Two-dimensional modelling of optical Hubble Space Telescope and infrared tip-tilt images of quasar host galaxies

Abstract

A description is given of the method used to extract quasar host-galaxy parameters from the deep Hubble Space Telescope (HST) quasar images presented by McLure, Dunlop and co-workers. We then give the results of extensive testing of this technique on a wide range of simulated quasar+host combinations spanning the redshift range of our HST study (0.1<z<0.3). These simulations demonstrate that, when applied to our deep HST images, our method of analysis can easily distinguish the morphological type of a given host galaxy, and determine its scalelength, luminosity, axial ratio and position angle to within an accuracy of a few per cent. We also present new infrared tip-tilt images of four of the most luminous quasars in our HST sample, along with the outcome of modelling these data in a similar manner. The results provide further confidence in the accuracy of the derived host-galaxy scalelengths, and allow an accurate determination of R-K colours for this subset of sources. All four of these quasar host galaxies have very similar red colours, R-K=2.9+/-0.2, indicative of a well-evolved stellar population.