IAU 304: Multiwavelegth AGN Surveys and Studies Proceedings IAU Symposium No. xxx, 2014 (cid:13)c 2014International AstronomicalUnion A. Mickaelian, F. Aharonian, D. Sanders Editor, eds. DOI:00.0000/X000000000000000X Obscured accretion from AGN surveys Cristian Vignali1,2 1 Dipartimento di Fisica e Astronomia, Universit`adi Bologna, Viale Berti Pichat 6/2, 40127 Bologna, Italy 2 INAF–Osservatorio Astronomico di Bologna, Via Ranzani1, 40127 Bologna, Italy email: [email protected] Abstract. Recent modelsof super-massiveblack hole(SMBH) andhost galaxy joint evolution 4 predict thepresenceof a keyphasewhere accretion, traced byobscured ActiveGalactic Nuclei 1 (AGN) emission, is coupled with powerful star formation. Then feedback processes likely self- 0 regulate the SMBH growth and quench the star-formation activity. AGN in this important 2 evolutionary phase have been revealed in the last decade via surveys at different wavelengths. n Ontheonehand,moderate-to-deep X-raysurveyshaveallowed a systematicsearch for heavily a obscured AGN, up to very high redshifts (z≈5). On the other hand, infrared/optical surveys J have been invaluable in offering complementary methods to select obscured AGN also in cases 0 where the nuclear X-ray emission below 10 keV is largely hidden to our view. In this review I 2 will present my personal perspective of thefield of obscured accretion from AGN surveys. ] A G 1. Introduction . h One ofthe mainscience goalsofmodernobservationalcosmologyis devotedto under- p stand how galaxies and SMBHs at their centers grow together. Their close link leaves - imprintsinseveralrelationsobservedinthelocalUniversebetweenthemassoftheblack o r holes and the properties of the host galaxies(e.g., their velocity dispersion;Gebhardt et st al. 2000; Ferrarese & Merritt 2000). The emerging picture is that AGN are the key to a understand the nature of such close connection, since the mass function of local SMBHs [ can be reasonably explained by the growth of seed black holes (whatever the origin of 1 such seeds is) during AGN phases (e.g., Soltan 1982; Marconi et al. 2004). v Theentirepicture,relatedtotheso-calledAGN-galaxyco-evolutionscenario,hasbeen 1 presentedinmanyworksoverthelastdecade,andhasbeenperfectlysynthesizedinFig.1 6 of Hopkins et al. (2008), along the path traced by the original suggestion of Sanders et 0 5 al. 1988 (see also Sanders & Mirabel 1996). Concisely, current quasar/host galaxy co- . evolutionmodels predict the existence of a dust-enshroudedphase associatedwith rapid 1 SMBH growth and active star formation, largely triggered by multiple galaxy mergers 0 4 andencounters(e.g.,Silk&Rees1998;DiMatteoetal.2005;Mencietal.2008;Zubovas 1 & King 2012; Lamastra et al. 2013). This phase is likely associated to obscured AGN : growth in strongly star-forming (sub-millimeter) galaxies (e.g., Alexander et al. 2005). v i Finally,massivequasar-drivenoutflowsblowawaymostofthecoldgasreservoir,creating X a population of “red-and-dead” gas-poor elliptical galaxies (e.g., Cattaneo et al. 2009). r Support to this scenario comes from observations of wide-angle molecular outflows a extending few kpc from the nucleus in some quasars hosted in ultra-luminous infrared galaxies; these systems, typically characterized by mass loss rates much larger than the ongoingstar-formationrate(e.g.,Feruglioetal.2010;Sturmetal.2011;Rupke&Veilleux 2013; Cicone et al. 2013), are observed up to very high redshifts (Maiolino et al. 2012; Borguet et al. 2013). Similarly, observations of powerful outflows in neutral and ionized gashavealsobeencollectedoverthepastfewyears(e.g.,Nesvadbaetal.2008;Alexander et al. 2010; Harrison et al. 2012). This feedback process ascribed to quasars is most 1 2 Cristian Vignali certainly related to radiation-driven winds and is often invoked to explain why SMBHs and galaxies stop growing at a certain phase of their life; for a more comprehensive discussion on this issue, see the review by C.M. Harrison (this Volume). Evidences for ultra-fast outflows (with velocities typically up to 0.1–0.4c)have been recently observed inX-raysinasizablesampleofAGN,bothinthelocalUniverse(e.g.,Tombesietal.2010, 2011,2012;Goffordet al.2013;Reeves et al.2003)and athigh redshift (e.g.,Chartas et al.2002,2007;Saezetal.2009).Theconnectionbetweenmolecularandhighlyionizedgas is, however,from from being assessed, and will