GCN: a gaseous Galactic halo stream?

TL;DR

This paper presents evidence that a string of HI clouds in the Galactic halo forms a gaseous stream likely originating from a dwarf galaxy, with detailed orbital analysis and no identified stellar counterpart.

Contribution

It introduces the identification and orbital modeling of a gaseous Galactic halo stream, suggesting a dwarf galaxy progenitor without a known stellar counterpart.

Findings

The HI clouds form a gaseous stream over 50 degrees in the Galactic halo.

The stream's orbit suggests a progenitor likely a dwarf galaxy at ~20 kpc.

No stellar counterpart has been identified for the stream.

Abstract

We show that a string of HI clouds that form part of the high-velocity cloud complex known as GCN is a probable gaseous stream extending over more than 50 deg in the Galactic halo. The radial velocity gradient along the stream is used to deduce transverse velocities as a function of distance, enabling a family of orbits to be computed. We find that a direction of motion towards the Galactic disk coupled with a mid-stream distance of ~20 kpc provides a good match to the observed sky positions and radial velocities of the HI clouds comprising the stream. With an estimated mass of 10^5 Msun, its progenitor is likely to be a dwarf galaxy. However, no stellar counterpart has been found amongst the currently known Galactic dwarf spheroidal galaxies or stellar streams and the exact origin of the stream is therefore currently unknown.