Where do z~2 Submillimeter-Emitting Galaxies Lie On the Black-Hole-Spheroid Mass Plane?

TL;DR

This study investigates the position of z~2 submillimeter-emitting galaxies on the black-hole-spheroid mass plane, revealing they have lower black-hole-to-spheroid mass ratios than local galaxies and quasars, suggesting different growth phases.

Contribution

The paper provides new black-hole mass estimates for SMGs at z~2 and compares their mass ratios to local and quasar populations, highlighting their distinct evolutionary stage.

Findings

SMGs have lower black-hole-spheroid mass ratios than local galaxies.

SMGs cannot reach the quasar black-hole-spheroid mass relationship without overproducing massive black holes.

Longer or more intense AGN activity is needed for SMGs to align with the local mass relationship.

Abstract

Submillimeter-emitting galaxies (SMGs) are z~2 bolometrically luminous systems hosting energetic starburst and AGN activity. SMGs may represent a rapid growth phase that every massive galaxy undergoes before lying on the well-established black-hole-spheroid mass relationship in the local Universe. Here we briefly discuss our recent results from Alexander et al. (2008) where we estimated the masses of the black holes in SMGs using the black-hole virial mass estimator, finding M_BH~6x10^7 M_solar for typical SMGs. We show that the black-hole-spheroid mass ratio for SMGs at z~2 was suggestively below that found for massive galaxies in the local Universe and more than an order of magnitude below the black-hole-spheroid mass ratio estimated for z~2 quasars and radio galaxies. We demonstrate that SMGs and their progeny cannot lie on the elevated z~2 black-hole-spheroid mass relationship of quasars-radio galaxies without overproducing the space density of the most massive black holes (M_BH~10^9 M_solar), unless the galaxy spheroid of SMGs is an order of magnitude lower than that typically assumed (M_SPH~10^10 M_solar). We also show that the relative black-hole-spheroid growth rates of typical SMGs appear to be insufficient to significantly increase the black-hole-spheroid mass ratio, without requiring long duty cycles (~10^9 years), and argue that a more AGN-dominated phase (e.g., an optically bright quasar) is required to significantly move SMGs (and their progeny) up the black-hole-spheroid mass plane.