constitute undoubtedly one of the prime science goals of the coming years using ALMA and IRAM facilities at long wavelengths and Chandra and XMM-Newton in the X-ray domain. Accordingtothescenariodescribedabove,themaintriggermechanismofBHaccretion andgrowthis ascribedto galaxymergersandinteractions,atleastinthe mostluminous and massive systems. Most of their mass is assembled in short periods (≈10–100 Myr) of “bursting” nuclear and star-forming activity, while the bulk of galaxies and SMBHs grow their mass in a secular (i.e., “smooth”) mode over timescales of Gyrs (e.g., Daddi et al. 2007a; Hickox et al. 2009). This picture has recently been confirmed by Herschel surveys, showing a distinction between the bulk of galaxies growing quietly (in the so- called“mainsequence”)andtheminorityofthegalaxypopulationwhosegrowthhappens mostly during events of mergers of gas-rich galaxies in the so-called “starburst mode” (e.g., Elbaz et al. 2011; Rodighiero et al. 2011; see also Rosario et al. 2013). As a natural consequence of the merger scenario,a key phase in the AGN and galaxy life iswhenlargeamountsofgasarefunneled tothe center,thusinducing bothobscured accretion and star formation (e.g., Treister et al. 2010). Significant efforts have been made recently to search for and characterize, as much as possible, the most heavily obscured AGN and quasars, dubbed Compton thick, characterized by column densities above1.5×1024cm−2 (seeComastri2004forareview);suchabsorbersstronglylimitthe possibility for these sources of being detected at energies below 10 keV (where sensitive X-ray imaging instruments are currently operative). Therefore, in order to provide a census as complete as possible of this source population, a multi-wavelength synergistic approach is needed. InthisreviewIwillfocusonsomeaspectsandmethodsofinvestigationthatIthinkare importantinthequestforheavilyobscuredAGN.Assuch,thisproceedingisnotmeantto provideanexhaustiveviewofthis topic.Furtherand,possibly,alternativeapproachesin this researchfieldandconsequencesfor AGNsynthesismodelsfor the X-raybackground (XRB) are addressed by other authors in this Volume (e.g., A. Barger, A. Del Moro, S. Juneau, A. Levenson, S. Mateos, L. Spinoglio, D. Stern, E. Treister, Y. Ueda). 2. Searching for heavily obscured AGN The problem of finding heavily obscured AGN and quasars can be tackled following various prescriptions and adopting different approaches. The bad news is that there is no way to obtain a complete census of this AGN population either using single-band observations or a unique selection method/criterion. The good news is that the multi- wavelength observing campaigns which characterize most of the current surveys offer a unique possibility to detect the most obscured AGN, up to very high redshifts. Adopt- ing several selection criteria and keeping in mind the observational biases intrinsic to each detection band are what we need in the future to infer the demographics of these elusive AGN and use them to provide “boundary” conditions and useful constraints to AGN/galaxy co-evolution models. In the following, I will try to elucidate some detection techniques adopted to find Obscured accretion from AGN surveys 3 Optical emission lines [OIII]5007Å,[NeV]3426Å NLR Mid-IR selection AGN disc emission reprocessed by the torus BLR torus torus accretion disc BLR NLR Figure1.SchematicviewofAGN(notinscale).Emphasisisgiventotheemissioncomponents which,at differentwavelengths, allow for thedetection ofobscured AGN.BLR andNLRstand for broad-line region and narrow-line region, respectively. obscuredAGN,whichareschematizedinFig.1.Inparticular,I amreferringtomethods related to X-ray (§2.1), mid-infrared (mid-IR; §2.2) and optical selection (§2.3). 