The distance to two hydrogen clouds: The HVC complex A and the Low-Latitude Intermediate-Velocity Arch

A lower limit of 4 kpc (Z > 3 kpc) is derived for the distance to the high-velocity cloud (HVC) complex A. The HVC is detected toward the Seyfert galaxy Mark 106 in Mg II lambda lambda 2796, 2803 absorption spectra taken with HST, proving that Mg II is present in the cloud. It is not detected in the Mg II spectra of two stars, PG 0859+593 (distance 4 kpc) and PG0906+597 (distance 0.7 kpc). The distances to the stars are derived by matching Stromgren photometry and intermediate-resolution spectroscopy with model stellar atmospheres; they are estimated to be accurate to within 25%. From a combination of Effelsberg data and Westerbork maps with 2 or 3 arcmin resolution we show that the H I column density and thus the Mg II abundance in the direction of the two stars is sufficiently high for the non-detections to imply that the HVC is behind the stars. This distance limit can be used to eliminate several recent models for complex A which placed it nearby.