The Cosmological Significance of High‐Velocity Cloud Complex H

Abstract

We have used new and archival infrared and radio observations to search for a dwarf galaxy associated with the high-velocity cloud (HVC) known as `complex H.' Complex H is a large (Ω>~400 deg2) and probably nearby (d=27 kpc) HVC whose location in the Galactic plane has hampered previous investigations of its stellar content. The H I mass of the cloud is 2.0×107(d/27 kpc)2 Msolar, making complex H one of the most massive HVCs if its distance is more than ~20 kpc. Virtually all similar H I clouds in other galaxy groups are associated with low surface brightness dwarf galaxies. We selected mid-infrared sources observed by the MSX satellite in the direction of complex H that appeared likely to be star-forming regions and observed them at the wavelength of the CO J=1-->0 rotational transition in order to determine their velocities. Of the 60 observed sources, 59 show emission at Milky Way velocities, and we detected no emission at velocities consistent with that of complex H. We use these observations to set an upper limit on the ongoing star formation rate in the HVC of