The Infrared Astronomy Satellite (IRAS) survey is used to study large scale properties and the origin of the diffuse emission of the Galaxy. A careful subtraction of the zodiacal light enables longitude profiles of the galactic emission at 12, 25, 60, and 100 microns to be presented

Boulanger, F.

Falgarone, E.

Perault, M.

Puget, J. L.

English

NASA Technical Reports Server

N87- 24279
DIFFUSE INFRARED EMISSION OF THE GALAXY: LARGE SCALE PROPERTIES
M. P_rault, F. Boulanger, E. Falgarone and J.L. Puget.
Radioastronomie, ENS, 24 rue Lhomond, 75231 Paris Cedex 05.
IPAC, Caltech 100-22, Pasadena CA 91125.
The IRAS survey is used to study large scale properties and the origin of the
diffuse emission of the Galaxy. A careful subtraction of the zodiacal light
enables us to present longitude profiles of the galactic emission at 12, 25, 60
and i00 #m.
I. About 2/3 of the power radiated in the I00 #m band comes from the diffuse
medium (atomic, molecular and ionized components), the other 1/3 coming from
well identified luminous sources (complexes of giant molecular clouds (GMC) and
HII regions). The sources have a much lower 12#m/25#m and a much higher
60#m/100#m emission ratio than the diffuse component, giving a very striking
anticorrelation in the colour-colour diagrams. The emission from the sources
will not be further discussed here.
2. The diffuse radiation observed at 12 and 25 #m represents a large fraction
of the far-infrared emission. The absence of any strong colour gradient across
the Galaxy implies that most of the emission up to 60 #m is produced by small
particles through temperature fluctuations.
3. The large scale galactic structure is clearly seen at b=l °, and even much
higher. The dust producing the strong emission excess for longitudes smaller
than 30 @ at b=l ° is located about 150pc above or below the galactic plane, a
height where the gas is mostly atomic hydrogen. The clear difference in gas
scale height between the narrower molecular component and the broader atomic
(and partially ionized) component is used to determine the contribution of each
of these two components to the diffuse emission. Using an axisymmetric
assumption for the infrared production rate, and taking optical depth effects
into account, we simultaneously invert the diffuse infrared longitude profiles
(successively for each of the 4 bands) at b=0 ° and b=l °.
4. At all radii in the Galaxy the diffuse emission is dominated by the broad
atomic component. In the "molecular ring" half of the 100#m emission is
produced by this broad component, half by the sum of the cool molecular
component and the bright sources.
In the solar neighbourhood the dust infrared emission per H atom is 4 times
larger in the atomic gas than in the quiet molecular clouds. For atomic gas
this emissivity is 6 to 8 times larger in the "molecular ring" than in the
solar neighbourhood. These numbers imply that dust in molecular clouds is
efficiently shielded from the external radiation, and that a significant part
of the luminosity of 0 and B stars is absorbed in the atomic component. The
emissivity in this widespread component measures the interstellar radiation
field energy density (with strong weight in the ultra-violet), which increases
almost by a factor of i0 between the solar neighbourhood and the "molecular
ring".
A paper is in preparation, and will be submitted to Astronomy and Astrophysics.
Carol J. Lonsdale Persson (Editor)
Star Formation in Galaxies 97
https://ntrs.nasa.gov/search.jsp?R=19870014846 2020-03-20T10:57:36+00:00Z


Diffuse infrared emission of the galaxy: Large scale properties

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