CHASING HIGHLY OBSCURED QSOs IN THE COSMOS FIELD

By F. Fiore, S. Puccetti, M. Brusa, Mara Salvato, G. Zamorani, Tom Aldcroft, H. Aussel, H. Brunner, P. Capak, N. Cappelluti and 34 more. 2008

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A large population of heavily obscured, Compton-thick active galactic nuclei (AGNs) is predicted by AGN synthesis models for the cosmic X-ray background and by the “relic” supermassive black hole mass function measured from local bulges. However, even the deepest X-ray surveys are inefficient to search for these elusive AGNs. Alternative selection criteria, combining mid-infrared with near-infrared, and optical photometry, have instead been successful in pinpointing a large population of Compton-thick AGNs. We take advantage of the deep Chandra and Spitzer coverage of a large area (more than 10 times the area covered by the Chandra deep fields, CDFs) in the Cosmic Evolution Survey (COSMOS) field to extend the search of highly obscured, Compton-thick active nuclei to higher luminosity. These sources have low surface density, and therefore large samples can be provided only through large area surveys, like the COSMOS survey. We analyze the X-ray properties of COSMOS MIPS sources with 24 μm fluxes higher than 550 μJy. For the MIPS sources not directly detected in the Chandra images, we produce stacked images in soft and hard X-rays bands. To estimate the fraction of Compton-thick AGN in the MIPS source population, we compare the observed stacked count rates and hardness ratios to those predicted by detailed Monte Carlo simulations, including both obscured AGN and star-forming galaxies. The volume density of Compton-thick QSOs (log L(2–10 keV) = 44–45 erg s−1, or logλLλ(5.8 μm) = 44.79–46.18 erg s−1 for a typical infrared to X-ray luminosity ratio) evaluated in this way is (4.8 ± 1.1) × 10−6 Mpc−3 in the redshift bin 1.2–2.2. This density is ∼44% of all X-ray-selected QSOs in the same redshift and luminosity bin, and it is consistent with the expectation of the most up-to-date AGN synthesis models for the cosmic X-ray background (Gilli et al. ). The density of lower luminosity Compton-thick AGNs (log L(2–10 keV) = 43.5–44) at z = 0.7–1.2 is (3.7 ± 1.1) × 10−5 Mpc−3, corresponding to ∼67% of X-ray-selected AGNs. The comparison between the fraction of infrared-selected, Compton-thick AGNs to the X-ray selected, unobscured, and moderately obscured AGNs at high and low luminosity suggests that Compton-thick AGNs follow a luminosity dependence similar to that discovered for Compton-thin AGNs, becoming relatively rarer at high luminosities. We estimate that the fraction of AGNs (unobscured, moderately obscured, and Compton thick) to the total MIPS source population is 49 ± 10%, a value significantly higher than that previously estimated at similar 24 μm fluxes. We discuss how our findings can constrain AGN feedback models.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/73df3a82079fc925e567c51b59047e79a07592d4

