We make new discoveries every day from finding new species here on Earth to identifying galaxies at the very edge of the universe. But with our current technology you would think that the age for discovering massive galaxies right next door to ours has passed. A discovery of this nature was made less than 18 months ago when an ambitious star mapping effort revealed a previously undetected galaxy right next door to the Milky Way.

Collision with Earth

The galaxy is Antlia 2 a relatively large unusually faint galaxy that was revealed during the European Space Agency’s second Gaia Mission in November 2018. It is over 10,000 light-years in diameter and sits at just under 400 thousand light-years from the earth. Virtually nothing on an intergalactic scale. While we’d not seen Antlia – 2 until now astronomers at the Rochester Institute for technology predicted that a galaxy like Antlia 2 might exist in that area in 2009 and they ran simulations on the galaxies and calculated that, it was involved in a collision with our galaxy the Milky Way less than a billion years ago. A collision which removed many of its vital stars and star forming gases, leaving it dim diffuse and dominated by dark matter. Now for the first time we can see it the remnants of a distorted tarnished dwarf galaxy trailing across the Gaia sky as a faint streak, a galactic echo, a ghost galaxy.

Satellite Galaxy

Antlia 2 is just one product of the chaotic interactions that before more than a half century of satellite galaxies, which orbit the Milky Way. Smaller dwarf galaxies that orbit our galaxy’s core akin to planets orbiting a star. Satellite galaxies have unique properties making them giant local laboratories for us to study star formation and galaxy formation with a surprising amount to learn in between. A satellite galaxies is a small companion galaxy orbiting within the gravitational field of a larger galaxy such as our own like planets orbiting stars. No less than 59 satellite galaxies orbit within about one-and-a-half million light-years of the Milky Way alone. They are commonplace within galaxies clusters and throughout the rest of the universe.

Dwarf Galaxies

Most satellite galaxies are classed as Dwarf Galaxies likely the first type of galaxies in the universe. According to the standard model of the universe’s evolution and future, matter in the universe has been forming from the ground up since the temperature of the universe fell below the threshold to allowed neutral hydrogen atoms to form. These atoms became dust in gas which form the first stars and then as the great fires of the universe began appearing they joined into small disordered groups of dwarf galaxies which themselves would collide to feed larger galaxies billions of years later. Galaxies super clusters and filaments now line the universe with trillions of galaxies.

Why They Form

Most of which hosts smaller satellite dwarf galaxies which circle their parents like ghosts in a cartoon. Unlike large galaxies which can remain relatively stable for billions of years satellite galaxies tend to lead turbulent and chaotic lives due to their unstable orbits and interactions with other satellites. As a satellite galaxy orbits the tidal gravitational forces of other satellites exert varying amounts of influence on the satellite disturbing its structure. In many cases this scatters the stars within and gives the satellite a tarnished deformed appearance. Others remain relatively structured but a still stripped of vital gases and thus their ability to produce new stars. For the satellite galaxies that aren’t ejected out of their systems altogether they will usually meet one of two phase. Either they will merge the satellites or they will be consumed by their host galaxy entirely. Let’s start with the latter.

Minor Merger

Most satellite galaxies will end up being consumed by the host galaxy. Satellite galaxies orbiting hosts are constantly spiralling inwards, as they orbit losing energy and distance from the parent as they experience gravitational drag aka the process of orbital decay. As the satellite veers closer to the parent the tidal gravitational forces acting upon it becomes stronger and the stars on the outer regions are stripped away and sucked into the parent. Usually along with important star forming gases because the vast majority of both the satellite galaxies and the parents volume is near empty space and interstellar gas stars that fall into the host usually arrive without colliding with anything. As such the process is comparable to the erosion of cliff edges by the Earth’s oceans bit by bit, piece by piece. Gradually mass is lost until the entire satellite is absorbed into the vastness of the host galaxy this process is called a minor merger. The merging of two galaxies with varying masses one large and the other small.

