HI holes and high-velocity clouds in the spiral galaxy NGC 6946

TL;DR

This study presents deep 21-cm observations of NGC 6946, revealing widespread high-velocity HI, numerous HI holes, and evidence of stellar feedback, tidal interactions, and gas accretion affecting the galaxy's gaseous halo and kinematics.

Contribution

It provides the deepest HI observations of NGC 6946, identifying high-velocity gas, HI holes, and linking these features to star formation, feedback, and external interactions.

Findings

Detection of high-velocity HI up to 100 km/s

Identification of 121 HI holes mainly in the inner regions

Evidence of tidal interactions and gas accretion

Abstract

We present a study of the distribution and kinematics of the neutral gas in the low-inclination Scd galaxy NGC 6946. The galaxy has been observed for 192 hours at 21-cm with the Westerbork Synthesis Radio Telescope. These are among the deepest observations ever obtained for a nearby galaxy. We detect widespread high-velocity HI (up to about 100 km/s) and find 121 HI holes, most of which are located in the inner regions where the gas density and the star formation rate are higher. Much of the high-velocity gas appears to be related to star formation and to be, in some cases, associated with HI holes. The overall kinematics of the high-velocity gas is characterized by a slower rotation as compared with the regular disk rotation. We conclude that the high-velocity gas in NGC 6946 is extra-planar and has the same properties as the gaseous halos observed in other spiral galaxies including the Milky Way. Stellar feedback (galactic fountain) is probably at the origin of most of the high-velocity gas and of the HI holes. There are also indications, especially in the outer regions, -an extended HI plume, velocity anomalies, sharp edges, and large-scale asymmmetries- pointing to tidal encounters and recent gas accretion.