2.1. Hard X-ray surveys According to the unified model for AGN (Antonucci 1993), the X-ray emission, once it interceptstheobscuringmaterial(i.e.,thetorus;seeFig.1),canbeprofoundlydepressed in the X-rayband. In particular,if the opticaldepth for Compton scattering(τ =NH× σT) does not exceed values of the order of “a few”, X-ray photons with energies higher than10–15keVareable topenetrate the obscuringmaterialandreachthe observer.For higher values of τ, the entire X-ray spectrum is depressed by Compton down-scattering and the X-ray photons are effectively trapped by the obscuring material irrespective of their energy. The former class of sources (mildly Compton thick) can be efficiently detected by X-ray instruments above 10 keV, while for the latter (heavily Compton thick) their nature may be inferred through indirect arguments, such as the presence of a strong iron Kα emission line over a flat reflected continuum. Mildly Compton- thick AGN are the most promising candidates to explain the residual (i.e., not resolved yet) spectrum of the cosmic XRB at its 30 keV peak (e.g., Worsley et al. 2005; Gilli et al. 2007; Ballantyne 2009; Treister et al. 2009; Moretti et al. 2012; Shi et al. 2013) but only a handful of them are known (i.e., have been classified as such beyond any reasonable doubt) outside the local Universe (e.g., Iwasawa et al. 2005). An unbiased censusofextremelyobscuredAGNwouldrequiretosurveythehardX-rayabove10keV with a fairly good sensitivity. A step forward in this direction is being provided by the Swift/BAT and Integral/IBIS surveys (e.g., Tueller et al. 2008; Beckmann et al. 2009; Vasudevan et al. 2013), which have covered a large portion of the sky though limited to relatively bright X-ray fluxes (≈ 10−11 erg cm−2 s−1), hence to low redshifts, and 4 Cristian Vignali have resolved less than 10% of the XRB. The spectral characterization of the heavily obscured AGN discovered in these shallow hard X-ray surveys often required follow-up observations with the more sensitive instruments onboard Chandra, XMM-Newton and Suzaku (e.g., Eguchi et al. 2009; Comastri et al. 2010; Winter et al. 2010; Severgnini et al. 2011;Burlon et al. 2011);this approachled to an estimate of a fractionof ≈ 10–20% of Compton-thick AGN among hard X-ray selected samples (e.g., Malizia et al. 2009; Burlon et al. 2011; Vasudevan et al. 2013). Data from the NuSTAR satellite, having imaging capabilities up to ≈ 80 keV, can shed new light on this topic at sensitivities more than a factor 100 better than those achieved by Integral and Swift (Alexander et al. 2013). Deep X-ray surveyswith sensitive imaging instruments (Chandra and XMM-Newton) can push the detection of Compton-thick AGN at considerably higher redshifts (e.g., z = 4.75, Gilli et al. 2011). Indications of Compton-thick material in AGN and quasars have been found by many authors, often coupled to powerful star formation (from few hundredto≈1000M⊙/yr),mostlyusingthedeepexposuresintheChandra DeepField- South(CDF-S)providedbybothChandra (currently4Ms–Xueetal.2011–closetobe extendedto7Ms)andXMM-Newton (≈ 3Ms;Ranallietal.2013);see,e.g.,Tozzietal. (2006);Georgantopoulosetal.(2009,2013);Comastrietal.(2011);Feruglioetal.(2011); Brightman & Ueda (2012); Vito et al. (2013). For a significant fraction of X-ray sources found indeep fields,the signal-to-noiseratio ofthe spectra is limited and does notallow for a proper characterizationof the source spectral complexities. Further constraints on the obscured AGN population may be derived using X-ray stacking techniques which take benefit of the good spatial resolution (primarily offered by Chandra) and allow explorationofconsiderablydeeperX-rayfluxes(e.g.,Xueetal.2012).However,eventhe deepest X-ray exposures currently available miss a significant number of very obscured AGN, hence a not negligible fraction of the accretion power in the Universe. Another interesting result which is emerging from deep X-ray surveys is related to the increasing fraction of heavily obscured quasars from z=0 to z≈3–4; a similar trend is apparently not observed in lower luminosity AGN (Iwasawa et al. 2012; Vito et al. 2013).Since the fractionofAGN in mergersseems to increasewith the bolometric lumi- nosity (Treister et al. 2012), we may expect that at high redshift, when the merger rate was higher, a larger gas fraction (producing obscuration) was available in galaxies. The planned extension of Chandra observations in the CDF-S, coupled to very deep infrared data (e.g., CANDELS), will hopefully allow us to explore this hypothesis at very high redshifts in a couple of years. 