VIRGOHI21
| VIRGOHI 21 | |
|---|---|
| Observation data (J2000 epoch) | |
| Constellation | Virgo |
| Right ascension | 12h 17m 53.6s |
| Declination | +14° 45′ 25″ |
| Redshift | 1966 km/s |
| Distance | 50 Mly |
| Apparent magnitude (V) | < 27 |
| Notable features | No stars |
VIRGOHI21 is an extended region of neutral hydrogen (HI) in the Virgo cluster discovered in 2005. Analysis of its internal motion indicates that it may contain a large amount of dark matter, as much as a small galaxy. Since VIRGOHI21 apparently contains no stars, this would make it one of the first detected dark galaxies. Skeptics of this interpretation argue that VIRGOHI21 is simply a tidal tail of the nearby galaxy NGC 4254.
VIRGOHI21 was detected through radio telescope observations of its neutral hydrogen 21 cm emissions. The detected hydrogen has a mass of about 100 million solar masses and is about 50 million light-years away. By analyzing the Doppler shift of the emissions, astronomers determined that the gas has a high velocity-profile width; that is, different parts of the cloud are moving at high speed relative to other parts. Follow-up Hubble space telescope deep observations of the region have detected very few stars (a few hundred).
If the high velocity-profile width of VIRGOHI21 is interpreted as rotation, it is far too fast to be consistent with the gravity of the detected hydrogen. Rather, it implies the presence of a dark matter halo with tens of billions of solar masses. Given the very small number of stars detected, this implies a mass-to-light ratio of about 500, far greater than that of a normal galaxy (around 50). The large gravity of the dark matter halo in this interpretation explains the perturbed nature of the nearby spiral galaxy NGC 4254 and the bridge of neutral hydrogen extending between the two entities.
Under this interpretation, VIRGOHI21 would be the first discovery of the dark galaxies anticipated by simulations of dark-matter theories. Although other dark-galaxy candidates have previously been observed, follow-up observations indicated that these were either very faint ordinary galaxies or tidal tails. VIRGOHI21 is considered the best current candidate for a dark galaxy.
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