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
|