2001MNRAS.322...13S

We present single-dish Arecibo 21-cm Hi observations, covering a 0°.675x0°.625 RA-Dec. grid, of the intermediate-velocity cloud (IVC) centred upon the M15 globular cluster. The velocity and positional structure of the IVC gas at V_LSR=70km.s^–1 are investigated; it is found to be clumpy and has a peak surface density N_Hi∼8x10¹⁹cm^–2. Additionally, we have performed a long Hi integration towards HD 203664, a Galactic halo star some 3°.1 from M15, in which optical IVC absorption has previously been detected. No Hi with a velocity exceeding 60km.s^–1 was found to a brightness temperature limit of 0.05K. However, additional pointings did detect IVC gas approximately mid-way between HD 203664 and M15. Finally, we present both Arecibo Hi pointings and low-resolution spectra in the Caii H and K lines towards 15 field stars in the general field towards M15, in an attempt to obtain the distance to the IVC. Intermediate-velocity Hi is detected towards seven sightlines. Stellar spectral types are derived for 12 of the sample. Assuming that these stars lie on the main sequence, their distances are estimated to lie in the range 150≤d≤1350pc. No Caii absorption is observed, either because the IVC is further away than ∼1350pc or more likely because the gas along these sightlines is of too low a density to be detected by the current observations.