Inferences on the Relations Between Central Black Hole Mass and Total
  Galaxy Stellar Mass in the high-redshift Universe
  At the highest redshifts, z>6, several tens of luminous quasars have been
detected. The search for fainter AGN, in deep X-ray surveys, has proven less
successful, with few candidates to date. An extrapolation of the relationship
between black hole (BH) and bulge mass would predict that the sample of z>6
galaxies host relatively massive BHs (>1e6 Msun), if one assumes that total
stellar mass is a good proxy for bulge mass. At least a few of these BHs should
be luminous enough to be detectable in the 4Ms CDFS. The relation between BH
and stellar mass defined by local moderate-luminosity AGN in low-mass galaxies,
however, has a normalization that is lower by approximately an order of
magnitude compared to the BH-bulge mass relation. We explore how this scaling
changes the interpretation of AGN in the high-z Universe. Despite large
uncertainties, driven by those in the stellar mass function, and in the
extrapolation of local relations, one can explain the current non-detection of
moderate-luminosity AGN in Lyman Break Galaxies if galaxies below 1e11 Msun are
characterized by the low-normalization scaling, and, even more so, if their
Eddington ratio is also typical of moderate-luminosity AGN rather than luminous
quasars. AGN being missed by X-ray searches due to obscuration or instrinsic
X-ray weakness also remain a possibility.
