Things Aren’t Like What They Used to Be

Especially in Galaxies

One of the hot topics in astronomy these days focuses on star formation in the early universe and the formation of galaxies. We do know that the first stars began to form a few hundred million years after the birth of the universe in the Big Bang. The first galaxies assembled themselves shortly thereafter. Astronomers are searching out those first galaxies to figure out their star-formation rates (essentially, how many stars were born in them in a given time period), and — just as importantly — what those galaxies were made of. The first stars were made of hydrogen (and some helium) that was created in the Big Bang. Stars like the Sun (which have more metals in them) weren’t around at that time. That’s because Sun-like stars are of a later generation than the first stars. Those early stars had to be born, live, and die before sunlike stars could exist.

Why is this? Because the first massive stars had to evolve through all the stages of stellar life and then explode as supernovae. As they evolved, they created heavier elements in their nuclear furnaces and when those stars exploded, they scattered those elements (plus a few that got cooked up in the explosion) out to space. Those elements mixed with hydrogen gas clouds and eventually, new generations of stars were born. THOSE stars had more heavy elements in them. The galaxies that contained them ALSO had more heavy elements in them (and by heavy elements, I mean heavier than hydrogen and helium and lithium, which were abundant from the Big Bang forward).

So, astronomers looking back at the earliest epochs they can see, can observe galaxies forming lots of stars, but those stars aren’t very metal-heavy. Fast-forward to today (in cosmic time), and they see that “current” galaxies aren’t forming stars at quite the rate the early ones did. Why is this? It’s been a mystery. Do earlier galaxies crank out stars more efficiently? Or, do they have more raw material in the form of gas and dust available to make more stars? And, if so, do huge rates of star formation in the early galaxies mean that when they get older, they’ve run out of fuel and therefore don’t make stars as much?

Viewed through the Hubble Space Telescope at visible light (left), a galaxy does not reveal its full secret underlying star formation. Only when observed using a combination of radio emission and infrared wavelengths, the galaxy reveals a massive, rotating disc measuring about 60,000 light years across (right). This disc consists of cold molecular gas and dust, the raw materials from which stars are born. Courtesy University of Arizona and Michael Cooper.

Astronomers at the University of Arizona, led by Michael Cooper, looked at swaths of the early universe. They used data from an earlier study, where they surveyed about 50,000 galaxies. They then winnowed out a group of “average” galaxies and looked at them through a number of telescopes, including Hubble and Spitzer, and radio telescope arrays in France and California. By using this “multiwavelength” method of studying the early galaxies, the astronomers were able to find the cold gas clouds that supply the “stuff” of stars. Their data tell them that the early galaxies that were ancestors to our own Milky Way had a much greater supply of gas than the Milky Way does today. This means that they have been making stars according to the same laws of physics that govern the star-making machinery in the Milky Way. But, they’re making more of them in a given time because they had a greater supply of material.

One typical galaxy, named EGS 1305123, seen in this image as it appeared only 5.5 billion years after the Big Bang, has a huge rotating disk that measures about 60,000 light years across. That disk is stuffed full of cold gas and dust (the stuff of stars). The galaxy looks like how the Milky Way probably appeared more than eight billion years ago.

So, typical galaxies in the early universe were crammed with three to ten times more molecular gas than today’s galaxies have. Over time, they gave birth to stars, thus depleting the starbirth nurseries of the building blocks of stars. Star birth formation rates slowed down to the rates we see in today’s galaxies because they are running out of gas and dust.

Want to read more about this finding?  Check out the University of Arizona news page!

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