A NICMOS imaging study of high-z quasar host galaxies

We present the first results from a major Hubble Space Telescope programme designed to investigate the cosmological evolution of quasar host galaxies from z similar or equal to 2 to the present day. Here we describe J and H-b and NICMOS imaging of two quasar samples at redshifts of 0.9 and 1.9 respec tively. Each sample contains equal numbers of radio-loud and radio-quiet qu asars, selected to lie within the same narrow range of optical absolute mag nitude (-24 - greater than or equal to M-v greater than or equal to -25). F ilter and target selection were designed to ensure that at each redshift th e images sample the same part of the object's rest-frame spectrum, longward s of 4000 Angstrom where starlight from the host galaxy is relatively promi nent, but avoiding potential contamination by [O III]lambda 5007 and H alph a emission lines. At z similar or equal to 1 we have been able to establish host-galaxy lumin osities and scalelengths with sufficient accuracy to demonstrate that the h osts of both radio-loud and radio-quiet quasars lie on the same Kormendy re lation described by 3CR radio galaxies at comparable redshift. Taken at fac e value the gap between the host luminosities of radio-loud and radio-quiet objects appears to have widened from only similar or equal to 0.4 mag at z similar or equal to 0.2 to similar or equal to 1 mag at z similar or equal to 1, a difference that cannot be a result of emission-line contamination, given the design of our study. However, within current uncertainties, simp le passive stellar evolution is sufficient to link these galaxies with the elliptical hosts of low-redshift quasars of comparable nuclear output, impl ying that the hosts are virtually fully assembled by z - 1. At z similar or equal to 2 the hosts have proved harder to characterize acc urately, and for only two of the nine z similar or equal to 2 quasars obser ved has it proved possible to properly constrain the scalelength of the hos t galaxy. However, the data are of sufficient quality to yield host-galaxy luminosities accurate to within a factor of 2. At this redshift the luminos ity gap between radio-loud and radio-quiet quasars appears to have widened further to similar or equal to 1.5 mag. Thus while the hosts of radio-loud quasars remain consistent with a formation epoch of z > 3, allowing for pas sive evolution implies that the hosts of radio-quiet quasars are similar or equal to 2-4 times less massive at z similar or equal to 2 than at z simil ar or equal to 0.2. If the relationship between black hole and spheroid mass is unchanged out t o redshift z similar or equal to 2, then our results rule out any model of quasar evolution which involves a substantial component of luminosity evolu tion. Rather, this study indicates that at z similar or equal to 2 there is a substantial increase in the number density of active black holes, along with a moderate increase in the fuelling efficiency of a typical observed q uasar. The fact that this latter effect is not displayed by the radio-loud objects in our sample might be explained by a selection effect arising from the fact that powerful radio sources are only produced by the most massive black holes.