59 research outputs found
Black hole variability and the star formation-active galactic nucleus connection : do all star-forming galaxies host an active galactic nucleus?
We investigate the effect of active galactic nucleus (AGN) variability on the observed connection between star formation and black hole accretion in extragalactic surveys. Recent studies have reported relatively weak correlations between observed AGN luminosities and the properties of AGN hosts, which has been interpreted to imply that there is no direct connection between AGN activity and star formation. However, AGNs may be expected to vary significantly on a wide range of timescales (from hours to Myr) that are far shorter than the typical timescale for star formation (gsim100 Myr). This variability can have important consequences for observed correlations. We present a simple model in which all star-forming galaxies host an AGN when averaged over ~100 Myr timescales, with long-term average AGN accretion rates that are perfectly correlated with the star formation rate (SFR). We show that reasonable prescriptions for AGN variability reproduce the observed weak correlations between SFR and L AGN in typical AGN host galaxies, as well as the general trends in the observed AGN luminosity functions, merger fractions, and measurements of the average AGN luminosity as a function of SFR. These results imply that there may be a tight connection between AGN activity and SFR over galaxy evolution timescales, and that the apparent similarities in rest-frame colors, merger rates, and clustering of AGNs compared to "inactive" galaxies may be due primarily to AGN variability. The results provide motivation for future deep, wide extragalactic surveys that can measure the distribution of AGN accretion rates as a function of SFR
Inferring a difference in the star-forming properties of lower versus higher X-ray luminosity AGNs
We explore the distribution of RMS ⥠SFR/SFRMS (where SFRMS is the star formation rate of âmain-sequenceâ star-forming galaxies) for AGN hosts at z = 1. We split our sample into two bins of X-ray luminosity divided at LX = 2 Ă 1043 ergâsâ1 to investigate whether the RMS distribution changes as a function of AGN power. Our main results suggest that, when the RMS distribution of AGN hosts is modelled as a log-normal distribution (i.e. the same shape as that of MS galaxies), galaxies hosting more powerful X-ray AGNs (i.e. LX > 2 Ă 1043 ergâsâ1) display a narrower RMS distribution that is shifted to higher values compared to their lower LX counterparts. In addition, we find that more powerful X-ray AGNs have SFRs that are more consistent with that of MS galaxies compared to lower LX AGNs. Despite this, the mean SFRs (as opposed to RMS) measured from these distributions are consistent with the previously observed flat relationship between SFR and LX. Our results suggest that the typical star-forming properties of AGN hosts change with LX, and that more powerful AGNs typically reside in more MS-like star-forming galaxies compared to lower LX AGNs
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 XMMâNewton 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 (2â8âkeV), 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 12âÎŒm luminosity to the observed 2â10âkeV 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 (64âperâcent; out of the 14, for which X-ray spectra could be fitted) based on the X-ray spectra, and seven (78âperâcent; out of the nine detected spectroscopically in the SDSS) based on the [OâIII] line fluxes. Combining all criteria, we determine the final number of heavily obscured AGNs to be 12â19, and the number of Compton-thick AGNs to be 2â5, 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 opticalâinfrared 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 (L2â10keV<1044ergsâ1), even though most of them are located in the âquasi stellar object (QSO) locusâ. However, a selection scheme based on a relatively low X-ray luminosity and mid-infrared colours characteristic of QSOs would not select âŒ25âperâcent of the heavily obscured AGNs of our sample
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 Ă (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%â6.5% (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 â , 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
Mid-infrared luminous quasars in the GOODSâHerschel fields: a large population of heavily obscured, Compton-thick quasars at z â 2
We present the infrared (IR) and X-ray properties of a sample of 33 mid-IR luminous quasars (ÎœL6âÎŒm â„ 6 Ă 1044âergâsâ1) at redshift z â 1â3, identified through detailed spectral energy distribution analyses of distant star-forming galaxies, using the deepest IR data from Spitzer and Herschel in the GOODSâHerschel fields. The aim is to constrain the fraction of obscured, and Compton-thick (CT, NH > 1.5 Ă 1024âcmâ2) quasars at the peak era of nuclear and star formation activities. Despite being very bright in the mid-IR band, â30 perâcent of these quasars are not detected in the extremely deep 2 and 4 Ms Chandra X-ray data available in these fields. X-ray spectral analysis of the detected sources reveals that the majority (â67 perâcent) are obscured by column densities NH > 1022âcmâ2; this fraction reaches â80 perâcent when including the X-ray-undetected sources (9 out of 33), which are likely to be the most heavily obscured, CT quasars. We constrain the fraction of CT quasars in our sample to be â24â48 perâcent, and their space density to be Ί = (6.7 ± 2.2) Ă 10â6âMpcâ3. From the investigation of the quasar host galaxies in terms of star formation rates (SFRs) and morphological distortions, as a sign of galaxy mergers/interactions, we do not find any direct relation between SFRs and quasar luminosity or X-ray obscuration. On the other hand, there is tentative evidence that the most heavily obscured quasars have, on average, more disturbed morphologies than the unobscured/moderately obscured quasar hosts, which preferentially live in undisturbed systems. However, the fraction of quasars with disturbed morphology amongst the whole sample is â40 perâcent, suggesting that galaxy mergers are not the main fuelling mechanism of quasars at z â 2
