3.3. X-ray Constraints on AGN in Infrared and Submillimeter Galaxies
Deep X-ray surveys arguably allow the best assessment of the AGN content of distant infrared and submillimeter (hereafter, submm) galaxies in which much of cosmic star formation took place. AGN are expected to be significant emitters at these observed wavelengths, owing to the reprocessing of their strong shorter wavelength emission by omnipresent dust and gas in their nuclei. Overall, however, the current evidence points to a picture where AGN emission is dominated by that from star-formation processes.
In the infrared band, only a minority of 15 µm galaxies are identified with X-ray detected AGN; the majority of 15 µm/X-ray matches appear to be starburst galaxies (e.g., Alexander et al. 2002; Fadda et al. 2002). AGN nuclear emission contributes 3-5% or less of the total background in most infrared bands, the rest coming from starburst and normal galaxies (e.g., Silva, Maiolino & Granato 2004). The fractional AGN contribution could be significantly underestimated if there is a large population of infrared-emitting AGN missed by deep X-ray surveys (see Section 2.4). For example, the elusive Compton-thick AGN may dominate the AGN contribution to the infrared background, although inclusion of these probably still leaves AGN as minority contributors to the infrared background. Spitzer should be able to detect the putative Compton-thick AGN missed by deep X-ray surveys (e.g., Rigby et al. 2004; Treister et al. 2004). However, unambiguously separating these from the numerous starburst galaxies detected may prove formidable, especially since many galaxies hosting Compton-thick AGN are also likely to host significant starburst activity.
Surveys at submm wavelengths have uncovered a large population of luminous, dust-obscured starburst galaxies at z 1.5-3 with star-formation rates (SFRs) of the order of 1000 M yr-1 (e.g., Blain et al. 2002; Chapman et al. 2003; and references therein). Optical spectral classification studies of most of these galaxies are difficult due to optical faintness, and thus deep X-ray surveys play a critical role in determining their AGN content. Early comparisons between submm surveys and 20-150 ks Chandra surveys yielded little ( 10%) source overlap, but in the exceptionally sensitive CDF-N about 85% of submm galaxies (850 µm flux densities of 4-12 mJy) with reliable positions are now detected by Chandra (e.g., Barger et al. 2001c; Alexander et al. 2003a, 2004; Borys et al. 2004; and references therein). The majority of these X-ray detected submm galaxies appear to contain moderate-luminosity AGN that are usually obscured (see Figure 9), based upon their X-ray spectral shapes and other properties. Although sample definition and selection effects are complex, the data suggest an AGN fraction in the submm galaxy population of at least 40% (Alexander et al. 2004). Thus the SMBH in submm galaxies are almost continuously growing during the observed phase of intense star formation. Even after correcting for the significant amount of X-ray absorption present, however, AGN are unlikely to contribute more than 10-20% of the bolometric luminosity of typical submm galaxies.
Figure 9. Far-infrared luminosity versus absorption-corrected X-ray luminosity for submm galaxies in the CDF-N. Red filled circles indicate submm galaxies believed to contain AGN, while smaller blue filled circles indicate submm galaxies that are plausibly pure starbursts. Labeled black squares denote well-known AGN-dominated and star-formation dominated sources from the literature as well as two distant well-studied submm galaxies containing AGN (SMM 02399-0134 and SMM 02399-0136). Slanted lines show ratios of constant X-ray to far-infrared luminosity. This ratio is typically 3-30% for AGN-dominated sources; many of the CDF-N submm sources appear to contain moderate-luminosity AGN, but these AGN are unlikely to dominate the bolometric luminosity. Adapted from Alexander et al. (2004).