Spectral and morphological properties of quasar hosts in smoothed particle hydrodynamics simulations of active galactic nucleus feeding by mergers

We present a method for generating virtual observations from smoothed particle hydrodynamics simulations. This method includes stellar population synthesis models and the reprocessing of starlight by dust to produce realistic galaxy images. We apply this method and simulate the merging of two identical giant Sa galaxies (M-disc 10(11) M-circle dot, M-spheroid = 2.5 x 10(10) M-circle dot). The merger remnant is an elliptical galaxy (M-spheroid similar or equal to 1.3 x 10(11)M(circle dot), M-disc similar or equal to 7.4 x 10(10) M-circle dot). The merger concentrates the gas content of the two galaxies into the nuclear region. The gas that flows into the nuclear region refuels the central black holes of the merging galaxies. We follow the refuelling of the black holes during the merger semi-analytically. In the simulation presented in this paper, the black holes grow from 3 x 10(7) to 1.8 x 10(8) M-circle dot, with a peak active galactic nucleus (AGN) luminosity of M-B similar to -23.7. We study how the morphological and spectral properties of the system evolve during the merger, and we work out the predictions of this scenario for the properties of host galaxies during the active phase. The peak of AGN activity coincides with the merging of the two galactic nuclei and occurs at a stage when the remnant looks like a lenticular galaxy. The simulation predicts the formation of a circumnuclear starburst ring/dusty torus with an opening angle of 30 degrees-40 degrees and made of clouds with n(H) = 10(24) cm(-2). The average optical depth of the torus is quite high, but the obscuring medium is patchy, so that there still exist lines of sight where the AGN is visible in a nearly edge-on view. For the same reason, there are lines of sight where the AGN is completely obscured in the face-on view.