We present new measurements of the evolution of the X-ray luminosity functions (XLFs) of unabsorbed and absorbed active galactic nuclei (AGNs) out to z ∼ 5. We construct samples containing 2957 ...sources detected at hard (2–7 keV) X-ray energies and 4351 sources detected at soft (0.5–2 keV) energies from a compilation of Chandra surveys supplemented by wide-area surveys from ASCA and ROSAT. We consider the hard and soft X-ray samples separately and find that the XLF based on either (initially neglecting absorption effects) is best described by a new flexible model parametrization where the break luminosity, normalization, and faint-end slope all evolve with redshift. We then incorporate absorption effects, separately modelling the evolution of the XLFs of unabsorbed (20 < log N
H < 22) and absorbed (22 < log N
H < 24) AGNs, seeking a model that can reconcile both the hard- and soft-band samples. We find that the absorbed AGN XLF has a lower break luminosity, a higher normalization, and a steeper faint-end slope than the unabsorbed AGN XLF out to z ∼ 2. Hence, absorbed AGNs dominate at low luminosities, with the absorbed fraction falling rapidly as luminosity increases. Both XLFs undergo strong luminosity evolution which shifts the transition in the absorbed fraction to higher luminosities at higher redshifts. The evolution in the shape of the total XLF is primarily driven by the changing mix of unabsorbed and absorbed populations.
Abstract
We use deep Chandra X-ray imaging to measure the distribution of specific black hole accretion rates (LX relative to the stellar mass of the galaxy) and thus trace active galactic nucleus ...(AGN) activity within star-forming and quiescent galaxies, as a function of stellar mass (from 108.5 to 1011.5 M⊙) and redshift (to z ∼ 4). We adopt near-infrared-selected samples of galaxies from the CANDELS and UltraVISTA surveys, extract X-ray data for every galaxy, and use a flexible Bayesian method to combine these data and to measure the probability distribution function of specific black hole accretion rates, λsBHAR. We identify a broad distribution of λsBHAR in both star-forming and quiescent galaxies – likely reflecting the stochastic nature of AGN fuelling – with a roughly power-law shape that rises towards lower λsBHAR, a steep cut-off at λsBHAR ≳ 0.1–1 (in Eddington equivalent units), and a turnover or flattening at $\lambda _\mathrm{sBHAR} \lesssim 10^{-3}\hbox{ {to} }10^{-2}$. We find that the probability of a star-forming galaxy hosting a moderate λsBHAR AGN depends on stellar mass and evolves with redshift, shifting towards higher λsBHAR at higher redshifts. This evolution is truncated at a point corresponding to the Eddington limit, indicating black holes may self-regulate their growth at high redshifts when copious gas is available. The probability of a quiescent galaxy hosting an AGN is generally lower than that of a star-forming galaxy, shows signs of suppression at the highest stellar masses and evolves strongly with redshift. The AGN duty cycle in high-redshift (z ≳ 2) quiescent galaxies thus reaches ∼20 per cent, comparable to the duty cycle in star-forming galaxies of equivalent stellar mass and redshift.
Abstract
We map the co-eval growth of galaxies and their central supermassive black holes in detail by measuring the incidence of active galactic nuclei (AGNs) in galaxies as a function of star ...formation rate (SFR) and redshift (to $z$ ∼ 4). We combine large galaxy samples with deep Chandra X-ray imaging to measure the probability distribution of specific black hole accretion rates (LX relative to stellar mass) and derive robust AGN fractions and average specific accretion rates. First, we consider galaxies along the main sequence of star formation. We find a linear correlation between the average SFR and both the AGN fraction and average specific accretion rate across a wide range in stellar mass ($\mathcal {M}_*\sim 10^{8.5-11.5}\, \mathcal {M}_\odot$) and to at least $z$ ∼ 2.5, indicating that AGNs in main-sequence galaxies are driven by the stochastic accretion of cold gas. We also consider quiescent galaxies and find significantly higher AGN fractions than predicted, given their low SFRs, indicating that AGNs in quiescent galaxies are fuelled by additional mechanisms (e.g. stellar winds). Next, we bin galaxies according to their SFRs relative to the main sequence. We find that the AGN fraction is significantly elevated for galaxies that are still star forming but with SFRs below the main sequence, indicating further triggering mechanisms enhance AGN activity within these sub-main-sequence galaxies. We also find that the incidence of high accretion rate AGN is enhanced in starburst galaxies and evolves more mildly with redshift than within the rest of the galaxy population, suggesting mergers play a role in driving AGN activity in such high-SFR galaxies.
