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BIB-VERSION:: AST-PP-v1.0
ID:: epreps.stsci//prep1206
ENTRY:: March 9, 1998
TITLE:: Cold Massive Molecular Clouds in the Inner Disk of M31
SUBTITLE::
AUTHOR:: Loinard, Laurent (1)(2)
AUTHOR:: Allen, Ronald J. (1)
AFFIL:: (1) Space Telescope Science Institute 3700 San Martin Drive Baltimore, MD 21218 USA
AFFIL:: (2) Observatoire de Grenoble, Laboratoire d'Astrophysique, Université Joseph Fourier, B.P. 53X, F-38041 Grenoble Cedex France
DATE:: December 1997
JOURNAL:: To appear in: The Astrophysical Journal
SUBMITTED:: 10 October 1997
ACCEPTED:: 9 December 1997
OTHER_ACCESS::
COPYRIGHT:: Copyright 1997 The Association of Universities for Research in Astronomy, Inc. All Rights Reserved.
LANGUAGE:: English
ABSTRACT::

We present new interferometric ¹²CO(1-0) and single-dish
¹²CO(3-2) observations of the central parts of D478, a
large (> 200 pc) dark dust cloud located in a
quiescent region in the inner disk of M31 where single-dish
¹²CO(1-0) and ¹²CO(2-1) observations were
previously obtained. Only a small fraction (< 15%) of the
¹²CO(1-0) flux previously detected in this region with the
single-dish telescope is recorded by the interferometer. Most of
the ¹²CO(1-0) emission must therefore have the appearance
of a smooth surface with very little structure on scales smaller than
~ 25" (85 pc). Together with the earlier
¹²CO(1-0) and ¹²CO(2-1) single-dish results
the new ¹²CO(3-2) data are in good agreement with LTE
predictions for optically thick lines at T_ex = T_kin = 3.5K. These results rule out
the conventional model for these clouds consisting of warm clumps
with a low filling factor (as would be the case if they resembled
Galactic GMCs) and confirm that large, massive, cold molecular clouds
exist in the inner disk of M31 with kinetic temperatures close to
that of the cosmic microwave background. Such extremely low
temperatures are likely to be a consequence of the low heating rate
in these particular regions of M31, where very little massive star
formation is occurring at present. From the ¹²CO line
profile widths we estimate the Virial mass surface density of
D478 to be 80 - 177 M pc^-2. This is a factor
7 - 16 times larger than the value obtained by multiplying
the ¹²CO profile integrals with the conventional
"X-factor".

END:: epreps.stsci//prep1206