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{{short description|Galaxy that orbits a larger galaxy due to gravitational attraction}}{{Use dmy dates|date=October 2019}}File:Satellite Galaxies.svg|thumb|Satellite galaxies of the Milky WayMilky WayA satellite galaxy is a smaller companion galaxy that travels on bound orbits within the gravitational potential of a more massive and luminous host galaxy (also known as the primary galaxy).BOOK, Galactic dynamics, Binney, James, 2008, Princeton University Press, Tremaine, Scott, 1950-, 9781400828722, 2nd, Princeton, 759807562, Satellite galaxies and their constituents are bound to their host galaxy, in the same way that planets within our own solar system are gravitationally bound to the Sun.WEB,weblink What Is a Satellite Galaxy?, NASA Spaceplace, en, 10 April 2016, While most satellite galaxies are dwarf galaxies, satellite galaxies of large galaxy clusters can be much more massive.WEB,weblink Dwarf Galaxies, www.cfa.harvard.edu, 2018-06-10, The Milky Way is orbited by about fifty satellite galaxies, the largest of which is the Large Magellanic Cloud.Moreover, satellite galaxies are not the only astronomical objects that are gravitationally bound to larger host galaxies (see globular clusters). For this reason, astronomers have defined galaxies as gravitationally bound collections of stars that exhibit properties that cannot be explained by a combination of baryonic matter (i.e. ordinary matter) and Newton's laws of gravity.JOURNAL, Willman, Beth, Strader, Jay, 2012-09-01, "Galaxy," Defined, 1203.2608, The Astronomical Journal, 144, 3, 76, 10.1088/0004-6256/144/3/76, 0004-6256, 2012AJ....144...76W, 250756695, For example, measurements of the orbital speed of stars and gas within spiral galaxies result in a velocity curve that deviates significantly from the theoretical prediction. This observation has motivated various explanations such as the theory of dark matter and modifications to Newtonian dynamics. Therefore, despite also being satellites of host galaxies, globular clusters should not be mistaken for satellite galaxies. Satellite galaxies are not only more extended and diffuse compared to globular clusters, but are also enshrouded in massive dark matter halos that are thought to have been endowed to them during the formation process.JOURNAL, Forbes, Duncan A., Kroupa, Pavel, Metz, Manuel, Spitler, Lee, 2009-06-29, Globular Clusters and Satellite Galaxies: Companions to the Milky Way,weblink Mercury, 38, 2, 24–27, 0906.5370, 2009arXiv0906.5370F, Satellite galaxies generally lead tumultuous lives due to their chaotic interactions with both the larger host galaxy and other satellites. For example, the host galaxy is capable of disrupting the orbiting satellites via tidal and ram pressure stripping. These environmental effects can remove large amounts of cold gas from satellites (i.e. the fuel for star formation), and this can result in satellites becoming quiescent in the sense that they have ceased to form stars.JOURNAL, Wetzel, Andrew R., Tollerud, Erik J., Weisz, Daniel R., 2015-07-22, Rapid Environmental Quenching of Satellite Dwarf Galaxies in the Local Group, 1503.06799, The Astrophysical Journal, 808, 1, L27, 10.1088/2041-8205/808/1/L27, 2041-8213, 2015ApJ...808L..27W, 33556186, Moreover, satellites can also collide with their host galaxy resulting in a minor merger (i.e. merger event between galaxies of significantly different masses). On the other hand, satellites can also merge with one another resulting in a major merger (i.e. merger event between galaxies of comparable masses). Galaxies are mostly composed of empty space, interstellar gas and dust, and therefore galaxy mergers do not necessarily involve collisions between objects from one galaxy and objects from the other, however, these events generally result in much more massive galaxies. Consequently, astronomers seek to constrain the rate at which both minor and major mergers occur to better understand the formation of gigantic structures of gravitationally bound conglomerations of galaxies such as galactic groups and clusters.WEB,weblink Our Galaxy and its Satellites Link for sharing this page on Facebook, Cseligman, en, 8 April 2016, WEB,weblink HubbleSite: News - Astronomers Pin Down Galaxy Collision Rate, hubblesite.org, 2018-06-14,