We model the luminosity-dependent projected two-point correlation function of DEEP2 ( unk similar to 1) and SDSS ( unk similar to 0) galaxies within the HOD framework. At both epochs, there is a ...tight correlation between central galaxy luminosity and halo mass, with the slope and scatter decreasing for larger halo masses, and the fraction of satellite galaxies decreases at higher luminosity. We find little evolution in the relation between mass scales of host halos for central galaxies and satellite galaxies above the same luminosity threshold. Combining these HOD results with theoretical predictions of the typical growth of halos, we establish an evolutionary connection between the galaxy populations at the two redshifts by linking unk similar to 0 central galaxies to z similar to 1 central galaxies that reside in their progenitor halos. We find that the stellar mass growth of galaxies depends on halo mass. On average, the majority of the stellar mass in central galaxies residing in z similar to 0 low-mass halos ( similar to 5 x 10 super(11)h super(-1)iM unk) and only a small fraction of the stellar mass in central galaxies of high-mass halos ( similar to 10 super(13) h super(-1)M unk) result from star formation between unk similar to 1 and unk similar to 0. In addition, the mass scale of halos where the star formation efficiency reaches a maximum is found to shift toward lower mass with time. Future work that combines HOD modeling of the clustering of galaxies at different redshifts with the assembly history and dynamical evolution of dark matter halos can lead to an understanding of the stellar mass growth due to both mergers and star formation as a function of host halo mass and provide powerful tests of galaxy formation theories.
We use deep Chandra 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 ... 1042 erg s-1 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 'X-ray main sequence' with a constant slope ...0.63 plus or minus 0.03 over ... and 0.1 ... z ... 4, with a normalization that increases with redshift as (1 + z)3.79 plus or minus 0.12. 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 ... SFR0.83 x (1 + z)1.3, 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 ...0.76 plus or minus 0.06 and a normalization that evolves with redshift as (1 + z)2.95 plus or minus 0.33. 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. (ProQuest: ... denotes formulae/symbols omitted.)
We present new observational determinations of the evolution of the 2–10 keV X-ray luminosity function (XLF) of active galactic nuclei (AGN). We utilize data from a number of surveys including both ...the 2 Ms Chandra Deep Fields and the AEGIS-X 200 ks 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 statistically account for X-ray sources without optical counterparts to correct for incompleteness in our samples. We also account for Poissonian effects on the X-ray flux estimates and sensitivities and thus correct for the Eddington bias. 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 that >50 per cent of black hole growth takes place at z > 1, with around half in LX < 1044 erg s−1 AGN.
In this the first of a series of Letters, we present a panchromatic data set in the Extended Groth Strip region of the sky. Our survey, the All-Wavelength Extended Groth Strip International Survey ...(AEGIS), aims to study the physical properties and evolutionary processes of galaxies at z 6 1. It includes the following deep, wide-field imaging data sets: Chandra/ACIS X-ray, GALEX ultraviolet, CFHT/MegaCam Legacy Survey optical, CFHT/CFH12K optical, Hubble Space Telescope/ACS optical and NICMOS near-infrared, Palomar/WIRC near-infrared, Spitzer/IRAC mid-infrared, Spitzer/MIPS far-infrared, and VLA radio continuum. In addition, mis region of the sky has been targeted for extensive spectroscopy using the Deep Imaging Multi-Object Spectrograph (DEIMOS) on the Keck II 10 m telescope. Our survey is compared to other large multiwavelength surveys in terms of depth and sky coverage.
