![]() |
Annu. Rev. Astron. Astrophys. 1992. 30:
575-611 Copyright © 1992 by Annual Reviews. All rights reserved |
If the radio energy source in a galaxy is not a supermassive black
hole or related nuclear ``monster'', then that galaxy is called ``normal''
in this review, regardless of its optical morphology. Nearly all of
the radio emission from normal galaxies is synchrotron radiation from
relativistic electrons and free-free emission from HII regions.
Thermal reradiation of starlight by dust quickly overwhelms these
components above ~ 200 GHz (
~ 1.5 mm), defining a practical
upper bound to the frequencies of ``radio'' observations. Typical
relative intensities of synchrotron radiation, free-free emission, and
dust reradiation are shown in the radio/far-infrared (FIR) spectrum of
M82 (Figure 1). The radio
continuum from a normal galaxy is clearly
just a tracer, accounting for < 10-4 of its bolometric luminosity.
Figure 1. The observed radio/FIR spectrum
of M82
(Klein et al. 1988, Carlstrom & Kronberg 1991) is the sum
(solid line) of synchrotron (dot-dash line), free-free
(dashed line), and dust (dotted line) components. The H II
regions in this bright starburst galaxy start to become opaque below
~ 1 GHz, reducing both the free-free and
synchrotron flux densities. The free-free component is largest only in
the poorly observed frequency range 30-200 GHz. Thermal reradiation from
T ~ 45 K dust with opacity proportional to
1.5 swamps the radio emission at higher
frequencies. Lower abscissa: frequency (GHz). Upper abscissa: wavelength
(cm). Ordinate: flux density (Jy).
Only stars more massive than M ~ 8 M
produce the Type II and Type Ib supernovae whose remnants (SNRs)
are thought to accelerate most of the relativistic electrons in
normal galaxies (see Section 4.3), and
these massive stars ionize the
H II regions as well. Such massive stars live
3 x 10 7 yr, and the
relativistic electrons
probably have lifetimes
108 yr. Radio observations are therefore
probes of very recent star-formation activity in normal galaxies, and
they are especially valuable for three reasons: (a) The radio emission
from normal galaxies is not overwhelmed by stellar populations older
than about 10 8 yr. (b) Radio maps can be made with
subarcsecond position accuracy and resolution, unambiguously
identifying the most luminous star-forming regions within galaxies and
resolving even the most compact ones. (c) Only at radio and FIR
wavelengths are the most intense ``starbursts'' transparent, so
that observed flux densities are accurately proportional to intrinsic
luminosities.
On the other hand, existing radio data alone are poor constraints for
quantitative models of star formation. The free-free emission emerges
directly from H II regions containing the ionizing stars and its
intensity is proportional to the production rate of Lyman continuum
photons, but isolating the free-free component and measuring its flux
density is difficult observationally because the flat-spectrum (
~ +0.1, where the spectral index
is defined
by S
-
, S
being the flux density and
the frequency)
free-free emission is
usually weaker than the steep-spectrum (
~
+0.8) synchrotron emission below
~ 30 GHz
(Figure 1). Most of the easily observed
synchrotron radiation in a typical galaxy arises from fairly old (
10 7 yr)
relativistic electrons
that have propagated significant
distances (
1
kpc) from their
short-lived (
10
5 yr) and now
defunct parent SNRs. Consequently the original sources of the
relativistic electrons have disappeared, and their detailed spatial
distribution has been smoothed beyond recognition. Finally, the steps
between star formation and synchrotron emission (supernova explosion,
acceleration of relativistic electrons in the SNR, propagation of
cosmic rays throughout the galaxy, energy loss, and escape) are poorly
understood, impeding quantitative interpretation of the observed
synchrotron spectra and brightness distributions.
Fortunately, the remarkably tight and ubiquitous correlation found
between the global FIR and (predominantly nonthermal) radio
luminosities of normal galaxies (see Figure 8 later) is emerging as an
almost miraculous constraint for models relating radio emission to
massive star formation. Except perhaps for galaxies with very low
star-formation rates, the FIR luminosity appears to be a good measure
of the bolometric luminosity produced by fairly massive (M 5 M
)
young stars. The FIR / radio correlation suggests that a one-parameter
model specifying the FIR and radio luminosities in terms of the recent
star-formation rate can describe most normal galaxies. Secondary
parameters apparently do not vary significantly from galaxy to galaxy
or are irrelevant. For example, the magnetic energy densities in
galaxies obeying the FIR / radio correlation span four orders of
magnitude, so the total synchrotron energy produced per SNR must be
almost independent of magnetic field strength.
This review emphasizes those aspects of radio emission and recent
star formation that appear to be shared by most normal galaxies and
their application to a simple model consistent with, if not uniquely
required by, the data. The range of source properties found in the
population of normal galaxies is outlined in Section 2. The
characteristics of free-free emission and synchrotron radiation that
are needed to interpret observations of normal galaxies are outlined
in Sections 3 and 4. Frequently used equations are presented
in
astronomically convenient units, and their consequences for real
observations are stressed. The textbooks by Pacholczyk (1970) and
Longair (1981) are recommended
for broader coverage of these topics.
Section 5 is devoted to the FIR / radio
correlation. Population-synthesis models relating the FIR / radio
emission to recent star formation are covered in Section 6. Section 7
summarizes the current situation and future hopes for understanding
the radio emission from normal galaxies.