# Presentation: 375948

## Multi-Wavelength Surveys of Obscured AGN

**Multi-Wavelength Surveys of Obscured AGN**
**Eleni T. Chatzichristou**
**Institute of Astronomy & Astrophysics, National Observatory of Athens, 15236 Penteli, Greece**

**Abstract****   **
**Several key goals require measuring the number of all AGN in the Universe, and the evolution of the ratio of obscured to un-obscured AGN with redshift. This reflects the structure of AGN and thus the development in the heart of all galaxies. Hard X-rays can penetrate most obscuring dust columns to reveal the AGN that remains hidden in all other wavelengths. Mid-IR surveys probe the thermal dust emission, that is, the continuum light from the central source after it is reprocessed by dust, and this emission dominates the bolometric luminosities of dusty high-redshift galaxies. Thus, combining deep mid-IR and hard X-ray surveys can provide us with accurate demographics of AGN especially at high redshifts. Multi-wavelength surveys aim to address these science goals by exploiting the unprecedented combination of great observatories such as HST, Chandra and Spitzer to survey the distant universe to the faintest flux limits across the broadest range of wavelengths. Here we present and discuss some of the results coming from multi-wavelength surveys placing particular focus on the systematic study of obscured AGN.**
**Introduction**
**AGN are traditionally classified by optical spectroscopy and UV excess/color selection techniques. These usually miss the highly obscured or weak AGN, being biased towards the highly luminous, unobscured AGN (e.g. Moran et al. 2002, ApJ 579, 71; Maiolino et al. 2003, MNRAS 344, L59). The obscured AGN can only be detected in hard X-rays (>2 keV) which can penetrate most obscuring columns to reveal the AGN, and in the mid- and far-IR wavelengths, where the majority of their bolometric luminosity is observed. Many deep multi-wavelength surveys such as GOODS (Giavalisco et al. 2004, ApJ 600, L93) are motivated by such a quest for a previously unknown population of obscured AGN. **
**They furthermore give a unique opportunity to test ideas about obscuration of AGN: **
***What are the mid-IR properties of X-ray selected AGN and vice-versa? ***
***What is the interplay between IR and X-ray backgrounds?***

**X-Ray Surveys**
**They have identified a population of high-z, heavily obscured luminous AGN (Stern et al. 2002, ApJ 568, 71; Norman et al. 2002, ApJ 571, 218), as well as a population of apparently normal galaxies with optical spectra dominated by starlight but hosting significant X-ray emission implying the presence of an AGN (Barger et al. 2001, AJ 121, 662; Hornschmeier et al. 2001, ApJ 554, 742; Stern et al. 2002, AJ 123, 2223). For instance, 43% of X-ray selected AGN in the CDFS are not classified as active by optical spectroscopy (Szokoly et al. 2004, ApJS 155, 271). X-ray AGN selection has the advantage of being reasonably efficient and reliable, but may miss some obscured sources (Peterson et al. 2006, AJ 131, 133), while IR and radio observations can identify the AGN missed in X-rays (Donley et al. 2005; ApJ 634, 169; Alonso-Herrero et al. 2006, ApJ 640, 167) and can help distinguish between different activity types for X-ray selected AGN.**
**Mid-IR Surveys**
**Mid-IR selected AGN show higher surface density relative to optically-selected AGN of comparable mid-IR flux (Stern et al. 2005, ApJ 631, 163; Gorjian et al. 2006, in preparation). Furthermore, among the most IR luminous galaxies at both intermediate and high redshifts, the AGN fraction can be very high, at least 40% (Alexander et al., 2005, Nature 434, 738; Yan et al. 2005, ApJ 628, 604). **
**Deep mid-IR imaging studies with Spitzer, as well as radio-selection criteria have begun to detect a population of type 2 QSOs which seem to be at least comparable in number density to the un-obscured type 1 population (Martinez-Sansigre et al. 2005, Nature 436, 666; Donley et al. 2005, ApJ 634, 169). Some of these show no hard X-ray counterpart, indicating that they are hidden by Compton-thick material even at high X-ray luminosities. Treister et al (2006, ApJ 640, 603) furthermore show that the contribution of AGN missed in X-rays to the IR background can add 45% more flux, 30% coming from obscured AGN. **

***Figure 1 from Stern et al. 2005, showing the success of Spitzer mid-IR colour criteria for separating quasars and Seyfert 1s from galaxies.***

