| Annu. Rev. Astron. Astrophys. 1992. 30:
575-611 Copyright © 1992 by . All rights reserved |
Impressive advances in both observation and interpretation have been
made in the time since the general review of radio emission from spiral
galaxies by van der Kruit & Allen
(1976). The VLA has provided the speed
to map hundreds of galaxies in complete samples, the sensitivity to detect
dozens of individual radio supernovae and SNRs in external galaxies, the
resolution to map nearby galaxies in detail and to resolve even the most
compact nuclear starbursts, and the ability to make maps at several
frequencies with similar (u, v)-plane coverage. Equally important is
IRAS, whose FIR sky survey provided complete samples of normal galaxies
ranging from the quiescent to the ultraluminous (LFIR
> 1011 L
) and
showed that the FIR/radio correlation is remarkably strong and universal.
The FIR/radio correlation is the single most important factor in the
widespread acceptance of massive stars as the energy source for both the FIR
and nonthermal radio emission from normal galaxies. It also suggests
that the star-formation rate SFR(M 
5 M) is the
dominant variable controlling the range of FIR and radio luminosities
observed. Other factors, such as magnetic field strength, either play a
minor role or do not vary significantly from ordinary disk galaxies to
luminous nuclear starbursts.
Models relating the thermal radio, nonthermal radio, and FIR
luminosities of galaxies to SFR(M 5 M) can now be
constructed that are consistent with, but not required by, the data. For
example, the empirical relation between the supernova rate and
nonthermal radio luminosity (Equation 18) does not explain how cosmic
rays are generated in supernova
remnants, propagate in the interstellar medium, and lose their energy or
escape from the galaxy; it merely sweeps these problems under the rug.
But such models do highlight the problems that remain to be solved by
better analysis and observation. Existing data on the strong nearby source
in M82 indicate what might be done with other normal
galaxies using
improved instruments in the near future. Direct estimates of the supernova
rate are badly needed to pin down the relation between nonthermal radio
luminosity and the SFR. The VLA is capable of monitoring the number
of SNRs in M82, their flux densities, and even their angular
sizes. Periodic
VLA observations of other nearby galaxies would be sensitive enough to
detect new radio supernovae and determine their average radio supernova
rate without the optical limitations of confusion by starlight and extinction
by dust. At most we have an outline of what might be; only the
future will bring the full picture of what must be.
ACKNOWLEDGEMENTS
The National Radio Astronomy Observatory is operated by Associated Universities, Inc., under cooperative agreement with the National Science Foundation. I thank Pat Crane, George Helou, Allan Sandage, Joan Wrobel, and especially Ko Hummel for many valuable suggestions and comments.