The impact of the warm outflow in the young (GPS) radio source and ULIRG PKS 1345+12 (4C 12.50)

We present new deep Very Large Telescope (VLT)/FORS optical spectra with intermediate resolution and large wavelength coverage of the compact radio source and ultra-luminous IR galaxy (ULIRG) PKS 1345+12 (4C 12.50; z = 0.122), taken with the aim of investigating the impact of the nuclear activity on the circumnuclear interstellar medium (ISM). PKS 1345+12 is a powerful quasar [L(H beta)(NLR) similar to 1042 erg s-1] and is also the best studied case of an emission line outflow in a ULIRG. Using the density-sensitive transauroral emission lines [S ii]4068,4076 and [O ii]7318,7319,7330,7331, we pilot a new technique to accurately model the electron density for cases in which it is not possible to use the traditional diagnostic [S ii]6716/6731, namely sources with highly broadened complex emission line profiles and/or high (N-e greater than or similar to 104 cm-3) electron densities. We measure electron densities of N-e = (2.94+0.71(-1.03)) x 103 cm-3, N-e = (1.47+0.60(-0.47)) x 104 cm-3 and N-e = (3.16+1.66(-1.01)) x 105 cm-3 for the regions emitting the narrow, broad and very broad components, respectively. We therefore calculate a total mass outflow rate of M-circle dot yr-1, similar to the range estimated for another compact radio source, PKS 1549-79. We estimate the total mass in the warm gas outflow is M-total = (8+3(-3)) x 105 M-circle dot with filling factors of epsilon = (4.4+1.8(-1.5)) x 10-4 and epsilon = (1.6+0.7(-0.5)) x 10-7 for the regions emitting the broad and very broad components, respectively. The total kinetic power in the warm outflow is erg s-1. Taking the black hole properties published by Dasyra et al., we find that only a small fraction of the available accretion power is driving the warm outflow in PKS 1345+12, which is significantly less than that currently required by the majority of quasar feedback models (similar to 5-10 per cent of L-bol), but similar to the recent suggestion of Hopkins & Elvis if a two-stage feedback model is implemented (similar to 0.5 per cent of L-bol). The models also predict that active galactic nuclei (AGN)-driven outflows will eventually remove the gas from the bulge of the host galaxy. Our observations show that the visible warm outflow in PKS 1345+12 is not currently capable of doing so. However, it is entirely possible that much of the outflow is either obscured by a dense and dusty natal cocoon and/or in cooler or hotter phases of the ISM. This result is important not just for studies of young Gigahertz-Peaked Spectrum/Compact Steep Spectrum radio sources, but for AGN in general.