Ionization cones and radio ejecta in active galaxies

A. S. Wilson, Z. I. Tsvetanov

Research output: Contribution to journalArticle

179 Citations (Scopus)

Abstract

We report radio mapping at three frequencies of the Seyfert 2 galaxy NGC 5252, which is known to exhibit a spectacular pair of "ionization cones" in optical emission-line images. The radio structure of the galaxy comprises an unresolved (<50 pc) source coincident with the optical nucleus, weak, narrow features extending ≃900 pc to north and south from the nucleus, and an unresolved radio source some 10 kpc from the nucleus. The inner parts of the extended radio structure and the off-nuclear source align well with the axis of the ionization cones. There are currently 11 Seyfert galaxies known to possess an ionization cone or a bi-cone; 8 of these galaxies also contain a linear (double, triple, or jet-like) nuclear radio structure. For this limited, incomplete sample, there is a tight alignment between cone and radio axes: the formal mean difference between the measured projections of these axes on the sky is only 6°, and the alignment may well be better than this at the location(s) closer to the nucleus where the collimation occurs. Although the degree of collimation is much worse for the ionizing photons than for the radio plasma, it is clear that they are collimated by the same, or coplanar, nuclear disks or tori. In particular, if the ionization cones result from absorption by dusty tori on the pc scale and the radio ejecta from accretion disks around the central black hole, the absence of differential precession indicates that either the gravitating mass distribution is close to spherical or the dusty torus has settled into a preferred plane. The cones currently known in late-type (but not early-type) spirals show a trend to align with the axis of the galaxy stellar disk. We argue that this alignment is either an observational selection effect or indicates that the gas accreted to power the nuclear activity has an internal origin in late-type spirals, but may have an external origin (e.g., a galaxy merger) in early-types.

Original languageEnglish
Pages (from-to)1227-1234
Number of pages8
JournalAstronomical Journal
Volume107
Issue number4
Publication statusPublished - 1 Apr 1994

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

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