Search for Diffuse Neutral Hydrogen and HI Clouds in the NGC 2403 Group

TL;DR

This study used the Green Bank Telescope to search for faint neutral hydrogen clouds in the NGC 2403 group, finding likely Milky Way high velocity clouds instead of group-associated clouds, and setting upper limits on their mass and abundance.

Contribution

First sensitive search for diffuse HI clouds in the NGC 2403 group, providing constraints on their presence and properties compared to cosmological model predictions.

Findings

Detected 3 new HI clouds likely part of the Milky Way HVC population.

No HI clouds were found that match the predictions for the NGC 2403 group.

HI content of dark matter halos around NGC 2403 is less than 1% of their total mass.

Abstract

We have observed the NGC 2403 group of galaxies using the Robert C. Byrd Green Bank Telescope (GBT) in a search for faint, extended neutral hydrogen clouds similar to the clouds found around the M81/M82 group, which is located approximately 250 kpc from the NGC 2403 group along the same filament of galaxies. For an HI cloud with a size < 10 kpc within 50 kpc of a group galaxy, our 7-sigma mass detection limit is 2.2 x 10^6 M_sun for a cloud with a linewidth of 20 km/s, over the velocity range from -890 to 1750 km/s. At this sensitivity level we detect 3 new HI clouds in the direction of the group, as well as the known galaxies. The mean velocity of the new clouds differs from that of the group galaxies by more than 250 km/s, but are in the range of Milky Way High Velocity Clouds (HVCs) in that direction. It is most likely that the clouds are part of the Milky Way HVC population. If HI clouds exist in the NGC 2403 group, their masses are less than 2.2 x 10^6 M_sun. We also compared our results to structures that are expected based on recent cosmological models, and found none of the predicted clouds. If NGC 2403 is surrounded by a population of dark matter halos similar to those proposed for the Milky Way in recent models, our observations imply that their HI content is less than 1% of their total mass.