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.

Optical spectroscopy of the M 15 intermediate velocity cloud

“…Some IVCs (such as the LLIV arch) have similar values to this mean, with others, such as previous measurements towards the M 15 IVC (Langer et al 1990;Kennedy et al 1998b;Meyer & Lauroesch 1999). The Na i/H i column density ratios for the current dataset (based upon H i data of resolution ∼1×2 arcmin), vary from ∼(1−3)×10 −8 , and by as much as a factor of ∼2 in sightlines only separated by 30 arcsec.…”

“…Over scales ranging from 30 arcsec to 1 arcmin, the Na i column density and the Na i/H i ratio varies by upto 70 per cent and a factor of ∼2, respectively. Combining the current sightlines with previously obtained Na i data from Kennedy et al (1998b), the Na i/H i column density ratio over cluster sightlines varies by upto a factor of ∼25, when using H i data of resolution ∼2×1 arcmin. One cluster star, M 15 ZNG-1, was also observed in the Ca i (λair = 4226.728Å) and Ca ii (λair = 3933.663Å) lines.…”

“…Assuming that the IVC is at a distance of 1 kpc, the separation between BBFP 456 and the other two stars is ∼0.15 pc and the approximate transverse cloud size at full width half maximum column density is thus ∼0.3 pc. Following Shaw et al (1996) and Kennedy et al (1998b), if this part of the cloud is spherically symmetric, and assuming a N (Na i)/N (H i) ratio of 4×10 −9 appropriate to high-latitude clouds (Ferlet et al 1985), the resulting volume density for log 10 (N (Na i cm −2 )) = 12.55is n HI ≈ 1000 cm −3 . Clearly if there is sheet-like structure in the IVC, this volume density is an upper limit, and hence we emphasize that given the difficulty in determining the N (Na i)/N (H i) ratio and unknown cloud geometry, this implied volume density is very uncertain.…”

Distances to Galactic High‐Velocity Clouds. I. Cohen Stream, Complex GCP, Cloud g1

“…8). Absorption is not detected toward HD 203699 (D ¼ 0:8 AE 0:1 kpc; Kennedy et al 1998), nor toward several A and F stars at distances between 0.2 and 1.4 kpc (Smoker et al 2001). The nondetections in the latter paper are usually not significant by our criteria, except maybe for their star 7, an A2 V star at a distance of 1.4 kpc.…”

“…It was detected in Ca ii and Na i absorption toward several stars in M15 (D ¼ 10 kpc ;Cohen 1979;Songaila & York 1980;Kennedy et al 1998;Lehner et al 1999). Intermediate-velocity absorption at the same velocity as cloud g1 was also seen toward the star HD 203664 (D ¼ 4:3 AE 0:4 kpc; Albert et al 1993; Little Ryans et al 1996), which lies 3 west of the main cloud core (see Fig.…”