History

Early 20th century

Prior to the 20th century, the notion that galaxies existed beyond our Milky Way was not well established. In fact, the idea was so controversial at the time that it led to what is now heralded as the "Shapley-Curtis Great Debate" aptly named after the astronomers Harlow Shapley and Heber Doust Curtis that debated the nature of "nebulae" and the size of the Milky Way at the National Academy of Sciences on April 26, 1920. Shapley argued that the Milky Way was the entire universe (spanning over 100,000 lightyears or 30 kiloparsec across) and that all of the observed "nebulae" (currently known as galaxies) resided within this region. On the other hand, Curtis argued that the Milky way was much smaller and that the observed nebulae were in fact galaxies similar to our own Milky Way.BOOK, Galactic astronomy, Binney, James, 1998, Princeton University Press, Merrifield, Michael, 1964-, 978-0691004020, Princeton, NJ, 39108765, This debate was not settled until late 1923 when the astronomer Edwin Hubble measured the distance to M31 (currently known as the Andromeda galaxy) using Cepheid Variable stars. By measuring the period of these stars, Hubble was able to estimate their intrinsic luminosity and upon combining this with their measured apparent magnitude he estimated a distance of 300 kpc, which was an order-of-magnitude larger than the estimated size of the universe made by Shapley. This measurement verified that not only was the universe much larger than previously expected, but it also demonstrated that the observed nebulae were actually distant galaxies with a wide range of morphologies (see Hubble sequence).

Modern times

Despite Hubble's discovery that the universe was teeming with galaxies, a majority of the satellite galaxies of the Milky Way and the Local Group remained undetected until the advent of modern astronomical surveys such as the Sloan Digital Sky Survey (SDSS) and the Dark Energy Survey (DES).JOURNAL, The DES Collaboration, Drlica-Wagner, A., Bechtol, K., Rykoff, E. S., Luque, E., Queiroz, A., Mao, Y.-Y., Wechsler, R. H., Simon, J. D., 2015-11-04, Eight Ultra-faint Galaxy Candidates Discovered in Year Two of the Dark Energy Survey, 1508.03622, The Astrophysical Journal, 813, 2, 109, 10.1088/0004-637X/813/2/109, 1538-4357, 2015ApJ...813..109D, 55909299, JOURNAL, Wang, Peng, Guo, Quan, Libeskind, Noam I., Tempel, Elmo, Wei, Chengliang, Kang, Xi, 2018-05-15, The shape alignment of satellite galaxies in galaxy pairs in SDSS, Monthly Notices of the Royal Astronomical Society, 484, 3, 4325–4336, 1805.06096, 10.1093/mnras/stz285, In particular, the Milky Way is currently known to host 59 satellite galaxies (see satellite galaxies of the Milky Way), however two of these satellites known as the Large Magellanic Cloud and Small Magellanic Cloud have been observable in the Southern Hemisphere with the unaided eye since ancient times. Nevertheless, modern cosmological theories of galaxy formation and evolution predict a much larger number of satellite galaxies than what is observed (see missing satellites problem).JOURNAL, Klypin, Anatoly, Kravtsov, Andrey V., Valenzuela, Octavio, Prada, Francisco, September 1999, Where Are the Missing Galactic Satellites?, 1999ApJ...522...82K, The Astrophysical Journal, 522, 1, 82–92, 10.1086/307643, 0004-637X, astro-ph/9901240, 12983798, ARXIV, Bullock, James S., 2010-09-22, Notes on the Missing Satellites Problem, 1009.4505, astro-ph.CO, However, more recent high resolution simulations have demonstrated that the current number of observed satellites pose no threat to the prevalent theory of galaxy formation.JOURNAL, Wetzel, Andrew R., Hopkins, Philip F., Kim, Ji-hoon, Faucher-Giguere, Claude-Andre, Keres, Dusan, Quataert, Eliot, 2016-08-11, Reconciling dwarf galaxies with LCDM cosmology: Simulating a realistic population of satellites around a Milky Way-mass galaxy, 1602.05957, The Astrophysical Journal, 827, 2, L23, 10.3847/2041-8205/827/2/L23, 2041-8213, 2016ApJ...827L..23W, 16245449, free, JOURNAL, Kim, Stacy Y., Peter, Annika H. G., Hargis, Jonathan R., 2018, There is No Missing Satellites Problem, Physical Review Letters, 121, 21, 211302, 1711.06267, 10.1103/PhysRevLett.121.211302, 30517791, 54484838, File:Milky_Way_Satellite_Discoveries.gif|alt=|center|thumb|600x600px|Animation illustrating the discovery history of satellite galaxies of the Milky Way over the last 100 years. The classical satellite galaxies are in blue (labeled with their names), SDSS-discoveries are in red, and more recent discoveries (mostly with DES) are in green.]]

