Detection of diffuse interstellar bands in M31

TL;DR

This study detects and analyzes diffuse interstellar bands in M31, revealing similarities to the Milky Way and suggesting a conducive environment for DIB carrier formation near OB78.

Contribution

First detection and comparison of DIB spectra in M31's interstellar medium, highlighting environmental differences from the Milky Way.

Findings

DIBs detected at velocities matching M31 Na I absorption.

DIB spectrum in M31 similar to Milky Way near MAG 63885.

Higher DIB equivalent widths per reddening near MAG 70817.

Abstract

We investigate the diffuse interstellar band (DIB) spectrum in the interstellar medium of M31. The DEIMOS spectrograph of the W. M. Keck observatory was used to make optical spectroscopic observations of two supergiant stars, MAG 63885 and MAG 70817, in the vicinity of the OB78 association in M31 where the metallicity is approximately equal to solar. The 5780, 5797, 6203, 6283 and 6613 DIBs are detected in both sightlines at velocities matching the M31 interstellar Na I absorption. The spectra are classified and interstellar reddenings are derived for both stars. Diffuse interstellar band (DIB) equivalent widths and radial velocities are presented. The spectrum of DIBs observed in M31 towards MAG 63885 is found to be similar to that observed in the Milky Way. Towards MAG 70817 the DIB equivalent widths per unit reddening are about three times the Galactic average. Compared to observations elsewhere in the Universe, relative to reddening the M31 ISM in the vicinity of OB78 is apparently a highly favourable environment for the formation of DIB carriers.