![]() | Annu. Rev. Astron. Astrophys. 1991. 29:
581-625 Copyright © 1991 by Annual Reviews. All rights reserved |
9.1 M82
M82 is one of the first galaxies in which CO was detected
(Rickard et al 1975,
Solomon & de Zafra
1975),
and the CO emission in this nearly
edge-on irregular galaxy has been mapped extensively (see
Table 2 for
references). The total H2 mass within the inner 6 kpc could
be as high
as 2 x 109 M (assuming the standard galactic CO to H2
conversion), in
which case, it is an order of magnitude greater than the HI mass
within the same region. It is one of the few galaxies in which the
molecular emission is sufficiently bright to enable studies of
molecules other than CO.
Rickard & Palmer (1977)
first detected HCN
and CS here. In addition, 13CO J = 1-0 is observed
with an intensity approximately 1/20 that of CO
(Stark & Carlson 1984,
Young & Scoville 1984),
and the CO J = 2-1 emission is significantly brighter than the
J = 1-0 emission at the central position
(Knapp et al 1980,
Sutton et al 1983).
Both of these characteristics have led a number of authors
to suggest that the CO emission in the center of M82 is optically
thin, an excitation regime seldom seen in galactic GMCs.
Carlstrom (1989)
has published high resolution aperture synthesis
maps of the CO, HCO+, HCN, and 3 mm continuum emission, as shown in
Figure 13 together with the 6 cm and 2 µm
continuum. The 2 µm
continuum arises mostly from late-type giant stars; the 6 cm continuum
is mainly synchrotron emission from high energy particles injected
into the ISM from supernovae; and the 3 mm continuum is predominately
free-free emission from H II regions, ionized by young stars. All of
the distributions show an elongation of the central galactic disk, and
within the disk all of the molecular line maps show a clear
double-peaked structure. The latter feature was first discovered by
Nakai et al (1987)
and is generally interpreted as an edge-on torus at
200 pc radius. The total mass of gas in this structure is 0.6-1 x
108
M, which is 10% of the
dynamical mass within the same region. The
central peak in the 2 µm continuum probably represents the nuclear
star cluster with a significant enrichment by super-giant stars
produced from the starburst. It is noteworthy that the distribution of
nonthermal radiation shown in the 6 cm continuum shows roughly the
same spatial extent as the molecular torus, but no evidence of the two
peaks seen in the molecular emission. Comparison of the 3 mm continuum
map with the distribution of gas seen in the CO, HCO+, and HCN
suggests that at present, the formation of high mass stars as probed
by the distribution of free-free emission is occurring predominately
to the west of the nucleus.
![]() |
Figure 13. The central disk of M82 with CO (5.5" resolution) on an R-band gray scale image; the 2 µm continuum (Telesco et al 1991); HCO+ (10" resolution); HCN (10" resolution); 6-cm radio continuum (0.35" resolution, Kronberg et a1 1985); and 3 mm continuum (6" resolution). The diamond denotes the position of the 2 µm ``stellar'' peak. The molecular images and the 3 mm continuum are from Carlstrom (1989). |
Based upon an excitation analysis of the HCN and HCO+ emission,
Carlstrom finds that the density of the gas in the emitting regions
probably exceeds 5 x 104 cm-3, that is, a factor
of 200 higher than
the mean volume densities of galactic GMCs. This dense gas has a
filling factor 0.1%. The
Lymann continuum production rate required
to explain the 3 mm free-free emission is 6 x 1053
sec-1
(Carlstrom 1989),
exceeding by a factor of four the Lyman continuum production
rate required for the entire Milky Way
(Mezger 1978).
The implied rate of star formation within the molecular torus is
6-10 M
yr-1
assuming a Miller-Scalo initial mass function with an upper mass
cutoff of 45 M
and
lower mass cutoffs of 0.1-1 M
(see
Scoville & Soifer
1991).
At this rate, the gas in the torus at 200 pc radius
would be entirely converted into stars in 107 years (if low
mass stars
do indeed form) and in order to maintain the nuclear starburst, gas
must flow in from further out in the galaxy. If low mass stars do not
form, the starburst can be sustained for a considerably longer time.