The Cosmological Significance of High‐Velocity Cloud Complex H

We have used infrared and radio observations to search for a dwarf galaxyassociated with the high-velocity cloud (HVC) known as Complex H. Complex H isa large (> 400 deg^2) and probably nearby (d = 27 kpc) HVC whose location inthe Galactic plane has hampered previous investigations of its stellar content.The HI mass of the cloud is 2.0 x 10^7 (d/27 kpc)^2 Msun, making Complex H oneof the most massive HVCs if its distance is > ~20 kpc. Virtually all similar HIclouds in other galaxy groups are associated with LSB dwarf galaxies. Weselected mid-IR MSX sources in the direction of Complex H that appeared likelyto be star-forming regions and observed them at the wavelength of the CO 1-0transition. 59 of the 60 observed sources show emission at Milky Wayvelocities, and we detected no emission at velocities consistent with that ofComplex H. We use these observations to set an upper limit on the ongoing starformation rate in the HVC of < 5 x 10^-4 Msun yr^-1. We also searched the 2MASSdatabase for evidence of any dwarf-galaxy-like stellar population in thedirection of the HVC and found no trace of a distant red giant population, withan upper limit on the stellar mass of ~10^6 Msun. Given the lack of evidencefor either current star formation or an evolved population, we conclude thatComplex H cannot be a dwarf galaxy with properties similar to those of knowndwarfs. Complex H is therefore one of the most massive known HI clouds withoutstars. If Complex H is self-gravitating, then it is one of the few known darkgalaxy candidates. These findings may offer observational support for the ideathat the Cold Dark Matter substructure problem is related to the difficulty offorming stars in low-mass dark matter halos; alternatively, Complex H could bean example of a cold accretion flow onto the Milky Way. [slightly abridged]Comment: 16 pages, 7 figures (2 in color), 1 table. Accepted for publication in Ap