13.2.1. Spiral, Seyfert, and Irregular Galaxies
For normal spiral galaxies, much of the radio emission comes primarily from the disk. In a few nearby spirals where the distribution of radio emission can be mapped, the spiral structure is clearly evident. This diffuse component of the nonthermal emission appears to be unrelated to that of the much more luminous radio galaxies and quasars but rather to the optical luminosity and morphology of the galaxy. High-resolution observations indicate that the characteristic size of radio nuclei of spirals is about 100 pc, but significant emission sometimes comes from a region less than a parsec across (Dressel and Condon 1978, Hummel 1980, van der Hulst et al. 1981, Heckman et al. 1983a).
These latter sources are probably related to the powerful compact sources found in AGNs and quasars. The discovery of flat-spectrum, compact, variable sources in the nuclei of M81 and M104 (de Bruyn et al. 1976) gave the first evidence of activity in the nuclei of normal spirals of the -type characteristic of quasars and AGNs. High-resolution VLBI observations of M81 show that the radio nucleus is only about 0.01 pc in extent. Based largely on the presence of narrow emission lines in the optical spectrum, M81 is sometimes classified as a Seyfert galaxy. But most Seyfert galaxies are not strong radio sources, although a few, such as NGC 1068, have prominent radio cores and jets which have been, extensively studied (e.g., Wilson 1982). Often the cores of Seyfert galaxies show a low-frequency spectral turnover due to free-free absorption by ionized gas in the emission-line region, with emission measures in the range of 104 to 106 cm2. Meurs and Wilson (1984) discuss the radio emission from Seyfert galaxies and their relation to other radio sources.
A small fraction of spiral and irregular galaxies show enhanced radio emission which is closely correlated with the 10 - µm infrared flux density and apparently corresponds to regions where there are bursts of star formation (e.g., Condon 1982). At one time the dramatic visual image of M82, particularly in H, was interpreted as evidence of explosive activity characteristic of the powerful radio galaxies. More recent work, however, has shown that the radio emission from M82 comes from a number of small (less than a few parsecs) discrete components probably related to supernovae events or to regions of intense star formation with intrinsic luminosities as much as 100 times that of Cas A. The strongest of these is known to be variable and is only a few hundred AU in size (Kronberg et al. 1981).
Observations by Stocke (1978) indicate enhanced radio emission from interacting pairs of galaxies, although, curiously, the excess emission comes primarily from a compact core, and not from the disk (e.g., Hummel 1980).