Combining
with bright end data from very wide surveys we also see marginal evidence for
continued positive evolution of the characteristic break luminosity L*. Our
data therefore support models of luminosity-dependent density evolution between
z=1 and z=3. A sharp upturn in the the XLF is seen at the very lowest
luminosities (Lx < 10^42.5 erg s^-1), most likely due to the contribution of
pure X-ray starburst galaxies at very faint fluxes.Comment: 16 pages, 9 figures, accepted for publication in MNRA
Evidence for a mass-dependent AGN Eddington ratio distribution via the flat relationship between SFR and AGN luminosity
The lack of a strong correlation between AGN X-ray luminosity (L X ; a proxy for AGN power) and the star formation rate (SFR) of their host galaxies has recently been attributed to stochastic AGNvariability. Studies using population synthesis models have incorporated this by assuming a broad, universal (i.e. does not depend on the host galaxy properties) probability distribution for AGN specific X-ray luminosities (i.e. the ratio of L X to host stellar mass; a common proxy for Eddington ratio). However, recent studies have demonstrated that this universal Eddington ratio distribution fails to reproduce the observed X-ray luminosity functions beyond z ~ 1.2. Furthermore, empirical studies have recently shown that the Eddington ratio distribution may instead depend upon host galaxy properties, such as SFR and/or stellarmass. To investigate this further, we develop a population synthesis model in which the Eddington ratio distribution is different for star-forming and quiescent host galaxies. We showthat, although this model is able to reproduce the observed X-ray luminosity functions out to z ~ 2, it fails to simultaneously reproduce the observed flat relationship between SFR and X-ray luminosity. We can solve this, however, by incorporating a mass dependency in the AGN Eddington ratio distribution for starforming host galaxies. Overall, our models indicate that a relative suppression of low Eddington ratios (λ Edd < ~0.1) in lower mass galaxies (M * < ~10 10-11 M⊙) is required to reproduce both the observed X-ray luminosity functions and the observed flat SFR/X-ray relationship
Optically faint X-ray sources in the CDFN: Spitzer constraints
We investigate the properties of the most optically faint sources in the
GOODS-N area (R > 26.5 AB). Such extremely optically faint populations present
an uncharted territory despite the fact that they represent an appreciable
fraction of the X-ray sources in the GOODS-N field. They are believed to
contain either red AGN at moderate redshifts or possibly QSO at very high
redshift. We compile our sample by first finding the 3.6um IRAC counterparts of
the X-ray sources and searching for the optical counterparts of the IRAC
sources. 35 sources do not have counterparts in the R-band Subaru optical
images. Of these, 18 have HST-ACS counterparts while the remaining have no
optical counterparts. The vast majority of our 35 sources are classified as
Extremely Red Objects (EROs) on the basis of their V-K lower limits. Their
photometric redshifts show that these populate moderate redshifts (median
z~2.8), being markedly different from the already spectroscopically identified
population which peaks at z~0.7. The Spitzer-IRAC mid-IR colours of the sources
which have no HST counterparts tend to lie within the mid-IR colour diagram AGN
"wedge", suggesting either QSO, ULIRG (Mrk231), or early-type galaxy templates
at z>3. A large fraction of our sources (17/35), regardless of whether they
have HST counterparts, can be classified as mid-IR bright/optically faint
sources (Dust Obscured Galaxies) a class which is believed to include many
heavily absorbed AGN. The co-added X-ray spectrum of the optically faint
sources is very flat having a spectral index of Gamma~0.87, significantly
flatter than the spectrum of the X-ray background. The optically faint R>26.5
X-ray sources constitute more than 50% of the total X-ray population at
redshifts z>2 bearing important implications for the luminosity function and
its evolution; considering X-ray sources with 2<z<4 we find good agreement with
a modified PLE model.Comment: Accepted for publication in A&
Bone mineral content after renal transplantation
Forearm bone mineral content (BMC), as evaluated by photonabsorption densitometry, was measured in 28 cadaver kidney donor recipients who entered the study 8 weeks postoperatively and were followed up for 18 months. BMC decreased signifiantly (p<0.05) but marginally in placebo-treated patients (n=14) (initial BMC 1.09±0.25 g/cm; final BMC 1.05±0.24). Fourteen patients were prophylactically given 1,25(OH)2vitamin D3 in a dose which avoided hypercalcemia and hypercalciuria (sim0.25 µg/day); under 1,25(OH)2 vitamin D3 prophylaxis a significant decrease of forearm BMC was observed no longer (initial BMC 0.94±0.21 g/cm; final BMC 0.95±0.21), but the difference between placebo and 1,25(OH)2 vitamin D3 narrowly missed statistical significance (p=0.066).
