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Annu. Rev. Astron. Astrophys. 1992. 30:
575-611 Copyright © 1992 by Annual Reviews. All rights reserved |
In principle, the relatively flat-spectrum (
0.1) thermal
radio emission should be distinguishable from the steeper-spectrum
(
~ 0.8) nonthermal emission via total flux
densities or maps obtained at two or more frequencies. In practice, most
normal galaxies are not bright enough to be detected at frequencies much
higher than
~ 10 GHz (
~ 3 cm), so the observed thermal fractions
ST / S, where S is the total flux density, are
small and most measured ST are uncertain by a factor
of two. Other indicators available for large samples of galaxies
(e.g. H
and FIR fluxes) suggest
that the rms scatter in ST / S from galaxy to
galaxy is not more than a factor of two (Kennicutt 1983b), except
possibly for dwarf galaxies. Thus, the average
< ST / S > ratio obtained from these samples is
likely a better estimate of individual ST / S
than the poorly observed values for many normal galaxies.
Klein & Emerson (1981) first
succeeded in measuring good 10.7-GHz
integrated flux densities for a small sample of nearby spiral galaxies with
the Effelsberg 100-m telescope. The steepest power-law fit to flux densities
measured at several lower frequencies was extrapolated to 10.7 GHz and
subtracted from the observed flux density of each galaxy to remove the
dominant nonthermal contribution, thereby determining
ST. They detected
no significant evidence for spectral flattening due to a thermal component
emerging at 10.7 GHz and set upper limits ST /
S 0.4 to
the thermal
fractions at 10.7 GHz. Likewise, Gioia et al. (1982) did not find
flattening at 10.7 GHz in a larger sample of normal spiral galaxies with
known 408
MHz and 4.9 GHz flux densities. They assumed a nonthermal spectral
index
= 0.8, subtracted the nonthermal
component to obtain ST, and also concluded
ST / S
0.4 at
10.7 GHz for most galaxies. Israel &
van der Hulst (1983) claimed a
marginal statistical detection, < ST / S ~ 0.3 >
at 10.7 GHz, in a large sample of galaxies observed with the OVRO 130-ft
telescope.
If integrated flux densities at only three or four frequencies are used to
fit the (unknown) nonthermal spectral index and the thermal fraction
simultaneously, the resulting nonthermal spectral index and thermal
fraction are strongly correlated. Consequently, the actual errors in
these quantities may be much larger than the formal uncertainties
estimated by 2 tests. The
(nonthermally dominated)
low-frequency spectral indices of most spiral galaxies actually fall
within a fairly narrow range: <
> = 0.74,
0.12 in the Gioia et al. (1982) sample, and <
> = 0.75,
= 0.10
(Condon 1983). Individual values
of ST derived from
fits implying nonthermal spectral indices outside the range 0.6
1.0 (cf
Duric et al. 1988, Skillman & Klein 1988, Klein et al. 1991) may be
affected by this correlation.
The thermal fraction is controversial even in the brightest and
best-observed normal galaxies such as M82
(Figure 1) because the
nonthermal component may not have a straight spectrum. By assuming a straight
nonthermal spectrum for M82, Klein
et al. (1988) calculated a thermal flux
density ST = 0.15 Jy at = 32
GHz and deduced an ionization rate of only Nuv ~ 2 x
1053 s-1. Using
practically the same data but assuming that the nonthermal spectrum
steepens at high frequencies, Carlstrom & Kronberg (1991) obtained
ST = 0.5 Jy at
= 92 GHz and
estimated Nuv ~ 9 x 1053
s-1. The difficulty of distinguishing nonthermal
spectral bends from different thermal fractions is illustrated by the
calculated spectra in Figure
6 shown
later (see Section 4.1). In the
case of M82, Nuv can be estimated
independently from the H53
radio
recombination-line flux. This high-frequency (43 GHz) line is dominated
by spontaneous emission and is unaffected by dust extinction. It
indicates Nuv ~ 1054 s-1 (Puxley
et al. 1989), in better agreement with a bent nonthermal
spectrum and large thermal fraction at 92 GHz. The prototypical
ultraluminous FIR galaxy Arp 220 may be similar. A power-law
extrapolation of its low-frequency nonthermal spectrum yields
ST / S < 0.3 at
= 110
GHz, leading Scoville et
al. (1991) to favor a monster
in an AGN as the energy source rather than a starburst whose initial
mass function (IMF) would have to be truncated above 25 M
to
produce the low observed Nuv / LFIR
ratio. However, a normal IMF starburst would be consistent with the data
if the high-frequency nonthermal spectrum of Arp 220 steepens and the
thermal flux is a factor of two larger.
