VIRGOHI 21 is the name given to an intriguing object that is located approximately 50 million light years away in the Virgo Cluster. The Virgo Cluster is a cluster consisting of between one to two thousand member galaxies. VIRGOHI 21 was discovered through radio telescope observations of the 21 centimeter wavelength radio emissions from its neutral hydrogen content. The total mass of hydrogen in VIRGOHI 21 is estimated to be around 100 million times the mass of our Sun. Observations of the motion of hydrogen gas within VIRGOHI 21 shows that the hydrogen gas is moving far too rapidly to be explained by the gravity from just the mass of the detected hydrogen alone. In fact, the total mass of VIRGOHI 21 is inferred to be as large as 100 billion times the mass of our Sun!
Deep observations by the Hubble Space Telescope revealed no optical counterpart to VIRGOHI 21 and this makes VIRGOHI 21 an excellent candidate for a dark galaxy since it has a mass of a galaxy but is entirely devoid of stars. Almost all of the mass which makes up VIRGOHI 21 is expected to be in the form of dark matter and less than a fraction of a percent of its mass is ordinary matter. Dark matter is basically matter whose existence can only be inferred from its gravitational effects due to the fact that dark matter does not scatter nor emit electromagnetic radiation. Interestingly, a paper entitled “Tidal Debris from High-Velocity Collisions as Fake Dark Galaxies: A Numerical Model of VIRGOHI 21” suggests that VIRGOHI 21 may not be a genuine dark galaxy and instead, it could be the result of a high-speed collision between two large galaxies.
Located half a million light years from VIRGOHI 21 is a large spiral galaxy called NGC 4254 and a filamentary structure of hydrogen gas connects VIRGOHI 21 with NGC 4254. This trail of hydrogen gas is almost devoid of stars and its velocity distribution is coherent with the outer disk of the spiral galaxy NGC 4254 to which it is morphologically connected. Furthermore, a tidal origin for this trail of hydrogen gas is unlikely since a counter trail is nonexistent in the opposite direction from the spiral galaxy NGC 4254. Instead, such a feature is consistent with a high speed collision between the spiral galaxy NGC 4254 and another galaxy since an event like this will cause little disturbance to the stars in the main disk of the spiral galaxy NGC 4254, resulting in the lack of stars in the trail of hydrogen gas that connects VIRGOHI 21 with NGC 4254. A high speed collision with another galaxy will also create a counter trail of hydrogen gas that is much fainter and shorter than the main trail. This counter trail will quickly fall back into the disk of the parent spiral galaxy and in a few hundred million years after the collision, a galaxy with just one trail of hydrogen gas will be observed.
The interloper galaxy which collided with the spiral galaxy NGC 4254 is probably a few million light years away by now since the collision is expected to occur at a velocity on the order of a thousand kilometers per second and it is estimated that a couple of billion years would have already elapsed since the collision. VIRGOHI 21 is located along the trail of hydrogen gas and the velocity distribution within VIRGOHI 21 differs remarkably from the rest of the trail. This can occur when denser parts of the trail contract and become self-gravitating. Eventually, a region like this can become an independent object with the mass of a dwarf galaxy, resulting in an object like VIRGOHI 21.
Observing the composition of the filamentary structure which connects VIRGOHI 21 to the spiral galaxy NGC 4254 can provide further evidence to proof if VIRGOHI 21 is a genuine dark galaxy or if it originated from a high speed collision between two galaxies. This is due to the assumption that genuine dark galaxies will be made up of pristine metal-poor gases as there will be no stars to fuse the hydrogen and helium into heavier elements. On the other hand, if VIRGOHI 21 formed out of matter spewed out from the spiral galaxy NGC 4254 after a high speed collision with another galaxy, VIRGOHI 21 will be observed to be enriched with elements heavier than hydrogen and helium from the many episodes of stellar fusion prevalent in the main stellar disk of NGC 4254. In conclusion, a high speed collision could provide an explanation for the origin of putative dark galaxies such as VIRGOHI 21.