The Milky Way has several smaller galaxies gravitationally bound to it, as part of the Milky Way subgroup, which is part of the local galaxy cluster, the Local Group. [1]
There are 61 small galaxies confirmed to be within 420 kiloparsecs (1.4 million light-years ) of the Milky Way, [2] but not all of them are necessarily in orbit, and some may themselves be in orbit of other satellite galaxies. The only ones visible to the naked eye are the Large and Small Magellanic Clouds, which have been observed since prehistory. Measurements with the Hubble Space Telescope in 2006 suggest the Magellanic Clouds may be moving too fast to be orbiting the Milky Way. [3] Of the galaxies confirmed to be in orbit, the largest is the Sagittarius Dwarf Spheroidal Galaxy, which has a diameter of 2.6 kiloparsecs (8,500 ly) [4] or roughly a twentieth that of the Milky Way.
Satellite galaxies that orbit from 1,000 ly (310 pc ) of the edge of the disc of the Milky Way Galaxy to the edge of the dark matter halo of the Milky Way at 980,000 ly (300 kpc) from the center of the galaxy, [a] are generally depleted in hydrogen gas compared to those that orbit more distantly. This is because of their interactions with the dense hot gas halo of the Milky Way that strip cold gas from the satellites. Satellites beyond that region still retain copious quantities of gas. [5] [6]
The Milky Way's satellite galaxies include the following: [7] [2]
Name | Diameter (kpc) | Distance (kpc) | Absolute visual magnitude | Type | Discovered |
---|---|---|---|---|---|
Large Magellanic Cloud | 4 | 48.5 | −18.1 | SBm | prehistoric |
Antlia 2 | 2.9 | 130 | −8.5 | Irr? | 2018 |
Sagittarius Dwarf | 2.6 | 20 | −13.5 | E | 1994 |
Crater II | 2.2 | 117.5 | −8.2 | dSph | 2016 [8] |
Small Magellanic Cloud | 2 | 61 | −16.8 | Irr | prehistoric |
Canes Venatici I | 1.1 | 220 | −8.6 | dSph | 2006 |
Canis Major Dwarf | 1.5 | 8 | −14.4 | Irr | 2003 |
Boötes III | 1.0 | 46 | −5.75 | dSph? | 2009 |
Sculptor Dwarf | 0.8 | 90 | −11.1 | dE3 | 1937 |
Draco Dwarf | 0.7 | 80 | −8.8 | dE0 | 1954 |
Hercules | 0.7 | 135 | −6.6 | dSph | 2006 |
Leo II | 0.7 | 210 | −9.8 | dE0 | 1950 |
Fornax Dwarf | 0.6 | 140 | −13.4 | dE2 | 1938 |
Eridanus II [9] | 0.55 | 366 | −7.1 | dSph | 2015 [10] [11] |
Sextans Dwarf Spheroidal | 0.5 | 90 | −9.3 | dE3 | 1990 |
Carina Dwarf Spheroidal | 0.5 | 100 | −9.1 | dE3 | 1977 |
Leo I | 0.5 | 250 | −12.0 | dE3 | 1950 |
Ursa Minor Dwarf | 0.4 | 60 | −8.8 | dE4 | 1954 |
Leo T | 0.34 | 420 | −8.0 | dSph/dIrr | 2006 |
Aquarius II | 0.32 | 108 | −4.2 | dSph | 2016 [12] |
Boötes I | 0.30 | 60 | −6.3 | dSph | 2006 |
Canes Venatici II | 0.30 | 155 | −4.9 | dSph | 2006 |
Leo IV | 0.30 | 160 | −5.8 | dSph | 2006 |
Tucana IV | 0.25 | 48 | −3.5 | dSph | 2015 [13] |
Columba I | 0.21 | 182 | −4.5 | dSph | 2015 [13] |
Ursa Major II Dwarf | 0.20 | 30 | −4.25 | dG D | 2006 |
Grus II | 0.19 | 53 | −3.9 | dSph | 2015 [13] |
Cetus III | 0.18 | 251 | −2.4 | dSph? | 2017 [14] |
Coma Berenices | 0.14 | 42 | −4.1 | dSph | 2006 |
Hydra II | 0.14 | 128 | −4.8 | dSph | 2015 [15] |
Reticulum III | 0.13 | 92 | −3.3 | dSph | 2015 [13] |
Pisces II | 0.12 | 180 | −5.0 | dSph | 2010 |
Pegasus III | 0.11 | 215 | −3.4 | dSph | 2015 [16] [17] |
Hydrus I | 0.10 | 28 | −4.7 | dSph | 2018 [18] |
Boötes II | 0.10 | 42 | −2.7 | dSph | 2007 |
Tucana III | 0.09 | 25 | −2.4 | dSph | 2015 [13] |
Virgo I | 0.09 | 91 | −0.3 | dSph? | 2016 [14] |
Horologium II | 0.09 | 78 | −2.6 | dSph | 2015 [19] |
Sagittarius II | 0.08 | 67 | −5.2 | dSph | 2015 [20] |
Leo V | 0.08 | 180 | −5.2 | dSph | 2007 |
Triangulum II | 0.07 | 30 | −1.8 | dSph | 2015 |
Segue 2 | 0.07 | 35 | −2.5 | dSph | 2007 |
Segue 1 | 0.06 | 23 | −1.5 | dSph | 2007 |
Draco II | 0.04 | 20 | −2.9 | dSph | 2015 [20] |
