# M31 circum-nuclear region: a molecular survey with the IRAM-interferometer

## TL;DR

This study presents a detailed molecular survey of the circum-nuclear region of Andromeda using IRAM interferometry, revealing quiescent, low-density molecular clumps with no current star formation, consistent with a passive galaxy core.

## Contribution

First high-resolution molecular survey of Andromeda's center, characterizing molecular clumps, their physical state, and star formation activity, highlighting their transient nature and quiescent conditions.

## Key findings

- 12 molecular clumps detected with total mass ~8.4 x 10^4 M_sun
- Clumps are non-virialized and likely transient structures
- Region shows no current star formation and is consistent with passive galaxy core

## Abstract

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.