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A satellite galaxy is a collection of gravitationally bound stars orbiting a larger, central galaxy. The satellite galaxy typically has a mass of $$10^7 M_\odot$$, with some as high as $$10^9 M_\odot$$, while the central galaxy has a typical mass of $$10^{11} - 10^{12} M_\odot$$. This relationship is distinct from two interacting galaxies of similar size, such as the Milky Way and Andromeda, which form a binary system. As satellite galaxies are diffuse and dim, they are difficult to observe even near the Milky Way; 12 of the 24 satellite galaxies of the Milky Way have been discovered since 2003.

Unlike the more stable central galaxies, satellite galaxies are frequently disrupted by gravitational effects, such as tidal stripping, and drag effects, such as ram-pressure stripping. This characteristic disruption of satellite galaxies is thought to explain many of their physical properties.

Satellite galaxy formation and evolution
The visible mass of galaxies is only a small fraction of the total mass of a given system. The non-visible dark matter is believed to form a spherical halo around the visible galaxy.



Simulations such as the Via Lactea project show that smaller dark matter halos form first, then merge into larger halos. The most massive halo is typically associated with the central galaxy (or galaxies, in the case of a cluster), while satellite galaxies are associated with independent dark matter "sub-halos", where they formed prior to capture.

In the central halo environment, the satellites interact with the parent galaxy and other nearby satellites. Ram pressure due to motion through the intergalactic gas in the central halo removes gas from satellite galaxies, a process called ram pressure stripping. Gravitational interactions with the central halo, such as tidal stripping, also remove gas and stars from satellite galaxies. The result of these interactions is the the outermost hot gas in the halo of a galaxy is stripped, cutting off the supply of the cool gas. This process is known as strangulation because star formation is suppressed, or "strangled", by the lack of raw material.

Eventually the orbits of the satellites decay due to dynamical friction, and the galaxies sink deeper into the potential well of the central galaxy where interactions are more likely. The additional interactions result in even more orbital decay, a runaway process which eventually ends with accretion onto the central galaxy. The more massive galaxy tears apart the satellite during this process, leaving observable tidal streams.

The role of satellites in central galaxy evolution
The formation of thick spiral disks from thin disks can be caused by the accretion of dwarf galaxies. During the accretion process of a dwarf galaxy into a spiral galaxy, the dwarf perturbs the orbits of stars in the central spiral. This introduces heat, and thus random motion which thickens the disk. There is evidence that same perturbation is responsible for the formation of disk globular clusters.

Luminosity distribution function of satellite galaxies
The luminosity function describes the distribution of luminosities in a sample of galaxies, which is indicative of the stellar mass content. It is important to note that the luminosity of a given galaxy is not constant and evolves both with stellar evolution and widespread galactic dynamical effects, and therefore the luminosity distribution function provides insight into galactic evolution.



Central galaxies are typically built up by the accretion of satellites and other galaxies, while satellites are accreted by their central galaxies. As they undergo different evolutionary paths, their predicted luminosity functions differ. It is important to understand the relative contribution to the total light observed from both the central galaxy and satellites, because satellites are not generally resolved against their central galaxy except in nearby systems and thus the luminosity of distant galaxies will be the total contributed from the parent and its satellites.

It has been observed that the properties of satellite galaxies depend on the properties of the central halo, but it is not yet known in detail how these effects culminate to form the total observed luminosity function. However, the satellite's contribution can be approximated from the naïve assumption that the stellar mass and luminosity of a satellite do not change after capture by the central galaxy. An explicit derivation in Galaxy Formation and Evolution by Mo, van der Bosch and White shows that the luminosity function for satellite galaxies is expected to have a sharp break at the bright end of the distribution. If the luminosity function is restricted to more massive halos, bright central host galaxies overwhelm the luminosity of the brightest satellites at this break. However, central galaxies in low mass halos may not be significantly brighter than the brightest satellite galaxies, and a significant gap can be observed between the different populations' luminosity distribution functions. This general behavior was observed by Yang, et al 2008. However, Yang et al. also observed significant deviation from the test case detailed above, and attributed this to luminosity evolution due to tidal disruption.

Star formation in satellite galaxies
As discussed above, dynamical effects, collectively known as quenching mechanisms, remove the satellite's gas reservoir. If one assumes that the original star formation rate per unit mass of satellites is similar to that of central galaxies of the same luminosity, quenching is thought to occur at very nearly the dynamical timescale of the central halo. For halos at redshift z ~ 1 (about 7.7 billion years ago), the gas will be fully stripped within two billion years. As a result, ignoring recent captures, satellite galaxies tend to be devoid of star-formation. Such satellite's luminosities are said to evolve "passively," without contributions from newly formed stars.

