X-ray galaxies

The currently identified classes of compact extragalactic X-ray sources comprise type 1 Seyfert galaxies, galaxies with active nuclei and relatively sharp emission lines, BL Lac objects and quasars. The known members of these categories are listed. In this paper, the properties of the first two classes are reviewed and discussed, with particular emphasis on their radio and optical properties and the relation between the various nuclear components. Electromagnetic spectra from radio to X-rays are presented for those galaxies detected in more than two wavebands. Optical observations of type 1 Seyferts indicate a wide range of scales for the emitting material. The continuum source and the broad wings on the Balmer lines probably originate in a region less than about 0.1 pc in extent, while the forbidden line region may occupy several hundred parsecs. From the point of view of their radio properties, galaxies with Seyfert or Seyfert-like spectra may be divided into four general categories, namely: (1) powerful (more than 10 25 W Hz -1 sr -1 at 1.4 GHz), very extended, double radio sources with steep spectra; (2) powerful (more than 10 24 W Hz -1 sr -1 at 1.4 GHz) compact radio sources with flat or inverted spectra, the radio emission originating from a small region in the nucleus; (3) weaker (10 20 -10 23 W Hz -1 sr -1 at 1.4 GHz) emission from the nucleus, usually with a steep non-thermal radio spectrum and occasionally an extended ( ca . 50 kpc) radio halo; and (4) no detected radio emission. The distinction between (3) and (4) is probably only a matter of observational selection and almost all Seyferts fall into these last two categories. When spatially resolved, the radio emitting region of category (3) galaxies usually has an approximately similar scale to the forbidden line region. The radio power is correlated with Seyfert type and forbidden line power, which may be interpreted in terms of a pressure balance between the filaments of thermal gas radiating the forbidden lines and the relativistic gas emitting the non-thermal radio radiation. The radio power is, however, uncorrelated with the thermal pressure, suggesting that the differences in radio and forbidden line power from galaxy to galaxy are dominated by variations in overall size or filling factor. Recent discussions of the infrared radiation have proposed a non thermal origin in type 1 Seyferts but a thermal mechanism in type 2 Seyferts. The infrared and radio powers seem to be correlated in both types. The X-rays originate in a region not more than 10 -2 pc in extent, and possibly much smaller. Type 1 Seyferts are intrinsically more powerful X-ray sources than type 2 Seyferts. The X-ray power appears to be correlated with the optical and infrared continuum luminosities and possibly with the Balmer line width. Studies of highly ionized species in the optical emission line spectrum are described with a view to their possible relation to the X-rays. Both [Fe x] λ 6374 Å and [Fe xi] λ 7892 Å are blue-shifted with respect to lines of lower ionization species by an amount ranging up to 300km s -1 . At least in NGC 3783, the line width is correlated with the ionization potential necessary to create the species in the sense that the higher the ionization, the greater the line width. This result fits into the scheme of a systematic decrease in density ( N e > 10 8 -10 3 cm -3 ) and decrease in velocity dispersion(< v > = 10 4 -3 x 10 2 km s -1 ) with increasing distance from the central object ( r ≾ 0.1-5 x 10 2 pc). It is proposed that the highly ionized species occur in a region of intermediate velocity spread (<v> ≈ 1.5 x 10 3 km s -1 ) and distance ( r ≈ 2 pc). X-ray emitting galaxies with active nuclei but relatively sharp emission lines have X-ray luminosities in the range 10 41 -10 43 ergs -1 (1 erg s -1 = 10 -7 W), weaker than type 1 Seyferts (10 43 -10 45 ergs -1 ) but stronger than relatively normal galaxies ( ca .10 39 ergs -1 ). Lines from species of both high and low ionization are found in the optical spectra, implying ionization by a non-thermal continuum or by hot stars. These galaxies have very strong infrared excesses and are often detected as radio sources. They contain appreciable quantities of dust and, presumably, neutral gas and are either a member of an interacting pair or lie in a small group. Possibly the gas has been accreted from the neighbour(s), triggering the X-ray activity.