# Dwarf spheroidal galaxy

NGC147 (left) and the Fornax Dwarf (right), two of the earliest known dwarf spheroidal galaxies.

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 to 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.

## Discovery

Despite the radii of dSphs being much larger than those of globular clusters, they are much more difficult to find due to their low luminosities and surface brightnesses. Dwarf spheroidal galaxies have a large range of luminosities, and known dwarf spheroidal galaxies span several orders of magnitude of luminosity.[1] Their luminosities are so low that Ursa Minor, Carina, and Draco, the known dwarf spheroidal galaxies with the lowest luminosities, have mass-to-light ratios (M/L) greater than that of the Milky Way.[2] Dwarf spheroidals also have little to no gas with no obvious signs of recent star formation.[3][4] When it comes to the Local Group, dSphs are primarily found near the Milky Way and M31.[5][6]

The first dwarf spheroidal galaxies discovered were Sculptor and Fornax in 1938.[2] The Sloan Digital Sky Survey has resulted in the discovery of 11 more dSph galaxies as of 2007[7] By 2015, many more ultra-faint dSphs were discovered, all satellites of the Milky Way.[8] Nine potentially new dSphs were discovered in the Dark Energy Survey in 2015.[9] Each dSph is named after constellations they are discovered in, such as the Sagittarius dwarf spheroidal galaxy, all of which consist of stars generally much older than 1-2 Gyr that formed over the span of many gigayears.[2]

For example, 98% of the stars in the Carina dwarf spheroidal galaxy are older than 2 Gyr, formed over the course of three bursts around 3, 7, and 13 Gyr ago.[2] The stars in Carina have also been found to be metal-poor.[10] This is unlike star clusters because, while star clusters have stars form at once, dwarf spheroidal galaxies experience multiple bursts of star formation.[2]

## Evidence of dark matter

Because of the faintness of the lowest-luminosity dwarf spheroidal galaxies and the nature of the stars contained within them, some astronomers suggest that dwarf spheroidal galaxies and globular clusters may not be clearly separate and distinct types of objects.[11] Other recent studies, however, have found a distinction in that the total amount of mass inferred from the motions of stars in dwarf spheroidals is many times that which can be accounted for by the mass of the stars themselves. Studies reveal that dwarf spheroidal galaxies have a dynamical mass of around 10${\displaystyle ^{7}}$ solar masses, which is very large despite the low luminosity of dSph galaxies.[1]

Although at fainter luminosities of dwarf spheroidal galaxies, it is not universally agreed upon how to differentiate between a dwarf spheroidal galaxy and a star cluster; however, many astronomers decide this depending on the object's dynamics: if it seems to have more dark matter, then it is likely that it is a dwarf spheroidal galaxy rather than a faint star cluster. In the current predominantly accepted Lambda cold dark matter cosmological model, the presence of dark matter is often cited as a reason to classify dwarf spheroidal galaxies as a different class of object from globular clusters, which show little to no signs of dark matter. Because of the extremely large amounts of dark matter in dwarf spheroidal galaxies, they may deserve the title "most dark matter-dominated galaxies."[12]

Further evidence of the prevalence of dark matter in dSphs includes the case of Fornax dwarf spheroidal galaxy, which can be assumed to be in dynamic equilibrium to estimate mass and amount of dark matter, since the gravitational effects of the Milky Way are small.[13] Unlike the Fornax galaxy, there is evidence that the UMa2, a dwarf spheroidal galaxy in the Ursa Major constellation, experiences strong tidal disturbances from the Milky Way.[9]