It is concluded that the decrease of forearm BMC is negligible in transplant recipients with low steroid regimens. The data suggest a trend for prophylaxis with 1,25(OH)2 vitamin D3 to slightly ameliorate forearm (cortical) BMC loss
The MOSFIRE Deep Evolution Field (MOSDEF) Survey: Rest-Frame Optical Spectroscopy for ~1500 H-Selected Galaxies at 1.37 < z < 3.8
In this paper we present the MOSFIRE Deep Evolution Field (MOSDEF) survey.
The MOSDEF survey aims to obtain moderate-resolution (R=3000-3650) rest-frame
optical spectra (~3700-7000 Angstrom) for ~1500 galaxies at 1.37<z<3.80 in
three well-studied CANDELS fields: AEGIS, COSMOS, and GOODS-N. Targets are
selected in three redshift intervals: 1.37<z<1.70, 2.09<z<2.61, and
2.95<z<3.80, down to fixed H_AB (F160W) magnitudes of 24.0, 24.5 and 25.0,
respectively, using the photometric and spectroscopic catalogs from the 3D-HST
survey. We target both strong nebular emission lines (e.g., [OII], Hbeta,
[OIII], 5008, Halpha, [NII], and [SII]) and stellar continuum and absorption
features (e.g., Balmer lines, Ca-II H and K, Mgb, 4000 Angstrom break). Here we
present an overview of our survey, the observational strategy, the data
reduction and analysis, and the sample characteristics based on spectra
obtained during the first 24 nights. To date, we have completed 21 masks,
obtaining spectra for 591 galaxies. For ~80% of the targets we derive a robust
redshift from either emission or absorption lines. In addition, we confirm 55
additional galaxies, which were serendipitously detected. The MOSDEF galaxy
sample includes unobscured star-forming, dusty star-forming, and quiescent
galaxies and spans a wide range in stellar mass (~10^9-10^11.5 Msol) and star
formation rate (~10^0-10^3 Msol/yr). The spectroscopically confirmed sample is
roughly representative of an H-band limited galaxy sample at these redshifts.
With its large sample size, broad diversity in galaxy properties, and wealth of
available ancillary data, MOSDEF will transform our understanding of the
stellar, gaseous, metal, dust, and black hole content of galaxies during the
time when the universe was most active.Comment: Accepted for publication in ApJS; 28 pages, 19 figures; MOSDEF
spectroscopic redshifts available at
http://mosdef.astro.berkeley.edu/Downloads.htm
The evolving AGN duty cycle in galaxies since z ∼ 3 as encoded in the X-ray luminosity function
We present a new modeling of the X-ray luminosity function (XLF) of active galactic nuclei (AGNs) out to z ~ 3, dissecting the contributions of main-sequence (MS) and starburst (SB) galaxies. For each galaxy population, we convolved the observed galaxy stellar mass (M sstarf) function with a grid of M sstarf-independent Eddington ratio (λ EDD) distributions, normalized via empirical black hole accretion rate (BHAR) to star formation rate (SFR) relations. Our simple approach yields an excellent agreement with the observed XLF since z ~ 3. We find that the redshift evolution of the observed XLF can only be reproduced through an intrinsic flattening of the λ EDD distribution and with a positive shift of the break λ*, consistent with an antihierarchical behavior. The AGN accretion history is predominantly made by massive (1010 44.36 + 1.28 × (1 + z). We infer that the probability of finding highly accreting (λ EDD > 10%) AGNs significantly increases with redshift, from 0.4% (3.0%) at z = 0.5%–6.5% (15.3%) at z = 3 for MS (SB) galaxies, implying a longer AGN duty cycle in the early universe. Our results strongly favor a M sstarf-dependent ratio between BHAR and SFR, as BHAR/SFR ∝ , supporting a nonlinear BH buildup relative to the host. Finally, this framework opens potential questions on super-Eddington BH accretion and different λ EDD prescriptions for understanding the cosmic BH mass assembly
A new method for determining the sensitivity of X-ray imaging observations and the X-ray number counts
We present a new method for determining the sensitivity of X-ray imaging
observations, which correctly accounts for the observational biases that affect
the probability of detecting a source of a given X-ray flux, without the need
to perform a large number of time consuming simulations. We use this new
technique to estimate the X-ray source counts in different spectral bands
(0.5-2, 0.5-10, 2-10 and 5-10keV) by combining deep pencil-beam and shallow
wide-area Chandra observations. The sample has a total of 6295 unique sources
over an area of and is the largest used to date to determine
the X-ray number counts. We determine, for the first time, the break flux in
the 5-10 keV band, in the case of a double power-law source count distribution.