Tucana V | 0.03 | 55 | −1.6 | dSph | 2015 [13] |
Cetus II | 0.03 | 30 | 0.0 | dSph? | 2015 [13] |
Reticulum II | 0.064 | 30 | −3.6 | dSph | 2015 [10] [11] |
Tucana II | 0.33 | 70 | −3.9 | dSph | 2015 [10] [11] |
Pisces Overdensity | 1.5 | 80 | −13 | dSph? | 2009 |
DES 1 | 0.02 | 82 | −3.05 | GC | 2016 [21] |
Eridanus III | 0.028 | 90 | −2.4 | dSph? [b] | 2015 [10] [11] |
Horologium I | 0.06 | 100 | −3.5 | dSph? [b] | 2015 [10] [11] |
Kim 2/Indus I | 0.074 | 100 | −3.5 | GC | 2015 [10] [11] |
Phoenix II | 0.0521 | 100 | −3.7 | dSph? [b] | 2015 [10] [11] |
Ursa Major I Dwarf | 0.64 | 100 | −5.5 | dG D | 2005 |
Pictoris I | 0.058 | 115 | −3.7 | dSph? [b] | 2015 [10] [11] |
Grus I | 0.12 | 120 | −3.4 | dSph | 2015 [10] |
Pegasus IV | 0.082 | 90 | −4.25 | dSph | 2022 [22] |
Carina II | 0.182 | 36 | −4.5 | dSph | 2018 [23] |
Carina III | 0.06 | 28 | −2.4 | GC? | 2018 [23] |
Boötes IV | 0.28 | 209 | −4.53 | dSph | 2019 [24] |
Centaurus I | 0.076 | 116 | −5.55 | dSph | 2020 [25] |
Pictor II | 0.046 | 46 | −3.2 | dSph | 2016 [26] |
Boötes V | - | 102 | −3.2 | - | 2022 [27] |
Leo Minor I | - | 82 | −2.4 | - | 2022 [27] |
Virgo II | - | 72 | −1.6 | - | 2022 [27] |
Willman 1 | 0.02 | 38 | −2.53 | dSph | 2018 [28] |
Ursa Major III | 0.003 | 10 | +2.2 | dSph | 2023 |
Leo K | - | 434 | −4.86 | dSph | 2024 [29] |
Leo M | - | 459 | −5.77 | dSph | 2024 [29] |
Sextans II | - | - | - | - | 2024 [30] |
Virgo III | - | 154 [31] | - | - | 2024 [30] |
The Sagittarius Dwarf Spheroidal Galaxy is currently in the process of being consumed by the Milky Way and is expected to pass through it within the next 100 million years. The Sagittarius Stream is a stream of stars in polar orbit around the Milky Way leeched from the Sagittarius Dwarf. The Virgo Stellar Stream is a stream of stars that is believed to have once been an orbiting dwarf galaxy that has been completely distended by the Milky Way's gravity.
The Local Group is the galaxy group that includes the Milky Way, where Earth is located. It has a total diameter of roughly 3 megaparsecs (10 million light-years; 9×1019 kilometres), and a total mass of the order of 2×1012 solar masses (4×1042 kg). It consists of two collections of galaxies in a "dumbbell" shape; the Milky Way and its satellites form one lobe, and the Andromeda Galaxy and its satellites constitute the other. The two collections are separated by about 800 kiloparsecs (3×10 6 ly; 2×1019 km) and are moving toward one another with a velocity of 123 km/s. The group itself is a part of the larger Virgo Supercluster, which may be a part of the Laniakea Supercluster. The exact number of galaxies in the Local Group is unknown as some are occluded by the Milky Way; however, at least 80 members are known, most of which are dwarf galaxies.
A dwarf galaxy is a small galaxy composed of about 1000 up to several billion stars, as compared to the Milky Way's 200–400 billion stars. The Large Magellanic Cloud, which closely orbits the Milky Way and contains over 30 billion stars, is sometimes classified as a dwarf galaxy; others consider it a full-fledged galaxy. Dwarf galaxies' formation and activity are thought to be heavily influenced by interactions with larger galaxies. Astronomers identify numerous types of dwarf galaxies, based on their shape and composition.
A dwarf spheroidal galaxy (dSph) is a term in astronomy applied to small, low-luminosity galaxies with very little dust and an older stellar population. They are found in the Local Group as companions to the Milky Way and as systems that are companions to the Andromeda Galaxy (M31). While similar to dwarf elliptical galaxies in appearance and properties such as little to no gas or dust or recent star formation, they are approximately spheroidal in shape and generally have lower luminosity.
