A dark galaxy is a hypothesized

galaxy A galaxy is a system of stars, stellar remnants, interstellar gas, dust, dark matter, bound together by gravity. The word is derived from the Greek ' (), literally 'milky', a reference to the Milky Way galaxy that contains the Solar System. ...

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 Dark matter is a hypothetical form of matter thought to account for approximately 85% of the matter in the universe. Dark matter is called "dark" because it does not appear to interact with the electromagnetic field, which means it does not ab ...

, 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 Dragonfly 44 is an ultra diffuse galaxy in the Coma Cluster. This galaxy is well-known because observations of the velocity dispersion in 2016 suggested a mass of about one trillion solar masses, about the same as the Milky Way. This mass was con ...

, an

ultra diffuse galaxy An ultra diffuse galaxy (UDG) is an extremely low luminosity galaxy, the first example of which was discovered in the nearby Virgo Cluster by Allan Sandage and Bruno Binggeli in 1984. These galaxies have been studied for many years prior to their ...

(UDG) with the

mass Mass is an intrinsic property of a body. It was traditionally believed to be related to the quantity of matter in a physical body, until the discovery of the atom and particle physics. It was found that different atoms and different elementar ...

of the

Milky Way The Milky Way is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye ...

, but with nearly no discernable

star A star is an astronomical object comprising a luminous spheroid of plasma (physics), plasma held together by its gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked ...

s or galactic structure, is made almost entirely of

Observational evidence

Large surveys with sensitive, but low resolution radio telescopes like

Arecibo Arecibo (; ) is a city and municipality on the northern coast of Puerto Rico, on the shores of the Atlantic Ocean, located north of Utuado and Ciales; east of Hatillo; and west of Barceloneta and Florida. It is about west of San Juan, the ...

or the

Parkes Telescope Parkes Observatory is a radio astronomy observatory, located north of the town of Parkes, New South Wales, Australia. It hosts Murriyang, the 64 m CSIRO Parkes Radio Telescope also known as "The Dish", along with two smaller radio telescopes. T ...

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

quasar A quasar is an extremely Luminosity, luminous active galactic nucleus (AGN). It is pronounced , and sometimes known as a quasi-stellar object, abbreviated QSO. This emission from a galaxy nucleus is powered by a supermassive black hole with a m ...

s. 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 galaxies

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

Structure

Dark galaxies are composed of

. Furthermore, dark galaxies are theoretically composed of hydrogen and

dust Dust is made of fine particles of solid matter. On Earth, it generally consists of particles in the atmosphere that come from various sources such as soil lifted by wind (an aeolian process), volcanic eruptions, and pollution. Dust in homes ...

. 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 (AGES) is a current study using the

Arecibo radio telescope The Arecibo Telescope was a spherical reflector radio telescope built into a natural sinkhole at the Arecibo Observatory located near Arecibo, Puerto Rico. A cable-mount steerable receiver and several radar transmitters for emitting signals wer ...

to search for dark galaxies, which are predicted to contain detectable amounts of

neutral hydrogen The hydrogen line, 21 centimeter line, or H I line is the electromagnetic radiation spectral line that is created by a change in the energy state of neutral hydrogen atoms. This electromagnetic radiation has a precise frequency of , w ...

. 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. Unfortunately, the decommissioning and subsequent catastrophic collapse of the Arecibo Radio Telescope on December 1st, 2020 has somewhat limited its efficacy in facilitating further data collection.

Alternative theories

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

redshift In physics, a redshift is an increase in the wavelength, and corresponding decrease in the frequency and photon energy, of electromagnetic radiation (such as light). The opposite change, a decrease in wavelength and simultaneous increase in f ...

z=0.285.

Hubble Space Telescope The Hubble Space Telescope (often referred to as HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the first space telescope, but it is one of the largest and most versa ...

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 The Triangulum Galaxy is a spiral galaxy 2.73 million light-years (ly) from Earth in the constellation Triangulum. It is catalogued as Messier 33 or NGC (''New General Catalogue)'' 598. With the D25 isophotal diameter of , the Triangulum Galax ...

. 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

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 (and apparently inconsistent with the predictions of the Modified Newtonian Dynamics (MOND) theory). 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 The Virgo Cluster is a large cluster of galaxies whose center is 53.8 ± 0.3 Mly (16.5 ± 0.1 Mpc) away in the constellation Virgo. Comprising approximately 1,300 (and possibly up to 2,000) member galaxies, the cluster forms the heart of the lar ...

References

External links

* Universe Today
Some Galaxies Are Made Almost Entirely of Dark Matter
{{Portal bar, Stars, Spaceflight, Outer space, Solar System Physical cosmology

Galaxy A galaxy is a system of stars, stellar remnants, interstellar gas, dust, dark matter, bound together by gravity. The word is derived from the Greek ' (), literally 'milky', a reference to the Milky Way galaxy that contains the Solar System. ...

Hypothetical galaxies Dark concepts in astrophysics