The distribution of the ratio S / F(H) provides an independent
constraint on the average thermal fraction of normal disk
galaxies. Kennicutt (1983b) found
a strong correlation between
S1.4 GHz and F(H
)
even though the total flux densities are completely dominated by
nonthermal emission at 1.4 GHz. The observed average < S1.4
GHz / F(H
) > corrected for extinction
indicates that normal disk galaxies have an average thermal fraction
< ST / S > ~ 0.1 at
=
1.4 GHz, and the scatter about this average is approximately a factor of
two. The FIR/radio correlation can also be used to estimate the average
thermal fraction for normal galaxies (Condon & Yin 1990). Discrete Galactic
IRAS sources coincide with thermal radio sources in luminous H II
regions, not supernova remnants or other nonthermal
radio sources (Haslam & Osborne
1987). The average FIR/radio flux
density ratio of these thermal radio sources is about a factor of ten higher
than the average FIR/radio ratio of normal galaxies at
~ 1.4 GHz, suggesting a fairly good upper limit <
ST / S >
0.1 at
= 1.4 GHz. Thus,
at frequencies sufficiently high to make the free-free opacity
small, the approximation (Condon & Yin 1990)
where ~ 0.8 is a typical nonthermal
spectral index, probably yields a better estimate of the global thermal
fraction than do existing radio spectra for most galaxies.
Spatial isolation of the brightest H II regions in radio maps is easier to
achieve than global spectral separation of the thermal and nonthermal
components. The radio emission from H II regions is much clumpier than
the rather smooth synchrotron disk and spiral arms, so their derived
distribution is fairly insensitive to the assumed nonthermal spectral index
of the subtracted smooth background (Klein et al. 1982). The Effelsberg
100-m telescope has been used to separate the thermal and nonthermal
components in maps of M33 (Buczilowski 1988), M81 (Beck et al. 1985),
M101
(Gräve et al. 1990),
and NGC 6946 (Klein et
al. 1982). Multifrequency single-dish maps of the Large Magellanic
Cloud (Klein et al. 1989) show
the relatively large contribution from H II regions typical of dwarf irregular
galaxies. Most of the moderately compact (D 20 pc) radio sources seen
in high-resolution maps of nearby spiral disks are H II regions; see the
maps of M31
(Braun 1990), M51
(van der Hulst et al. 1988), and
NGC 4736
(Duric & Dittmar 1988).
Scaled arrays of the VLA can map the brightest sources in normal
galaxies with similar (u, v)-plane coverage and high
resolution at three
frequencies 1.5, 5, and 15 GHz. Fits for the nonthermal spectral index
and the thermal flux density at each resolution element in the map yield
separate maps of the thermal emission, the nonthermal emission, and the
nonthermal spectral index. Duric et
al. (1986) mapped NGC 3310 in this
way with 4".5 FWHM resolution. They deduced a thermal fraction
ST / S ~ 0.1 at = 1.5
GHz and obtained an impressive correlation of
the thermal radio brightness and H
brightness distributions. The large
reported spectral-index gradient (from
~
0.3 near the center to
~ 1.5
at the edge) of the smoother nonthermal disk is more sensitive to errors
introduced by limited (u,v) coverage, primary-beam
corrections, etc. The
total flux density integrated over any large-scale interferometer map is
actually zero, and every source is surrounded by a negative bowl that
depresses the bowl-shaped positive emission from a disk. The negative
bowl is wide and shallow if short baselines are present, and it may be
largely removed by the CLEAN deconvolution algorithm (Cornwell &
Braun 1989). Even so, observers can miss a lot of extended flux without
realizing it, as Hummel & Gräve
(1990) have pointed out in their paper
containing excellent multifrequency maps of IC 342 made with the 100-m
telescope and the VLA. Other spectral maps combining single-dish and
interferometer data include those of M51 (Tilanus et al. 1988) and M83
(Sukumar et al. 1987).