Are Newly Discovered HI High Velocity Clouds Minihalos in the Local Group?

TL;DR

This study investigates HI high velocity clouds in the Local Group, suggesting they may be isolated minihalos consistent with LambdaCDM, with specific properties and alternative explanations considered.

Contribution

It presents new HI cloud observations from ALFALFA that align with minihalo models and discusses their properties within the LambdaCDM framework.

Findings

HI masses range from 5x10^4 to 10^6 solar at 1 Mpc distance

HI radii are between 0.4 and 1.6 kpc at 1 Mpc distance

Total halo masses estimated between 10^8 and 10^9 solar

Abstract

A set of HI 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 (1999), the HI clouds found by ALFALFA do not violate any structural requirements or halo scaling laws of the LambdaCDM structure paradigm, nor would they have been detected by extant HI surveys of nearby galaxy groups other than the LG. At a distance of d Mpc, their HI masses range between $5 x 10^4 d^2 and 10^6 d^2 solar and their HI radii between <0.4d and 1.6 d kpc. If they are parts of gravitationally bound halos, the total masses would be on order of 10^8--10^9 solar, their baryonic content would be signifcantly 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.