Dust in the outskirts of M31 and M33

TL;DR

This study maps dust distribution in the outskirts of M31 and M33 using stellar data, revealing extensive dust beyond optical radii and complex structures, which enhances understanding of dust cycling in spiral galaxies.

Contribution

The paper presents a high-resolution dust reddening map of M31 and M33 outskirts, combining Gaia and LAMOST data, revealing dust distributions extending beyond optical radii.

Findings

Dust detected in M31 halo up to 100 kpc.

Dust in M31 and M33 disks extends to about 2.5 times their optical radii.

Dust distribution fits exponential disk models with specific scale lengths.

Abstract

M31 and M33 serve as ideal places to study distributions of dust in the outskirts of spiral galaxies. In this letter, using about 0.2 million stars selected from the LAMOST data and combining precise photometry and parallaxes from the Gaia DR2, we have constructed a two-dimensional foreground dust reddening map towards the M31 and M33 region ($111.2{\deg} {\le} gl {\le} 136.2{\deg}, -36.5{\deg} {\le} gb {\le} -16.5{\deg}$). The map has a typical spatial resolution of about 12 arc-minute and precision of 0.01 mag. The complex structure of dust clouds towards the M31 is revealed. By carefully removing the foreground extinction from the dust reddening map of Schlegel et al. (1998), we thus have obtained a residual map to study dust distributions in the outskirts of M31 and M33. A large amount of dust is detected in the M31 halo out to a distance of over 100 kpc. Dust in the M31 disk is found to extend out to about 2.5 times its optical radius, whose distribution is consistent with either an exponential disk of scale length of 7.2 kpc or two disks with scale length of 11.1 kpc within its optical radius and 18.3 kpc beyond its optical radius. Dust in the disk of M33 is also found to extend out to about 2.5 times its optical radius, its distribution beyond one optical radius is consistent with an exponential disk of scale length of 5.6 kpc. Our results provide new clues on the distributions and cycling of dust in galaxies.