The co-evolution of the obscured quasar PKS1549-79 and its host galaxy: evidence for a high accretion rate and warm outflow

Abstract

ABRIDGED: We use deep optical, IR and radio observations to explore the symbiosis between the nuclear activity and galaxy evolution in the southern compact radio source PKS1549-79 (z=0.1523). The optical imaging observations reveal the presence of tidal tail features which provide strong evidence that the host galaxy has undergone a major merger in the recent past. This is further supported by the detection of a young stellar population which was formed 50-250Myr ago and makes up 1-30% of the total stellar mass. Despite the core-jet structure of the radio source we detect HI absorption associated with both the core and the jet. The luminous, quasar-like AGN (M_V < -23.5) is highly extinguished (A_v > 4.9) at optical wavelengths and shows many properties in common with narrow line Seyfert 1 galaxies (NLS1), including relatively narrow permitted lines (FWHM ~ 1940 km/s), highly blueshifted [OIII] lines (~680 km/s), and evidence that the putative supermassive black hole is accreting at a high Eddington ratio. This suggests that accretion at high Eddington ratio does not prevent the formation of powerful jets. Our observations lend strong support to the predictions of some recent numerical simulations of galaxy mergers in which the black hole grows rapidly through merger-induced accretion following the coalescence of the nuclei of two merging galaxies; the major growth phase largely hidden by the natal gas and dust. Although the models predict that AGN-driven outflows will eventually remove the gas from the bulge of the host galaxy, the visible warm outflow in PKS1549-79 is not currently capable of doing so. ABRIDGED