The head-tail structure of high-velocity clouds - A survey of the northern sky

TL;DR

This survey analyzes high-velocity clouds in the northern sky, revealing a head-tail structure in nearly 20% of the clouds, which suggests interaction with ambient medium and provides insights into their physical properties.

Contribution

The study introduces a comprehensive catalog of HVCs with head-tail structures, highlighting their prevalence and correlation with physical parameters, advancing understanding of HVC interactions.

Findings

Approximately 20% of HVCs exhibit head-tail structures.

A linear correlation exists between head-tail fraction and peak column density.

Head-tail HVCs are likely interacting with ambient medium, showing material stripping.

Abstract

We present new observational results on high-velocity clouds (HVCs) based on an analysis of the Leiden/Dwingeloo HI survey. We cataloged all HVCs with NHI > 1 10^19 cm^-2 and found 252 clouds that form a representative flux limited sample. The detailed analysis of each individual HVC in this sample revealed a significant number of HVCs (nearly 20%) having simultaneously a velocity and a column density gradient. These HVCs have a cometary appearance in the position-velocity representation and are called henceforward head-tail HVCs (HT HVCs). The head is the region with the highest column density of the HVC, while the column density of the tail is in general much lower (by a factor of 2 - 4). The absolute majority of the cataloged HVCs belongs to the well known HVC complexes. With exception of the very faint HVC complex L, all HVC complexes contain HT HVCs. The HT HVCs were analyzed statistically with respect to their physical parameters like position, velocity (v_LSR,v_GSR), and column density. We found a linear correlation between the fraction of HVCs having a head-tail structure and the peak column density of the HVCs. While there is no correlation between the fraction of HT HVCs and v_LSR, we found a dependence of the fraction of HT HVCs and v_GSR. There is no significant correlation between the fraction of HT HVCs and the parameters galactic longitude and latitude. The HT HVCs may be interpreted as HVCs that are currently interacting with their ambient medium. In the context of this model the tails represent material that is stripped off from the HVC core. We discuss the implications of this model for galactic and extragalactic HVCs.