THE ORIGIN OF THE FAR INFRARED LUMINOSITY WITHIN THE SPIRAL GALAXY NGC
6946
ABSTRACT
High resolution, 45", measurements of the 160mm
and Ha luminosity have been examined at 158
locations within the spiral galaxy
NGC 6946 in order to elucidate the origin of the far infrared luminosity.
After correcting the Ha luminosity for extinction
and extrapolating
the 160mm luminosity to a 40 - 1000mm
far infrared luminosity we find that the far infrared luminosity is comparable
to that expected
from the O and B stars which are required to ionize the hydrogen gas.
The
results show that the far infarred luminosity is in
quantitativeagreement with that expected from O and B stars at all
locations within NGC 6946. Further, the IRAS
HiRes 60 and 100mm images of NGC 6946 indicate
that the temperature of the dust primarily responsible for the far infrared
luminosity is ~ 33 K ( l-1 )
and similar to that expexted for HII regions over the entire star forming
disk. The mass of dust required
to radiate the 60 and 100mm emission, measured
for NGC 6946 by IRAS, is about a factor of 10 lower than the dust content
of NGC 6946
estimated from the atomic and molecular gas masses and a Galactic gas/dust
ratio. The most plausible explanation for the discrepency is
that the majority (~ 90% ) of the dust within NGC 6946 is radiating
at wavelengths longward of 100mm, and must therefore
be cooler
than the ~ 33K dust that dominates the emission measured by IRAS. The
170 and 160mm measurements of NGC 6946, obtained
by Smith (1984) and Engargiola (1991), provide independent support
for large quantities of cold dust, as both the 160 and
170mm measurements are significantly higher
than would be expected if all the dust in NGC 6946 were radiating at the
same
temperature as that measured by IRAS. The mass of warm dust is used
to constrain the mass of dust that radiates longward of
100mm, and the shape of the far infrared
spectral energy distribution is used to constrain the temperatureof
the dust. The
observations require the bulk, ~ 90%, of the dust mass within NGC 6946
to be cold with a temperature < 17 K. It has long been
known that molecular gas dominates the interstellar medium within NGC
6946 and the temperature derived for the dust is consistent
with that expected for molecular clouds. The far infrared luminosityof
NGC 6946 is dominated by thermal emission from warm,
~33 K, dust even though most (~ 90% ) of the dust mass within
NGC 6946 is much colder with a temperature of ~ 17 K. The
ability for a small mass of warm dust to dominate the luminosity of
significantly greater quantities of colder dust arises as a natural
consequence of the strong temperature dependence of thermal emission.
More
details appear in the Astronomical Journal, Devereux & Young,
1993, Vol 106, 948.