A topic of research is how much the internal dynamics of dwarf spheroidal galaxies are affected by the gravitational tidal dynamics of the galaxy they are orbiting. In other words, dwarf spheroidal galaxies could be prevented from achieving equilibrium due to the gravitational field of the Milky Way or other galaxy that they orbit.[2] For example, the Sextans dwarf spheroidal galaxy has a velocity dispersion of 7.9±1.3 km/s, which is a velocity dispersion that could not be explained solely by its stellar mass according to the Virial Theorem. Similar to Sextans, previous studies of Hercules dwarf spheroidal galaxy reveal that its orbital path does not correspond to the mass contained in Hercules.[14] Furthermore, there is evidence that the UMa2, a dwarf spheroidal galaxy in the Ursa Major constellation, experiences strong tidal disturbances from the Milky Way.[9]

## References

1. ^ a b Strigari, Louis E.; Bullock, James S.; Kaplinghat, Manoj; Simon, Joshua D.; Geha, Marla; Willman, Beth; Walker, Matthew G. (2008-08-28). "A common mass scale for satellite galaxies of the Milky Way". Nature. 454 (7208): 1096–1097. arXiv:0808.3772. Bibcode:2008Natur.454.1096S. doi:10.1038/nature07222. ISSN 0028-0836. PMID 18756252.
2. Sparke, L.S.; Gallagher, III, J.S. (2016). Galaxies in the Universe. United Kingdom: Cambridge University Press. pp. 162–165. ISBN 978-0-521-67186-6.
4. ^ McConnachie, Alan W. (2012-06-05). "THE OBSERVED PROPERTIES OF DWARF GALAXIES IN AND AROUND THE LOCAL GROUP". The Astronomical Journal. 144 (1): 4. arXiv:1204.1562. Bibcode:2012AJ....144....4M. doi:10.1088/0004-6256/144/1/4. ISSN 0004-6256.
6. ^ K., Grebel, E. (1998). "Star Formation Histories of Local Group Dwarf Galaxies". Highlights of Astronomy. 11: 125. arXiv:astro-ph/9806191. Bibcode:1998HiA....11..125G.
7. ^ Simon, Josh; Geha, Marla (November 2007), "The Kinematics of the Ultra-faint Milky Way Satellites: Solving the Missing Satellite Problem", The Astrophysical Journal, 670 (1): 313–331, arXiv:0706.0516, Bibcode:2007ApJ...670..313S, doi:10.1086/521816
8. ^ Sergey E. Koposov; Vasily Belokurov; Gabriel Torrealba; N. Wyn Evans (10 March 2015). "Beasts of the Southern Wild. Discovery of a large number of Ultra Faint satellites in the vicinity of the Magellanic Clouds". The Astrophysical Journal. 805 (2): 130. arXiv:1503.02079. Bibcode:2015ApJ...805..130K. doi:10.1088/0004-637X/805/2/130.
9. ^ a b c Bonnivard, V.; Combet, C.; Daniel, M.; et al. (2015). "Dark matter annihilation and decay in dwarf spheroidal galaxies: the classical and ultrafaint dSphs". Monthly Notices of the Royal Astronomical Society. 453 (1): 849–867. arXiv:1504.02048. Bibcode:2015MNRAS.453..849B. doi:10.1093/mnras/stv1601.
10. ^ Bono, G.; Stetson, P. B.; Walker, A. R.; Monelli, M.; Fabrizio, M.; Pietrinferni, A.; Brocato, E.; Buonanno, R.; F. Caputo (2010-01-01). "On the Stellar Content of the Carina Dwarf Spheroidal Galaxy". Publications of the Astronomical Society of the Pacific. 122 (892): 651. arXiv:1004.2559. Bibcode:2010PASP..122..651B. doi:10.1086/653590. ISSN 1538-3873.
11. ^ van den Bergh, Sidney (November 2007), "Globular Clusters and Dwarf Spheroidal Galaxies", MNRAS (Letters), in Press, 385 (1): L20–L22, arXiv:0711.4795, Bibcode:2008MNRAS.385L..20V, doi:10.1111/j.1745-3933.2008.00424.x
12. ^ Strigari, Louie; Koushiappas, Savvas M.; Bullock, James S.; Kaplinghat, Manoj; Simon, Joshua D.; Geha, Marla; Willman, Beth; et al. (2008), "The Most Dark Matter Dominated Galaxies: Predicted Gamma-ray Signals from the Faintest Milky Way Dwarfs", The Astrophysical Journal, 678 (2): 614–620, arXiv:0709.1510, Bibcode:2008ApJ...678..614S, doi:10.1086/529488
13. ^ Battaglia, Giuseppina; Sollima, Antonio; Nipoti, Carlo (2015). "The effect of tides on the Fornax dwarf spheroidal galaxy". Monthly Notices of the Royal Astronomical Society. 454 (3): 2401–2415. arXiv:1509.02368. Bibcode:2015MNRAS.454.2401B. doi:10.1093/mnras/stv2096.
14. ^ Roderick, T.A.; Jerjen, H.; Da Costa, G.S.; Mackey, A.D. (2016). "Structural analysis of the Sextans dwarf spheroidal galaxy". Monthly Notices of the Royal Astronomical Society. 460 (1): 30–43. arXiv:1604.06214. Bibcode:2016MNRAS.460...30R. doi:10.1093/mnras/stw949.
Andromeda III

