NICMOS Imaging of the Host Galaxies of z ~ 2-3 Radio-quiet Quasars

We have made a deep NICMOS imaging study of a sample of five z ~ 2-3 radio-quiet quasars with low absolute nuclear luminosities, and we have detected apparent host galaxies in all of these. Most of the hosts have luminosities approximately equal to present-day L*, with a range from 0.2L* to about 4L*. These host galaxies have magnitudes and sizes consistent with those of the Lyman break galaxies at similar redshifts and at similar rest wavelengths but are about 2 mag fainter than high-z powerful radio galaxies. The hosts of our high-z sample are comparable to or less luminous than the hosts of the low-z radio-quiet quasars with similar nuclear absolute magnitudes. However, the high-z galaxies are more compact than the hosts of the low-z quasars and probably have only 10%-20% of the stellar mass of their low-z counterparts. In one host, we find a residual component that is not centered on the quasar nucleus, and several hosts have apparent companions within a projected distance of ~10 kpc, indications that these systems are possibly in some phase of a merger process. Application of the Mbulge/MBH relation found for present-day spheroids to the stellar masses implied for the high-z host galaxies would indicate that they contain black holes with masses around 108 M☉. Comparison to their nuclear magnitudes implies accretion rates that are near or at the Eddington limit. Although these high-z hosts already contain supermassive black holes, the galaxies will need to grow significantly to evolve into present-day L* galaxies. These results are basically consistent with theoretical predictions for the hierarchical buildup of the galaxy host and its relation to the central supermassive black hole.