1,290 research outputs found
X-rays across the galaxy population â I. Tracing the main sequence of star formation
We use deep imaging to measure the distribution of X-ray luminosities (LX) for samples of star-forming galaxies as a function of stellar mass and redshift, using a Bayesian method to push below the nominal X-ray detection limits. Our luminosity distributions all show narrow peaks at LX âČ 10 erg s that we associate with star formation, as opposed to AGN that are traced by a broad tail to higher LX. Tracking the luminosity of these peaks as a function of stellar mass reveals an âX-ray main sequenceâ with a constant slope â0.63 ± 0.03 over 8.5âČlogMâ/MââČ11.5 and 0.1 âČ z âČ 4, with a normalization that increases with redshift as (1 + z). We also compare the peak X-ray luminosities with UV-to-IR tracers of star formation rates (SFRs) to calibrate the scaling between LX and SFR. We find that LX â SFR Ă (1 + z), where the redshift evolution and non-linearity likely reflect changes in high-mass X-ray binary populations of star-forming galaxies. Using galaxies with a broader range of SFR, we also constrain a stellar-mass-dependent contribution to LX, likely related to low-mass X-ray binaries. Using this calibration, we convert our X-ray main sequence to SFRs and measure a star-forming main sequence with a constant slope â0.76 ± 0.06 and a normalization that evolves with redshift as (1 + z). Based on the X-ray emission, there is no evidence for a break in the main sequence at high stellar masses, although we cannot rule out a turnover given the uncertainties in the scaling of LX to SFR.JA acknowledges support from ERC Advanced Grant FEEDBACK 340442. ALC acknowledges support from NSF CAREER award AST-1055081. AG acknowledges the THALES project 383549 that is jointly funded by the European Union and the Greek Government in the framework of the programme âEducation and lifelong learningâ. This work is based in part on observations taken by the 3D-HST Treasury Program (GO 12177 and 12328) with the NASA/ESA HST, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. Based in part on data obtained with the European Southern Observatory Very Large Telescope, Paranal, Chile , under Large Program 185.A-0791, and made available by the VUDS team at the CESAM data center, Laboratoire dâAstrophysique de Marseille, France. The scientific results reported in this article are based to a significant degree on observations made by the Chandra X-ray Observatory
Investigating evidence for different black hole accretion modes since redshift z~1
Chandra data in the COSMOS, AEGIS-XD and 4Ms CDFS are combined with
optical/near-IR photometry to determine the rest-frame U-V vs V-J colours of
X-ray AGN hosts at mean redshifts 0.40 and 0.85. This combination of colours
(UVJ) provides an efficient means of separating quiescent from star-forming,
including dust reddened, galaxies. Morphological information emphasises
differences between AGN split by their UVJ colours. AGN in quiescent galaxies
are dominated by spheroids, while star-forming hosts are split between bulges
and disks. The UVJ diagram of AGN hosts is then used to set limits on the
accretion density associated with evolved and star-forming systems. Most of the
black hole growth since z~1 is associated with star-forming hosts.
Nevertheless, ~15-20% of the X-ray luminosity density since z~1, is taking
place in the quiescent region of the UVJ diagram. For the z~0.40 subsample,
there is tentative evidence (2sigma significance), that AGN split by their UVJ
colours differ in Eddington ratio. AGN in star-forming hosts dominate at high
Eddington ratios, while AGN in quiescent hosts become increasingly important as
a fraction of the total population toward low Eddington ratios. At higher
redshift, z~0.8, such differences are significant at the 2sigma level only at
Eddington ratios >1e-3. These findings are consistent with scenarios in which
diverse accretion modes are responsible for the build-up of SMBHs at the
centres of galaxies. We compare our results with the GALFORM semi-analytic
model, which postulates two black hole fuelling modes, the first linked to
star-formation and the second occuring in passive galaxies. GALFORM predicts a
larger fraction of black hole growth in quiescent galaxies at z<1, compared to
the data. Relaxing the strong assumption of the model that passive AGN hosts
have zero star-formation rate could reconcile this disagreement.Comment: MNRAS accepte
The incidence of AGN in galaxies with different stellar population ages
It has been argued that recycled gas from stellar mass loss in galaxies might
serve as an important fuelling source for black holes (BHs) in their centers.
Utilizing spectroscopic samples of galaxies from the Sloan Digital Sky Survey
(SDSS) at and the Large Early Galaxy Astrophysics Census (LEGA-C)
survey at that have X-ray coverage from XMM-Newton or Chandra, we
test this stellar mass loss fuelling scenario by investigating how AGN activity
and BH growth vary with the break strength at 4000 ,
(which is closely related to the age of stellar populations), as younger
galaxies are considered to have higher stellar mass loss rates. We found that
when controlling for host-galaxy properties, the fraction of log / > 32 (which roughly corresponds to Eddington ratios %)
AGN and sample-averaged black hole accretion rate ()
decrease with among 1.9 galaxies,
suggesting a higher level of AGN activity among younger galaxies, which
supports the stellar mass loss fuelling scenario. For the oldest and most
massive galaxies at , this decreasing trend is not present anymore.
