Figure 1.6: Fraction of detected AGN with increasing column density (Tozzi et al., 2006), lower detected fraction at higher z as these objects tend to be lower luminosity. Dashed line is the average of all samples depicted.
et al. 2011; Feruglio et al. 2011; Gilli et al. 2011) but these objects only represent a small percentage of the total population. Stacking the X-ray spectra of weakly detected AGN which are thought to be heavily obscured to improve their signal-to-noise ratios has also proved effective (e.g. Alexander et al. 2011). But the large population of Compton thick AGN at high redshift predicted from observations of the XRB hard X-ray deficit is yet to be confirmed observationally.
1.5
Identifying obscured AGN without X-rays
One of the main drivers of the research presented in this thesis is the identification of obscured and Compton thick AGN at high redshifts. Such sources can be serendipitously identified in deep X-ray surveys, but the limited coverage of such surveys makes it difficult to constrain the extent of these populations. Additionally the majority of high redshift Compton thick AGN are too X-ray faint and remain completely undetected in typical X-ray surveys (Comastri, 2004). More and more frequently, alternative identification methods using wavelengths (e.g. optical and IR) with wider coverage are implemented to identify obscured AGN candidate sources. The accuracy of these techniques can be checked with follow up X-ray observations through a combination of X-ray spectral fitting and X-ray stacking analyses. If these techniques prove accurate, they may be vital in identifying the large, heavily obscured AGN population predicted from the hard XRB deficit (e.g. Gilli et al. 2007).
1.5 Identifying obscured AGN without X-rays 35
1.5.1 Mid-IR selection
The majority of the Infrared (IR) emission of AGN is due to thermal emission from dust, hence is observed to be blackbody, or a superposition of many blackbodies. The strongest IR emitters are typically regions of intense star-formation, where high concentrations of gas and dust are in close proximity to young stars, absorbing the UV and optical emission and re-emitting it in the IR. AGN are only a small fraction of all the sources selected in IR surveys, compared to say normal and starburst galaxies, and can be indistinguishable from non-AGN if only a few infrared bands are available (Polletta et al., 2008). But a major benefit of observing in the infrared is that, like X-rays, it experiences far less attenuation than optical and UV when transmitting through gas and dust. This is particularly useful when observing obscured AGN and can in principle reveal AGN in which even the X-ray emission is suppressed. Figure 1.6 illustrates the drop-off in the X-ray detected fraction as NH increases, with a sharp drop-off as AGN enter moderate-to-heavy obscuration regime
(NH > 1023 cm−2). Using Infrared emission, which can be less heavily attenuated, tech-
niques for identifying X-ray undetected heavily obscured and Compton thick AGN activity have been developed.
IR emission in Compton thick AGN is thought to be produced by the reprocessing of UV and optical emission from the accretion disk by the dust torus into IR wavelengths. It can be difficult to distinguish between obscured AGN and starbursts without additional characteristic AGN emission features, such as hard X-ray emission or strong radio emission. However, AGN have been observed to possess distinctive mid-infrared colours compared to star-forming galaxies (Lacy et al., 2004; Stern et al., 2005; Hatziminaoglou et al., 2010). Alonso-Herrero et al. (2001) examine the source of non-stellar emission in Seyfert galaxies, typically seen at λ > 3µm and conclude mid-IR emission can be used as a measure of AGN luminosity. Therefore galaxies exhibiting excessive amounts of mid-IR emission (mid-IR excess) appear likely to contain AGN. This proves especially useful for investigating heavily obscured AGN populations which are faint or undetected in X-ray surveys. LIRG and ULIRG (Sanders and Mirabel, 1996) galaxies are among the most IR luminous objects in the universe, containing vast quantities of molecular dust in their central regions that is ideal for the fuelling of intense starbursts and building/fuelling of AGN. They are likely to be chosen as mid-IR excess galaxies, but it is often unclear whether this is due to an obscured AGN or an obscured starburst.
