Probing the M_BH-σ_* Relation in the Non-local Universe Using Red QSOs

Abstract

We describe a method to measure the M_BH-σ_* relation in the non-local universe using dust-obscured QSOs. We present results from a pilot sample of nine Two Micron All Sky Survey red QSOs with redshifts 0.14 < z < 0.37. We find that there is an offset (0.8 dex, on average) between the position of our objects and the local relation for active galactic nuclei (AGNs), in the sense that the majority of red QSO hosts have lower velocity dispersions and/or more massive black holes (BHs) than local galaxies. These results are in agreement with recent studies of AGNs at similar and higher redshifts. This could indicate an unusually rapid growth in the host galaxies since z ~ 0.2, if these objects were to land in the local relation at present time. However, the z > 0.1 AGNs (including our sample and those of previous studies) have significantly higher M_BH than those of local AGNs, so a direct comparison is not straightforward. Further, using several samples of local and higher-z AGNs, we find a striking trend of an increasing offset with respect to the local M_BH-σ_* relation as a function of AGN luminosity, with virtually all objects with log(L ₅₁₀₀/erg s^-1) >43.6 falling above the relation. Given the relatively small number of AGNs at z > 0.1 for which there are direct measurements of stellar velocity dispersions, it is impossible at present to determine whether there truly is evolution in M_BH-σ_* with redshift. Larger, carefully selected samples of AGNs are necessary to disentangle the dependence of M_BH-σ_* on mass, luminosity, accretion rates, and redshift.