The Boötes Dwarf Galaxy is a galaxy discovered in 2006, which appears faint, with a luminosity of 100,000 L☉ and an absolute magnitude of –5.8. It lies about 197,000 light-years away in the constellation Boötes. This dwarf spheroidal galaxy appears to be tidally disrupted by the Milky Way Galaxy, which it orbits, and has two stellar tails that cross over to form a cross. Tidally disrupted galaxies usually only form one tail. The galaxy appears to be significantly elongated, with an ellipticity of ε = 0.68 ± 0.15.
Coma Berenices or Com is a dwarf spheroidal galaxy situated in the Coma Berenices constellation and discovered in 2006 in data obtained by the Sloan Digital Sky Survey. The galaxy is located at the distance of about 44 kpc from the Sun and moves away from the Sun with the velocity of about 98 km/s. It is classified as a dwarf spheroidal galaxy (dSph) meaning that it has an elliptical shape with the half-light radius of about 70 pc.
Pisces II is a dwarf spheroidal galaxy situated in the Pisces constellation and discovered in 2010 in the data obtained by the Sloan Digital Sky Survey. The galaxy is located at the distance of about 180 kpc (kiloparsecs) from the Sun. It is classified as a dwarf spheroidal galaxy (dSph) meaning that it has an elongated shape with the half-light radius of about 60 pc and ratio of the axis of about 5:3.
The Pisces Overdensity is a clump of stars in the Milky Way's halo, which may be a disrupted dwarf spheroidal galaxy. It is situated in the Pisces constellation and was discovered in 2009 by analysis of distribution of RR Lyrae stars in the data obtained by the Sloan Digital Sky Survey's data. The galaxy is located at the distance of about 80 kpc from the Sun and moves towards it with a speed of about 75 km/s.
An ultra diffuse galaxy (UDG), or dark galaxy, 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 renaming in 2015. Their lack of luminosity is due to the lack of star-forming gas, which results in these galaxies being reservoirs of very old stellar populations.
Triangulum II is a dwarf galaxy close to the Milky Way Galaxy. Like other dwarf spheroidal galaxies, its stellar population is very old: the galaxy was quenched before 11.5 billion years ago. It contains only 1000 stars, yet is quite massive, having a solar mass to light ratio of 3600. This is an unusually high mass for such a small galaxy.
Laevens 1 is a faint globular cluster in the constellation Crater that was discovered in 2014. It is also known as Crater, the Crater cluster and PSO J174.0675-10.8774.
Reticulum II is a dwarf galaxy in the Local Group. Reticulum II was discovered in 2015 by analysing images from the Dark Energy Survey. It is a satellite of the Magellanic Clouds and was probably captured relatively recently. Like other dwarf spheroidal galaxies, its stellar population is old: the galaxy was quenched before 11.5 billion years ago.
Crater 2 is a low-surface-brightness dwarf satellite galaxy of the Milky Way, located approximately 380,000 ly from Earth. Its discovery in 2016 revealed significant gaps in astronomers' understanding of galaxies possessing relatively small half-light diameters and suggested the possibility of many undiscovered dwarf galaxies orbiting the Milky Way. Crater 2 was identified in imaging data from the VST ATLAS survey.
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 consistent with a count of about 90 and 70 globular clusters observed around Dragonfly 44 in two different studies.
The Eridanus II Dwarf is a low-surface brightness dwarf galaxy in the constellation Eridanus. Eridanus II was independently discovered by two groups in 2015, using data from the Dark Energy Survey. This galaxy is probably a distant satellite of the Milky Way. Eridanus II contains a centrally located globular cluster; and is the smallest, least luminous galaxy known to contain a globular cluster. Crnojević et al., 2016. Eridanus II is significant, in a general sense, because the widely accepted Lambda CDM cosmology predicts the existence of many more dwarf galaxies than have yet been observed. The search for just such bodies was one of the motivations for the ongoing Dark Energy Survey observations. Eridanus II has special significance because of its apparently stable globular cluster. The stability of this cluster, near the center of such a small, diffuse, galaxy places constraints on the nature of dark matter.
The Gaia Sausage or Gaia Enceladus is the remains of a dwarf galaxy that merged with the Milky Way about 8–11 billion years ago. At least eight globular clusters were added to the Milky Way along with 50 billion solar masses of stars, gas and dark matter. It represents the last major merger of the Milky Way.
Antlia II is a low-surface-brightness dwarf satellite galaxy of the Milky Way at a galactic latitude of 11.2°. It spans 1.26° in the sky just southeast of Epsilon Antliae. The galaxy is similar in size to the Large Magellanic Cloud, despite being 1/10,000 as bright. Antlia II has the lowest surface brightness of any galaxy discovered and is ~ 100 times more diffuse than any known ultra diffuse galaxy. The large size of the galaxy suggests that it is currently being tidally disrupted, and is in the process of becoming a stellar stream. The southeast side of Antlia II is farther away than the northwest side, likely due to the tidal disruption. It was discovered using data from the European Space Agency's Gaia spacecraft in November 2018.