Major Merger

On the other hand a satellite galaxy could merge with another satellite called a major merger. As two galaxies with a similar mass combine to make a larger galaxy. This new satellite will then continue its journey around the parent to destined to be consumed by the host galaxy or perhaps be ejected out of its system entirely. Life for satellite galaxies is short and unpredictable and it rarely ends well.

First Discovery

Satellite galaxies are a recent and constantly evolving area of study evidenced by the discovery of Antlia 2. Among other things we didn’t even properly understand that there were other galaxies out there until the 1920s. Most of the Milky Way satellites are so faint that we didn’t start discovering them or understanding them until partway through the century in 1938 Harlow Shapley discovered the sculptor dwarf galaxy and from there we discovered a new one every five or ten years. However from 2000 onwards complex astronomical surveys became more common. In particular the Sloan Digital Sky Survey and the Dark Energy survey. With those we went from discovering a handful of satellites in 50 years to more than 50 satellites in a handful of years and today we’ve identified 59 around the Milky Way and dozens more around Andromeda with Antlia. Proving that there are probably still plenty more out there waiting to be discovered.

Types

There’s a lot we still don’t know about these galaxies but we know that they fall into a few main morphological types.

Dwarf Irregular

First there are Dwarf Irregular types. These are small fledgling satellite galaxies that have been pulled and warped by the gravitational influences around it. They have high star formation rays as they usually still have their star forming gases. The universe is dotted with irregular type galaxies each brimming with massive stars and productive nebulae.

Dwarf Spheroidal

Second, the Dwarf Spheroidal. These are larger and more common than their irregular counterparts. They appear diffuse and have low surface brightness and high mass to light ratios. They make up the vast majority of the Milky Way Satellites with the closest confirmed being.

The Sagittarius dwarf spheroidal Galaxies discovered in 1994. It’s about ten thousand light-years in diameter, making it the third largest of the Milky Way’s satellites. It currently sits at about seventy thousand light years away. But approaches as close as 50 thousand light years during its polar orbit of our galaxy. The Sagittarius Dwarf Spheroidal galaxies appears to be an old one with a large number of early population two stars and four identified magnificent globular clusters within, with one situated at its core. In contrast Dwarf elliptical galaxies are much more struck.

Dwarf Elliptical

Dwarf spiral galaxies do exist as a classification but it is believed that most of them are galaxies in their own right which just happened to be within the field of a larger galaxy at the time we exist. To observe them giving them the illusion of being a satellite Dwarf Elliptical on the other hand are much more common, they often form after collisions with other satellites leaving the bursting with stars but lacking star forming gases resulting in an aging and dying yellowish stellar population and low gas fractions.

One example of a Dwarf Elliptical is Messier 110 also known as NGC 205. It sits just outside Andromeda and unlike most Dwarf elliptical. It does have patches of dust and faint young blue stars near its core. The reason for this inconsistency is unknown. In any case, however these stars have some spectacular nightly views though the night sky is somewhat obscured by the light from its core. Anything or anyone that exists inside could look up at the night sky and be able to see Andromeda in all of its glory hanging overhead.

Transitional

Just imagine what religious implications seeing an entire massive galaxy in the sky every night which disappeared during the day would have for a primitive sentient civilization. Not every satellite galaxy has an explicit type. They can be in between types also known as transitional. Due to the effects of mergers and tidal gravitational forces some satellites cannot be attributed to a particular type as they undergo profound structural changes like Antlia, currently recovering from a collision with the Milky Way. Antlia is classified as a transitional as is the only Milky Way satellite larger that of all the satellite galaxies which orbit the Milky Way. One has caught the attention of mankind and has been visible to humans as a faint smudge in the southern hemisphere sky since prehistoric times.

Large Magellanic Cloud

The Large Magellanic Cloud at 14,000 light-years in diameter is the largest confirmed satellite galaxy and fourth largest overall galactic object in the local group. The large Magellanic Cloud is considered to be in transition to an irregular type galaxy and it has a unique ghostly appearance in the night sky. Whereas the vast majority of the Milky way’s Satellites were discovered in the last century. Astronomers have been looking at this thing for thousands of years. The galaxy was first noted over a millennium ago in Abd al-Rahman al-Sufi Shirazi work, ‘The book of fixed stars' which was written in 964 AD.