The effect of active galactic nuclei on the cold interstellar medium in distant star-forming galaxies
In the framework of a systematic study with the ALMA interferometer of IR-selected main-sequence and starburst galaxies at zââŒâ1â
ââ
1.7 at typical âŒ1âł resolution, we report on the effects of mid-IR- and X-ray-detected active galactic nuclei (AGN) on the reservoirs and excitation of molecular gas in a sample of 55 objects. We find widespread detectable nuclear activity in âŒ30% of the sample. The presence of dusty tori influences the IR spectral energy distribution of galaxies, as highlighted by the strong correlation among the AGN contribution to the total IR luminosity budget (fAGNâ=âLIR,ââAGN/LIR), its hard X-ray emission, and the Rayleigh-Jeans to mid-IR (S1.2âmm/S24âÎŒm) observed color, with evident consequences on the ensuing empirical star formation rate estimates. Nevertheless, we find only marginal effects of the presence and strength of AGN on the carbon monoxide CO (Jâ=â2,â4,â5,â7) or neutral carbon ([C I](3P1ââ
ââ
â3P0), [C I](3P2ââ
ââ
â3P1)) line luminosities and on the derived molecular gas excitation as gauged by line ratios and the full spectral line energy distributions. The [C I] and CO emission up to Jâ=â5,â7 thus primarily traces the properties of the host in typical IR luminous galaxies. However, our analysis highlights the existence of a large variety of line luminosities and ratios despite the homogeneous selection. In particular, we find a sparse group of AGN-dominated sources with the highest LIR,ââAGN/LIR,ââSFR ratios, âł3, that are more luminous in CO (5â4) than what is predicted by the LâČCO(5-4)âLIR,âSFR relation, which might be the result of the nuclear activity. For the general population, our findings translate into AGN having minimal effects on quantities such as gas and dust fractions and star formation efficiencies. If anything, we find hints of a marginal tendency of AGN hosts to be compact at far-IR wavelengths and to display 1.8 times larger dust optical depths. In general, this is consistent with a marginal impact of the nuclear activity on the gas reservoirs and star formation in average star-forming AGN hosts with LIRâ>â5â
Ăâ
1011âLâ, typically underrepresented in surveys of quasars and submillimeter galaxies
X-Ray Spectral Constraints for z â 2 Massive Galaxies: The Identification of Reflection-dominated Active Galactic Nuclei
We use the 4 Ms Chandra Deep Field-South (CDF-S) survey to place direct constraints on the ubiquity of z 2 heavily obscured active galactic nuclei (AGNs) in K 10 keV observatories. On the basis of these analyses, we estimate the space density for typical (intrinsic X-ray luminosities of L 2-10 keV 1043 erg sâ1) heavily obscured and Compton-thick AGNs at z 2. Our space-density constraints are conservative lower limits but they are already consistent with the range of predictions from X-ray background models
NuSTAR J033202-2746.8: Direct Constraints on the Compton Reflection in a Heavily Obscured Quasar at z â 2
We report Nuclear Spectroscopic Telescope Array (NuSTAR) observations of NuSTAR J033202-2746.8, a heavily obscured, radio-loud quasar detected in the Extended Chandra Deep Field-South, the deepest layer of the NuSTAR extragalactic survey (~400 ks, at its deepest). NuSTAR J033202-2746.8 is reliably detected by NuSTAR only at E > 8 keV and has a very flat spectral slope in the NuSTAR energy band (; 3-30 keV). Combining the NuSTAR data with extremely deep observations by Chandra and XMM-Newton (4 Ms and 3 Ms, respectively), we constrain the broad-band X-ray spectrum of NuSTAR J033202-2746.8, indicating that this source is a heavily obscured quasar ( cmâ2) with luminosity L 10-40 keV â 6.4 Ă 1044 erg sâ1. Although existing optical and near-infrared (near-IR) data, as well as follow-up spectroscopy with the Keck and VLT telescopes, failed to provide a secure redshift identification for NuSTAR J033202-2746.8, we reliably constrain the redshift z = 2.00 ± 0.04 from the X-ray spectral features (primarily from the iron K edge). The NuSTAR spectrum shows a significant reflection component (), which was not constrained by previous analyses of Chandra and XMM-Newton data alone. The measured reflection fraction is higher than the R ~ 0 typically observed in bright radio-loud quasars such as NuSTAR J033202-2746.8, which has L 1.4 GHz â 1027 W Hzâ1. Constraining the spectral shape of active galactic nuclei (AGNs), including bright quasars, is very important for understanding the AGN population, and can have a strong impact on the modeling of the X-ray background. Our results show the importance of NuSTAR in investigating the broad-band spectral properties of quasars out to high redshift
The NuSTAR Serendipitous Survey: Hunting for the Most Extreme Obscured AGN at >10 keV
We identify sources with extremely hard X-ray spectra (i.e., with photon indices of ) in the 13 deg2 NuSTAR serendipitous survey, to search for the most highly obscured active galactic nuclei (AGNs) detected at . Eight extreme NuSTAR sources are identified, and we use the NuSTAR data in combination with lower-energy X-ray observations (from Chandra, Swift XRT, and XMM-Newton) to characterize the broadband (0.5â24 keV) X-ray spectra. We find that all of the extreme sources are highly obscured AGNs, including three robust Compton-thick (CT; cmâ2) AGNs at low redshift () and a likely CT AGN at higher redshift (z = 0.16). Most of the extreme sources would not have been identified as highly obscured based on the low-energy ( keV) X-ray coverage alone. The multiwavelength properties (e.g., optical spectra and X-rayâmid-IR luminosity ratios) provide further support for the eight sources being significantly obscured. Correcting for absorption, the intrinsic rest-frame 10â40 keV luminosities of the extreme sources cover a broad range, from to 1045 erg sâ1. The estimated number counts of CT AGNs in the NuSTAR serendipitous survey are in broad agreement with model expectations based on previous X-ray surveys, except for the lowest redshifts (), where we measure a high CT fraction of . For the small sample of CT AGNs, we find a high fraction of galaxy major mergers (50% ± 33%) compared to control samples of "normal" AGNs
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