Ca II and Na I absorption signatures from extraplanar gas in the halo of the Milky Way

Abstract

We analyse absorption characteristics and physical conditions of extraplanar intermediate- and high-velocity gas to study the distribution of the neutral and weakly ionised Milky Way halo gas and its relevance for the evolution of the Milky Way and other spiral galaxies. We combine optical absorption line measurements of CaII/NaI and 21 cm emission line observations of HI along 103 extragalactic lines of sight towards quasars (QSOs) and active galactic nuclei (AGN). The archival optical spectra were obtained with UVES at the ESO VLT, while the 21 cm HI observations were carried out using the 100-m radio telescope at Effelsberg. The analysis of the UVES spectra shows that single and multi-component CaII/NaI absorbers at intermediate and high velocities are present in about 35% of the sight lines, indicating the presence of neutral extraplanar gas structures. In some cases the CaII/NaI absorption is connected with HI intermediate- or high-velocity gas, while other absorbers show no associated HI emission. Our study suggests that the Milky Way halo is filled with a large number of neutral gaseous structures whose high column density tail represents the population of common HI high-velocity clouds seen in 21 cm surveys. The CaII column density distribution follows a power-law which is comparable to the distribution found for intervening metal-line systems toward QSOs. Many of the statistical and physical properties of the CaII absorbers resemble those of strong MgII absorbing systems observed in the circumgalactic environment of other galaxies, suggesting that both absorber populations may be closely related.