Discovery of a large HI ring around the quiescent galaxy AGC 203001

TL;DR

This paper reports the discovery of an enormous, off-centered HI ring around a quiescent galaxy, with faint optical emission, challenging traditional collision-based formation models.

Contribution

It presents the first detailed observation of a large HI ring around a quenched galaxy without significant star formation, suggesting alternative formation scenarios.

Findings

Discovered a 115 kpc HI ring around AGC 203001

The ring shows faint optical emission at low surface brightness

Contradicts typical collision-induced ring galaxy models

Abstract

Here we report the discovery with the Giant Metrewave Radio Telescope of an extremely large ($\sim$115 kpc in diameter) HI ring off-centered from a massive quenched galaxy, AGC 203001. This ring does not have any bright extended optical counterpart, unlike several other known ring galaxies. Our deep $g$, $r$, and $i$ optical imaging of the HI ring, using the MegaCam instrument on the Canada-France-Hawaii Telescope, however, shows several regions with faint optical emission at a surface brightness level of $\sim$28 mag/arcsec$^2$. Such an extended HI structure is very rare with only one other case known so far -- the Leo ring. Conventionally, off-centered rings have been explained by a collision with an "intruder" galaxy leading to expanding density waves of gas and stars in the form of a ring. However, in such a scenario the impact also leads to large amounts of star formation in the ring which is not observed in the ring presented in this paper. We discuss possible scenarios for the formation of such HI dominated rings.