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Observational evidence for intermediate-mass black holes (2017, background): “…be explained by a low BH occupation fraction, by smaller BHs predicted by the MBH −M∗ relation (Reines & Volonteri 2015; Volonteri & Reines 2016) than by the MBH −Mbulge relations, or by heavy obscuration (e.g., Gilli et al. 2007; Fiore et al. 2009; Treister et al. 2009; Cappelluti et al. 2017).”
OBSCURATION-DEPENDENT EVOLUTION OF ACTIVE GALACTIC NUCLEI (2015, background): “…Akylas & Georgantopoulos 2009; Gilli et al. 2010; Vignali et al. 2010; Jia et al. 2013; Mignoli et al. 2013; Vignali et al. 2014), infrared (e.g. Fiore et al. 2008, 2009; Alexander et al. 2011; Brightman & Nandra 2011a,b) and hard X-ray (e.g. Sazonov et al. 2008; Burlon et al. 2011; Alexander…”
Cosmic X-ray surveys of distant active galaxies (2015, background): “High 24 µm-to-optical flux ratios and red optical/NIR colors (e.g., Fiore et al 2008, 2009; Treister et al 2009a).”
Enhanced star formation rates in AGN hosts with respect to inactive galaxies from PEP-Herschel observations (2012, background): “A sizeable fraction of very MIR bright galaxies, usually selected from their 24µm excess, are likely to be highly obscured AGNs, which are too dust enshrouded to be detected by X-ray surveys (e.g. Daddi et al. 2007a; Georgantopoulos et al. 2008; Fiore et al. 2008, 2009).”
A mature cluster with X-ray emission at z = 2.07 (2010, background): “On the other hand, the 24µm emission (which atz ∼ 2 corresponds to 8µm rest-frame) might be due to extremely obscured AGN activity (Fiore et al. 2009, Daddi et al. 2007b), an interpretation supported by the presence among the MIPS 24 µm sources of the Chandra-detected AGN.”
THE XMM-NEWTON WIDE-FIELD SURVEY IN THE COSMOS FIELD (XMM-COSMOS): DEMOGRAPHY AND MULTIWAVELENGTH PROPERTIES OF OBSCURED AND UNOBSCURED LUMINOUS ACTIVE GALACTIC NUCLEI (2010, background): “Given the high X–ray column density, most of these sources may remain undetected even in the deepest X–ray exposures and their AGN nature can be unvealed only through stacking analysis (Daddi et al. 2007, Fiore et al. 2008, 2009, Lanzuisi et al. 2009).”
A physical model for z∼ 2 dust-obscured galaxies (2009, background): “…these galaxies as dust-obscured galaxies (DOGs).1 DOGs exhibit a range of luminosities, though AGN-dominated galaxies tend to preferentially dominate the bright end of the 24- μm flux density distribution (Brand et al. 2006; Weedman et al. 2006a,b; Fiore et al. 2008, 2009; Sacchi et al. 2009; D08).”
IDENTIFYING LUMINOUS ACTIVE GALACTIC NUCLEI IN DEEP SURVEYS: REVISED IRAC SELECTION CRITERIA (2012, background): “While many MIR-based selection criteria are therefore designed to specifically target heavily obscured AGN (e.g., Daddi et al. 2007b; Fiore et al. 2008, 2009), Spitzer IRAC selection is sensitive to the hot dust signature present in ∼> 80 − 95% of luminous AGN regardless of obscuration (Hao et al.…”
TOWARD THE STANDARD POPULATION SYNTHESIS MODEL OF THE X-RAY BACKGROUND: EVOLUTION OF X-RAY LUMINOSITY AND ABSORPTION FUNCTIONS OF ACTIVE GALACTIC NUCLEI INCLUDING COMPTON-THICK POPULATIONS (2014, methodology): “More recent results reported by Fiore et al. (2009) from the COSMOS data (filled squares) at z = 0.7− 1.2 (log LX > 43.5) and atz = 1.2− 2.2 (log LX > 44) are within a factor of∼2 from our baseline model, which would be acceptable by considering possible uncertainties in the luminosity range of the…”
X-shooter reveals powerful outflows in z ∼ 1.5 X-ray selected obscured quasi-stellar objects (2014, background): “…the entire XMM-COSMOS sample, on the basis of their observed-frame mid–infrared (flux24µm/O> 1000), near–infrared (R-K> 5 or R-[3.6]> 4), optical and X-ray (X/O> 10) properties (following Fiore et al. 2003; Brusa et al. 2005; Mignoli et al. 2004; Fiore et al. 2009, Melini et al. in preparation).”
What drives the growth of black holes (2011, background): “…where the absorbing column densities are so high that not even X-ray photons can escape (e.g., Lacy et al., 2004; Stern et al., 2005; Donley et al., 2005, 2008; Alonso-Herrero et al., 2006; Daddi et al., 2007a; Hickox et al., 2007; Fiore et al., 2008, 2009; Bauer et al., 2010; Luo et al., 2011).”
THE SPACE DENSITY OF COMPTON-THICK ACTIVE GALACTIC NUCLEUS AND THE X-RAY BACKGROUND (2009, methodology): “This is in agreement with the conclusions of Fiore et al. (2009), who found a similar decrease in CT AGN with increasing luminosity in their high-redshift IR-selected sample.”
GOODS-Herschel: dust attenuation properties of UV selected high redshift galaxies (2012, methodology): “To check this issue, we use the F(24)/F(R) flux ratio: highly obscured Compton thick AGN are expected to exhibit F(24)/F(R) flux ratios larger than few hundreds (Fiore et al., 2009).”
Defining the intrinsic AGN infrared spectral energy distribution and measuring its contribution to the infrared output of composite galaxies (2011, methodology): “…rely on either observed AGN SEDs that likely include considerable amounts of host-galaxy contamin tion at infrared wavelengths or predictions from radiative transfer models which are often poorly constrained (e.g. Polletta et al.2007; Fiore et al. 2008, 2009; Pozzi et al. 2010; Hatziminaoglou etal.”
Galaxy Interactions Trigger Rapid Black Hole Growth: an unprecedented view from the Hyper Suprime-Cam Survey (2017, methodology): “…luminosities for our sample are computed by fitting powerlaw slopes to the mid-IR photometry of the AGN, and using the best-fit slope to predict the rest-frame 6µm continuum luminosity, which is shown to be a robust indicator of LAGN (e.g., Lutz et al. 2004; Fiore et al. 2009; Chen et al. 2017).”