We found that, among these most massive galaxies at low redshift, the fraction
of low specific-accretion-rate (31 log / 32) AGNs
increases with , which may be associated with additional
fuelling from hot halo gas and/or enhanced accretion capability.Comment: 24 pages, 28 figures. Accepted for publication in MNRA
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
Constraints on the X-ray luminosity function of AGN at z = 5.7â6.4 with the Extragalactic Serendipitous Swift Survey
X-ray luminosity functions (XLFs) of Active Galactic Nuclei (AGN) trace the
growth and evolution of supermassive black hole populations across cosmic time,
however, current XLF models are poorly constrained at redshifts of z>6, with a
lack of spectroscopic constraints at these high redshifts. In this work we
\redit{place limits} on the bright-end of the XLF at z=5.7-6.4 using
high-redshift AGN identified within the Extragalactic Serendipitous Swift
Survey (ExSeSS) catalogue. Within ExSeSS we find one serendipitously X-ray
detected z>6 AGN, ATLAS J025.6821-33.4627, with an X-ray luminosity of
and
making it the highest redshift, spectroscopically confirmed,
serendipitously X-ray detected quasar known to date. We also calculate an upper
limit on the space density at higher luminosities where no additional sources
are found, enabling us to place constraints on the shape of the XLF. Our
results are consistent with the rapid decline in the space densities of
high-luminosity AGN toward high redshift as predicted by extrapolations of
existing parametric models of the XLF. We also find that our X-ray based
measurements are consistent with estimates of the bolometric quasar luminosity
function based on UV measurements at , although they require a large
X-ray to bolometric correction factor at these high luminosities.Comment: 10pages, 6 figures. Resubmitted to MNRAS, following referee comment
The X-ray luminosity function of Active Galactic Nuclei in the redshift interval z=3-5
We combine deep X-ray survey data from the Chandra observatory and the
wide-area/shallow XMM-XXL field to estimate the AGN X-ray luminosity function
in the redshift range z=3-5. The sample consists of nearly 340 sources with
either photometric (212) or spectroscopic (128) redshift in the above range.
The combination of deep and shallow survey fields provides a luminosity
baseline of three orders of magnitude, Lx(2-10keV)~1e43-1e46erg/s at z>3. We
follow a Bayesian approach to determine the binned AGN space density and
explore their evolution in a model-independent way. Our methodology accounts
for Poisson errors in the determination of X-ray fluxes and uncertainties in
photometric redshift estimates. We demonstrate that the latter is essential for
unbiased measurement of space densities. We find that the AGN X-ray luminosity
function evolves strongly between the redshift intervals z=3-4 and z=4-5. There
is also suggestive evidence that the amplitude of this evolution is luminosity
dependent. The space density of AGN with Lx<1e45erg/s drops by a factor of 5
between the redshift intervals above, while the evolution of brighter AGN
appears to be milder. Comparison of our X-ray luminosity function with that of
UV/optical selected QSOs at similar redshifts shows broad agreement at bright
luminosities, Lx>1e45erg/s. The faint-end slope of UV/optical luminosity
functions however, is steeper than for X-ray selected AGN. This implies that
the type-I AGN fraction increases with decreasing luminosity at z>3, opposite
to trends established at lower redshift. We also assess the significance of AGN
in keeping the hydrogen ionised at high redshift. Our X-ray luminosity function
yields ionising photon rate densities that are insufficient to keep the
Universe ionised at redshift z>4. A source of uncertainty in this calculation
is the escape fraction of UV photons for X-ray selected AGN.Comment: MNRAS accepte
The Nustar Extragalactic Surveys: Initial Results and Catalog from the Extended Chandra Deep Field South
We present initial results and the source catalog from the NuSTAR survey of the Extended Chandra Deep Field South (hereafter, ECDFS) - currently the deepest contiguous component of the NuSTAR extragalactic survey program. The survey covers the full ~30 arcmin x 30 arcmin area of this field to a maximum depth of ~360 ks (~220 ks when corrected for vignetting at 3-24 keV), reaching sensitivity limits of ~1.3 x 10^-14 erg/cm2/s (3-8 keV), ~3.4 x 10^-14 erg/cm2/s (8-24 keV) and ~3.0 x 10^-14 erg/cm2/s (3-24 keV). Fifty four (54) sources are detected over the full field, although five of these are found to lie below our significance threshold once contaminating flux from neighboring (i.e., blended) sources is taken into account. Of the remaining 49 that are significant, 19 are detected in the 8-24 keV band. The 8-24 keV to 3-8 keV band ratios of the twelve sources that are detected in both bands span the range 0.39-1.7, corresponding to a photon index range of Gamma ~ 0.5-2.3, with a median photon index of 1.70 +/- 0.52. The redshifts of the 49 sources in our main sample span the range z = 0.21-2.7, and their rest-frame 10-40 keV luminosities (derived from the observed 8-24 keV fluxes) span the range L(10-40 keV) ~ (0.7-300) x 10^43 erg/s, sampling below the knee of the X-ray luminosity function out to z ~ 0.8-1. Finally, we identify one NuSTAR source that has neither a Chandra nor an XMM-Newton counterpart, but that shows evidence of nuclear activity at infrared wavelengths, and thus may represent a genuine, new X-ray source detected by NuSTAR in the ECDFS
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