Satellite galaxy

A 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).[1] 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.[2] While most satellite galaxies are dwarf galaxies, satellite galaxies of large galaxy clusters can be much more massive.[3]

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.[4] 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.[1] 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.[5]

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.[6] 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, 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.[7][8]

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.[9] 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).[9]

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).[10][11] 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).[12][13] 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.[14][15]

Milky Way Satellite Discoveries
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.[14][16][17]

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.[18]

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.[19] 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.

Large.mc.arp.750pix
The Large Magellanic Cloud, the Milky Way's largest satellite galaxy, and fourth largest in the Local Group. This satellite is also classified as a transition type between a dwarf spiral and dwarf irregular.

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.[19][20]

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.[1] 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.[19]

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.[19]

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.[21]

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.[22]

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).[23] Among other things, the EPS formalism can be used to infer the fraction of mass that originated from collapsed objects of a specific mass at an earlier time by applying the statistics of Markovian random walks to the trajectories of mass elements in -space, where and 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 at is equal to the mass fraction at time that is incorporated in halos with masses ".[24] Consequently, this ansatz ensures that each trajectory will upcross the barrier given some arbitrarily large , and as a result it guarantees that each mass element will ultimately become part of a halo.[24]

Furthermore, the fraction of mass that originated from collapsed objects of a specific mass at an earlier time can be used to determine average number of progenitors at time within the mass interval that have merged to produce a halo of at time . This is accomplished by considering a spherical region of mass with a corresponding mass variance and linear overdensity , where is the linear growth rate that is normalized to unity at time and is the critical overdensity at which the initial spherical region has collapsed to form a virialized object.[24] Mathematically, the progenitor mass function is expressed as:

Various comparisons of the progenitor mass function with numerical simulations have concluded that good agreement between theory and simulation is obtained only when is small, otherwise the mass fraction in high mass progenitors is significantly underestimated, which can be attributed to the crude assumptions such as assuming a perfectly spherical collapse model and using a linear density field as opposed to a non-linear density field to characterize collapsed structures.[25][26] Nevertheless, the utility of the EPS formalism is that it provides a computationally friendly approach for determining properties of dark matter halos.

Halo merger rate

Another utility of the EPS formalism is that it can be used to determine the rate at which a halo of initial mass M merges with a halo with mass between M and M+ΔM.[24] This rate is given by

where , . In general the change in mass, , is the sum of a multitude of minor mergers. Nevertheless, given an infinitesimally small time interval it is reasonable to consider the change in mass to be due to a single merger events in which transitions to .[24]

Galactic cannibalism (minor mergers)

Remnants of a minor merger can be observed in the form of a stellar stream falling onto the galaxy NGC5907.

Throughout their lifespan, satellite galaxies orbiting in the dark matter halo experience dynamical friction and consequently descend deeper into the gravitational potential of their host as a result of orbital decay. Throughout the course of this descent, stars in the outer region of the satellite are steadily stripped away due to tidal forces from the host galaxy. This process, which is an example of a minor merger, continues until the satellite is completely disrupted and consumed by the host galaxies.[27] Evidence of this destructive process can be observed in stellar debris streams around distant galaxies.

Orbital decay rate

As satellites orbit their host and interact with each other they progressively lose small amounts of kinetic energy and angular momentum due to dynamical friction. Consequently, the distance between the host and the satellite progressively decreases in order to conserve angular momentum. This process continues until the satellite ultimately mergers with the host galaxy. Furthermore, If we assume that the host is a singular isothermal sphere (SIS) and the satellite is a SIS that is sharply truncated at the radius at which it begins to accelerate towards the host (known as the Jacobi radius), then the time that it takes for dynamical friction to result in a minor merger can be approximated as follows:

An edge-on photo of the Needle Galaxy (NGC 4565) that demonstrates the observed thick disk and thin disk components of satellite galaxies.

