When our Milky Way galaxy was still young, billions of years ago, it was smaller and more chaotic than it is now. It also contained huge amounts of cold molecular gas – the stuff that stars are born from. At least, that’s the conclusion that astronomers draw from new ALMA observations.
Of course, ALMA cannot see what the Milky Way galaxy looked like some 8 to 10 billion years ago. Still, just like other large telescopes, ALMA is something like a time machine. By looking far out into space, astronomers also look far back in time. That’s because the light from very distant objects needs billions of years to reach Earth.
ALMA has now studied four galaxies at some 9 billion light-years. That means we’re seeing these galaxies as they were some 9 billion years ago, when the universe was much younger than it is now. The four galaxies are smaller than our Milky Way, and they have very irregular shapes. Still, astronomers believe that these objects will later evolve into beautiful spiral galaxies like our own.
So instead of studying the youth of the Milky Way, ALMA studied the youth of other galaxies. It’s as if you want to know what your grandfather looked like when he was a teenager. Looking at photos of other teenagers at least gives you an idea.
The four young galaxies turned out to contain a lot of carbon dioxide. ALMA’s detectors are very good at detecting this molecular gas. And wherever there’s carbon monoxide, there are also much larger amounts of molecular hydrogen gas. In fact, the total mass of all the cold molecular gas in the four galaxies is larger than the total mass of all their stars.
Most astronomers already thought that grand spiral galaxies like our own Milky Way started out as much smaller, irregularly-shaped systems. The birth rate of new stars in these ‘ancestor galaxies’ would be many times higher than in the Milky Way. The new ALMA observations beautifully support this view.
The four distant galaxies studied by ALMA are known by their catalogue numbers as ZFOURGE CDFS 467, 4409, 6497 and 8193. They are located in the southern constellation Fornax, the Oven, at a distance of some 9 billion light-years. The galaxies (together with thousands of others) were first studied by the Hubble Space Telescope and by a large ground-based telescope at the Las Campanas Observatory in Chile. At millimeter and sub-millimeter wavelengths, ALMA was able to map the distribution of carbon monoxide gas in the four galaxies.
The ALMA observations of the four distant galaxies were carried out by a team of astronomers, led by Casey Papovich of the Texas A&M University in College Station. Casey worked together with twelve other scientists from the United States, the Netherlands, Australia and France. The new results have been published in the professional journal Nature Astronomy.