Dark galaxy (astronomy)

A dark galaxy is a hypothesized galaxy with no, or very few, stars. They received their name because they have no visible stars, but may be detectable if they contain significant amounts of gas. Astronomers have long theorized the existence of dark galaxies, but there are no confirmed examples to date. Dark galaxies are distinct from intergalactic gas clouds caused by galactic tidal interactions, since these gas clouds do not contain dark matter, so they do not technically qualify as galaxies. Distinguishing between intergalactic gas clouds and galaxies is difficult; most candidate dark galaxies turn out to be tidal gas clouds. The best candidate dark galaxies to date include HI1225+01, AGC229385, and numerous gas clouds detected in studies of quasars.

On 25 August 2016, astronomers reported that Dragonfly 44, an ultra diffuse galaxy (UDG) with the mass of the Milky Way galaxy, but with nearly no discernable stars or galactic structure, may be made almost entirely of dark matter.

Large surveys with sensitive, but low resolution radio telescopes like Arecibo or the Parkes Telescope look for 21 cm emission from atomic hydrogen in galaxies. These surveys are then matched to optical surveys to identify any objects with no optical counterpart, i.e. sources with no stars. However, significant follow-up observations are required to show they have detected a dark galaxy. Astronomers need to demonstrate that the gas is rotating like a galaxy (due to the gravitational influence of dark matter), and that the galaxy actually doesn't have any stars.

Another way astronomers search for dark galaxies is to look for hydrogen absorption lines in the spectra of background quasars. This technique has revealed many intergalactic clouds of hydrogen, but following up candidate dark galaxies is difficult, since these sources tend to be too far away, and are often optically drowned out by the bright light from the quasar.

In 2000, astronomers found gas cloud VIRGOHI21 and attempted to determine what it was and why it caused such a gravitational pull on galaxy NGC 4254. After years of running out of other explanations, some have concluded that VIRGOHI21 is a dark galaxy, due to the massive effect it had on NGC 4254.

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