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Two become one: These two galaxies are gravitationally attracted to each other, and will merge into one in a few 100 million years.

Photo: ESA / Hubble & NASA, Acknowledgment: Judy Schmidt (Geckzilla)

How are galaxies formed?

Astrophysicist Peter Laursen takes you on a step-by-step journey through galaxy formation – from primordial collapse to the formation of spiral arms.

Peter Laursen PH.D, ASTROPHYSICIST AND SCIENCE COMMUNICATOR, AT THE COSMIC DAWN CENTER, NIELS BOHR INSTITUTE/UNIVERSITY OF COPENHAGEN AND DTU SPACE

Published 24 November 2020 - 08:00

A recent ScienceNordic article explained what galaxies – the fundamental building blocks of our Universe – actually are, which types exist, and what they consist of. In this follow-up article, we will look further into the origin of these spectacular conglomerations of stars.

The realization that other galaxies exist outside of our own Milky Way is but 100 years old. In the years around 1920, several astronomers – of whom Edwin Hubble arguably is the most famous – established the distances to some nebulous objects and showed that they were far outside the Milky Way.

At this time, the theory of gravity was already well in place, both the Newtonian law of gravity from the late 17th century, and Einstein’s elegant and more correct interpretation known as the general theory of relativity from 1915.

It was clear that gravity played an important role in making giant gas clouds collapse under their own weight and form stars.

However, conceiving the full story and all its glorious details took time and is still far from being completely in place. For example, a central question was whether large clouds first condensed into galaxies which then began to form stars, or whether the stars formed first and then clumped together in galaxies.

A famous article from 1962 argued for the first scenario but the reality, we think, is more complex. In the prevailing theory, the formation of a galaxy can be divided into three stages:

1. A ‘primordial collapse’ of matter into a clump in the early, expanding Universe.

2. Hierarchical structure formation of larger clumps from smaller clumps.

3. A smoother evolution through internal processes such as the formation of spiral arms.

Gravity paves the way

To begin with, thanks go out to gravity.

In the Universe’s youth, its gas was almost completely evenly distributed. But not entirely!

Ever so small inhomogeneities – the origin of which can be traced back to the quantum mechanical uncertainty principle – implied that some regions were just a wee bit denser than other regions, and thus had just a little more gravity to pull the surrounding matter.

These small inhomogeneities can actually be observed in the earliest light we can see – the so-called cosmic microwave background radiation.

When this light was emitted, less than 400,000 years after the Big Bang, the dense regions were only about one hundred thousandth denser than the dilute regions (see here, here and here).

By attracting more matter, the clumps gradually grew in an earliest, ‘primordial’ collapse. But in the beginning, the expansion of the Universe was tremendously high, and a race between expansion and collapse began.

Dark matter to the rescue! If there had only been ‘normal’ matter, the clumps would not have been able to grow sufficiently before expansion had pulled matter too far apart.

And in that case, neither galaxies, nor stars, planets, or us would ever have formed.

Primordial collapse: Tiny fluctuations in density grow over time, and if they can clump enough together before the Universe has expanded too much, roughly spherical halos are created from both dark and ordinary matter.

Galaxy hierarchy

Luckily, things turned out less bleak. Dark matter collapsed, and gas followed along. So, which structures were formed first? Did small clumps turn into stars that later clustered together into galaxies? Or did colossal clumps form galaxies that later fragmented into stars?

Theoretical calculations and computer simulations suggest that the first structures ‘detaching’ from the expansion of the Universe were relatively small; perhaps about the size of globular clusters, that is, with masses around 100,000 times that of the Sun.

Observationally, this scenario is consistent with the fact that globular clusters contain some of the oldest stars in the Universe.

As these clouds began to form the first stars, they merged into larger clumps which, in turn, merged to even larger clumps, and so on. The largest galaxies thus predominantly formed at late times, while galaxy clusters formed even later.

This organizational structure of the Universe is called 'hierarchical', and is in contrast to the claim of the above mentioned article from 1962, namely that the galaxies formed in one big, 'monolithic' collapse.