Colors of satellite galaxies
Star-formation and metallicity are the major factors that contribute to a galaxy's overall color. Since satellites generally tend to be metal-poor, the dominant effect in determining a satellite's color is star formation. Active star formation results in "blue" young stars dominating the color of the galaxy, while less star formation leads to a larger color contribution from cooler, longer-lasting "red" stars.

Because of their lack of significant star-formation, satellite galaxies are typically redder than an associated central galaxy of the same luminosity. However, observations show that this color difference is not strong as models predict, indicating that the processes limiting star formation are not as efficient as predicted. This behavior points to strangulation as the dominant disruptor of star-forming gas, as strangulation typically takes longer than the dynamical time scale of the host galaxy. A longer scale for quenching than the crossing time of the satellite galaxy is consistent with reduced but not completely quenched star formation.

Satellite galaxies as a probe of dark matter
Satellite galaxies can be described as a collection of luminous matter in the center of a small halo of dark matter. It follows that some properties of small halos can be described by their associated satellite galaxies. Satellite galaxies of the Milky Way have been observed to have a typical mass of about $$10^7 M_\odot$$ solar masses, setting a limit for the minimum mass of dark matter halos. In the warm dark matter model, a dark matter mass of about 1 keV would result in minimum halo masses of about $$10^9 M_\odot$$ solar masses. Therefore the existence of satellite galaxies as observed in the Milky Way constrains dark matter models to those that allow halos of $$10^7 M_\odot$$ solar masses or smaller to form.

Satellite galaxies' orbits can also be used as tracers for the extent of the dark matter halo of the central galaxy. Assuming that satellite galaxies have primarily radial orbits, the extent of the dark matter halo of the Galaxy is not far beyond the edge of visible galactic features at $$r \approx 30 \rm{kpc}$$. This conflicts with measurements made using the Magellanic Stream and Local Group timing, although the difference can be reconciled if an unknown formation mechanism caused satellite galaxies to have primarily tangential orbits.

Satellites of the Milky Way


The Large Magellanic Cloud (LMC) is the fourth largest of the galaxies in the Local Group. Although it is currently an irregular galaxy, it is thought to have once been a barred spiral galaxy, as it appears to exhibit the remnants of a prominent bar-like feature. The LMC still exhibits significant star formation compared to other satellites. It is thought that its intrinsically high mass relative to other satellites (about 1/10th that of the Milky Way) allows the LMC to gravitationally retain its gas and consequently form stars. It is also thought to be making its first pass around the Milky Way, and thus has not had a significant amount of gas removed by dynamical stripping processes. The visibly irregular nature of the LMC is attributed to disruption from the Milky Way, which is also believed to have caused the Magellanic Stream, a large tidally stripped tail originating from the Magellanic Clouds.

The Small Magellanic Cloud (SMC) is also thought to have been a barred spiral galaxy until tidal disruption effects shaped the SMC into the irregular galaxy seen today. It displays a tidal tail of gas, dust, and newly formed stars. There appears to be some triggered star-formation in the tidally stripped tail, illustrating that while the general result of tidal interactions is quenched star-formation there can be intermediary burst events as well. In addition to the tail, "wings" from the disrupted bar are commonly observed. There is some speculation as to whether or not the SMC and LMC are gravitationally bound to the Milky Way due to their high tangential velocity, although the Milky Way's tidal influence has undoubtedly left its mark on both. It is also postulated that the LMC will eventually accrete the SMC, and there is evidence for interactions between the two satellites.

There are 24 known satellites of the Milky Way, many of which were originally discovered as stellar over-densities because of their low surface brightness. As an example, Boötes I was discovered by Vasily Belokurov in 2006. The figure to the left shows the original Sloan Digital Sky Survey (SDSS) image in which Boötes I was first identified, along with an "enhanced" image with faint stars within the galaxy identified by color, artificially enhanced in brightness, and over-plotted on the SDSS image. This second image shows a more representative stellar population of the galaxy.



Extragalactic satellite galaxies
In general, satellite galaxies are difficult to observe, as one must discern faint objects very near to the far more luminous parent galaxy. They have only been directly observed out to the Coma Cluster, approximately 100 Mpc away. |The satellites of the Andromeda Galaxy are the best-known of the extragalactic satellite galaxies, and include Messier 32 (M32) and the Triangulum Galaxy.

The Andromeda Galaxy provides an active example of violent tidal disruption of its companion dwarfs. A number of its satellites show significant disruption; of these, M32 is of particular interest. As M32 contains a supermassive black hole, it may have once been the bulge of a spiral galaxy. Research suggests that strong tidal disruptions from the interaction between the Andromeda Galaxy and M32 changed it from a spiral galaxy to a compact elliptical (cE) galaxy.

Another prominent tidal feature of the Andromeda Galaxy is the Giant Southern Stream, which extends over 150 kpc in length. It is thought be the remnant of one of the Local Group's most massive galaxies, accreted less than 1 billion years ago.