21 cm Synthesis Observations of VIRGOHI 21—A Possible Dark Galaxy in the Virgo Cluster

Abstract

Many observations indicate that dark matter dominates the extragalactic 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 À1 ) 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 yr ago with a perturber of $10 11 M . 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 H i and find none down to an I-band surface brightness limit of 31:1 AE 0:2 mag arcsec À2 .

H i observations of galaxies in the southern filament of the Virgo Cluster with the Square Kilometre Array Pathfinder KAT-7 and the Westerbork Synthesis Radio Telescope

“…The dark galaxy VIRGOHI 21 was observed in the Virgo Cluster to have a similar H I tail; it is a starless H I cloud, apparently connected by a faint H I bridge to NGC 4254, a spiral galaxy located ∼20 arcmin away (Minchin et al 2007). Later numerical simulations by Duc & Bournaud (2008) showed that the long tail of the cloud may have resulted from a high-velocity collision, probably with the parent galaxy NGC 4254.…”

A deep Parkes H i survey of the Sculptor group and filament: H i mass function and environment

“…Potential detections, such as HIPASS J0731−69 (Ryder et al 2001), are more likely to be the result of tidal or rampressure stripping, in particular as they tend to occur near luminous galaxies with obvious signs of recent interaction. More recently, several 'dark galaxy' candidates have been reported (e.g., Minchin et al 2007;Oosterloo et al 2013), in particular as part of the ALFALFA survey Haynes et al 2011;Janowiecki et al 2015). While the nature of some of these objects is still uncertain, many other 'dark galaxy' candidates have since either been identified as tidal debris Duc & Bournaud 2008;Cannon et al 2015;Taylor et al 2017), or faint stellar counterparts have been detected in deep optical data .…”

Tides in Colliding Galaxies

“…According to a more exotic scenario, star-less intergalactic HI clouds might reveal the presence of so-called "dark galaxies", i.e. galaxies embedded in a massive dark matter halo that would contain very few baryons, only in the form of gas (see the proceedings of IAU symposium 244 dedicated to dark galaxies and Minchin et al, 2007). An example of such objects is VirgoHI21, near the spiral NGC 4254 shown on Figure 8 (bottom panel).…”