Recent observations have revealed that starburst galaxies can drive molecular gas outflows through stellar radiation pressure. Molecular gas is the phase of the interstellar medium from which stars ...form, so these outflows curtail stellar mass growth in galaxies. Previously known outflows, however, involve small fractions of the total molecular gas content and have typical scales of less than a kiloparsec. In at least some cases, input from active galactic nuclei is dynamically important, so pure stellar feedback (the momentum return into the interstellar medium) has been considered incapable of rapidly terminating star formation on galactic scales. Molecular gas has been detected outside the galactic plane of the archetypal starburst galaxy M82 (refs 4 and 5), but so far there has been no evidence that starbursts can propel substantial quantities of cold molecular gas to the same galactocentric radius (about 10 kiloparsecs) as the warmer gas that has been traced by metal ion absorbers in the circumgalactic medium. Here we report observations of molecular gas in a compact (effective radius 100 parsecs) massive starburst galaxy at redshift 0.7, which is known to drive a fast outflow of ionized gas. We find that 35 per cent of the total molecular gas extends approximately 10 kiloparsecs, and one-third of this extended gas has a velocity of up to 1,000 kilometres per second. The kinetic energy associated with this high-velocity component is consistent with the momentum flux available from stellar radiation pressure. This demonstrates that nuclear bursts of star formation are capable of ejecting large amounts of cold gas from the central regions of galaxies, thereby strongly affecting their evolution by truncating star formation and redistributing matter.
We analyze star formation (SF) as a function of stellar mass (M sub(*)) and redshift z in the All-Wavelength Extended Groth Strip International Survey. For 2905 field galaxies, complete to 10 ...super(10)(10 super(10.8)) M sub((.)) at z < 0.7(1), with Keck spectroscopic redshifts out to z = 1.1, we compile SF rates (SFRs) from emission lines, GALEX, and Spitzer MIPS 24 km photometry, optical-NIR M sub(*) measurements, and HST morphologies. Galaxies with reliable signs of SF form a distinct "main sequence" (MS), with a limited range of SFRs at a given M sub(*) and z (1 s c0.3 dex), and log(SFR) approximately proportional to log M sub(*). The range of log (SFR) remains constant to z > 1, while the MS as a whole moves to higher SFR as z increases. The range of the SFR along the MS constrains the amplitude of episodic variations of SF and the effect of mergers on the SFR. Typical galaxies spend 667%(95%) of their lifetime since z = 1 within a factor of 2(4) of their average SFR at a given M sub(*) and z. The dominant mode of the evolution of SF since z 6 1 is apparently a gradual decline of the average SFR in most individual galaxies, not a decreasing frequency of starburst episodes, or a decreasing factor by which SFRs are enhanced in starbursts. LIRGs at z 6 1 seem to mostly reflect the high SFR typical for massive galaxies at that epoch. The smooth MS may reflect that the same set of few physical processes governs SF prior to additional quenching processes. A gradual process like gas exhaustion may play a dominant role.
We analyze star formation (SF) as a function of stellar mass (M sub(*)) and redshift z in the All-Wavelength Extended Groth Strip International Survey, for star-forming field galaxies with M sub(*) ...10 super(10) M sub((.)) out to z = 1.1. The data indicate that the high specific SF rates (SFRs) of many less massive galaxies do not represent late, irregular or recurrent, starbursts in evolved galaxies. They rather seem to reflect the onset (initial burst) of the dominant SF episode of galaxies, after which SF gradually declines on gigayear timescales to z = 0 and forms the bulk of a galaxy's M sub(*). With decreasing mass, this onset of major SF shifts to decreasing z for an increasing fraction of galaxies (staged galaxy formation). This process may be an important component of the "downsizing" phenomenon. We find that the predominantly gradual decline of SFRs described by Noeske et al. can be reproduced by exponential SF histories (t models), if less massive galaxies have systematically longer e-folding times t, and a later onset of SF (z sub(f)). Our model can provide a first parameterization of SFR as a function of M sub(*) and z, and quantify mass dependences of t and z sub(f) from direct observations of M sub(*) and SFRs up to z> 1. The observed evolution of SF in galaxies can plausibly reflect the dominance of gradual gas exhaustion. The data are also consistent with the history of cosmological accretion onto dark matter halos.
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