Motivations to study satellite galaxies

Spectroscopic, photometric and kinematic observations of satellite galaxies have yielded a wealth of information that has been used to study, among other things, the formation and evolution of galaxies, the environmental effects that enhance and diminish the rate of star formation within galaxies and the distribution of dark matter within the dark matter halo. As a result, satellite galaxies serve as a testing ground for prediction made by cosmological models.JOURNAL, Li, Zhao-Zhou, Jing, Y. P., Qian, Yong-Zhong, Yuan, Zhen, Zhao, Dong-Hai, 2017-11-22, Determination of Dark Matter Halo Mass from Dynamics of Satellite Galaxies, 1710.08003, The Astrophysical Journal, 850, 2, 116, 10.3847/1538-4357/aa94c0, 1538-4357, 2017ApJ...850..116L, 59388535, free, JOURNAL, Wojtak, Radoslaw, Mamon, Gary A., 2013-01-21, Physical properties underlying observed kinematics of satellite galaxies, 1207.1647, Monthly Notices of the Royal Astronomical Society, 428, 3, 2407–2417, 10.1093/mnras/sts203, 1365-2966, 2013MNRAS.428.2407W,

Classification of satellite galaxies

As mentioned above, satellite galaxies are generally categorized as dwarf galaxies and therefore follow a similar Hubble classification scheme as their host with the minor addition of a lowercase "d" in front of the various standard types to designate the dwarf galaxy status. These types include dwarf irregular (dI), dwarf spheroidal (dSph), dwarf elliptical (dE) and dwarf spiral (dS). However, out of all of these types it is believed that dwarf spirals are not satellites, but rather dwarf galaxies that are only found in the field.JOURNAL, Schombert, James M., Pildis, Rachel A., Eder, Jo Ann, Oemler, Augustus Jr., November 1995, Dwarf Spirals, 1995AJ....110.2067S, The Astronomical Journal, 110, 2067, 10.1086/117669, 0004-6256, free,

Dwarf irregular satellite galaxies

Dwarf irregular satellite galaxies are characterized by their chaotic and asymmetric appearance, low gas fractions, high star formation rate and low metallicity.BOOK, Galaxies in the universe : an introduction, Sparke, Linda Siobhan, Linda Sparke, 2007, Cambridge University Press, Gallagher, John S., 978-0521855938, 2nd, Cambridge, 74967110, Three of the closest dwarf irregular satellites of the Milky Way include the Small Magellanic Cloud, Canis Major Dwarf, and the newly discovered Antlia 2.