Fiore et al. (2008) observe that F (24µm)/F (R) is tightly correlated to the X-ray to optical flux ratio (itself correlated to X-ray luminosity). They plot this ratio versus the R − K colour, because X-ray obscured AGN tend to have red R − K colours (Brusa et al., 2005), for a sample of X-ray sources. They postulate that sources with F (24µm)/F (R) > 1000 and R − K > 4.5 are powered by highly obscured AGN based on template fitting and stacking analysis. Polletta et al. (2008) select AGN with featureless, red power-law like IR SEDs which exhibit excessive mid-IR emission as obscured candidates, identifying 61 unobscured sources and 120 obscured sources. The 2:1 ratio of obscured to unobscured
1.5 Identifying obscured AGN without X-rays 36
Figure 1.7: Stern plots from Georgantopoulos et al. (2008). Left plot depicts distribution of mid-IR excess galaxies chosen using the criteria of Fiore et al. (2008). Right plot tracks redshift evolution of for ULIRGs, one of which is a Seyfert 2 (Mrk 273; solid magenta) and the other a starburst (Arp 220; dashed green) for 0 ≤ z ≤ 3.
objects is similar to previous studies of a similar nature (e.g. Alonso-Herrero et al. 2006), but is somewhat at odds with the 3:1 ratio which is observed in the local universe and predicted by most XRB models. Using Mid-IR selection, Polletta et al. (2008) estimate a surface density of Compton thick AGN that is 10% of the expected value derived from analysis of the XRB. This suggests that mid-IR excess selection techniques for Compton thick AGN are not as comprehensive as some research has suggested, and any conclusions drawn regarding their contribution to the XRB may require further investigation.
Stacking X-ray undetected mid-IR excess sources may hold the solution to the per- ceived deficit in obscured AGN activity. Studies in which X-ray undetected mid-IR excess sources are stacked frequently produce hard stacked emission indicative of significant ob- scured and Compton thick AGN activity (Daddi et al., 2007; Luo et al., 2011). Georgan- topoulos et al. (2008) compared the properties of X-ray undetected 24µm excess (Fiore et al., 2008) and IRAC colour selected AGN using X-ray stacking analysis. They found the Stern wedge selected objects (Stern et al., 2005) exhibit soft X-ray emission suggest- ing contamination by normal galaxies. The 24µm excess objects have a much harder stacked continuum indicative of either Compton thick AGN or low luminosity AGN with NH ∼ 1022 cm−2. Most of these objects lie in a region outside the Stern wedge more
commonly associated with star-forming ULIRGs (see Figure 1.7). Donley et al. (2005) look at radio excess relative to 24 µm emission for X-ray undetected AGN, finding no Compton thick NHs for X-ray weak AGN, but 6 X-ray undetected sources that could be
Compton thick. This corresponds to ∼22% of the sample being Compton thick which is consistent with the lower end of predictions from the XRB. Georgakakis et al. (2010) studied the X-ray luminosity of high IR-optical ratio objects selected by the Fiore et al. (2008) technique and observe many possess low X-ray luminosities (LX < 1043 erg s−1).
1.5 Identifying obscured AGN without X-rays 37
Figure 1.8: Plot of Kα iron line EW versus T-ratio from Bassani et al (1999). The results presented suggest that sources with T≤0.1 are Compton thick AGN.
of Compton thin AGN, low luminosity AGN and starbursts in addition to Compton thick candidates.
1.5.2 Optical emission line diagnostics
The optical emission line diagnostic of Baldwin et al. (1981) and Veilleux and Osterbrock (1987) identify Type 2 AGN with high accuracy out to z < 0.4 based on the ratio of optical emission line strengths (e.g. [O III]/Hβ versus [N II]/Hα). This selection technique (BPT selection) categorises different sources according to the excitation processes using narrow line emission. The AGN are selected based on their dominant [O III] emission, with preferential selection in high metallicity sources. In the case of AGN this emission occurs in narrow line region clouds located away from the central source, therefore suffers less attenuation when the AGN is obscured by the dust torus. The majority of Type 2 AGN in the local universe are obscured (e.g. Kirhakos and Steiner 1990) and contain a high fraction of Compton thick AGN (e.g. Risaliti et al. 1999). Thus the BPT technique can be used to select heavily obscured and Compton thick AGN candidates which remain undetected in X-rays. While the optical window limits the redshifts to which this technique can be implemented in optical spectroscopy (z < 0.4), BPT selection can be replicated at higher redshifts using NIR spectroscopy (Trump et al., 2011, 2013). Recent attempts to produce pseudo-BPT diagrams that loosen the redshift constraints to z ∼ 1 by replacing the longer wavelength emission lines have also proved effective (Weiner et al. 2007; Yan et al. 2011; Juneau et al. 2011; see Chapter 5).