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Register a new userThe Angular Clustering of Infrared-Selected Obscured and Unobscured Quasars
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Recent studies of luminous infrared-selected active galactic nuclei (AGN) suggest that the reddest, most obscured objects display a higher angular clustering amplitude, and thus reside in higher-mass dark matter halos. This is a direct contradiction to the prediction of the simplest unification-by-orientation models of AGN and quasars. However, clustering measurements depend strongly on the mask that removes low-quality data and describes the sky and selection function. We find that applying a robust, conservative mask to WISE-selected quasars yields a weaker but still significant difference in the bias between obscured and unobscured quasars. These findings are consistent with results from previous Spitzer surveys, and removes any scale dependence of the bias. For obscured quasars with $langle z rangle = 0.99$ we measure a bias of $b_q = 2.67 pm 0.16$, corresponding to a halo mass of $log (M_h / M_{odot} h^{-1}) = 13.3 pm 0.1$, while for unobscured sources with $langle z rangle = 1.04$ we find $b_q = 2.04 pm 0.17$ with a halo mass $log (M_h / M_{odot} h^{-1} )= 12.8 pm 0.1$. This improved measurement indicates that WISE-selected obscured quasars reside in halos only a few times more massive than the halos of their unobscured counterparts, a reduction in the factor of $sim$10 larger halo mass as has been previously reported using WISE-selected samples. Additionally, an abundance matching analysis yields lifetimes for both obscured and unobscured quasar phases on the order of a few 100 Myr ($sim$ 1% of the Hubble time) --- however, the obscured phase lasts roughly twice as long, in tension with many model predictions.
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Clustering measurements of obscured and unobscured quasars show that obscured quasars reside in more massive dark matter halos than their unobscured counterparts. These results are inconsistent with simple unified (torus) scenarios, but might be explained by models in which the distribution of obscuring material depends on Eddington ratio or galaxy stellar mass. We test these possibilities by constructing simple physical models to compare to observed AGN populations. We find that previously observed relationships between obscuration and Eddington ratio or stellar mass are not sufficient reproduce the observed quasar clustering results ($langle log M_{text{halo}}/M_{odot} rangle = 12.94 ^{+ 0.10}_{- 0.11}$ and $langle log M_{text{halo}}/M_{odot} rangle = 12.49 ^{+ 0.08}_{- 0.08}$ for obscured and unobscured populations, respectively) while maintaining the observed fraction of obscured quasars (30-65$%$). This work suggests that evolutionary models, in which obscuration evolves on the typical timescale for black hole growth, are necessary to understand the observed clustering of mid-IR selected quasars.
We calculate the angular correlation function for a sample of 170,000 AGN extracted from the Wide-field Infrared Survey Explorer (WISE) catalog, selected to have red mid-IR colors (W1 - W2 > 0.8) and 4.6 micron flux densities brighter than 0.14 mJy). The sample is expected to be >90% reliable at identifying AGN, and to have a mean redshift of z=1.1. In total, the angular clustering of WISE-AGN is roughly similar to that of optical AGN. We cross-match these objects with the photometric SDSS catalog and distinguish obscured sources with (r - W2) > 6 from bluer, unobscured AGN. Obscured sources present a higher clustering signal than unobscured sources. Since the host galaxy morphologies of obscured AGN are not typical red sequence elliptical galaxies and show disks in many cases, it is unlikely that the increased clustering strength of the obscured population is driven by a host galaxy segregation bias. By using relatively complete redshift distributions from the COSMOS survey, we find obscured sources at mean redshift z=0.9 have a bias of b = 2.9 pm 0.6 and are hosted in dark matter halos with a typical mass of log(M/M_odot)~13.5. In contrast, unobscured AGN at z~1.1 have a bias of b = 1.6 pm 0.6 and inhabit halos of log(M/M_odot)~12.4. These findings suggest that obscured AGN inhabit denser environments than unobscured AGN, and are difficult to reconcile with the simplest AGN unification models, where obscuration is driven solely by orientation.
