Are newly discovered HI high-velocity clouds minihalos in the local group?

Abstract

A set of H I sources extracted from the north Galactic polar region by the ongoing ALFALFA survey has properties that are consistent with the interpretation that they are associated with isolated minihalos in the outskirts of the Local Group (LG). Unlike objects detected by previous surveys, such as the compact high-velocity clouds of Braun & Burton, the H I clouds found by ALFALFA do not violate any structural requirements or halo scaling laws of the ΛCDM structure paradigm, nor would they have been detected by extant H I surveys of nearby galaxy groups other than the LG. At a distance of d Mpc, their H I masses range between 5 × 104 d 2 and 10 6 d 2 M · and their H I radii between <0.4d and 1.6d kpc. If they are parts of gravitationally bound halos, the total masses would be on the order of 108-109 M ·, their baryonic content would be significantly smaller than the cosmic fraction of 0.16 and present in a ionized gas phase of mass well exceeding that of the neutral phase. This study does not however prove that the minihalo interpretation is unique. Among possible alternatives would be that the clouds are shreds of the Leading Arm of the Magellanic Stream. © 2010. The American Astronomical Society. All rights reserved..