Detailed maps of interstellar clouds in front of ω Centauri: Small-scale structures in the Galactic disc-halo interface

The multiphase interstellar medium (ISM) is highly structured, on scales from the size of the Solar System to that of a galaxy. In particular, small-scale structures are difficult to study and hence are poorly understood. We used the multiplex capabilities of the AAOmega spectrograph at the Anglo-Australian Telescope to create a half-square-degree map of the neutral and low-ionized ISM in front of the nearby (∼5 kpc), most massive Galactic globular cluster, ω Centauri. Its redshifted, metal-poor and hot horizontal branch stars probe the medium-strong Ca ii K and Na i D₂ line absorption, and weak absorption in the λ5780 and λ5797 diffuse interstellar bands (DIBs), on scales around a parsec. The kinematical and thermodynamical picture emerging from these data is that we predominantly probe the warm neutral medium and weakly ionized medium of the Galactic Disc-Halo interface, ∼0.3-1 kpc above the mid-plane. A comparison with Spitzer Space Telescope 24 μm and DIRBE/IRAS maps of the warm and cold dust emission confirms that both Na i and Ca ii trace the overall column density of the warm neutral and weakly ionized medium. Clear signatures are seen of the depletion of calcium atoms from the gas phase into dust grains. Curiously, the coarse DIRBE/IRAS map is a more reliable representation of the relative reddening between sightlines than the Na i and Ca ii absorption-line measurements, most likely because the latter are sensitive to fluctuations in the local ionization conditions. The behaviour of the DIBs is consistent with the λ5780 band being stronger than the λ5797 band in regions where the ultraviolet radiation level is relatively high, as in the Disc-Halo interface. This region corresponds to a σ-type cloud in which Ca i and small diatomic molecules such as CH and CN are usually absent. In all, our maps and simple analytical model calculations show in unprecedented detail that small-scale density and/or ionization structures exist in the extra-planar gas of a spiral galaxy.