Probing the extended gaseous regions of M31 with quasar absorption lines

Abstract

We present Hubble Space Telescope Cosmic Origins Spectrograph spectra of 10 quasars located behind M31, selected to investigate the properties of gas associated with its extended disc and high-velocity clouds (HVCs). The sightlines have impact parameters ranging between b = 13 and 112 kpc. No absorption is detected in the four sightlines selected to sample any extended disc (or halo) gas that might be present in the outer regions of M31 beyond an impact parameter of b > 57 kpc. Of the six remaining sightlines, all of which lie at b < 32 kpc and within the NHI = 2 × 1018 cm-2 boundary of the HI disc of M31, we detect low-ionization absorption at M31 velocities along four of them (three of which include MgII absorption). We also detect MgII absorption from an HVC. This HVC sightline does not pass through the 21 cm disc of M31, but we detect additional MgII absorption at velocities distinct from the HVC that presumably arises in the halo.We find that along sightlines where both are detected, the velocity location of the low-ion gas tracks the peak in 21 cm emission. High-ionization absorption is detected along the three inner sightlines, but not along the three outer sightlines, for which C IV data exist. As inferred from high-resolution 21 cm emission-line maps of M31's disc and extended regions, only one of the sightlines may be capable of harbouring a damped Lyα system, i.e. with NHI ≥ 2 × 1020 cm-2. This sightline has impact parameter b = 17.5 kpc, and we detect both low- and high-ion absorption lines associated with it. The impact parameters of our observed sightlines through M31 are similar to the impact parameters of galaxies identified with MgII absorbers at redshifts 0.1 < z<1.0 in a 2011 study by Rao et al. However, even if we only count cases where absorption due to M31 is detected, the Mg II λ2796 rest equivalent width values are significantly smaller. In comparison, moderate-to-strong MgII absorption from Milky Way gas is detected along all 10 sightlines. Thus, this study indicates that M31 does not present itself as an absorbing galaxy which is typical of higher-redshift galaxies inferred to give rise to moderate-strength quasar absorption lines. M31 also appears not to possess an extensive large gaseous cross-section, at least not along the direction of its major axis. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.