The Relationship Between Stellar and Black-Hole Mass in Submillimeter Galaxies

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Abstract
We analyze deep X-ray, optical and mid-infrared Spitzer observations of the CDF-N/GOODS-N region to study 13 submillimeter-detected galaxies (SMGs) with spectroscopic redshifts (median z=2.2). We find a 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. 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. 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 SMGs pass through a subsequent accretion-dominated phase, where they would appear as optically bright quasars. (ABRIDGED)

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