The velocity structure of the interstellar gas towards stars in the globular cluster Omega Centauri

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Abstract
High-resolution spectra of Na I interstellar lines are presented for 16 stars in the globular cluster ω Cen and for a closer star (HD 116226) approximately 1° from the cluster. Towards the cluster stars there are four identified groups of components at the approximate velocities VLSR = –3, –15, –34 and –42 km s–1. There is variation in the detailed velocity structure of the gas across the face of the cluster on an angular scale ≤ 1 arcmin, especially in the higher velocity components. From our own data and from interstellar gas surveys towards nearby stars, it appears that the bulk of the gas towards the distant cluster stars lies beyond the large expanding Sco–Cen shell and the flow of local interstellar material discussed by Crutcher. The origin of the observed absorption features remains uncertain. However, we suggest that the components having low LSR radial velocities are produced by gas associated with the local ridge of molecular clouds mapped by Dame et al., and that the higher velocity components arise in the outlying gas in the Carina spiral arm.

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