Detection of diffuse interstellar bands in M 31

Abstract

Aims.We investigate the diffuse interstellar band (DIB) spectrum in the interstellar medium of M 31. Methods. 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 M 31 where the metallicity is approximately equal to solar. Results. The 5780, 5797, 6203, 6283 and 6613 DIBs are detected in both sightlines at velocities matching the M 31 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. Conclusions. The spectrum of DIBs observed in M 31 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 M 31 ISM in the vicinity of OB78 is apparently a highly favourable environment for the formation of DIB carriers. © 2008 ESO.