Minor merger driven star formation

In 1978, pioneering work involving the measurement of the colors of merger remnants by the astronomers Beatrice Tinsley and Richard Larson gave rise to the notion that mergers enhance star formation. Their observations showed that an anomalous blue color was associated with the merger remnants. Prior to this discovery, astronomers had already classified stars (see stellar classifications) and it was known that young, massive stars were bluer due to their light radiating at shorter wavelengths. Furthermore, it was also known that these stars live short lives due to their rapid consumption of fuel to remain in hydrostatic equilibrium. Therefore, the observation that merger remnants were associated with large populations of young, massive stars suggested that mergers induced rapid star formation (see starburst galaxy).[28] Since this discovery was made, various observations have verified that mergers do indeed induce vigorous star formation.[27] Despite major mergers being far more effective at driving star formation than minor mergers, it is known that minor mergers are significantly more common than major mergers so the cumulative effect of minor mergers over cosmic time is postulated to also contribute heavily to burst of star formation.[29]

Minor mergers and the origins of thick disk components

Observations of edge-on galaxies suggest the universal presence of a thin disk, thick disk and halo component of galaxies. Despite the apparent ubiquity of these components, there is still ongoing research to determine if the thick disk and thin disk are truly distinct components.[30] Nevertheless, many theories have been proposed to explain the origin of the thick disk component, and among these theories is one that involves minor mergers. In particular, it is speculated that the preexisting thin disk component of a host galaxy is heated during a minor merger and consequently thin disk expands to form a thicker disk component.[31]

See also

References

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Andromeda I

Andromeda I is a dwarf spheroidal galaxy(dSph) about 2.40 million light-years away in the constellation Andromeda. Andromeda I is part of the local group of galaxies and a satellite galaxy of the Andromeda Galaxy (M31). It is roughly 3.5 degrees south and slightly east of M31. As of 2005, it is the closest known dSph companion to M31 at an estimated projected distance of ~40 kpc or ~150,000 light-years.

Andromeda I was discovered by Sidney van den Bergh in 1970 with the Mount Palomar Observatory 48-inch telescope. Further study of Andromeda I was done by the WFPC2 camera of the Hubble Space Telescope. This found that the horizontal branch stars, like other dwarf spheroidal galaxies were predominantly red. From this, and the abundance of blue horizontal branch stars, along with 99 RR Lyrae stars detected in 2005, lead to the conclusion there was an extended epoch of star formation. The estimated age is approximately 10 Gyr. The Hubble telescope also found a globular cluster in Andromeda I, being the least luminous galaxy where such a cluster was found.

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 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).

Antlia 2

Antlia 2 (Ant 2) 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 10,000 times fainter. Antlia 2 has the lowest surface brightness of any galaxy discovered and is ~ 100 times more diffuse than any known ultra diffuse galaxy. It was discovered by the European Space Agency's Gaia spacecraft in November 2018.

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.

Cassiopeia Dwarf

The Cassiopeia Dwarf (also known as Andromeda VII) is a dwarf spheroidal galaxy about 2.58 Mly away in the constellation Cassiopeia. The Cassiopeia Dwarf is part of the Local Group and a satellite galaxy of the Andromeda Galaxy (M31).

The Cassiopeia Dwarf was found in 1998, together with the Pegasus Dwarf, by a team of astronomers (Karachentsev and Karachentseva) in Russia and Ukraine. The Cassiopeia Dwarf and the Pegasus Dwarf are farther from M31 than its other known companion galaxies, yet still appear bound to it by gravity. Neither galaxy contains any young, massive stars or shows traces of recent star formation. Instead, both seem dominated by very old stars, with ages of up to 10 billion years.

Crater 2 Dwarf

Crater 2 is a low-surface-brightness dwarf satellite galaxy of the Milky Way, located approximately 380,000 ly from Earth. Crater 2 was identified in imaging data from the VST ATLAS survey.The galaxy has a half-light radius of ∼1100 pc, making it the fourth largest satellite of the Milky Way. It has an angular size about double of that of the moon.

Draco Dwarf

The Draco Dwarf is a spheroidal galaxy which was discovered by Albert George Wilson of Lowell Observatory in 1954 on photographic plates of the National Geographic Society's Palomar Observatory Sky Survey (POSS). It is part of the Local Group and a satellite galaxy of the Milky Way galaxy. The Draco Dwarf is situated in the direction of the Draco Constellation at 34.6° above the galactic plane.

