21-cm synthesis observations of VIRGOHI 21 - a possible dark galaxy in the Virgo Cluster

TL;DR

This study presents evidence that VIRGOHI 21 in the Virgo Cluster is a potential dark galaxy, characterized by a neutral hydrogen disk, interaction with a luminous galaxy, and absence of stars, supporting the existence of dark matter-dominated structures.

Contribution

First imaging of VIRGOHI 21 in neutral hydrogen revealing a possible dark galaxy with a spinning disk, and evidence of its interaction with NGC 4254, supporting its dark matter nature.

Findings

VIRGOHI 21 has a broad line-width (~200 km/s) without visible stars.

Imaging shows a dark, edge-on, spinning disk of neutral hydrogen.

Interaction with NGC 4254 suggests VIRGOHI 21's significant mass.

Abstract

Many observations indicate that dark matter dominates the extra-galactic Universe, yet no totally dark structure of galactic proportions has ever been convincingly identified. Previously we have suggested that VIRGOHI 21, a 21-cm source we found in the Virgo Cluster using Jodrell Bank, was a possible dark galaxy because of its broad line-width (~200 km/s) unaccompanied by any visible gravitational source to account for it. We have now imaged VIRGOHI 21 in the neutral-hydrogen line and find what could be a dark, edge-on, spinning disk with the mass and diameter of a typical spiral galaxy. Moreover, VIRGOHI 21 has unquestionably been involved in an interaction with NGC 4254, a luminous spiral with an odd one-armed morphology, but lacking the massive interactor normally linked with such a feature. Numerical models of NGC 4254 call for a close interaction ~10^8 years ago with a perturber of ~10^11 solar masses. This we take as additional evidence for the massive nature of VIRGOHI 21 as there does not appear to be any other viable candidate. We have also used the Hubble Space Telescope to search for stars associated with the HI and find none down to an I band surface brightness limit of 31.1 +/- 0.2 mag/sq. arcsec.