Dense gas tracing the collisional past of Andromeda

Abstract

The central kiloparsec region of the Andromeda galaxy is relatively gas poor, while the interstellar medium appears to be concentrated in a ring-like structure at about 10 kpc radius. The central gas depletion has been attributed to a possible head-on collision 200 Myr ago, supported by the existence of an offset inner ring of warm dust. We present new IRAM 30 m radio telescope observations of the molecular gas in the central region, and the detection of CO and its isotopes 13 CO(2−1) and C 18 O(2−1), together with the dense gas tracers, HCN(1−0) and HCO+(1−0). A systematic study of the observed peak temperatures with non-local thermal equilibrium simulations shows that the detected lines trace dense regions with n H 2 in the range 2.5 × 10 4 −5.6 × 10 5 cm −3 , while the gas is very clumpy with a beam filling factor of 0.5−2 × 10 −2 . This is compatible with the dust mass derived from the far-infrared emission, assuming a dust-to-gas mass ratio of 0.01 with a typical clump size of 2 pc. We also show that the gas is optically thin in all lines except for 12 CO(1-0) and 12 CO(2-1), CO lines are close to their thermal equilibrium condition at 17-20 K, the molecular hydrogen density is larger than critical, and HCN and HCO+ lines have a subthermal excitation temperature of 9 K with a density smaller than critical. The average 12 CO/ 13 CO line ratio is high (∼21), and close to the 12 CO/C 18 O ratio (∼30) that was measured in the north-western region and estimated in the south-east stacking. The fact that the optically thin 13 CO and C 18 O lines have comparable intensities means that the secondary element 13 C is depleted with respect to the primary 12 C, as is expected just after a recent star formation. This suggests that there has been a recent starburst in the central region, supporting the head-on collision scenario.

Evidence for non-axisymmetry in M 31 from wide-field kinematics of stars and gas

“…However, the bar in NGC 2683 is only at an angle of 5 • to the disk major axis, so it is seen quite side-on, whereas the bar in M31 is at an angle of 17 • to the disk major axis and it is seen more end-on, which can explain the differences according to Kuzio de Naray et al (2009). Melchior & Combes (2011) also measure molecular emission lines in CO with two peaks, in Melchior & Combes (2016) sometimes even three. While the molecular gas is not expected to follow the ionized gas, it is still interesting that they also observe a line splitting.…”

Exhaustion of the gas next to the supermassive black hole of M31

“…This is much larger than the apparent mean density n app ∼ (M H 2 /m P )/[4/3πR app 3 ] ∼ 150 cm −3 . This would correspond to a volume filling factor smaller than 3.5 × 10 −5 , which is well below the volume filling factor (∼ 1%) estimated in Melchior & Combes (2016) in the gas present along the minor axis in the inner bulge. This suggests that the gas is clumped at a smaller scale, as observed in the Galaxy (e.g.…”

M 31 circum-nuclear region: A molecular survey with the IRAM interferometer

“…As described in Sect. 4.1 and in Melchior & Combes (2016), η b f provides the beam filling factor computed for a brightness temperature of 5 K relying on the RADEX results presented in Sect 4.1 and 30 K assuming it is thermalised with the dust. T C and T W correspond to the cold and warm dust components, M dust and M star are the dust and stellar masses scaled to the prorata of the 12 CO FWHM beam.…”