Andromeda III is a dwarf spheroidal galaxy about 2.44 million light-years away in the constellation Andromeda. It is part of the Local Group and is a satellite galaxy of the Andromeda Galaxy (M31) and was discovered by Sidney van den Bergh on photographic plates taken in 1970 and 1971.

Andromeda IX

Andromeda IX (And 9) is a dwarf spheroidal satellite of the Andromeda Galaxy. It was discovered in 2004 by resolved stellar photometry from the Sloan Digital Sky Survey (SDSS), by Zucker et al. (2004). At the time of its discovery, it was the galaxy with the lowest known surface brightness, ΣV ≃ 26.8mags arcsec−2 and the faintest galaxy known from its intrinsic absolute brightness.It was found from data acquired within an SDSS scan along the major axis of M31, on October 5, 2002. Its distance was estimated to be almost exactly the same as that of M31 by McConnacrchie et al. (2005).

Andromeda V

Andromeda V is a dwarf spheroidal galaxy about 2.52 Mly away in the constellation Andromeda.Andromeda V was discovered by Armandroff et al. and published in 1998 after their analysis of the digitized version of the second Palomar Sky Survey.The metallicity of Andromeda V is above the average metallicity to luminosity ratio of the Local Group's dwarf galaxies.

Andromeda X

Andromeda X (And 10) is a dwarf spheroidal galaxy about 2.9 million light-years away from the Sun in the constellation Andromeda. Discovered in 2005, And X is a satellite galaxy of the Andromeda Galaxy (M31).

Andromeda XI

Andromeda XI (And 11) is a dwarf spheroidal galaxy about 2.6 million light-years away from the Sun in the constellation Andromeda. Discovered in 2006, And XI is a satellite galaxy of the Andromeda Galaxy (M31).

Andromeda XXI

Andromeda XXI (And 21, And XXI) is a moderately bright dwarf spheroidal galaxy about 859 ± 51 kiloparsecs (2.80 ± 0.17 Mly) away from the Sun in the constellation Andromeda. It is the fourth largest Local Group dwarf spheroidal galaxy.

This large satellite of the Andromeda Galaxy (M31) has a half-light radius of nearly 1 kpc.The discovery arose from the first year data of a photometric survey of the M31/M33 subgroupings of the Local Group by the Pan-Andromeda Archaeological Survey (PAndAS). This survey was conducted with the Megaprime/MegaCam wide-field camera mounted on the Canada-France-Hawaii Telescope.

Andromeda XXI appears as a spatial overdensity of stars. It has red giant branches at the distance of M31/M33, and follows metal-poor, [Fe/H]=-1.8 when plotted in a color-magnitude diagram.

Although moderately bright (MV=-9.9 ± 0.6), it has low surface brightness. This indicates that numerous relatively luminous M31 satellites remain undiscovered.

