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The missing satellite problem

Missing Satellites: The non-visible neighbors of the Milky Way Galaxy

Felipe Farías Alvarado (Ingeniería civil en obras civiles, tercer año, 2018)

The universe is the home of many celestial objects, each holding different kinds of clues to understand the universe. Dwarf galaxies are a particular kind of objects given the fact that only a few of them can be detected by artefacts on earth. Observing everything the universe possesses might be the key to understand what happened at the beginning of time and what could happen in the future.

The Milky Way as one of many galaxies

The Milky Way, the galaxy we currently live in, as we know it, was formed around 13.6 billion years ago. The Big Bang happened around 13.8 billion years ago, meaning that our galaxy is roughly the same age as the Universe, fact that can be validated by measuring the age of the oldest stars within it.

Along with the formation of our galaxy, countless more were born as well, thanks to the abundance of baryonic and dark matter in the early universe. Galaxies of different shapes and sizes were also formed, some as large as the Andromeda galaxy, while others were as small as Segue 2[1], one of the smallest galaxies orbiting the Milky Way.

Dark matter and the formation of galaxies

According to the “Lambda Cold Dark Matter” model of the Universe (λCDM), baryonic matter, otherwise known as ordinary matter, is barely 4.9% of the total mass of the universe. While dark matter and dark energy (unknown form of energy) are 26.8% and 68.3% respectively[2], this is what caused the formation of the galaxies. Clumps of cold dark matter, a type of matter that, as of today, can only be detected through gravity, so far there is no other way to see it.

Dark matter is what scientists believe is and has induced the formation of galaxies throughout the universe. Because of the gravity that this matter creates, it makes baryonic matter come together, as the Universe has been expanding since the Big Bang. All the big galaxies that were formed do not live by themselves. There was also the formation of numerous small satellite galaxies known as dwarf galaxies, with far less stars in them, orbiting a large galaxy. With this in mind, it is troubling to not find all the dwarf galaxies the model says are going to orbit our Milky Way. Instead, astronomers have only been able to observe less than half of them.

Where are the lost galaxies?

A possible explanation for the current state of the invisible dwarf galaxies is, that after the dark ages of the Universe the first stars and galaxies that were created, sent out strong light influencing the rest of the matter in space. A period called the reionization, particularly affecting matter of dwarf galaxies being born[3], stopping their formation. Because these galaxies did not finish their processes, they are mainly dark matter with almost no baryonic matter. And as it was said before, dark matter can only be detected today by the gravitational pull it has on baryonic matter in close proximity, therefore, without ordinary matter, dark matter stays undetected.

Not being capable of observing these objects is especially problematic, because if scientists cannot do it with all the equipment that exists nowadays. Technology quite developed if you take a look at what the Hubble telescope can do, it has instruments that can absorb 40.000 times more light than a human eye, enabling it to have a more complete view of the universe. It can absorb and interpret three forms of light: near-UV, visible and near-infrared light. When it comes to visible light, the satellite can separate it into individual wavelengths, meaning it can make spectrographs for every star it can reach[4]. And soon, another satellite will be launched to orbit our planet, the James Webb Space Telescope (JWST). This satellite will greatly improve the data scientists have, due to the fact that this artifact is going to have amplified reach and more sensitive instruments compared to the Hubble telescope[5].

All this technological advancement however, still does not solve the mystery as to why not all dwarf galaxies can be seen. It is because these improvements apply over methods and instruments that are known to not detect dark matter, meaning that regardless of how advanced machines are, dark matter will remain undetected if the universe is being observed using the same methods.

Having said this, the answer to the problem is both simple and complicated at the same time, what needs to be done is to find an effective way to observe dark matter. The catch is that science may be decades or even centuries away from discovering a way to effectively detect it, since the rise of technology begun barely a century ago and the rise of human kind (homo-sapiens) begun around 300.000 years ago.

Once a valid, reliable way to detect dark matter comes to light, the understanding of science and astronomy may take a drastic turn, given that the universe is almost pure dark matter and dark energy. But, regarding astronomy, the ability to detect dwarf galaxies and dark matter could give the necessary evidence to understand why dwarf galaxies are the way they are. If science can understand what surrounds us, it is likely to understand the rest of the universe. If a timeline of the past can be modeled, it could be extrapolated to other galaxies and could even help build a model of what the future of the universe would look like, all this by just understanding a tiny bit.

References

[1]: “The tiniest galaxy in the universe, Liz Kruesi (2014)”, url: discovermagazine.com/2014/jan-feb/90-the-tiniest-galaxy-in-the-universe

[2]: Wikipedia, Dark matter

[3]: “Ultra-faint Dwarf Galaxies”, url: www.youtube.com/watch?v=EAFjrASWPuQ

[4]: “What is the Hubble Space Telescope”, Flint Wild (2018), url: www.nasa.gov/audience/forstudents/5-8/features/nasa-knows/what-is-the-hubble-space-telecope-58.html

[5]: www.jwst.nasa.gov