2.2. Mid-infrared selection The mid-IR regime offers much potential for discovery of heavily obscured AGN, since anyprimaryAGNcontinuum(i.e.,discemission)thatis absorbedmustultimatelycome out at these wavelengths after being thermally reprocessed by the torus (see Fig. 1). Thus, sources with weak emission in the optical band (because of extinction) and rela- tivelybrightmid-IRemissioncanbecountedasheavilyobscuredAGNcandidates,unless a significant contribution in the mid-IR comes from star-formation processes (PAH fea- tures and continuum emission). This probably “basic” high mid-IR/optical flux-ratio selection method found support in many works in the era of the Spitzer observatory (e.g., Mart´ınez-Sansigre et al. 2005; Houck et al. 2005; Weedman et al. 2006), and al- lowed Dey et al. (2008) to define a new class of sources at z ≈ 2, the dusty obscured galaxies (DOGs), having F24µm/FR > 1000. Among these, we may expect some of the most obscured AGN, especially if a selection at F24µm > 1 mJy is adopted to limit the contamination from star-forming galaxies (e.g., Sacchi et al. 2009). This selection is Obscured accretion from AGN surveys 5 different from those allowed by the widely adopted mid-IR color-color diagrams (e.g., Lacy et al. 2004; Stern et al. 2005; see also Donley et al. 2012), where separating the most heavily obscured AGN from the remaining source populations is not a trivial job (e.g., Castell´o-Mor et al. 2013).However,only X-ray data have been able to provide the smokinggunofthetrulyCompton-thicknatureforafractionofthe highmid-IR/optical flux-ratio sources (e.g., Polletta et al. 2006; Lanzuisi et al. 2009; Georgantopoulos et al. 2011;seealsoSevergninietal.2012).Furthermore,X-raystackinganalyseshaveallowed to place observational constraints, for the first time, to the space density of Compton- thickAGNathighredshifts(z ≈2−3;Daddietal.2007b;Fioreetal.2008,2009;Bauer et al. 2010; Alexander et al. 2011; but see also Georgakakis et al. 2010). Extension of the mid-IR search for heavily obscured AGN is within the capabilities offered by WISE, as shown by Mateos et al. (2013) and D. Stern (this Volume). 2.3. Optical selection The selection of obscured AGN using optical spectroscopy proceeds primarily through thedetectionofhigh-ionizationemissionlines,e.g.,[O iii]5007˚Aand[Ne v]3426˚A.These lines,beingproducedinthenarrow-lineregion(NLR),donotsufferfromextinctionfrom the torus and are considered good proxies of the nuclear intrinsic power (see Fig. 1). Applying the relationbetween [O iii] and2–10keVemission(e.g., Mulchaey et al.1994; Heckman et al. 2005; Panessa et al. 2006) to the sample of narrow-line AGN from the SloanDigital Sky Survey of Zakamska et al. (2003)led some authors (e.g., Vignaliet al. 2006, 2010; Ptak et al. 2006) to the discovery of about a dozen of Compton-thick AGN candidates. These studies allowed a first estimate of the space density of this obscured AGN population at z ≈ 0.3−0.8. According to Gilli et al. (2007, 2013) XRB models, the fraction of XRB emission at 20 keV produced by Compton-thick AGN and still “missing” has a peak at z ≈0.7 and is mostly due to Seyfert-like objects, with intrinsic 2–10 keV luminosity below 1044 erg s−1. Moving these investigations to slightly higher redshiftsrequirestheuseofthe[Ne v]emissionline,whichhastheadvantageofbeingan unambiguous marker of AGN (with a ionization potential of 97 eV vs. 54 eV of [O iii]) butis≈9timesweakerthan[O iii]andsuffersfromstrongerextinction.Calibratingthe X-ray-to-[Ne v] luminosity ratio on a sample of local AGN, Gilli et al. (2010) show that values < 15 are highly indicative of Compton-thick obscuration. How effective this line is in finding Compton-thick AGN has been recently confirmed by Mignoli et al. (2013), where narrow-line AGN were selected from the zCOSMOS survey and X-ray coverage was provided by Chandra (Vignali et al., in preparation). About 40% of the original ≈ 70 candidates are consistent with being Compton thick (in line with Gilli et al. 2007 model). We note, however, that optical spectroscopy, because of extinction within the NLR, is far from offering a complete census of obscured AGN (see §3.3 of Mignoli et al. 2013). Further insights into the properties of these [Ne v]-selected Compton-thick AGN will come out by using their mid-IR emission as another proxy of the nuclear emission (e.g., Gandhi et al. 2009) to be compared to the observed X-ray luminosity. 3. Conclusions Obscured AGN growth is a key phase in SMBH/galaxy co-evolution models. As the census of such objects is difficult, especially at high redshifts, a multi-wavelength syn- ergistic approach is needed, requiring deep X-ray exposure, mid-IR data and, possibly, optical/near-IR spectroscopy. 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