Boötes II (dwarf galaxy)

Bootes II or Boo II is a dwarf spheroidal galaxy situated in the Bootes constellation and discovered in 2007 in the data obtained by Sloan Digital Sky Survey. The galaxy is located at the distance of about 42 kpc from the Sun and moves towards the Sun with the speed of 120 km/s. It is classified as a dwarf spheroidal galaxy (dSph) meaning that it has an approximately round shape with the half-light radius of about 51 pc.Bootes II is one of the smallest and faintest satellites of the Milky Way—its integrated luminosity is about 1,000 times that of the Sun (absolute visible magnitude of about −2.7), which is much lower than the luminosity of the majority of globular clusters. However the mass of the galaxy is substantial corresponding to the mass to light ratio of more than 100.The stellar population of Bootes II consists mainly of moderately old stars formed 10–12 billion years ago. The metallicity of these old stars is low at [Fe/H]=−1.8, which means that they contain 80 times less heavy elements than the Sun. Currently there is no star formation in Bootes II. The measurements have so far failed to detect any neutral hydrogen in it—the upper limit is only 86 solar masses.Bootes II is located only 1.5 degrees (~1.6 kpc) away from another dwarf galaxy—Boötes I, although they are unlikely to be physically associated because they move in opposite directions relative to the Milky Way. Their relative velocity—about 200 km/s is too high. It is more likely associated with the Sagittarius Stream and, therefore, with the Sagittarius Dwarf Elliptical Galaxy (SagDEG). Bootes II may be either a satellite galaxy of SagDEG or one of its star clusters torn from the main galaxy 4–7 billion years ago.

Carina Dwarf Spheroidal Galaxy

The Carina Dwarf Spheroidal Galaxy is a dwarf galaxy in the Carina constellation. It was discovered in 1977 with the UK Schmidt Telescope by Cannon et al. The Carina Dwarf Spheroidal galaxy is a satellite galaxy of the Milky Way and is receding from it at 230 km/s. The diameter of the galaxy is about 1600 light-years, which is 75 times smaller than the Milky Way. Most of the stars in the galaxy formed 7 billion years ago, although it also experienced bursts of star formation about 13 and 3 billion years ago. It is also being tidally disrupted by the Milky Way galaxy.

Messier 54

Messier 54 (also known as M54 or NGC 6715) is a globular cluster in the constellation Sagittarius. It was discovered by Charles Messier in 1778 and subsequently included in his catalog of comet-like objects.

M54 is easily found in the sky, being close to the star ζ Sagittarii. It is, however, not resolvable into individual stars even with larger amateur telescopes.

Previously thought to belong to the Milky Way at a distance from Earth of about 50,000 light-years, it was discovered in 1994 that M54 most likely belongs to the Sagittarius Dwarf Elliptical Galaxy (SagDEG), making it the first globular cluster formerly thought to be part of our galaxy reassigned to extragalactic status, even if not recognized as such for nearly two and a quarter centuries. As it is located on SagDEG's center, some authors think it actually may be its core; however others have proposed that it is a real globular cluster that fell to the center of this galaxy due to decay of its orbit caused by dynamical friction.Modern estimates now place M54 at a distance of some 87,000 light-years, translating into a true radius of 150 light-years across. It is one of the denser of the globulars, being of class III (I being densest and XII being the least dense). It shines with the luminosity of roughly 850,000 times that of the Sun and has an absolute magnitude of −10.0. In July 2009, a team of astronomers reported that they had found evidence of an intermediate-mass black hole in the core of M54.

