Dust in the Eye of Andromeda

TL;DR

This study uses high-resolution Herschel images and Bayesian analysis to map dust properties in Andromeda, revealing a nuclear bar with warm dust and evidence of gas inflow, suggesting a nested bar structure similar to other spiral galaxies.

Contribution

First detailed Bayesian dust mapping of M31 revealing a nuclear bar with warm dust and inflow patterns, indicating a nested bar system.

Findings

Nuclear region contains a ~500 pc bar with warm dust (~30 K).

Evidence of gas inflow along the bar towards the nucleus.

Confirmation of radial variation in dust opacity index.

Abstract

We present new Herschel-derived images of warm dust in the Andromeda Galaxy, M31, with unprecedented spatial resolution (~ 30 pc), column density accuracy, and constraints on the three-dimensional distributions of dust temperature and dust opacity index (hence grain size and composition), based on the new PPMAP Bayesian analysis procedure. We confirm the overall radial variation of dust opacity index reported by other recent studies, including the central decrease within ~ 3 kpc of the nucleus. We also investigate the detailed distribution of dust in the nuclear region, a prominent feature of which is a ~ 500 pc bar-like structure seen previously in H{\alpha}. The nature of this feature has been the subject of some debate. Our maps show it to be the site of the warmest dust, with a mean line-of-sight temperature ~ 30 K. A comparison with the stellar distribution, based on 2MASS data, provides strong evidence that it is a gravitationally induced bar. A comparison with radial velocity maps suggests the presence of an inflow towards the nucleus from opposite directions along this bar, fed presumably by the nuclear spiral with which it appears to connect. Such behaviour is common in large-scale bars in spiral galaxies, as is the phenomenon of nested bars whereby a subkiloparsec nuclear bar exists within a large-scale primary bar. We suggest that M31 represents an example of such nesting.