Hierarchical structure: The smallest galactic halos are formed first. Some of them survive to later epochs, while others merge to form ever larger structures. The color of the halos indicates the time when they formed. Halos of a given size are typically formed at the same epoch (though there is great variation).

The galaxy relaxes

Dark matter cannot collide, neither with itself nor with normal matter. Hence, clumping together takes a long time for the dark matter, as the colliding particles simply pass straight through each other.

For the gas, the story is different: when gas becomes dense enough, another process begins to dominate gravity, namely hydrodynamics.

Hot gas has a high pressure and therefore cannot sink to the center of the halo. The atoms can collide, but if they're unable to get rid of their energy, pressure just increases as the gas is compressed, making it harder and harder for the atomic gas to contract.

Luckily, the atoms are able: when two particles collide, they have a certain kinetic energy. Some of this energy can go into kicking an atom's electron up into a higher energetic state. The atoms are said to be excited.

After a while, the electron 'falls down' again, and the energy is now released as a light particle (i.e. a photon), which can escape. In this way, the gas loses energy and cools.

The gas also has viscosity (i.e. a frictional resistance to flow) and can develop turbulence, which helps to 'slow it down', so that it can eventually settle down in the center of the dark matter halo.

Cooling: Hot gas implies fast particles. A fast atom and a fast electron collide (left). Part of their kinetic energy goes into exciting an electron in the atom. The two particles therefore leave the crime scene at a lower speed, and have thus cooled to a lower temperature. Shortly afterwards, the electron decays spontaneously and the excess energy leaves the atom as a photon that can escape (right). Click here to see an animated version.

A star is born, another star dies

Galaxies evolve not only by merging with other galaxies, but also through 'internal' processes. This evolution is called secular, as it typically takes much longer than the external processes. Secular processes take a few billion years, while external processes take a few 100 million years.

Stars die and return some of their gas to the interstellar medium (the dilute space between the stars), but now contaminated with metals, that is, elements heavier than hydrogen and helium.

New stars are born, both from the recycled gas, but also from new gas that is slowly supplied to the galaxy from the intergalactic medium (the even more dilute space that lies between the galaxies).

Galaxies can have their shape and structure altered by whizzing close past other galaxies. And perhaps most prominently, the galaxies' gaseous disks develop the well-known spiral structures, as well as 'bars'; elongated structures of gas and stars in galactic centers.

As you can read about in the previous article, there are many different types of galaxies. The two most notable types are spiral galaxies and elliptical galaxies.

In the following, we’ll take a closer look at how these are formed:

Gas clouds spinning like an ice skater

When a halo of gas and dark matter begins to form, the gravity of the neighboring halos makes it rotate to some degree (see here and here). Some pull one way, some pull the other way, and the direction into which most neighbors pull determines which direction the halo is going to rotate.

Origin of the rotation: So-called tidal forces from neighboring halos pull in the halo.

The gas cools and collapses further, and just like an ice skater rotating faster as they pull their arms in, the gas cloud rotates faster and faster as it collapses.

This is called conservation of momentum and is one of the fundamental laws of conservation in the Universe.

Whereas the gas cloud easily collapses along its axis of rotation, centrifugal forces impede the collapse in the plane of its rotation. Therefore, a disk of gas eventually forms – the basic shape of a spiral galaxy.

The formation of a galaxy's gaseous disk: As the gas cools and collapses it rotates faster, and the centrifugal force causes the gas to form a disk.

The spiral arms are full of young stars

Stars are born with a range of different sizes. Massive stars shine many times brighter than small stars, and with much higher energy. Therefore, their light is a bluish white, and they burn out rather quickly – over the course of a few million years.

Small stars, on the other hand, glow with an orange-red light, and live for billions of years.

A star-forming region in a galaxy will hence appear bluish, while a region where star formation has stalled eventually will appear reddish.

Although it may look like almost all the stars in a galaxy’s disk are in the blue spiral arms, the density is actually only 2-3 times higher here. The reason the spiral arms are so conspicuous is not because they contain more stars, but rather that this is where stars are currently being formed.

So why don’t the spiral arms gradually turn red? This is because the arms do not consist of the same stars all the time. Rather, it is a kind of pattern, or density waves, that propagate through the disk.

