Incidence of ionized gas in E/SO radio galaxies

Abstract

SEVERAL nearby E and SO galaxies are known to contain weak (L_rad∼10³⁹erg s⁻¹) radio sources^1–3. Heeschen⁴ found that these sources fall into two distinct classes: ‘extended’ sources with steep radio spectra and sizes comparable to their parent galaxies, and ‘compact’ sources with flat spectra, which are centred on the optical nucleus and have sizes smaller than a few arc s. Although ionised gas is generally inconspicuous in E/SO galaxies, spectroscopy of ∼24 E/SO radio sources^3,5,6 suggests a high incidence of optical emission lines in galaxies with compact radio structure. The two best-known examples, NGC1052 and NGC4278, have, in addition to unusually strong emission lines, abnormal amounts of neutral hydrogen^7–9, and this has led to the speculation¹⁰ that radio outbursts in early type galaxies are fuelled by accretion of neutral gas, possibly from intergalactic clouds. As part of an optical study of E/SO sources with flux densities higher than 20 mJy from the deep Arecibo radio survey of Dressel and Condon¹¹, we have recently obtained spectrophotometry of 57 objects which makes the radio structure–optical line emission correlation more definitive. The 2,380 MHz Arecibo survey of bright Uppsala General Catalogue galaxies included 2,095 objects with 0°≤δ≤+37°. 24% of the E, and 12% of the SO galaxies were detected at the 5σ level (S₂₃₈₀>15 mJy). Interferometer observations¹² of the brighter sources allowed separation into the compact (CRS) or extended (ERS) categories; continuing work with higher resolution indicates that several ERS objects have a well-defined double structure (J. J. Condon, personal communication). The optical observations were made with the image dissector scanner on the Kitt Peak National Observatory 2.1 m telescope in December 1977. The scanner was operated with an 8.4-arc s diameter entrance aperture, which yielded an effective resolution of ∼ 11 Å. Scans covered λλ 3400–5200. We confine our attention here to [O II] λ 3727, which is expected to be ∼ 10 times stronger against the galaxy background in objects with spectra like NGC1052 (ref. 8) than is Hβ.