The Inner Disk of M31 is Full of Cold Molecular Gas

Previous abstract Next abstract

Session 76 -- Spirals I
Display presentation, Wednesday, 11, 1995, 9:20am - 6:30pm

[76.09] The Inner Disk of M31 is Full of Cold Molecular Gas

R.J. Allen (STScI), J. Lequeux (Obs. de Paris), L. Loinard (Obs. de Grenoble)

The Andromeda galaxy is conspicuously devoid of the classical tracers of star formation inside a radius of about 8 kpc. This ``hole'' shows up in the distribution of HI, of HII regions, and of CO. The IRAS infrared and radio continuum also show similar ``donut''-shaped distributions, although they have additional nuclear sources.

The conclusion that the inner disk of M31 contains little or no interstellar gas is, however, quite incorrect. We have carried out a sensitive CO survey with the 30m IRAM millimeter radio telescope at more than two dozen randomly-chosen points in the ``hole'' of M31, and find faint CO emission characteristic of giant molecular clouds at more than half of them, often (but not always) coincident with known dust patches. The CO is more than a factor of 10 under-luminous when compared to Galactic GMC's, a consequence of its very low excitation temperature. Estimates of the mass surface density of molecular gas can be made using the virial theorem, leading to values of about 10 solar masses per square parsec averaged over the inner disk.

We conclude that THE ``HOLE'' IN M31 IS FULL OF COLD MOLECULAR GAS. The implications of this for our current views of the distribution of the ISM in galaxies will be discussed.

Wednesday program listing