The B-band luminosities of quasar host galaxies

We report on the analysis of B-band imaging data of 57 low-redshift quasi-stellar objects (quasars/QSOs) and Seyfert 1 galaxies selected from the Hamburg/ESO Survey, for which host-galaxy-dependent selection biases are greatly reduced compared with other optical surveys. Only one object in the sample is known to be radio-loud. We adopted a procedure to remove the active galactic nuclei (AGN) contribution by subtracting a scaled point spread function from each QSO image. To reclaim the integrated host galaxy flux we correct for oversubtraction based on simulations. This method is quite insensitive to the host galaxy morphological type, which we can unambiguously establish for 15 of the 57 objects. The quasar host galaxies are detected in all cases. The hosts are very luminous, ranging in absolute magnitude M-B from -19.0 to -23.8, with an average of M-B,M-gal = -21.5, considerably above L* for field galaxies. For the luminous QSO subsample with M-B < -23 the average host absolute magnitude is M-B,M-gal = -23.0, while for the complementary low-luminosity AGN we obtain M-B,M-gal = -21.2, roughly equal to V. The luminous host galaxies in the sample are typically similar to 1 mag brighter than expected when inferring B-band luminosities from studies of similar objects at longer wavebands. We argue that this mismatch is not likely to be explained by selection effects, but favours host galaxy colours significantly bluer than those of inactive galaxies. Although published B-band data are scant, this result and the findings of other authors are in good agreement.