Satellite galaxy

Some galaxies are small. They travel near big galaxies.

They stay close like planets near a sun. Our big galaxy has many small friends.

These small friends move in circles. They stay near the big galaxy. The big galaxy can pull on them.

Sometimes the small ones crash into the big one. This can make a much bigger galaxy.

It is fun to look at the stars.

A satellite galaxy is a small companion. It travels near a much larger host galaxy.

These small galaxies stay close because of gravity. This is like how planets orbit the Sun.

Our Milky Way has about 59 satellite galaxies. The largest one is the Large Magellanic Cloud.

People could see it with just their eyes. Many other small ones were found later. New tools like the Sloan Digital Sky Survey helped us find them.

Life for a satellite can be rough. The big galaxy can pull gas away from the small one. This is called ram pressure stripping. Without gas, the small galaxy cannot make new stars. Sometimes, a small galaxy hits the big one. This is called a minor merger.

These crashes help make even bigger galaxies over time. Scientists study these small neighbors to learn how the universe grows.

A satellite galaxy is a small companion to a much larger galaxy. These smaller neighbors travel on orbits around a massive host galaxy. They stay close because of gravity, just like planets orbit the Sun.

Most satellite galaxies are dwarf galaxies, which means they are quite small. However, satellites in huge galaxy clusters can be much more massive. They are different from globular clusters because they are more spread out. They also live inside huge clouds of dark matter.

Life for a satellite galaxy can be very chaotic. The large host galaxy can pull material away from the smaller one. This happens through things called tidal stripping or ram pressure stripping. These forces can remove cold gas from the satellite. Since gas is the fuel for making stars, the satellite might stop making them. This makes the galaxy quiescent, or quiet.

Sometimes, a satellite will crash into its host galaxy. This event is called a minor merger. If two satellites hit each other, it is a major merger.

For a long time, people did not know other galaxies existed. In 1920, two astronomers had a famous debate. Harlow Shapley thought the Milky Way was the whole universe. Heber Curtis argued that other galaxies lived far beyond us.

The debate ended in 1923 thanks to Edwin Hubble. He used Cepheid Variable stars to measure the distance to the Andromeda galaxy. He found it was 300 kiloparsecs away. This proved the universe was much larger than Shapley thought.

Today, we know our Milky Way has about 59 satellite galaxies.

Two of them, the Large and Small Magellanic Clouds, are easy to see. People have watched them in the sky since ancient times.

Modern tools like the Sloan Digital Sky Survey help us find more. We can group these small galaxies into different types. Dwarf irregulars look messy and make many stars. Dwarf ellipticals look like ovals and have old stars. Dwarf spheroidals are very spread out and have lots of dark matter.

Scientists believe these small galaxies helped build the large ones we see today. This idea comes from the $\Lambda$CDM model of the universe. In this model, small things clump together to form bigger things. First, tiny fluctuations in matter began to grow. These grew into dark matter halos that gathered gas. As these halos merged, they became larger and larger systems. Eventually, they formed giant structures like the Virgo Supercluster.

Studying satellites helps us understand how these huge structures grew.

A satellite galaxy is a smaller companion galaxy that travels on a bound orbit around a more massive, luminous host galaxy. This relationship is governed by gravity, much like how planets in our Solar System are gravitationally bound to the Sun. While most satellite galaxies are classified as dwarf galaxies, those found within large galaxy clusters can reach much higher masses. It is important to distinguish these from globular clusters, which are also gravitationally bound to larger galaxies. Satellite galaxies are more extended and diffuse than globular clusters. Furthermore, they are enshrouded in massive dark matter halos, which are thought to be endowed to them during the galaxy formation process.

Life for a satellite galaxy is often tumultuous due to chaotic interactions. The host galaxy can disrupt its orbiting satellites through processes known as tidal stripping or ram pressure stripping. These environmental effects can forcibly remove large amounts of cold gas from the satellite. Because cold gas serves as the essential fuel for star formation, its removal can cause a satellite to become quiescent. A quiescent galaxy is one that has effectively ceased forming new stars. Additionally, satellites may collide with their host galaxy, an event called a minor merger. If two satellites of comparable mass collide, it is known as a major merger.

Astronomers categorize satellite galaxies into several distinct types using the Hubble classification scheme. Dwarf irregular galaxies (dI) are characterized by a chaotic, asymmetric appearance and high star formation rates. Examples near the Milky Way include the Small Magellanic Cloud and the newly discovered Antlia 2. Dwarf elliptical galaxies (dE) appear oval on the sky and consist of older stellar populations with low gas fractions. Some of these, like NGC 147, orbit the Andromeda galaxy. Dwarf spheroidal galaxies (dSph) are highly diffuse and possess a high mass-to-light ratio, meaning they are dominated by dark matter. These make up the largest population of known satellites for the Milky Way.

Some galaxies exist as transitional types due to mergers or environmental changes. For instance, the Phoenix and LGS3 galaxies are intermediate types transitioning from dwarf irregulars to dwarf spheroidals. The Large Magellanic Cloud is also considered to be transitioning from a dwarf spiral to a dwarf irregular. These transitions show how the environment of a host galaxy actively reshapes its companions. By studying these different morphologies, scientists can better understand the evolutionary paths of small galaxies.

Our understanding of these objects has changed significantly since the early 20th century. Before this era, the existence of galaxies beyond the Milky Way was highly controversial. In 1920, Harlow Shapley and Heber Curtis engaged in the "Shapley-Curtis Great Debate." Shapley argued the Milky Way spanned 100,000 lightyears and contained all known nebulae. Curtis argued that the Milky Way was smaller and that nebulae were actually distant galaxies. The debate was settled in 1923 when Edwin Hubble measured the distance to the Andromeda galaxy (M31). Using Cepheid Variable stars, Hubble estimated a distance of 300 kiloparsecs. This proved the universe was much larger than Shapley had imagined.

Modern technology has revealed that the Milky Way hosts approximately 59 known satellite galaxies. While the Large and Small Magellanic Clouds have been visible to the naked eye since ancient times, many others remained hidden. Modern surveys like the Sloan Digital Sky Survey (SDSS) and the Dark Energy Survey (DES) have detected many more. These tools allow astronomers to use spectroscopic and kinematic observations to study galaxy evolution. They also help map the distribution of dark matter within the dark matter halo. These studies serve as a vital testing ground for modern cosmological models.

The formation of these galaxies is explained by the $\Lambda$CDM model of cosmology. This model suggests a "bottom-up" hierarchical process began after the recombination epoch. During this time, fluctuations in baryonic matter density grew until they matched dark matter density fluctuations. These fluctuations became nonlinear and reached gravitational equilibrium, or virialization. Small mass fluctuations clustered into increasingly larger systems, forming cold dark matter halos. These halos grew by accumulating gas through accretion and coalescing with other halos. In this view, dwarf galaxies are the fundamental building blocks of the universe. The satellites we see today are simply the dwarfs that have not yet been consumed by their hosts.