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 with the mass of the Milky Way galaxy, but with nearly no discernable stars or galactic structure, is made almost entirely of dark matter.
Observational evidence
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. 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.
Nature of dark galaxy
Origin
In 2005, 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.
Size
The actual size of dark galaxies is unknown because they cannot be observed with normal telescopes. There have been various estimations, ranging from double the size of the Milky Way to the size of a small quasar.
Structure
Dark galaxies are composed of dark matter. Furthermore, dark galaxies are theoretically composed of hydrogen and dust. Some scientists support the idea that dark galaxies may contain stars. Yet the exact composition of dark galaxies is unknown because there is no conclusive way to spot them so far. However, astronomers estimate that the mass of the gas in these galaxies is approximately 1 billion times that of the Sun.
Methodology to observe dark bodies
Dark galaxies contain no visible stars, and are not visible using optical telescopes. The Arecibo Galaxy Environment Survey is a current study using the Arecibo radio telescope to search for dark galaxies, which are predicted to contain detectable amounts of neutral hydrogen. The Arecibo radio telescope is useful where others are not because of its ability to detect the emission from this neutral hydrogen, specifically the 21 cm line.
Alternative theories
Scientists do not have much explanation for some astronomic events, so some use the idea of a dark galaxy to explain these events. Little is known about dark galaxies, and some scientists believe a dark galaxy is actually a newly forming galaxy. One such candidate is in the Virgo cluster. This candidate contains very few stars. Scientists classify this galaxy as a newly forming galaxy, rather than a dark galaxy. Scientists say that the galaxies we see today only began to create stars after dark galaxies. Based on numerous scientific assertions, dark galaxies played a big role in many of the galaxies astronomers and scientists see today. Martin Haehnel, from Kavli Institute for Cosmology at the University of Cambridge, claims that the precursor to the Milky Way galaxy was actually a much smaller bright galaxy that had merged with dark galaxies nearby to form the Milky Way we currently see. Multiple scientists agree that dark galaxies are building blocks of modern galaxies. Sebastian Cantalupo of the University of California, Santa Cruz, agrees with this theory. He goes on to say, "In our current theory of galaxy formation, we believe that big galaxies form from the merger of smaller galaxies. Dark galaxies bring to big galaxies a lot of gas, which then accelerates star formation in the bigger galaxies." Scientists have specific techniques they use to locate these dark galaxies. These techniques have the capability of teaching us more about other special events that occur in the universe; for instance, the “cosmic web”. This “web” is made of invisible filaments of gas and dark matter believed to permeate the universe, as well as “feeding and building galaxies and galaxy clusters where the filaments intersect.”
Potential dark galaxies
HE0450-2958
HE0450-2958 is a quasar at redshift z=0.285. Hubble Space Telescope images showed that the quasar is located at the edge of a large cloud of gas, but no host galaxy was detected for the quasar. The authors of the Hubble study suggested that one possible scenario was that the quasar is located in a dark galaxy. However, subsequent analysis by other groups found no evidence that the host galaxy is anomalously dark, and demonstrated that a normal host galaxy is probably present, so the observations do not support the dark galaxy interpretation.
HVC 127-41-330
HVC 127-41-330 is a cloud rotating at high speed between Andromeda and the Triangulum Galaxy. Astronomer Josh Simon considers this cloud to be a dark galaxy because of the speed of its rotation and its predicted mass.
Smith's Cloud is a candidate to be a dark galaxy, due to its projected mass and survival of encounters with the Milky Way.
VIRGOHI21
Initially discovered in 2000, VIRGOHI21 was announced in February 2005 as a good candidate to be a true dark galaxy. It was detected in 21-cm surveys, and was suspected to be a possible cosmic partner to the galaxy NGC 4254. This unusual-looking galaxy appears to be one partner in a cosmic collision, and appeared to show dynamics consistent with a dark galaxy. However, further observations revealed that VIRGOHI21 was an intergalactic gas cloud, stripped from NGC4254 by a high speed collision. The high speed interaction was caused by infall into the Virgo cluster.