NGC 147

NGC 147 (also known as DDO3 or Caldwell 17) is a dwarf spheroidal galaxy about 2.58 Mly away in the constellation Cassiopeia. NGC 147 is a member of the Local group of galaxies and a satellite galaxy of the Andromeda Galaxy (M31). It forms a physical pair with the nearby galaxy NGC 185,

another remote satellite of M31. It was discovered by John Herschel in September 1829. Visually it is both fainter and slightly larger than NGC 185 (and therefore has a considerably lower surface brightness). This means that NGC 147 is more difficult to see than NGC 185, which is visible in small telescopes. In the Webb Society Deep-Sky Observer's Handbook, the visual appearance of NGC 147 is described as follows:

Large, quite faint, irregularly round; it brightens in the middle to a stellar nucleus.

The membership of NGC 147 in the Local Group was confirmed by Walter Baade in 1944 when he was able to resolve the galaxy into individual stars with the 100-inch (2.5 m) telescope at Mount Wilson near Los Angeles.

NGC 185

NGC 185 (also known as Caldwell 18) is a dwarf spheroidal galaxy located 2.08 million light-years from Earth, appearing in the constellation Cassiopeia. It is a member of the Local Group, and is a satellite of the Andromeda Galaxy (M31). NGC 185 was discovered by William Herschel on November 30, 1787, and he cataloged it "H II.707". John Herschel observed the object again in 1833 when he cataloged it as "h 35", and then in 1864 when he cataloged it as "GC 90" within his General Catalogue of Nebulae and Clusters. NGC 185 was first photographed between 1898 and 1900 by James Edward Keeler with the Crossley Reflector of Lick Observatory. Unlike most dwarf elliptical galaxies, NGC 185 contains young stellar clusters, and star formation proceeded at a low rate until the recent past. NGC 185 has an active galactic nucleus (AGN) and is usually classified as a type 2 Seyfert galaxy, though its status as a Seyfert is questioned. It is possibly the closest Seyfert galaxy to Earth, and is the only known Seyfert in the Local Group.

NGC 5634

NGC 5634 is a globular cluster in the constellation Virgo (constellation), located about 82,000 light years (27 kiloparsecs) away. NGC 5634 has an apparent magnitude of about 10 and a diameter of 4 or 5 arcminutes. Its Shapley–Sawyer Concentration Class is IV, meaning the cluster shows intermediate rich concentrations. with stars 19th magnitude. The star on near the upper right is the eleventh-magnitude UCAC2 29844847. There is also a bright orange giant, HD 127119, about 1.3 arcminutes away from the cluster.

NGC 5634 was likely a member of the Sagittarius Dwarf Spheroidal Galaxy. The galaxy itself is being pulled apart by tidal forces from the Milky Way, similar to how NGC 5634 was pulled from the Dwarf Spheroidal Galaxy.

Palomar 12

Palomar 12 is a globular cluster in the constellation Capricornus.

First discovered on the National Geographic Society – Palomar Observatory Sky Survey plates by Robert George Harrington and Fritz Zwicky,

it was catalogued as a globular cluster. However Zwicky came to believe this was actually a nearby dwarf galaxy in the Local Group. It is a relatively young cluster, being about 30% younger than most of the globular clusters in the Milky Way. It is metal-rich with a metallicity of [Fe/H] ~= -0.8. It has an average luminosity distribution of Mv = -4.48.Based on proper motion studies, this cluster was first suspected in 2000 to have been captured from the Sagittarius dwarf galaxy about 1.7 Ga ago. It is now generally believed to be a member of that galaxy. It is estimated to be 6.5 Gyr old.

Pegasus Dwarf Spheroidal Galaxy

The Pegasus Dwarf Spheroidal (also known as Andromeda VI or Peg dSph for short) is a dwarf spheroidal galaxy about 2.7 million light-years away in the constellation Pegasus. The Pegasus Dwarf is a member of the Local Group and a satellite galaxy of the Andromeda Galaxy (M31).

