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

Abstract

A B S T R A C TWe present new spectroscopic observations of the southern radio galaxy PKS1549 2 79 ðz ¼ 0:152Þ. 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]ll5007, 4959, are unusually broad for a narrow-line radio galaxy ðFWHM , 1350 km s 21 Þ, and are blueshifted by 600 km s 21 relative to the lowionization lines such as [O II]ll3726,3729. The [O II] lines are also considerably narrower ðFWHM , 650 kms 21 Þ 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 PKS1549 2 79 is a radio source in an early stage of evolution.

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Evolution of [O Iii] Λ5007 Emission-Line Profiles in Narrow Emission-Line Galaxies

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