Distant, Ancient Galaxies Reveal Formation
The Universe is a big place, and the most distant galaxies we can see date to a time when it was very different. With sensitive radio observations, astronomers in Australia have now peered into the birthplace of two galaxy groups from a time when the Universe was a fourth of its present age.
Using the Australia Telescope Compact Array, or ATCA, astronomers detected carbon monoxide (CO) gas in several galaxies 10 billion light years away. CO is a molecular gas and a tracer of the more abundant molecular hydrogen gas. It is from this gas that stars form in galaxies.
The Spiderweb Galaxy is a complex region where molecular gas was found in quantities of 60 billion times the mass of the sun. (For reference, our Milky Way Galaxy, a product of a later time in the Universe’s history, is 500 to 1000 billion times the mass of the sun.) Much of the mass is concentrated in a central galaxy, while other clumps are the sites of smaller, satellite galaxies. There is so much mass going into star formation in this region that astronomers infer that the burst of star formation will continue for another 40 million years. That is not unlike the bright starburst galaxies that we see with infrared telescopes.
Two more galaxies were observed thanks to gravitational lensing. This occurs when a nearer galaxy bends the light of a distant galaxy around it and towards us on Earth. This allows the more distant object to be seen by our telescopes. These galaxies were also detected with CO, the cold molecular gas that forms stars. These also hold billions of solar masses of star-forming material.
These two galaxies were detected in broad-spectrum radio light in addition to the emission lines of CO, and these galaxies were also detected in infrared surveys. Astronomers use this information to determine the star formation rate in these galaxies since the radio emission is dominated by emission near star forming regions and from cosmic rays accelerated by supernovae, the explosions of young, large stars.
These galaxies are forming stars at a rate of over 400 solar masses per year. Compare that to our Milky Way’s one solar mass per year of star formation. These distant galaxies are forming stars at a prodigious rate, not unlike, once again, those starburst galaxies we see in the infrared.
ATCA has recently undergone a sensitivity upgrade, allowing it to see more distant and dim emission from the spectral lines of carbon monoxide. This allows it to complement other facilities, such as the Atacama Large Millimeter/Submillimeter Array (ALMA) in Chile and the Very Large Array (VLA) in New Mexico. ATCA complements those arrays both in frequency and in location on Earth, being of overlapping frequencies with the VLA but available to see the Southern sky.
These galaxies and those like them can be probed further with instruments like the VLA and ALMA, giving us a greater understanding of the formation and evolution of galaxies in the earlier days of the Universe. This is the story of where galaxies like our own came from.
Images: Top – Hubble image of the Spiderweb Galaxy. The inset shows the carbon monoxide gas as blue. Credits - NASA, ESA, George Miley and Roderik Overzier (Leiden Observatory), B. Emonts et al (CSIRO/ATCA). Bottom – 5 of the 6 22-meter antennas in the Australia Telescope Compact Array.