Probing the inner halo and IVC gas through the Local Interstellar Chimney

Abstract

We present an absorption study of the interstellar gas at high positive galactic latitudes in the direction close to the axis of the Local Chimney (LC), which is an extension of the rarefied local cavity that reaches out from the galactic disk to a z-distance of at least 250 pc into the lower halo region. Our study includes high-resolution (R ∼ 1. 7k m s −1 ) spectral observations of the interstellar NaI and CaII absorption lines seen towards 6 early-type stars with distances ranging from 225 to 500 pc contained within a radius of ∼6 ◦ along the sight-line (l = 160 ◦ , b =+ 55 ◦ ). These visible data are supplemented with far-ultraviolet absorption measurements of the interstellar sight-lines towards two hot white dwarf stars, RE J1043+490 (d = 230 pc) and RE J1059+512 (d = 315 pc), taken with the NASA Far Ultraviolet Spectroscopic Explorer (FUSE) satellite. Our observations reveal interstellar gas clouds with velocities in the −20 to −60 km s −1 range that appear to be falling towards the galactic disk. In particular, we have detected absorption with a velocity of Vhelio ∼− 55 km s −1 towards two sight-lines (HD 89501 and HD 88545) that can be associated with an intermediate velocity (IV) cloud called the IV Arch. Our observations place a probable z-distance to this IVC of 275-320 pc, this being much nearer than previously thought. The far UV spectra of the two hot white dwarfs show only a few (∼10) interstellar absorption lines, which is indicative of the very low density phase of the interstellar gas contained within the LC region. We have derived relative elemental abundance ratios for C, N, O, Si, Ar and Fe for these two sight-lines and find that the abundance patterns are very similar to those found for other sight-lines in the local cavity. The high ionization line of OVIλ1032 A has been detected towards RE J1043+490 with a column density of 7 × 10 12 cm −2 , which is consistent with the average space density of this ion recently found in the Local Bubble region. Finally, we (tentatively) propose a possible connection between the formation of the Gould Belt, the Local Bubble cavity, the Local Chimney and the overlying IV Arch clouds.