We present a spectroscopically complete sample of 147 infrared-color-selected AGN down to a 22 $mu$m flux limit of 20 mJy over the $sim$270 deg$^2$ of the SDSS Stripe 82 region. Most of these sources are in the QSO luminosity regime ($L_{rm bol} gtrsim 10^{12} L_odot$) and are found out to $zsimeq3$. We classify the AGN into three types, finding: 57 blue, unobscured Type-1 (broad-lined) sources; 69 obscured, Type-2 (narrow-lined) sources; and 21 moderately-reddened Type-1 sources (broad-lined and $E(B-V) > 0.25$). We study a subset of this sample in X-rays and analyze their obscuration to find that our spectroscopic classifications are in broad agreement with low, moderate, and large amounts of absorption for Type-1, red Type-1 and Type-2 AGN, respectively. We also investigate how their X-ray luminosities correlate with other known bolometric luminosity indicators such as [O III] line luminosity ($L_{rm [OIII]}$) and infrared luminosity ($L_{6 mu{rm m}}$). While the X-ray correlation with $L_{rm [OIII]}$ is consistent with previous findings, the most infrared-luminous sources appear to deviate from established relations such that they are either under-luminous in X-rays or over-luminous in the infrared. Finally, we examine the luminosity function (LF) evolution of our sample, and by AGN type, in combination with the complementary, infrared-selected, AGN sample of Lacy et al. (2013), spanning over two orders of magnitude in luminosity. We find that the two obscured populations evolve differently, with reddened Type-1 AGN dominating the obscured AGN fraction ($sim$30%) for $L_{5 mu{rm m}} > 10^{45}$ erg s$^{-1}$, while the fraction of Type-2 AGN with $L_{5 mu{rm m}} < 10^{45}$ erg s$^{-1}$ rises sharply from 40% to 80% of the overall AGN population.
We present our statistical study of near infrared (NIR) variability of X-ray selected Active Galactic Nuclei (AGN) in the COSMOS field, using UltraVISTA data. This is the largest sample of AGN light curves in YJHKs bands, making possible to have a global description of the nature of AGN for a large range of redshifts, and for different levels of obscuration. To characterize the variability properties of the sources we computed the Structure Function. Our results show that there is an anti-correlation between the Structure Function $A$ parameter (variability amplitude) and the wavelength of emission, and a weak anti-correlation between $A$ and the bolometric luminosity. We find that Broad Line (BL) AGN have a considerably larger fraction of variable sources than Narrow Line (NL) AGN, and that they have different distributions of the $A$ parameter. We find evidence that suggests that most of the low luminosity variable NL sources correspond to BL AGN, where the host galaxy could be damping the variability signal. For high luminosity variable NL, we propose that they can be examples of True type II AGN or BL AGN with limited spectral coverage which results in missing the Broad Line emission. We also find that the fraction of variable sources classified as unobscured in the X-ray is smaller than the fraction of variable sources unobscured in the optical range. We present evidence that this is related to the differences in the origin of the obscuration in the optical and X-ray regimes.
We present the first measurement of the spatial clustering of mid-infrared selected obscured and unobscured quasars, using a sample in the redshift range 0.7 < z < 1.8 selected from the 9 deg^2 Bootes multiwavelength survey. Recently the Spitzer Space Telescope and X-ray observations have revealed large populations of obscured quasars that have been inferred from models of the X-ray background and supermassive black hole evolution. To date, little is known about obscured quasar clustering, which allows us to measure the masses of their host dark matter halos and explore their role in the cosmic evolution of black holes and galaxies. In this study we use a sample of 806 mid-infrared selected quasars and ~250,000 galaxies to calculate the projected quasar-galaxy cross-correlation function w_p(R). The observed clustering yields characteristic dark matter halo masses of log (M_halo [h^-1 M_sun]) = 12.7^+0.4_-0.6 and 13.3^+0.3_-0.4 for unobscured quasars (QSO-1s) and obscured quasars (Obs-QSOs), respectively. The results for QSO-1s are in excellent agreement with previous measurements for optically-selected quasars, while we conclude that the Obs-QSOs are at least as strongly clustered as the QSO-1s. We test for the effects of photometric redshift errors on the optically-faint Obs-QSOs, and find that our method yields a robust lower limit on the clustering; photo-z errors may cause us to underestimate the clustering amplitude of the Obs-QSOs by at most ~20%. We compare our results to previous studies, and speculate on physical implications of stronger clustering for obscured quasars.
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