M31 circum-nuclear region: a molecular survey with the
  IRAM-interferometer
  We analyse molecular observations performed at IRAM interferometer in CO(1-0)
of the circum-nuclear region (within 250 pc) of Andromeda, with 2.9 arcsec = 11
pc resolution. We detect 12 molecular clumps in this region, corresponding to a
total molecular mass of (8.4 +- 0.4) x 10**4 Msol. They follow the Larson's
mass-size relation, but lie well above the velocity-size relation. We discuss
that these clumps are probably not virialised, but transient agglomerations of
smaller entities that might be virialised. Three of these clumps have been
detected in CO(2-1) in a previous work, and we find temperature line ratio
below 0.5. With a RADEX analysis, we show that this gas is in non local thermal
equilibrium with a low excitation temperature (Tex = 5-9 K). We find a surface
beam filling factor of order 5 percent and a gas density in the range 60-650
cm**{-3}, well below the critical density. With a gas-to-stellar mass fraction
of 4 x 10**{-4} and dust-to-gas ratio of 0.01, this quiescent region has
exhausted his gas budget. Its spectral energy distribution is compatible with
passive templates assembled from elliptical galaxies. While weak dust emission
is present in the region, we show that no star formation is present and support
the previous results that the dust is heated by the old and intermediate
stellar population. We study that this region lies formally in the low-density
part of the Kennicutt-Schmidt law, in a regime where the SFR estimators are not
completely reliable. We confirm the quiescence of the inner part of this galaxy
known to lie on the green valley.
