Optically Unseen HI Detections towards the Virgo Cluster detected in the Arecibo Legacy Fast ALFA Survey

TL;DR

This paper reports the discovery of eight optically unseen HI clouds in the Virgo Cluster via the ALFALFA survey, revealing new features of the cluster's gaseous environment, including potential dark galaxy candidates and tidal features.

Contribution

The study presents the first detection of multiple optically unseen HI features in the Virgo Cluster, including a candidate dark galaxy and tidal structures, expanding understanding of cluster gas dynamics.

Findings

Eight HI features detected without optical counterparts.

Some clouds associated with known galaxies, others isolated.

Detection of VIRGOHI21 as a tidal feature.

Abstract

We report the discovery by the Arecibo Legacy Fast ALFA (ALFALFA) survey of eight HI features not coincident with stellar counterparts in the Virgo Cluster region. All of the HI clouds have cz < 3000 km/s and, if at the Virgo distance, HI masses between 1.9 x 10**7 and 1.1 x 10**9 solar masses. Four of the eight objects were reported or hinted at by previous studies and "rediscovered" by ALFALFA. While some clouds appear to be associated with optical galaxies in their vicinity, others show no clear association with a stellar counterpart. Two of them are embedded in relatively dense regions of the cluster and are associated with M49 and M86; they were previously known. The others are mostly located in peripheral regions of the cluster. Especially notable are a concentration of objects towards the so-called M cloud, 3 to 5 degrees to the NW of M87, and a complex of several clouds projected roughly halfway between M87 and M49. The object referred to as VIRGOHI21 and proposed to be a "dark galaxy" is also detected and shown to be a tidal feature associated with NGC 4254.