Billions of years ago, the Universe experienced a baby boom of new stars. Back then, the number of stars that were born in galaxies was much higher than it is now. New observations by ALMA have revealed new information about those early galaxies. In particular, ALMA has found ‘pregnant’’ galaxies. They contain large amounts of gas from which stars can be born. But the formation of new stars hasn’t started yet.
Light from a distant galaxy takes time to arrive on Earth. In some cases, the galaxy may be so far away that its light takes 10 billion years to reach us. That means we’re seeing light that was emitted 10 billion years ago. We don’t see the galaxy as it is right now, but as it was back then. So by looking far away into space, astronomers also look far back in time.
In 2004, a small area on the sky was observed for a very long time by the Hubble Space Telescope. Hubble discovered thousands of remote galaxies, at look-back times of many billions of years. Large ground-based telescopes have studied the same part of the sky. By combining all those observations, astronomers discovered that early galaxies produced a lot of stars. The real baby boom occurred around 10 billion years ago.
Now, ALMA has also observed this Hubble Ultra Deep Field. ALMA sees other things than optical telescopes like Hubble see. For instance, it detects the millimeter radiation from cold clouds of dust and molecular gas – the raw material from which new stars will form.
The ALMA observations revealed ‘pregnant’ galaxies. They hardly produce any normal, visible light, so they are very faint when observed by Hubble. However, they emit a lot of radiation at the millimeter wavelengths that ALMA can see. These galaxies must contain large amounts of gas (including the carbon monoxide that ALMA detected). They’re about to give birth to large numbers of stars.
Thanks to the new ALMA observations, astronomers now have a much better view of the baby boom period of the Universe. In the future, ALMA will study the Hubble Ultra Deep Field again, and for much longer, to find more of the pregnant galaxies that populated the early Universe.
The Hubble Ultra Deep Field (HUDF) is a small area on the sky in the southern constellation Fornax the Oven. It is about a hundred times smaller than the apparent area of the Full Moon, but it contains thousands of very distant galaxies. The Hubble Space Telescope first studied the HUDF in 2004. ALMA has now also studied the Hubble Ultra Deep Field. Because the galaxies are so remote and faint, ALMA has to observe the HUDF for 50 hours in total. For most of this time, ALMA focused on a small part of the HUDF, 700 hundred smaller than the apparent size of the full Moon.
Two teams of astronomers have carried out the ALMA observations of the Hubble Ultra Deep Field. The first team was led by Jim Dunlop of the University of Edinburgh in the United Kingdom. The other team was led by Manuel Aravena of the Universidad Diego Portales in Santiago de Chile and Fabian Walter of the Max Planck Institute for Astronomy in Heidelberg, Germany. The results of the ALMA observations were presented at the ‘Half a Decade of ALMA’ conference in Palm Springs, California, on 22 September. They will also be published in a series of seven articles in Monthly Notices of the Royal Astronomical Society.