A team of astronomers – led by Dr Roberto Maiolino from the University of Cambridge, UK – has detected the most distant clouds of star-forming gas yet found in ‘normal’ galaxies in the early Universe.
This ALMA/VLT image shows the extremely distant galaxy BDF 3299. The bright red cloud just to the lower left is the ALMA detection of a vast cloud of material that is in the process of assembling the very young galaxy. Image credit: ESO / R. Maiolino.
When the first galaxies started to form a few hundred million years after the Big Bang, the Universe was full of a fog of hydrogen gas. But as more and more brilliant sources started to shine, they cleared away the mist and made the Universe transparent to UV light.
Astronomers call this the Epoch of Reionization, but little is known about these first galaxies, and up to now they have just been seen as very faint blobs. But now observations using ESO’s Atacama Large Millimeter/submillimeter Array (ALMA) are starting to change this.
Dr Maiolino and his colleagues used ALMA to observe distant galaxies that are seen when the Universe was less than 800 million years old.
The scientists were not looking for the light from stars, but instead for the faint glow of ionized carbon coming from the clouds of gas from which the stars were forming.
They were also not looking for the extremely brilliant rare objects – such as quasars – that had been seen up to now. Instead they concentrated on rather less dramatic, but much more common, galaxies that reionized the Universe and went on to turn into the bulk of the galaxies that we see around us now.
From one of the galaxies, named BDF 3299, ALMA could pick up a faint but clear signal from the glowing carbon. However, this glow wasn’t coming from the center of the galaxy, but rather from one side.
“This is the most distant detection ever of this kind of emission from a ‘normal’ galaxy, seen less than one billion years after the Big Bang. It gives us the opportunity to watch the build-up of the first galaxies,” said team member Dr Andrea Ferrara of the Scuola Normale Superiore in Pisa, Italy.
“For the first time we are seeing early galaxies not merely as tiny blobs, but as objects with internal structure.”
The scientists think that the off-center location of the glow is because the central clouds are being disrupted by the harsh environment created by the newly-formed stars, while the carbon glow is tracing fresh cold gas that is being accreted from the intergalactic medium.
By combining the new observations with computer simulations, it has been possible to understand in detail key processes occurring within the first galaxies.
The effects of the radiation from stars, the survival of molecular clouds, the escape of ionizing radiation and the complex structure of the interstellar medium can now be calculated and compared with observation.
BDF 3299 is likely to be a typical example of the galaxies responsible for reionization.
“We have been trying to understand the interstellar medium and the formation of the reionization sources for many years. Finally to be able to test predictions and hypotheses on real data from ALMA is an exciting moment and opens up a new set of questions,” Dr Ferrara said.
The results of this study were published online in the Monthly Notices of the Royal Astronomical Society (arXiv.org preprint).
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R. Maiolino et al. 2015. The assembly of ‘normal’ galaxies at z ~ 7 probed by ALMA. MNRAS 452 (1): 54-68; doi: 10.1093/mnras/stv1194
