Emission-line outflows in PKS1549-79: the effects of the early stages of radio-source evolution?

We present new spectroscopic observations of the southern radio galaxy PKS1549 - 79 (z = 0.152). Despite the flat-spectrum character of the radio emission from this source, our optical spectra show no sign of the broad permitted lines and non-stellar continuum characteristic of quasar nuclei and broad-line radio galaxies. However, the high-ionization forbidden lines, including [O III]lambda lambda 5007, 4959, are unusually broad for a narrow-line radio galaxy (FWHM similar to 1350 km s(-1)), and are blueshifted by 600 km s(-1) relative to the low-ionization lines such as [O II]lambda lambda 3726,3729. The [O II] lines are also considerably narrower (FWHM similar to 650 kms(-1)) than the [O III] lines, and have a redshift consistent with that of the recently detected H I 21-cm absorption-line system. Whereas the kinematics of the [O III] emission lines are consistent with outflow in an inner narrow-line region, the properties of the [O II] emission lines suggest that they are emitted by a more extended and quiescent gaseous component. We argue that, given the radio properties of the source, our line of sight is likely to be lying close to the direction of bulk outflow of the radio jets. In this case it is probable that the quasar nucleus is entirely obscured at optical wavelengths by the material responsible for the H I absorption-line system. The unusually broad [O III] emission lines suggest that the radio source is intrinsically compact. Overall, our data are consistent the idea that PKS 1549 - 79 is a radio source in an early stage of evolution.