We present dark-matter minihalo models for the Ultra-Compact High Velocity HI Clouds ( UCHVCs ) recently discovered in the 21 cm ALFALFA survey .
We assume gravitational confinement of 10 ^ { 4 } K HI gas by flat-cored dark-matter subhalos within the Local Group .
We show that for flat cores , typical ( median ) tidally-stripped cosmological subhalos at redshift z = 0 have dark-matter masses of \sim 10 ^ { 7 } M _ { \odot } within the central 300 pc ( independent of total halo mass ) , consistent with the “ Strigari mass scale ” observed in low-luminosity dwarf galaxies .
Flat-cored subhalos also resolve the mass-discrepancy between simulated and observed satellites around the Milky Way .
For the UCHVCs we calculate the photoionization-limited hydrostatic gas profiles for any distance-dependent total observed HI mass and predict the associated ( projected ) HI half-mass radii , assuming the clouds are embedded in distant ( d \gtrsim 300 kpc ) and unstripped subhalos .
For a typical UCHVC ( 0.9 Jy km \leavevmode \nobreak s ^ { -1 } ) we predict physical HI half-mass radii of 0.18 to 0.35 kpc ( or angular sizes of 0.6 to 2.1 arcmin ) for distances ranging from 300 kpc to 2 Mpc .
As a consistency check we model the gas-rich dwarf galaxy Leo T , for which there is a well-resolved HI column density profile and a known distance ( 420 kpc ) .
For Leo T we find that a subhalo with { M _ { 300 } } = 8 \leavevmode \nobreak ( \pm 0.2 ) \times 10 ^ { 6 } M _ { \odot } best fits the observed HI profile .
We derive an upper limit of { P _ { HIM } } \lesssim 150 cm ^ { -3 } K for the pressure of any enveloping hot IGM gas at the distance of Leo T. Our analysis suggests that some of the UCHVCs may in fact constitute a population of 21-cm-selected but optically-faint dwarf galaxies in the Local Group .