Volume 625, May 2019
|Number of page(s)||21|
|Published online||29 May 2019|
M 31 circum-nuclear region: A molecular survey with the IRAM interferometer⋆
LERMA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités, UPMC Univ. Paris 06, 75014 Paris, France
e-mail: firstname.lastname@example.org, email@example.com, firstname.lastname@example.org
2 Collège de France, 11, Place Marcelin Berthelot, 75005 Paris, France
Accepted: 13 February 2019
We analysed molecular observations performed at IRAM interferometer in CO(1-0) of the circum-nuclear region (within 250 pc) of Andromeda with 2.9″ = 11 pc resolution. We detected 12 molecular clumps in this region, corresponding to a total molecular mass of (8.4 ± 0.4)×104 M⊙. These clumps follow Larson’s mass-size relation, but lie well above the velocity-size relation. We discuss the possibility that these clumps are probably not virialised, but are 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 a temperature line ratio below 0.5 in this work. With a radiative transfer 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% and a gas density in the range 60 − 650 cm−3, which is well below the critical density. With a gas-to-stellar mass fraction of 4 × 10−4 and dust-to-gas ratio of 0.01, this quiescent region has exhausted its 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 the possibility that this region lies formally in the low-density part of the Kennicutt-Schmidt law in a regime where the star formation rate estimators are not completely reliable. We confirm the quiescence of the inner part of this galaxy known to lie on the green valley.
Key words: galaxies: individual: M 31 / galaxies: kinematics and dynamics / submillimeter: ISM / molecular data
The reduced spectra are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (18.104.22.168) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/625/A148
© J. Dassa-Terrier et al. 2019
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