The Relationship between Stellar and Black-hole Mass in Submillimeter Galaxies

We analyze deep X-ray, optical and mid-infrared Spitzer observations of the CDF-N/GOODS-N region to study a sample of 13 submillimeter-detected galaxies with spectroscopic redshifts (median z = 2.2). These galaxies are among the most active and massive at this epoch. We find evidence for a power-law correlation between the estimated stellar and X-ray luminosity, implying that masses of the black holes may be related to the stellar masses of their host galaxies. We derive the rest-frame UV–near-infrared spectral energy distributions for these galaxies, believed to be young spheroids, and fit them with model templates. Using the rest-frame near-infrared luminosities, which are relatively insensitive to uncertainties in stellar ages and reddening in these young dusty galaxies, and theoretical mass-to-light ratios, we can estimate their stellar masses. Although the submillimeter emission implies that these galaxies are undergoing an epoch of intense star-formation, the Spitzer data reveal a massive stellar population already in place. We find that our submillimeter galaxies have a median stellar mass of ∼ 10M⊙, which is roughly ten times more massive than typical UV-selected star-forming systems at similar redshifts. These stellar masses are then compared to previously published black hole mass estimates derived from the X-ray luminosities under the assumption of Eddington-limit accretion. We find that the black hole masses for our high-redshift sample are approximately 1–2 orders of magnitude smaller than galaxies of comparable stellar mass in the local Universe. Although our estimates of black hole masses will increase if the accretion is sub-Eddington, and our stellar masses will decrease if we assume a much younger stellar population or a different initial mass function, we find that only through a combination of effects is it possible to shift the high redshift galaxies such that they lie on the local relation. This suggests that the black holes need to grow substantially between z = 2.2 and the present-day, with much of the black hole growth occurring after the current obscured, far-infrared luminous phase of activity which is likely associated with the formation of the spheroid. This interpretation supports a scenario where submillimeter galaxies pass through a subsequent accretion-dominated phase, where they would appear as optically bright quasars. Subject headings: galaxies: evolution – galaxies: formation – galaxies: starburst