AGN-driven outflows and the AGN feedback efficiency in young radio
  galaxies
  Active galactic nuclei (AGN) feedback operated by the expansion of radio jets
can play a crucial role in driving gaseous outflows on galaxy scales. Galaxies
hosting young radio AGN, whose jets are in the first phases of expansion
through the surrounding interstellar medium (ISM), are the ideal targets to
probe the energetic significance of this mechanism. In this paper, we
characterise the warm ionised gas outflows in a sample of nine young radio
sources from the 2Jy sample, combining X-shooter spectroscopy and Hubble Space
Telescope (HST) imaging data. We find that the warm outflows have similar
radial extents (~0.06-2 kpc) as radio sources, consistent with the idea that
`jet mode' AGN feedback is the dominant driver of the outflows detected in
young radio galaxies. Exploiting the broad spectral coverage of the X-shooter
data, we have used the ratios of trans-auroral emission lines of [SII] and
[OII] to estimate the electron densities, finding that most of the outflows
have gas densities ($\log( n_e~cm^{-3})~3-4.8 $), which we speculate could be
the result of compression by jet-induced shocks. Combining our estimates of the
emission-line luminosities, radii, and densities, we find that the kinetic
powers of the warm outflows are a relatively small fraction of the energies
available from the accretion of material onto the central supermassive black
hole (SMBH), reflecting AGN feedback efficiencies below 1% in most cases.
Overall, the warm outflows detected in our sample are strikingly similar to
those found in nearby ultraluminous infrared galaxies (ULIRGs), but more
energetic and with a high feedback efficiencies on average than the general
population of nearby AGN of similar bolometric luminosity; this is likely to
reflect a high degree of coupling between the jets and the near-nuclear ISM in
the early stages of radio source evolution.