Sagittarius Dwarf Spheroidal Galaxy

The Sagittarius Dwarf Spheroidal Galaxy (Sgr dSph), also known as the Sagittarius Dwarf Elliptical Galaxy (Sgr dE or Sag DEG), is an elliptical loop-shaped satellite galaxy of the Milky Way. It consists of four globular clusters, the main cluster having been discovered in 1994. Sgr dSph is roughly 10,000 light-years in diameter, and is currently about 70,000 light-years from Earth, travelling in a polar orbit (i.e. an orbit passing over the Milky Way’s galactic poles) at a distance of about 50,000 light-years from the core of the Milky Way (about 1/3 the distance of the Large Magellanic Cloud). In its looping, spiraling path, it has passed through the plane of the Milky Way several times in the past. In 2018 the Gaia project of the European Space Agency showed that Sgr dSph had caused perturbations in a set of stars near the Milky Way's core, causing unexpected rippling movements of the stars triggered when it sailed past the Milky Way between 300 and 900 million years ago.

Sagittarius Stream

In astronomy, the Sagittarius Stream is a long, complex, structure made of stars that wrap around the Milky Way galaxy in an almost polar orbit. It consists of tidally stripped stars from the Sagittarius Dwarf Elliptical Galaxy resulting from the process of merging with the Milky Way over a period of billions of years.

Sculptor Dwarf Galaxy

The Sculptor Dwarf Galaxy (also known as Sculptor Dwarf Elliptical Galaxy or the Sculptor Dwarf Spheroidal Galaxy) is a dwarf spheroidal galaxy that is a satellite of the Milky Way. The galaxy lies within the constellation Sculptor. It was discovered in 1937 by American astronomer Harlow Shapley using the 24-inch Bruce refractor at Boyden Observatory. The galaxy is located about 290,000 light-years away from the Solar System. The Sculptor Dwarf contains only 4 percent of the carbon and other heavy elements in our own galaxy, the Milky Way, making it similar to primitive galaxies seen at the edge of the universe.

Segue 2

Segue 2 is a dwarf spheroidal galaxy situated in the constellation Aries and discovered in 2009 in the data obtained by Sloan Digital Sky Survey. The galaxy is located at the distance of about 35 kpc (35,000 parsecs (110,000 ly)) from the Sun and moves towards the Sun with the speed of 40 km/s. It is classified as a dwarf spheroidal galaxy (dSph) meaning that it has an approximately round shape with the half-light radius of about 34 pc.The name is due to the fact that it was found by the SEGUE program, the Sloan Extension for Galactic Understanding and Exploration.

Segue 2 is one of the smallest and faintest satellites of the Milky Way—its integrated luminosity is about 800 times that of the Sun (absolute visible magnitude of about −2.5), which is much lower than the luminosity of the majority of globular clusters. However, the mass of the galaxy—about 550,000 solar masses—is substantial, corresponding to the mass to light ratio of about 650.The stellar population of Segue 2 consists mainly of old stars formed more than 12 billion years ago. The metallicity of these old stars is also very low at [Fe/H] < −2, which means that they contain at least 100 times less heavy elements than the Sun. The stars of Segue 2 were probably among the first stars to form in the Universe. Currently, there is no star formation in Segue 2.Segue 2 is located near the edge of Sagittarius Stream and at the same distance. It may once have been a satellite of Sagittarius Dwarf Elliptical Galaxy or its star cluster.In June 2013 The Astrophysical Journal reported that Segue 2 was bound together with dark matter.Circa 1,000 stars are supposed to exist within the galaxy.

Terzan 7

Terzan 7 is a sparse and young globular cluster that is believed to have originated in the Sagittarius Dwarf Spheroidal Galaxy (Sag DEG) and is physically associated with it. It is relatively metal rich with [Fe/H = -0.6 and an estimated age of 7.5 Gyr. Terzan 7 has low levels of nickel ([Ni/Fe] = -0.2) which supports its membership in the Sag DEG system since it has a similar chemical signature. It has a rich population of blue stragglers that are strongly concentrated toward the center of Terzan 7. It has an average luminosity distribution of Mv = -5.05. It has a half-light radius (Rh) of 6.5pc.

Morphology
Structure
Active nuclei
Energetic galaxies
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