On top of that, these patterns are quite short-lived; a spiral arm probably typically lasts only a single or few revolutions before it dissolves and new ones are formed.

Origin and evolution of the spiral arms – still a bit of a mystery

Because galaxies to our short-lived human eyes seem 'frozen in time', we cannot directly observe the rotation and the emergence of spiral arms; consequently their lifetime is still quite an open question.

The stars revolve around galaxies at typical speeds of a few hundred km/s. Within a certain distance from the center (for the Milky Way, this point happens to be approximately where the Sun is) the stars overtake the patterns that the arms form, while further out the patterns overtake the stars.

Whether the Sun moves in and out of the spiral arms of the Milky Way, or whether we remain fairly still, is an ongoing debate, and there is currently no concrete evidence.

Exactly how spiral arms arise and evolve is also not entirely well-established, but has to do with the fact that instabilities in the disk collapse and form stars, and that the most massive stars quickly explode as supernovae – this creates pressure waves that propagate through the interstellar medium, which can then make other regions collapse and form stars.

Spiral arms: The spiral arms are not fixed objects, but temporary patterns of slightly increased density in the galactic disk. When a gas cloud overtakes (or is overtaken by) the density wave, it is slightly compressed due to the higher pressure, causing the cloud to fragment, collapse and form stars. The most massive – and thus bluish-white and bright – of these stars give the arm its color, but die quickly. The smaller – and thus reddish-orange and faint – stars leave the arm, and are not so clearly seen between the arms.

When gas clouds crash

Now we know how the spiral galaxies form. But how then are the giant elliptical galaxies formed?

From time to time galaxies collide. When that happens, stars hardly ever collide because they are so immensely far from each other, compared to their size. Not true for the gas clouds, which collide, collapse, and form new stars.

In general, gravity tends to thoroughly distort the galaxies.

Eventually, the merged galaxy settles down. If there is a great difference in size, the largest does not change much; the small one is simply swallowed whole by the large one. But if two spiral galaxies are roughly equal in size, the end result is not bound to be a disk again – it can then instead form an elliptical galaxy.

In a so-called starburst, almost all the gas is consumed, either forming stars, but also being blown out of the galaxy by stellar winds. For this reason, elliptical galaxies hardly form any new stars, and hence only the small, long-lived, and thus red-orange stars remain, providing these galaxies their characteristic color.

This is one way to form elliptical galaxies. Another is in the early Universe, as a result of several collisions of smaller proto-galaxies.

If star formation occurs rapidly, the proto-galaxy is simply depleted of gas before having time to form a disk. Thus, it fails to 'collapse' into a disk, the stars instead retaining their original, random orbits. When billions of stars have random orbits, the result is a more spherical distribution than the disk of spiral galaxies.

Elliptical galaxy formation: Several small proto-galaxies collide and form stars quickly, in a starburst. This evacuates the galaxy of gas to form new stars. If the process happens fast enough, the galaxy fails to form a gas disk until it's too late and the stars retain their original, random orbits. This gives the elliptical galaxies their shape. The short-lived, bluish-white stars disappear after a few (tens of) million years, while the long-lived, red-orange stars hang out for billions of years. This gives the galaxies their color.

Still many unsolved mysteries

The most massive galaxies in the Universe are elliptical. Whether they formed early or late in the history of the Universe has been debated for a long time, but recently a team of astronomers from the Niels Bohr Institute at the Cosmic Dawn Center and the National Astronomical Observatory of Japan found the most distant elliptical galaxy to date.

Because it takes time for light to travel down to us, 'distant' also means 'long time ago', and we see this galaxy all the way back from when the Universe was but 1/10 of its current age.

Already at this time we see it as 'extinct'; that is, it has ceased forming new stars.

This confirms the notion that elliptical galaxies formed early. However, there are still plenty of mysteries for astronomers to take on, such as the exact mechanism by which galaxies ‘shut down’ their star formation.

This so-called quenching is one of the hottest astronomical topics at the moment.

Read a Danish version of this article at our Danish sistersite, Forskerzonen.