MUSE three-dimensional spectroscopy and kinematics of the gigahertz peaked spectrum radio galaxy PKS 1934-63: interaction, recently triggered active galactic nucleus and star formation

Abstract

We observe the radio galaxy PKS 1934-63 (at z = 0.1825) using the Multi-Unit Spectroscopic Explorer (MUSE) on the Very Large Telescope (VLT). The radio source is a gigahertz peaked spectrum source and is compact (0.13 kpc), implying an early stage of evolution (≤10⁴ yr). Our data show an interacting pair of galaxies, with projected separation 9.1 kpc and velocity difference Δ(v) = 216 km s^-1. The larger galaxy is a M_* ≃ 10¹¹ M_⊙ spheroidal with the emission-line spectrum of a high-excitation young radio active galactic nucleus (AGN; e.g. strong [O I]6300 and [O III]5007). Emission-line ratios indicate a large contribution to the line luminosity from high-velocity shocks (≃ 550 km s^-1). The companion is a non-AGN disc galaxy, with extended Hα emission from which its star formation rate is estimated as 0.61 M_⊙ yr^-1. Both galaxies show rotational velocity gradients in Hα and other lines, with the interaction being prograde-prograde. The SE-NW velocity gradient of the AGN host is misaligned from the E-W radio axis, but aligned with a previously discovered central ultraviolet source, and a factor of 2 greater in amplitude in Hα than in other (forbidden) lines (e.g. [O III]5007). This could be produced by a fast rotating (100-150 km s^-1) disc with circumnuclear star formation. We also identify a broad component of [O III]5007 emission, blueshifted with a velocity gradient aligned with the radio jets, and associated with outflow. However, the broad component of [O I]6300 is redshifted. In spectral fits, both galaxies have old stellar populations plus ∼0.1 per cent of very young stars, consistent with the galaxies undergoing first perigalacticon, triggering infall and star formation from ∼40 Myr ago followed by the radio outburst.