3.3. X-ray Emitting AGN in Luminous Submillimeter Galaxies

The deepest Chandra and XMM-Newton surveys have finally provided the necessary X-ray sensitivity to complement the most sensitive surveys at submillimeter and infrared wavelengths. One notable instance where obtaining the highest possible X-ray sensitivity has been essential is in studies of the AGN content of distant submillimeter galaxies detected with the SCUBA instrument on the James Clerk Maxwell Telescope. Most of these galaxies are thought to contain intense starbursts with star-formation rates of 100 M yr^-1, yet they are not notable in optical galaxy surveys due to dust obscuration of the corresponding starlight. The SCUBA galaxy population is thought to be mostly at z 1.5-3, and such galaxies were ~ 1000 times more common at z ~ 2 than in the local universe. The obscured starlight in submillimeter galaxies is re-radiated in the rest-frame infrared (observed-frame submillimeter).

What fraction of submillimeter galaxies contains actively accreting supermassive black holes? Sensitive X-ray studies play an important role in addressing this question, since they allow effective searching for AGN in the majority of submillimeter galaxies that are optically faint (and thus challenging to study in detail with optical spectroscopy). Early comparisons between 20-150 ks Chandra surveys and submillimeter surveys yielded little ( 10%) source overlap. However, the latest analysis of the 2 Ms CDF-N data reveals that seven of the 13 ( 54%) bright submillimeter galaxies (with 850 µm flux densities of > 5 mJy) in the CDF-N central region mapped with SCUBA have X-ray counterparts (see Figure 8); these counterparts have 15-200 counts in the full Chandra bandpass. Five of the seven X-ray detected submillimeter galaxies likely host obscured AGN based upon their observed X-ray luminosities, X-ray spectral shapes, and X-ray-to-submillimeter flux ratios (see Figure 9). The remaining two have X-ray emission properties consistent with those expected from star formation activity, although it is possible that they host weak AGN as well. If the latter two sources are indeed powered mainly by star formation, they would be the most X-ray luminous ( 4 × 10⁴² erg s^-1) starburst galaxies known.

Figure 8. Map at 850 µm of the central region of the CDF-N; the map is 6' on a side. Sources at 850 µm with X-ray detections are enclosed by dotted circles. The three clustered 850 µm/X-ray sources near the upper-right corner are also coincident with an extended X-ray source, perhaps a high-redshift cluster. Adapted from D.M. Alexander, F.E. Bauer, W.N. Brandt, et al., 2003, AJ, 125, 383 and C. Borys, S. Chapman, M. Halpern, et al., 2003, MNRAS, 344, 385.

Do the X-ray emitting AGN found in many submillimeter galaxies make a significant contribution to these galaxies' total energy output? Answering this question requires assessment of the amount of X-ray absorption present since, for a given observed X-ray flux, a Compton-thick AGN can be much more luminous than a Compton-thin AGN (see Section 3.1). Basic X-ray spectral fitting suggests that three of the five submillimeter galaxies hosting AGN in the CDF-N central region have Compton-thin absorption, while only one is likely to have Compton-thick absorption (the final object has poor X-ray spectral constraints). Armed with this knowledge, consideration of the observed X-ray-to-submillimeter flux ratios (see Figure 9) suggests that 10% of the total energy output from these submillimeter galaxies is ultimately due to an AGN. Star-formation is apparently the dominant power source for the infrared/submillimeter emission, even when an AGN is also present.

Figure 9. Submillimeter-to-X-ray spectral index (sx) versus redshift. Submillimeter sources in the central part of the CDF-N with (without) X-ray detections are shown as solid (open) circles. The five circles with overlaid crosses are likely AGN according to their X-ray properties, while those with overlaid "U" are of unknown X-ray type. Dashed curves show sx values for 3C 273, NGC 6240, and Arp 220 adopting their observed amounts of X-ray absorption; alternative dotted curves show an AGN like NGC 6240 but with less internal absorption (NH = 5 × 1023 cm-2) and a smaller scattered flux fraction (fsc = 0.01). Adapted from D.M. Alexander, F.E. Bauer, W.N. Brandt, et al., 2003, AJ, 125, 383.