The high-velocity cloud complex Galactic Center Negative as seen by EBHIS and GASS. I. Cloud catalog and global properties

TL;DR

This study uses high-resolution surveys to resolve the Galactic Center Negative high-velocity cloud complex into numerous tiny clumps, revealing its substructure and suggesting it may be an example of warm-gas accretion onto the Milky Way.

Contribution

The paper provides a detailed catalog of 243 HVC clumps with higher resolution data, uncovering subpopulations and challenging previous notions of diffuse counterparts.

Findings

Resolved GCN into 243 tiny clumps.

Found only 5% of clumps have cold-line components.

Suggests GCN is an example of warm-gas accretion.

Abstract

Using Milky Way data of the new Effelsberg-Bonn HI Survey (EBHIS) and the Galactic All-Sky Survey (GASS), we present a revised picture of the high-velocity cloud (HVC) complex Galactic Center Negative (GCN). Owing to the higher angular resolution of these surveys compared to previous studies (e.g., the Leiden Dwingeloo Survey), we resolve Complex GCN into lots of individual tiny clumps, that mostly have relatively broad line widths of more than 15 km/s. We do not detect a diffuse extended counterpart, which is unusual for an HVC complex. In total 243 clumps were identified and parameterized which allows us to statistically analyze the data. Cold-line components (i.e., w < 7.5 km/s) are found in about 5% only of the identified cloudlets. Our analysis reveals that Complex GCN is likely built up of several subpopulations that do not share a common origin. Furthermore, Complex GCN might be a prime example for warm-gas accretion onto the Milky Way, where neutral HI clouds are not stable against interaction with the Milky Way gas halo and become ionized prior to accretion.