**Lack of correspondence between IR and X-rays?**
**Eventhough mid-IR colour criteria identify most of the X-ray selected AGN when both the X-ray and IR wavelengths are dominated by the central source, they are less successful when the host galaxy dominates the IR emission (e.g. Barmby et al (2006, ApJ 642, 126). The inverse is also true: among the CDFS AGN selected by IR power-law criteria only 50% are detected in at least one of the Chandra bands, and these represent only 30% of the hard X-ray sources with mid-IR counterparts (Alonso-Herrero et al. 2004, ApJS 154, 155 and 2006). Furthermore, it becomes more and more evident that the mid-IR properties do not directly correspond to X-ray hardness, and inversely, X-ray selected AGN  have a variety of optical/IR spectral types (e.g. Franceschini et al. 2005, AJ 129, 2074). **
***What is the reason for this lack of correspondence between IR and X-rays? ***
***Variations of the gas/dust ratio? A broad range of intrinsic AGN properties? ***

**Trends with Luminosity**
**Hard X-ray studies have shown a decreasing fraction of type 2 AGN with increasing X-ray luminosities.**** **
***Could this indicate that higher-luminosity AGN clean out their environment? Or rather that the torus opening angle is larger in more luminous objects? ***
**If these explanations are true, one would expect that the ratio of mid-IR/X-ray emission should be lower in type 2 QSOs than in Seyfert 2s. Is this what we indeed see?**
**In fact, reality seems to be more complex. Although some correlation between mid-IR emission and QSO 2 luminosity can indeed be seen (e.g. Lutz et al. 2004, A&A 418,465), and the above ratio is found to decrease with X-ray luminosity, there seems to be no correlation with hard X-ray or IR luminosity (Treister et al. 2006). Furthermore, it seems that in general the mid-IR spectra of luminous QSO2s are significantly different from those of lower luminosity type 2 AGN, in fact they are more similar to type 1 AGN, probably because the contribution of QSO 2 host galaxies to the mid-IR spectra is small (e.g. Sturm et al. (2006, ApJ 642, 81).**** *****If this is true, it could mean that the QSO2 contribution to the peak IR background is less important than predicted. ***

***Figure 7 from Barmby et al. 2006, showing (a) hard X-ray vs. 24 micron fluxes and (b) and (c) full band X-ray vs. IRAC 8.0 and 3.6 micron fluxes.***

***Figure 1 from Alonso-Herrero et al. 2006, showing mid-IR vs. hard X-ray fluxes, for X-ray selected sources in the CDFS. The lightly shaded area is the extrapolation of values for local AGN***

**Is there a good indicator of obscured AGN?**
**Obscured AGN locally seem to have higher mid-IR/hard-X-ray flux ratios compared to unobscured AGN (e.g. Fadda et al. 2002, A&A 383, 838; Silva et al. 2004, MNRAS 355, 973; Alonso-Herrero et al. 2006). One would thus expect to find a large population of X-ray hard, IR-bright AGN. However, most recent studies indicate that this flux ratio does not depend on X-ray hardness or column density N****H****, and does not differ between optically-classified broad-line and narrow-line AGN (e.g. Lutz et al. 2004; Treister et al. 2006). There are even findings of X-ray hard AGN with very low mid-IR/X-ray flux ratios (Rigby et al. 2004, ApJS 154, 160). **
**This is not expected within the unification paradigm.**** *****Could it be interpreted as indicating an intrinsic difference between high-z and local AGN, in their dust properties or absorbing geometries? Or, that very hard sources are also very faint IR (and maybe also X-ray) emitters?***** **
**Alternatively, one could simply suggest that, while there must be certainly some connection between X-ray hardness ratio and**** *****mid-IR spectral indices, the latter is not a particularly good indicator of the AGN type, but rather indicating the degree to which the mid-IR is dominated by the AGN or the host galaxy emission.***

***Figure 4 from Treister et al. 2006, showing hard X-ray to mid-IR luminosity ratio as a function of X-ray (left) and IR (right) luminosities. Triangles are GOODS AGN and circles are local AGN. Filled/open symbols denote obscured/unobscured sources, respectively.  ***

**The question that is posed is then: *****If indeed hard X-ray and mid-IR selection techniques find few Compton-thick AGN, what is a better way to search?***** **
**Improving statistics with larger, more uniform samples and cross identification with other wavelengths is certainly needed. Furthermore it is essential to reach a better understanding of their nature in order to be able to uncover the elusive population of obscured AGN. All of these issues are and will continue to be in the focus of the Spitzer and Herschel Space Observatories.**