The Cosmological Significance of High-Velocity Cloud Complex H

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 M⊙, 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 ≲5 × 10-4 M⊙ yr-1. We also searched the 2MASS database for evidence of any dwarf-galaxy-like stellar population in the direction of the HVC and found no trace of a distant red giant population, with an upper limit on the stellar mass of ∼ 106 M⊙. Given the lack of evidence for either current star formation or an evolved population, we conclude that complex H cannot be a dwarf galaxy with properties similar to those of known dwarfs. Complex H is therefore one of the most massive known H I clouds that does not contain any stars. If complex H is self-gravitating, then this object is one of the few known dark galaxy candidates. These findings may offer observational support for the idea that the cold dark matter substructure problem is related to the difficulty of forming stars in low-mass dark matter halos; alternatively, complex H could be an example of a cold accretion flow onto the Milky Way. © 2006. The American Astronomical Society. All rights reserved.