Radio H I and optical absorption-line spectra of an intermediate-velocity cloud in the general direction of the M15 globular cluster

Abstract

Using H I spectra obtained with the Lovell telescope (FWHM ∼ 12 arcmin) we present maps showing the H I distribution and velocity structure of an intermediate-velocity cloud (IVC; vLSR ∼ 70 km s−1) which is observed in the general direction of the globular cluster M15. The gas is shown to be clumpy in nature and we examine its position and velocity structure. The IVC is detected in absorption in the Ca II K line towards five cluster stars in intermediate resolution spectra obtained with ISIS/WHT and in high resolution UES/WHT Na I D line spectra of two cluster stars (II-75; IV-38). The clumpy nature of the gas is indicated by the Na I and K I spectra obtained in the II-75 and IV-38 sightlines, which have angular separation ∼ 3.5 arcmin. The IVC is detected in K I in the higher column density II-75 sightline; this appears to be the first detection of IVC or HVC gas in K I. The IVC gas towards M15 has a similar velocity to that observed towards HD 203664, some 3.1° away from the cluster. Similarities in the IVC gas velocity suggest a gas structure that extends across both sightlines, although gas column densities are considerably higher towards M15. For a common feature, this would place the M15 IVC at a height above the Galactic plane (z-distance) of ≲ 1.5 kpc based on the Little et al. estimate of the HD 203664 distance. From the fine-scale structure and column density observations, estimates are made of the space density of the small-scale concentrations. However, these remain uncertain and the present observations emphasize the need for higher spatial and spectral resolution studies to provide firmer estimates of cloud properties. We report also on a radio H I and Ca II line survey towards a sample of 24 stars over a wider field. This was carried out in an attempt to detect any wider distribution of the IVC gas and to place better limits on its distance. Although these observations are of sufficient spectral quality, no new optical detections are reported.