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Register a new userThe X-ray luminosity function of AGN at z~3
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James Aird
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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.
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We present the first clustering results of X-ray selected AGN at z~3. Using Chandra X-ray imaging and UVR optical colors from MUSYC photometry in the ECDF-S field, we selected a sample of 58 z~3 AGN candidates. From the optical data we also selected 1385 LBG at 2.8<z< 3.8 with R<25.5. We performed auto-correlation and cross-correlation analyses, and here we present results for the clustering amplitudes and dark matter halo masses of each sample. For the LBG we find a correlation length of r_0,LBG = 6.7 +/- 0.5 Mpc, implying a bias value of 3.5 +/- 0.3 and dark matter (DM) halo masses of log(Mmin/Msun) = 11.8 +/- 0.1. The AGN-LBG cross-correlation yields r_0,AGN-LBG = 8.7 +/- 1.9 Mpc, implying for AGN at 2.8<z<3.8 a bias value of 5.5 +/- 2.0 and DM halo masses of log(Mmin/Msun) = 12.6 +0.5/-0.8. Evolution of dark matter halos in the Lambda CDM cosmology implies that today these z~3 AGN are found in high mass galaxies with a typical luminosity of 7+4/-2 L*.
We present a new modeling of the X-ray luminosity function (XLF) of Active Galactic Nuclei (AGN) out to z$sim$3, dissecting the contribution of main-sequence (MS) and starburst (SB) galaxies. For each galaxy population, we convolved the observed galaxy stellar mass (M$_{star}$) function with a grid of M$_{star}$-independent Eddington ratio ($lambda_{rm EDD}$) distributions, normalised via empirical black hole accretion rate (BHAR) to star formation rate (SFR) relations. Our simple approach yields an excellent agreement with the observed XLF since z$sim$3. We find that the redshift evolution of the observed XLF can only be reproduced through an intrinsic flattening of the $lambda_{rm EDD}$ distribution, and with a positive shift of the break $lambda^{*}$, consistent with an anti-hierarchical behavior. The AGN accretion history is predominantly made by massive (10$^{10}<$M$_{star}<$10$^{11}$ M$_{odot}$) MS galaxies, while SB-driven BH accretion, possibly associated with galaxy mergers, becomes dominant only in bright quasars, at $log$(L$_{rm X}$/erg s$^{-1}$)$>$44.36 + 1.28$cdot$(1+z). We infer that the probability of finding highly-accreting ($lambda_{rm EDD}>$ 10%) AGN significantly increases with redshift, from 0.4% (3.0%) at z=0.5 to 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$_{star}$-dependent ratio between BHAR and SFR, as BHAR/SFR $propto$ M$_{star}^{0.73[+0.22,-0.29]}$, supporting a non-linear BH buildup relative to the host. Finally, this framework opens potential questions on super-Eddington BH accretion and different $lambda_{rm EDD}$ prescriptions for understanding the cosmic BH mass assembly.
The X-ray luminosity function of distant (3<z<5.1) unabsorbed quasars has been measured. A sample of distant high-luminosity quasars ($10^{45} leq L_{{rm X},2-10} < 7.5 times 10^{45}$ erg/s in the 2--10 keV energy band) from the catalog given in Khorunzhev et al. (2016) compiled from the data of the 3XMM-DR4 catalog of the XMM-Newton serendipitous survey and the Sloan Digital Sky Survey (SDSS) has been used. This sample consists of 101 sources. Most of them (90) have spectroscopic redshifts $z_{spec}geqslant 3$. The remaining ones are quasar candidates with photometric redshift estimates $z_{phot}geqslant 3$. The spectroscopic redshifts of eight sources have been measured with AZT-33IK and BTA telescopes. Owing to the record sky coverage area ($simeq 250$ sq. deg at X-ray fluxes $sim 10^{-14}$ erg/s/cm$^{2}$ in the 0.5-2 keV), from which the sample was drawn, we have managed to obtain reliable estimates of the space density of distant X-ray quasars with luminosities $L_{{rm X},2-10} > 2 times 10^{45}$ erg/s for the first time. Their comoving space density remains constant as the redshift increases from z=3 to z=5 to within a factor of 2. The power-law slope of the X-ray luminosity function of high-redshift quasars in its bright end (above the break luminosity) has been reliably constrained for the first time. The range of possible slopes for the quasar luminosity and density evolution model is $gamma_2=2.78^{+0.00}_{-0.04}pm0.20$, where initially the lower and upper boundaries of $gamma_2$ with the remaining uncertainty in the detection completeness of X-ray sources in SDSS, and subsequently the statistical error of the slope are specified.
We present new observational determinations of the evolution of the 2-10keV X-ray luminosity function (XLF) of AGN. We utilise data from a number of surveys including both the 2Ms Chandra Deep Fields and the AEGIS-X 200ks survey, enabling accurate measurements of the evolution of the faint end of the XLF. We combine direct, hard X-ray selection and spectroscopic follow-up or photometric redshift estimates at z<1.2 with a rest-frame UV colour pre-selection approach at higher redshifts to avoid biases associated with catastrophic failure of the photometric redshifts. Only robust optical counterparts to X-ray sources are considered using a likelihood ratio matching technique. A Bayesian methodology is developed that considers redshift probability distributions, incorporates selection functions for our high redshift samples, and allows robust comparison of different evolutionary models. We find that the XLF retains the same shape at all redshifts, but undergoes strong luminosity evolution out to z~1, and an overall negative density evolution with increasing redshift, which thus dominates the evolution at earlier times. We do not find evidence that a Luminosity-Dependent Density Evolution, and the associated flattening of the faint-end slope, is required to describe the evolution of the XLF. We find significantly higher space densities of low-luminosity, high-redshift AGN than in prior studies, and a smaller shift in the peak of the number density to lower redshifts with decreasing luminosity. The total luminosity density of AGN peaks at z=1.2+/-0.1, but there is a mild decline to higher redshifts. We find >50% of black hole growth takes place at z>1, with around half in Lx<10^44 erg/s AGN.
We searched for high-z quasars within the X-ray source population detected in the contiguous $sim 140^2$ eFEDS field observed by eROSITA during the performance verification phase. We collected the available spectroscopic information in the field, including the sample of all currently known optically selected z>5.5 quasars and cross-matched secure Legacy DR8 counterparts of eROSITA-detected X-ray point-like sources with this spectroscopic sample. We report the X-ray detection of an eROSITA source securely matched to the well-known quasar SDSS J083643.85+005453.3 (z=5.81). The soft X-ray flux of the source derived from eROSITA is consistent with previous Chandra observations. In addition, we report the detection of the quasar with LOFAR at 145 MHz and ASKAP at 888 MHz. The reported flux densities confirm a spectral flattening at lower frequencies in the emission of the radio core, indicating that the quasar could be a (sub-) gigahertz peaked spectrum source. The inferred spectral shape and the parsec-scale radio morphology of SDSS J083643.85+005453.3 suggest that it is in an early stage of its evolution into a large-scale radio source or confined in a dense environment. We find no indications for a strong jet contribution to the X-ray emission of the quasar, which is therefore likely to be linked to accretion processes. The detection of this source allows us to place the first constraints on the XLF at z>5.5 based on a secure spectroscopic redshift. Compared to extrapolations from lower-redshift observations, this favours a relatively flat slope for the XLF at $zsim 6$ beyond $L_*$. The population of X-ray luminous AGNs at high redshift may be larger than previously thought. From our XLF constraints, we make the conservative prediction that eROSITA will detect $sim 90$ X-ray luminous AGNs at redshifts 5.7<z<6.4 in the full-sky survey (De+RU).
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