Extended neutral hydrogen filamentary network in NGC 2403

TL;DR

This study reveals an extensive filamentary neutral hydrogen network in NGC 2403, likely caused by a recent galaxy interaction with DDO 44, challenging previous outflow-based explanations.

Contribution

It provides new high-resolution HI observations showing an extended filamentary structure linked to galaxy interaction, not outflows.

Findings

The HI complex extends over 20 kpc, with a total mass of 2×10^7 M_sun.

The filament is likely caused by a recent galaxy interaction, not galactic fountains.

A stellar stream connecting NGC 2403 and DDO 44 overlaps with the HI filament.

Abstract

We present new neutral hydrogen (HI) observations of the nearby galaxy NGC 2403 to determine the nature of a low-column density cloud that was detected earlier by the Green Bank Telescope. We find that this cloud is the tip of a complex of filaments of extraplanar gas that is coincident with the thin disk. The total HI mass of the complex is $2\times10^{7}\text{ M}_\odot$ or 0.6% of the total HI mass of the galaxy. The main structure, previously referred to as the 8-kpc filament, is now seen to be even more extended, along a 20 kpc stream. The kinematics and morphological properties of the filaments are unlikely to be the result of outflows related to galactic fountains. It is more likely that the 20 kpc filament is related to a recent galaxy interaction. In this context, a $\sim$ 50 kpc long stellar stream has been recently detected connecting NGC 2403 with the nearby dwarf satellite DDO 44. Intriguingly, the southern tip of this stream overlaps with that of 20 kpc HI filament. We conclude that the HII anomalies in NGC 2403 are the result of a recent ($\sim2\text{ Gyr}$) interaction with DDO 44 leading to the observed filamentary complex.