Dwarf elliptical satellite galaxies

Dwarf elliptical satellite galaxies are characterized by their oval appearance on the sky, disordered motion of constituent stars, moderate to low metallicity, low gas fractions and old stellar population. Dwarf elliptical satellite galaxies in the Local Group include NGC 147, NGC 185, and NGC 205, which are satellites of our neighboring Andromeda galaxy.JOURNAL, Hensler, Gerhard, 2011, The Morphological Origin of Dwarf Galaxies, 1103.1116, EAS Publications Series, 48, 383–395, 10.1051/eas/1148086, 2011EAS....48..383H, 118353978, 1633-4760,

Dwarf spheroidal satellite galaxies

Dwarf spheroidal satellite galaxies are characterized by their diffuse appearance, low surface brightness, high mass-to-light ratio (i.e. dark matter dominated), low metallicity, low gas fractions and old stellar population. Moreover, dwarf spheroidals make up the largest population of known satellite galaxies of the Milky Way. A few of these satellites include Hercules, Pisces II and Leo IV, which are named after the constellation in which they are found.

Transitional types

As a result of minor mergers and environmental effects, some dwarf galaxies are classified as intermediate or transitional type satellite galaxies. For example, Phoenix and LGS3 are classified as intermediate types that appear to be transitioning from dwarf irregulars to dwarf spheroidals. Furthermore, the Large Magellanic Cloud is considered to be in the process of transitioning from a dwarf spiral to a dwarf irregular.

Formation of satellite galaxies

According to the standard model of cosmology (known as the ΛCDM model), the formation of satellite galaxies is intricately connected to the observed large-scale structure of the Universe. Specifically, the ΛCDM model is based on the premise that the observed large-scale structure is the result of a bottom-up hierarchical process that began after the recombination epoch in which electrically neutral hydrogen atoms were formed as a result of free electrons and protons binding together. As the ratio of neutral hydrogen to free protons and electrons grew, so did fluctuations in the baryonic matter density. These fluctuations rapidly grew to the point that they became comparable to dark matter density fluctuations. Moreover, the smaller mass fluctuations grew to nonlinearity, became virialized (i.e. reached gravitational equilibrium), and were then hierarchically clustered within successively larger bound systems.JOURNAL, Blumenthal, George R., Faber, S. M., Primack, Joel R., Rees, Martin J., October 1984, Formation of galaxies and large-scale structure with cold dark matter, Nature, 311, 5986, 517–525, 10.1038/311517a0, 0028-0836, 1984Natur.311..517B, 1447148, 4324282, The gas within these bound systems condensed and rapidly cooled into cold dark matter halos that steadily increased in size by coalescing together and accumulating additional gas via a process known as accretion. The largest bound objects formed from this process are known as superclusters, such as the Virgo Supercluster, that contain smaller clusters of galaxies that are themselves surrounded by even smaller dwarf galaxies. Furthermore, in this model dwarfs galaxies are considered to be the fundamental building blocks that give rise to more massive galaxies, and the satellites that are observed around these galaxies are the dwarfs that have yet to be consumed by their host.JOURNAL, Kravtsov, Andrey V., 2010, Dark matter substructure and dwarf galactic satellites, 0906.3295, Advances in Astronomy, 2010, 281913, 10.1155/2010/281913, 1687-7969, 2010AdAst2010E...8K, 14595577, free,

Accumulation of mass in dark matter halos

A crude yet useful method to determine how dark matter halos progressively gain mass through mergers of less massive halos can be explained using the excursion set formalism, also known as the extended Press-Schechter formalism (EPS).JOURNAL, Bond, J. R., Cole, S., Efstathiou, G., Kaiser, N., October 1991, Excursion set mass functions for hierarchical Gaussian fluctuations, 1991ApJ...379..440B, The Astrophysical Journal, 379, 440, 10.1086/170520, 0004-637X, free, Among other things, the EPS formalism can be used to infer the fraction of mass M_2 that originated from collapsed objects of a specific mass at an earlier time t_1 < t_2 by applying the statistics of Markovian random walks to the trajectories of mass elements in (S,delta)-space, where S = sigma^2(M) and delta = {rho(x) - bar{rho} over bar{rho} } represent the mass variance and overdensity, respectively.In particular the EPS formalism is founded on the ansatz that states "the fraction of trajectories with a first upcrossing of the barrier delta_S = delta_{critical}(t) at S > S_1 = sigma^2(M_1) is equal to the mass fraction at time t that is incorporated in halos with masses M