In 1519 Ferdinand Magellan sighted the ghost galaxy on a voyage and brought knowledge of it to Western society of us. Both the Large Magellanic cloud and it’s smaller counterpart named after him. While it is small compared to the Milky Way its proximity makes it a perfect floating observatory for us to study star formation. Unlike many satellite galaxies around us the Large Magellanic Cloud is rich in vigorously productive gases and new stars are forming within as if the entire galaxy were a star factory

It’s estimated to contain over 30 billion stars including more than 60 globular clusters and more than 700 open clusters and hundreds of thousands of supergiant stars contained within. Tarantula nebula the most active star-forming region in the entire local group over a thousand light-years in diameter.

This star cradle is r136a1, not a very intriguing name, but a very intriguing star. It is a wolf-rayet star with the highest estimated mass and luminosity of any known star. Over 310 times heavier than the Sun, this makes the star massive but the largest that title might belong to the red hyper giant star WHO G64.

One of the biggest stars ever discovered, its diameter is estimated to be at least 1,500 times that of our Sun and could be as large as two and a half thousand times. Put that at the center of the solar system and it would be huge enough to engulf Jupiter comfortably. The reason we can’t ascertain a more specific figure for its diameter is because the star is surrounded by an envelope of stellar wind over a Light-year wide giving it an unparalleled luminosity allowing it to stand out as a bright red spot among a dark blue field from an entire galaxy away.

Speaking of envelopes of gas the Large Magellanic Cloud shares an envelope of neutral hydrogen with its smaller companion named the small Magellanic Cloud. This dwarf irregular type satellite is about half the size of its sibling around seven thousand light-years in diameter but it still contains hundreds of millions of stars. The two clouds also share a faint bridge of gas which is evidence of gravitational interactions between the two satellites in the past. Meaning that the small Magellanic Cloud was likely a former satellite of the large cloud and both are now satellites of the Milky Way.

Triangulum Galaxy

As mentioned the Large Magellanic Cloud is the fourth largest object in the local group, the top to being Andromeda and the Milky Way. But we have a somewhat overlooked third galactic neighbour in our local group, the Triangulum galaxy. This is a small spiral galaxy about 60,000 light-years in diameter. It gets its name from its position in the constellation Triangulum but it is also known as the pinwheel galaxy containing over 40 billion stars. The Triangulum Galaxy is an active star-forming galaxy several times more productive than its neighbour Andromeda at its core a nebula rich in gas spanning over 1,500 Light-years across a little under eight hundred thousand light-years from Andromeda. We are still uncertain as to whether the Triangulum galaxy is a galaxy in its own right or a companion of Andromeda.

Admittedly it would be exceptionally large and mature compared to other satellite galaxies in the local group. But the two galaxies are linked by streams of neutral hydrogen and renegade stars which suggests at the very least that they had a close encounter in the past and it’s highly likely that the two will interact again in the future. As the galaxies of the local group shift over the next few billion years the fate of the Triangulum galaxy will be revealed it will either be torn apart and consumed by Andromeda or perhaps it will evade Andromeda but fall into the collision, all perhaps it will be ejected from the local group.

Conclusion

All together satellite galaxies or no satellite galaxies both its past and its future appear to be heavily intertwined with its neighbour and its fate may end up being the same as the other satellite galaxies in the region. While galaxies like the Large Magellanic Cloud have supergiant stars and high amounts of radiation which make them less suitable candidates for the emergence of life. somewhere like the Triangulum galaxy has plenty of potential from rich stars and habitable planets, another place in which anything that was sentient could look up at the stars and see a massive separate galaxy hanging overhead life is treacherous for a Satellite Galaxy and four dwarf galaxies. In general, they are pulled apart eroded and eventually consumed in some kind of cosmic process of natural selection but for what it’s worth their night-time views couldn’t be better. 

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