Copyright © 1992 by Annual Reviews. All rights reserved
|
Annu. Rev. Astron. Astrophys. 1992. 30:
575-611 Copyright © 1992 by Annual Reviews. All rights reserved |
The thermal (free-free) radio luminosity of a normal galaxy indicates the total photoionization rate and hence the current number of the most massive short-lived stars. At low radio frequencies, free-free absorption affects both the thermal and synchrotron sources in normal galaxies, determining their radio spectra and limiting their maximum brightness temperatures to Tb ~ 10 5 K at GKz frequencies.
The free-free absorption coefficient 
of an
H II region is well approximated by
where ne is the electron density, Te
the electron temperature, and 
the
radio frequency expressed in the units indicated. The opacity 
= 
dl
integrated along the line of sight is often written in terms of the emission
measure EM 
ne2 dl (cm -6 pc). The free-free
emission coefficient is
= B , with B =
2kTe2/c2 at
radio frequencies. At sufficiently high
frequencies such that 
1, the thermal spectral luminosity
LT of an H II
region photoionized by hot stars is proportional to the production rate
Nuv of Lyman continuum photons
and varies only weakly with Te (Rubin
1968). The value of Nuv can be estimated from
The inequality allows for possible absorption of Lyman continuum photons
by dust. Values for the unknown Te range from 5 x
103 K
to 104 K in our galaxy (Downes et al. 1980) and are thought to be
comparable in other normal galaxies. The thermal luminosity of our galaxy is
LT ~ 2.5 x 1020 W Hz-1 near = 1 GHz
(Berkhuijsen 1984). The thermal
luminosity LT = 4
d2ST of
M82 at distance d = 3.2 Mpc based on the
thermal flux density ST ~ 0.7 Jy at 1 GHz
(Figure 1) is
LT ~ 1021 W Hz-1,
indicating Nuv ~ 6 x 1053 s-1.
The thermal flux density ST may be compared with other
measures of
Nuv such as the (extinction corrected) H 
line flux F(H ). If N(He+) /
N(H+)~ 0.08, then (Caplan & Deharveng 1986)
The ratios of other frequently observed optical and near-infrared
recombination line fluxes to F(H ) are
These approximations, based on the tables of Hummer & Storey (1987),
are good to ~1% for 102 cm-3 <
ne < 104 cm-3 and 5 x
103 K < Te < 104 K. The flux
F(Br 
) ~ 2 x 10-11 erg
cm-2 s-1 of M82 (Willner et al. 1977)
is consistent with the observed thermal flux density, based on Equations
3 and 4b.
The merits of radio continuum and optical/infrared recombination lines for detecting H II regions, measuring extinction, and deriving Nuv were compared by Kennicutt & Pogge (1990). Radio observations are generally best for extremely luminous H II regions or compact starbursts with high dust extinction even at near-infrared wavelengths. However, care must be taken to avoid contamination by nonthermal emission in giant H II regions (Condon & Yin 1990, Viallefond 1991).