Satellite galaxy

[3] The Milky Way is orbited by about fifty satellite galaxies, the largest of which is the Large Magellanic Cloud.

[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.

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.

For example, the host galaxy is capable of disrupting the orbiting satellites via tidal and ram pressure stripping.

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.

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.

[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.

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).

[10][11] In particular, the Milky Way is currently known to host 59 satellite galaxies (see satellite galaxies of the Milky Way), of which two known as the Large Magellanic Cloud and Small Magellanic Cloud have been observable in the Southern Hemisphere with the unaided eye since ancient times.

[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.

As a result, satellite galaxies serve as a testing ground for prediction made by cosmological models.

A few of these satellites include Hercules, Pisces II and Leo IV, which are named after the constellation in which they are found.

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.

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.

[22] 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).

In particular the EPS formalism is founded on the ansatz that states "the fraction of trajectories with a first upcrossing of the barrier

is the critical overdensity at which the initial spherical region has collapsed to form a virialized object.

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.

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.

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.

respectively representing the maximum impact parameter, the half-mass radius and the typical relative velocity.

Using the Faber-Jackson relation, the velocity dispersion of satellites and their host can be estimated individually from their observed luminosity.

[28] Since this discovery was made, various observations have verified that mergers do indeed induce vigorous star formation.