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The co-evolution of the obscured quasar PKS 1549-79 and its host galaxy: evidence for a high accretion rate and warm outflow
We use deep optical, infrared and radio observations to explore thesymbiosis between nuclear activity and galaxy evolution in the southerncompact radio source PKS 1549-79 (z = 0.1523). The optical imagingobservations reveal the presence of tidal tail features which providestrong evidence that the host galaxy has undergone a major merger in therecent past. The merger hypothesis is further supported by the detectionof a young stellar population (YSP), which, on the basis of spectralsynthesis modelling of our deep Very Large Telescope (VLT) opticalspectra, was formed 50-250 Myr ago and makes up a significant fractionof the total stellar mass (1-30 per cent). Despite the core-jetstructure of the radio source, which is consistent with the idea thatthe jet is pointing close to our line of sight, our HI 21-cmobservations reveal significant HI absorption associated with both thecore and the jet. Moreover, the luminous, quasar-like active galacticnucleus (AGN) (MV < -23.5) is highly extinguished(Av > 6.4) at optical wavelengths and show many propertiesin common with narrow-line Seyfert 1 galaxies (NLS1), includingrelatively narrow permitted lines [full width at half-maximum (FWHM) ~1940 km s-1], highly blueshifted [OIII]λλ5007,4959 lines (ΔV ~ 680 km s-1) andevidence that the putative supermassive black hole is accreting at ahigh Eddington ratio (0.3 < Lbol/Ledd < 11).The results suggest that accretion at high Eddington ratio does notprevent the formation of powerful relativistic jets.Together, the observations lend strong support to the predictions ofsome recent numerical simulations of galaxy mergers in which the blackhole grows rapidly through merger-induced accretion following thecoalescence of the nuclei of two merging galaxies, and the major growthphase is largely hidden at optical wavelengths by the natal gas anddust. Although the models also predict that AGN-driven outflows willeventually remove the gas from the bulge of the host galaxy, the visiblewarm outflow in PKS 1549-79 is not currently capable of doing so.However, much of the outflow may be hidden by the material obscuring thequasar and/or tied up in hotter or cooler phases of the interstellarmedium.By combining our estimates of the reddening of the quasar with the HIcolumn derived from the 21-cm radio observations, we have also made thefirst direct estimate of the HI spin temperature in the vicinity of aluminous AGN: Tspin > 3000 K.
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