The enduring mystery of how galaxies grow up

Astronomers have discovered that there are two different types of galaxies in the Universe: elliptical galaxies and spiral galaxies. Elliptical galaxies are dead galaxies full of very old, red stars that move on chaotic random orbits around the centre of their galaxy in such a way that makes their shape look like fluffy footballs. On the other hand, live spiral galaxies contain both old stars and many new, freshly formed stars. They appear as beautiful spiral galaxies as their stars and gas move on nearly circular paths around a common centre in a thin rotating disk. These spiral galaxies extend to distances from the centre of 30 thousand light years or more, whereas elliptical galaxies have a typical radius of 10 thousand light years. When counting all of the galaxies that are heavier than about 10 billion suns, only 3% are dead, while the very vast majority today are spiral galaxies full of young stars. Our Milky Way is a spiral galaxy such as this and our Sun lies about 24 thousand light years from its centre.

Our own Milky Way galaxy has about 50 billion stars and continuously forms more of them. Around four suns are born every year, and this phenomenon has been going on for 12 billion years. There have been ups and downs however, sometimes the Milky Way has made more stars and sometimes it has made less depending upon whether neighbouring galaxies came close. The nearest big galaxy to us, Andromeda, which is about 2.2 million light years away and falling towards us, contains more stars but is otherwise quite similar.

Stars form from the gas which such galaxies contain, but if we calculate all of this gas, it can be estimated that a galaxy such as ours would run out of gas within 3 billion years. Our Sun is 5 billion years old, so, since the time it and our Earth formed, the Milky Way should have completely run out of gas. And yet, astronomers see star-forming gas clouds only about 300 light years away, suggesting that the Milky Way is making stars today as if it was not consuming its gas. When considering this fascinating situation, it is important for astronomers to develop a theory to understand how the gas is replenished and solve this deep cosmological problem.

Astronomers and physicists today are convinced that there was a hot Big Bang about 13.8 billion years ago which made all of the matter and catalysed the continual expansion of the Universe. As the gas cooled after the Big Bang, the first galaxies condensed, thereby merging with others to make the large galaxies we see today. According to this theory, the rate at which new stars are born in a typical galaxy ought to have increased to a maximum value about 8 billion years ago. The production of stars should have gradually decreased over time as the gas in the galaxies was consumed, as the supply of new gas from the surroundings dried up, and as newly merging galaxies became more rare in the increasingly diluted Universe. In addition to this, dark energy, a largely unknown form of energy, appears to be ripping the Universe apart ever more rapidly. So, in theory, galaxies should stop growing up.

A team of astronomers from the University of Bonn and Charles University in Prague have now made a strange discovery: they took the many hundreds of galaxies that are well observed as they are close by, being within 30 million light years, and tested the above theory. The galaxies in this small chunk of the Universe were found to behave very differently to the aforementioned theory as they have not changed the rates at which their new stars are being born. A heavy galaxy today, for example, is making ten stars per year, and when it was born 12 billion light years ago, it was also making ten stars per year. A smaller galaxy may be making only one star every ten years today, a rate that has not changed since it had been born. This shocking discovery upsets the previously supposed theory and makes little sense, since this means that when a galaxy was born by making its first star cluster, the gas it was receiving then and the gas it receives today may conspire to be about the same. There is no indication that the galaxies follow the above theory. If anything, the telescope observations appear to suggest a weak trend according to which the galaxies are, if anything, slightly increasing the rate with which new stars are born within them.

This now reveals a new mystery: how can galaxies continue to grow up? How can the environment in which a galaxy lives arrange this amount of gas to be supplied to the galaxy at the exact right rate so that the number of stars being born in the galaxy over its whole lifetime of 12 billion years remains quite the same? Neither astronomers nor physicists know the answer to this question. In fact, since this question is brand new, much original thinking and research will need to be done in order to try to answer it. Some astronomers will try to answer this question by enforcing it into the above theory, but it is not at all clear how this can work since the same theory would need to give different results for the nearby galaxies and there is nothing in the theory that can arrange for this. Others might try to develop entirely new theories of how the surrounding space feeds gas into its galaxy. The outcome of this remains wide open, but it seems assured that some of our theories of physics may change fundamentally.

Featured image: NGC 7331 Galaxy by Edward Conway, Nathan Horleston, and Steve Maddox (University of Nottingham) and Nik Szymanek (University of Hertfordshire) via Issac Newton Group of Telescopes