Galactic tide

A galactic tide is a tidal force experienced by objects subject to the gravitational field of a galaxy such as the Milky Way. Particular areas of interest concerning galactic tides include galactic collisions, the disruption of dwarf or satellite galaxies, and the Milky Way's tidal effect on the Oort cloud of the Solar System.

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 2100

NGC 2100 is an open cluster in the Large Magellanic Cloud, a small satellite galaxy of the Milky Way. These clusters have a lifespan measured in tens or hundreds of millions of years, as they eventually disperse through gravitational interaction with other bodies. As its format is approximately round, it is sometimes mistaken as a globular cluster.

NGC 4242

NGC 4242 is a spiral galaxy in the northern constellation of Canes Venatici. The galaxy is about 18 million light years (5.5 megaparsecs) away. It was discovered on 10 April 1788 by William Herschel, and it was described as "very faint, considerably large, irregular, round, very gradually brighter in the middle, resolvable" by John Louis Emil Dreyer, the compiler of the New General Catalogue.NGC 4242's galaxy morphological type is SABdm. This means that it is an intermediate spiral galaxy, with loosely wound spiral arms and is generally irregular in appearance. It was photographed by the Hubble Space Telescope in 2017. The image shows an asymmetric center and a small galactic bar. NGC 4242 has a relatively low surface brightness and rate of star formation. NGC 4242 may be a satellite galaxy of Messier 106 and is a member of the Canes II Group.SN 2002bu was detected in NGC 4242, brightening to its peak magnitude of 15.5 in 2002. It was originally classified as a type II supernova, but it may be a supernova impostor, like SN 2008S.

NGC 7752 and NGC 7753

NGC 7752 and NGC 7753 are a pair of galaxies approximately 272 million light-years away in the constellation Pegasus.

NGC 7753 is the primary galaxy. It is a barred spiral galaxy with a small nucleus. NGC 7752 is the satellite galaxy of NGC 7753. It is a barred lenticular galaxy that is apparently attached to one of NGC 7753's spiral arms. They resemble the Whirlpool Galaxy (M51A) and its satellite NGC 5195 (M51B).

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).

Pisces

Pisces may refer to:

Pisces, an obsolete taxonomic term for fish

Pisces (astrology), an astrological sign

Pisces (constellation), a constellation

Pisces I (dwarf galaxy), an overdensity of stars in the Milky Way's halo that is situated in the Pisces constellation

Pisces II (dwarf galaxy), a satellite galaxy of the Milky Way

Pisces Dwarf, a satellite galaxy of the Triangulum Galaxy

Pisces (Chinese astronomy), the division of the sky in traditional Chinese uranography that lies across the modern constellation Pisces

PISCES (Personal Identification Secure Comparison and Evaluation System), a border control database system administered by the United States Department of State

Pisces (album), a 1961 album by Art Blakey & the Jazz Messengers

Pisces (comics), a Marvel Comics character

Pisces class DSV, a class of three-person research deep-submergence vehicles

NOAAS Pisces (R 226), an American fisheries and oceanographic research ship in commission in the National Oceanic and Atmospheric Administration since 2009

OZ-09MMS Pisces, a fictional mecha in the Gundam Wing anime

Pisces (band), a psychedelic rock band

Ursa Minor Dwarf

The Ursa Minor Dwarf is a dwarf spheroidal galaxy, discovered by A.G. Wilson of the Lowell Observatory, in the United States, during the Palomar Sky Survey in 1955. It appears in the Ursa Minor constellation, and is a satellite galaxy of the Milky Way. The galaxy consists mainly of older stars and seems to house little to no ongoing star formation. Its centre is around 225,000 light years distant from Earth.

Virgo I

Virgo I is an extremely faint satellite galaxy of the Milky Way. It was discovered in the Subaru Strategic Survey. Virgo I has an absolute visual magnitude of -0.8 making it the least luminous galaxy confirmed thus far. The galaxy has a radius of 124 light years, (half light radius 38 pc) meaning that it is too big to be a globular cluster. Cetus II is dimmer, but too small to be classed as a galaxy. Virgo I is dimmer than Segue I, the previous dimmest known. The distance to Virgo I is 87 kiloparsecs (280,000 ly).

Morphology
Structure
Active nuclei
Energetic galaxies
Low activity
Interaction
Lists
See also

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