A Lower Limit to the Distance of High-Velocity Cloud Complex H

Abstract

We derive a lower limit for the distance of the high-velocity cloud (HVC) complex H, which is a structure covering 480 square degrees on the sky and is centered on l = 131°, b = 1°. Considering the uncertainties in the derivation of stellar distances, we find that the distance to the HVC is certainly larger than 3.4 kpc, probably larger than 5 kpc, and possibly larger than 6.5 kpc. This distance limit is based on the result that we do not find absorption associated with the HVC in IUE spectra of 17 OB stars. The three most distant of these stars were observed by us; we used the IUE archives to analyze the spectra of the other 14 stars. We do not have conclusive evidence that heavy elements are present in this HVC. This would require a detection of absorption in the spectrum of an extragalactic background source. However, the nondetections can still be considered secure, as the column density detection limits for the Mg II λλ2796, 2802, C II λ1334, and O I λ1302 lines are a factor of 30-4100 below the column density values expected for normal interstellar medium gas phase abundances. Our lower limit to the distance is used to discuss possible origins of HVC complex H. It seems unlikely that it is associated with a superbubble at large Galactocentric radii, an infalling dwarf galaxy, or the outer arm. It might be an unusual Galactic fountain cloud or an intergalactic cloud.