Two primordial spiral galaxies observed by the ALMA radio telescope

Artist's concept of primordial spiral galaxy with quasar in the background (Image A. Angelich (NRAO/AUI/NSF))
Artist’s concept of primordial spiral galaxy with quasar in the background (Image A. Angelich (NRAO/AUI/NSF))

An article published in the journal “Science” describes a research about two very young galaxies that probably look very wimilar to the Milky Way 12 billion years ago. Called ALMA J081740.86+135138.2 and ALMA J120110.26+211756.2, they were observed by a team of astronomers using the ALMA radio telescope as a follow-up of previous research conducted using the light from a quasar behind the two galaxies. The new research showed that the two galaxies are surrounded by a halo of gas and that their star formation rate is rather high.

The problem in the study of these two galaxies is that despite the size comparable to that of the Milky Way their distance of 12 billion light years makes their electromagnetic emissions estreamemente dim. The quasars behind them, that is about 12.5 billion light years away from Earth, is extremely bright thanks to a supermassive black hole that is very active because it’s surrounded by a disk of material that get strongly heated.

The quasar’s electromagnetic emissions are filtered when they pass through the two galaxies and continue their long journey to reach Earth. This filtered light allows to obtain information on the two galaxies’ composition, in particular on the presence of hydrogen, but it’s much more intense than the light they emit and this makes it difficult to get more information.

The ALMA radio telescope, inaugurated in March 2013, made it possible to directly observe the emissions at millimeter wavelengths from the two galaxies. In particular, they originate from the ionized carbon contained in dust-filled region inside them where stars form.

The researchers got a surprise when they realized that the carbon emissions are offset from the gas detected by the quasar’s light: 137,000 years light from a galaxy and 59,000 light years from the other. This suggests that the gas extends well beyond the galactic discs and that the two galaxies are shrouded in massive neutral hydrogen halos.

Another discovery is that the two galaxies are already rotating, a distinctive element for spiral galaxies like the Milky Way. Stars are forming within them at a fairly high rate: more than 100 solar masses per year in one galaxy about 25 solar masses for year in the other one. The gas present in the halos, more extented than expected, could contain materials which subsequently contributed to the growth of the two galaxies.

This type of research is providing answers to decade-old questions regarding galaxy formation. In particular, the two galaxy studied with ALMA are providing information on what were most likely the first stages of the evolution of the Milky Way.

False-color view of one of the galaxies studied and the quasar (Image ALMA (ESO/NAOJ/NRAO), M. Neeleman & J. Xavier Prochaska; Keck Observatory)
False-color view of one of the galaxies studied and the quasar (Image ALMA (ESO/NAOJ/NRAO), M. Neeleman & J. Xavier Prochaska; Keck Observatory)

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