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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 (3×106 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.
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 18.74 kiloparsecs (61,100 light-years), the Triangulum Galaxy is the third-largest member of the Local Group of galaxies, behind the Andromeda Galaxy and the Milky Way.
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.
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.
Andromeda II is a dwarf spheroidal galaxy about 2.22 Mly away in the constellation Pisces. While part of the Local Group, it is not quite clear if it is a satellite of the Andromeda Galaxy or the Triangulum Galaxy.
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.
NGC 4449, also known as Caldwell 21, is an irregular Magellanic type galaxy in the constellation Canes Venatici, being located about 13 million light-years away. It is part of the M94 Group or Canes Venatici I Group that is relatively close to the Local Group hosting our Milky Way galaxy.
Canes Venatici II or CVn II is a dwarf spheroidal galaxy situated in the Canes Venatici constellation and discovered in 2006 in data obtained by the Sloan Digital Sky Survey. The galaxy is located at a distance of about 150 kpc from the Sun and moves towards the Sun with the velocity of about 130 km/s. It is classified as a dwarf spheroidal galaxy (dSph) meaning that it has an elliptical shape with a half-light radius of about 74+14
−10 pc.
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.
Hercules, or Her, is a dwarf spheroidal galaxy situated in the Hercules constellation and discovered in 2006 in data obtained by the Sloan Digital Sky Survey. The galaxy is located at a distance of about 140 kpc from the Sun and moves away from the Sun with a velocity of about 45 km/s. It is classified as a dwarf spheroidal galaxy (dSph). It has a noticeably elongated shape with a half-light radius of about 350 pc. This elongation may be caused by tidal forces acting from the Milky Way galaxy, meaning that Her is being tidally disrupted now. Her also shows some gradient of velocities across the galaxy's body and is embedded into a faint stellar stream, which also points towards its ongoing tidal disruption.
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.
Galaxy X is a postulated dark satellite dwarf galaxy of the Milky Way Galaxy. If it exists, it would be composed mostly of dark matter and interstellar gas with few stars. Its proposed location is some 90 kpc (290 kly) from the Sun, behind the disk of the Milky Way, and some 12 kpc (39 kly) in extent. Galactic coordinates would be (l= -27.4°,b=-1.08°).
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.
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.
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.
References
- 1 2 3 4 5 6 Laevens, Benjamin P. M.; Martin, Nicolas F.; Ibata, Rodrigo A.; Rix, Hans-Walter; Bernard, Edouard J.; Bell, Eric F.; Sesar, Branimir; Ferguson, Annette M. N.; Schlafly, Edward F.; Slater, Colin T.; et al. (31 March 2015). "A New Faint Milky Way Satellite Discovered in the PAN-STARRS1 3π Survey" (PDF). The Astrophysical Journal. 802 (2): L18. arXiv: 1503.05554 . Bibcode:2015ApJ...802L..18L. doi:10.1088/2041-8205/802/2/L18. S2CID 54179272.
- ↑ Sacchi, Elena; Richstein, Hannah; Kallivayalil, Nitya; Van Der Marel, Roeland; Libralato, Mattia; Zivick, Paul; Besla, Gurtina; Brown, Thomas M.; Choi, Yumi; Deason, Alis; Fritz, Tobias; Geha, Marla; Guhathakurta, Puragra; Jeon, Myoungwon; Kirby, Evan; Majewski, Steven R.; Patel, Ekta; Simon, Joshua D.; Tony Sohn, Sangmo; Tollerud, Erik; Wetzel, Andrew (2021). "Star Formation Histories of Ultra-faint Dwarf Galaxies: Environmental Differences between Magellanic and Non-Magellanic Satellites?". The Astrophysical Journal Letters. 920 (1): L19. arXiv: 2108.04271 . Bibcode:2021ApJ...920L..19S. doi: 10.3847/2041-8213/ac2aa3 . S2CID 236965965.
- 1 2 3 4 Kirby, Evan N.; Cohen, Judith G.; Simon, Joshua D.; Guhathakurta, Puragra (16 November 2015). "Triangulum II: Possibly a Very Dense Ultra-Faint Dwarf Galaxy". The Astrophysical Journal. 814 (1): L7. arXiv: 1510.03856 . Bibcode:2015ApJ...814L...7K. doi:10.1088/2041-8205/814/1/L7. S2CID 13600560.
- 1 2 Dajose, Lori (18 November 2015). "Dark matter dominates in nearby dwarf galaxy". Phys.org. Retrieved 19 November 2015.; Lori Dajose (November 18, 2015). "Dark Matter Dominates in Nearby Dwarf Galaxy". Caltech . Retrieved December 16, 2015.
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