We also find an upturn in the 0.5-2keV counts at fluxes below about
6e-17erg/s/cm2. We show that this can be explained by the emergence of normal
star-forming galaxies which dominate the X-ray population at faint fluxes. The
fraction of the diffuse X-ray background resolved into point sources at
different spectral bands is also estimated. It is argued that a single
population of Compton thick AGN cannot be responsible for the entire unresolved
X-ray background in the energy range 2-10keV.Comment: Accepted for publication in MNRAS. Data products available at
http://astro.imperial.ac.uk/research/xray
A wide search for obscured active galactic nuclei using XMM-Newton and WISE
Heavily obscured and Compton-thick active galactic nuclei (AGNs) are missing even in the deepest X-ray surveys, and indirect methods are required to detect them. Here we use a combination of the XMM–Newton serendipitous X-ray survey with the optical Sloan Digital Sky Survey (SDSS), and the infrared WISE all-sky survey in order to check the efficiency of the low X-ray-to-infrared luminosity selection method in finding heavily obscured AGNs. We select the sources which are detected in the hard X-ray band (2–8 keV), and also have a redshift determination (photometric or spectroscopic) in the SDSS catalogue. We match this sample with the WISE catalogue, and fit the spectral energy distributions of the 2844 sources which have three, or more, photometric data points in the infrared. We then select the heavily obscured AGN candidates by comparing their 12 μm luminosity to the observed 2–10 keV X-ray luminosity and the intrinsic relation between the X-ray and the mid-infrared luminosities. With this approach, we find 20 candidate heavily obscured AGNs and we then examine their X-ray and optical spectra. Of the 20 initial candidates, we find nine (64 per cent; out of the 14, for which X-ray spectra could be fitted) based on the X-ray spectra, and seven (78 per cent; out of the nine detected spectroscopically in the SDSS) based on the [O III] line fluxes. Combining all criteria, we determine the final number of heavily obscured AGNs to be 12–19, and the number of Compton-thick AGNs to be 2–5, showing that the method is reliable in finding obscured AGNs, but not Compton thick. However, those numbers are smaller than what would be expected from X-ray background population synthesis models, which demonstrates how the optical–infrared selection and the scatter of the Lx-LMIR relation limit the efficiency of the method. Finally, we test popular obscured AGN selection methods based on mid-infrared colours, and find that the probability of an AGN to be selected by its mid-infrared colours increases with the X-ray luminosity. The (observed) X-ray luminosities of heavily obscured AGNs are relatively low (L2−10keV<1044ergs−1), even though most of them are located in the ‘quasi stellar object (QSO) locus’. However, a selection scheme based on a relatively low X-ray luminosity and mid-infrared colours characteristic of QSOs would not select ∼25 per cent of the heavily obscured AGNs of our sample
The galaxy’s gas content regulated by the dark matter halo mass results in a superlinear M BH–M ⋆ Relation
Supermassive black holes (SMBHs) are tightly correlated with their hosts, but the origin of such connection remains elusive. To explore the cosmic buildup of this scaling relation, we present an empirically motivated model that tracks galaxy and SMBH growth down to z = 0. Starting from a random mass seed distribution at z = 10, we assume that each galaxy evolves on the star-forming "main sequence" (MS) and each BH follows the recently derived stellar mass (M sstarf) dependent ratio between BH accretion rate and star formation rate, going as . Our simple recipe naturally describes the BH–galaxy buildup in two stages. At first, the SMBH lags behind the host that evolves along the MS. Later, as the galaxy grows in M sstarf, our M sstarf-dependent BHAR/SFR induces a superlinear BH growth, as . According to this formalism, smaller BH seeds increase their relative mass faster and earlier than bigger BH seeds, at fixed M sstarf, thus setting along a gradually tighter M BH–M sstarf locus toward higher M sstarf. Assuming reasonable values of the radiative efficiency epsilon ~ 0.1, our empirical trend agrees with both high-redshift model predictions and intrinsic M BH–M sstarf relations of local BHs. We speculate that the observed nonlinear BH–galaxy buildup is reflected in a twofold behavior with dark matter halo mass (M DM), displaying a clear turnover at M DM ~ 2 × 1012 M ⊙. While supernovae-driven feedback suppresses BH growth in smaller halos (), above the M DM threshold cold gas inflows possibly fuel both BH accretion and star formation in a similar fashion ()
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