The extended HI halo of NGC 4945 as seen by MeerKAT

TL;DR

This study uses MeerKAT observations to reveal a large, previously undetected HI halo around NGC 4945, showing complex gas dynamics likely driven by starburst activity and past interactions.

Contribution

First detailed MeerKAT HI observations of NGC 4945 revealing a significant, extended HI halo and complex gas kinematics, including warps and nuclear outflows.

Findings

Detected 6.8% of HI mass in the halo, previously missed.

Identified a warp in the galaxy's HI distribution.

Observed broad, asymmetric HI absorption lines indicating nuclear outflows.

Abstract

Observations of the neutral atomic hydrogen (HI) in the nuclear starburst galaxy NGC 4945 with MeerKAT are presented. We find a large amount of halo gas, previously missed by HI observations, accounting for 6.8% of the total HI mass. This is most likely gas blown into the halo by star formation. Our maps go down to a $3\sigma$ column density level of $5\times10^{18} cm^{-2}$ . We model the HI distribution using tilted-ring fitting techniques and find a warp on the galaxy's approaching and receding sides. The HI in the northern side of the galaxy appears to be suppressed. This may be the result of ionisation by the starburst activity in the galaxy, as suggested by a previous study. The origin of the warp is unclear but could be due to past interactions or ram pressure stripping. Broad, asymmetric HI absorption lines extending beyond the HI emission velocity channels are present towards the nuclear region of NGC 4945. Such broad lines suggest the existence of a nuclear ring moving at a high circular velocity. This is supported by the clear rotation patterns in the HI absorption velocity field. The asymmetry of the absorption spectra can be caused by outflows or inflows of gas in the nuclear region of NGC 4945. The continuum map shows small extensions on both sides of the galaxy's major axis that might be signs of outflows resulting from the starburst activity.