A possible double phase of intense star formation in a primordial galaxy

The galaxy MACS0416_Y1 seen by ALMA and Hubble (Image ALMA (ESO/NAOJ/NRAO), NASA/ESA Hubble Space Telescope, Tamura, et al.)
The galaxy MACS0416_Y1 seen by ALMA and Hubble (Image ALMA (ESO/NAOJ/NRAO), NASA/ESA Hubble Space Telescope, Tamura, et al.)

An article published in “The Astrophysical Journal” describes a study on the galaxy MACS0416_Y1. A team of researchers led by Professor Yoichi Tamura of the Japanese University of Nagoya used the ALMA radio telescope to observe a galaxy we see as it was about 13.2 billion years ago. The surprising discovery is the considerable amount of interstellar dust present within it, explained by two intense periods of star formation that took place around 300 million and 600 million years after the Big Bang with a quiet phase between them.

The study of the oldest stars in the universe also includes that of the dust that formed after they exploded into supernovae ejecting heavy elements into interstellar space. That dust and elements such as oxygen end up forming new generations of stars but also of planets, to detect the oldest ones when they’re present in interstellar clouds they need to observe primordial galaxies from which electromagnetic frequencies such as radio waves are detectable. For this reason an instrument such as the ALMA (Atacama Large Millimeter/submillimeter Array radio telescope, inaugurated in March 2013, with its power and sensitivity is ideal for studies of this type.

In March 2017 an article published in “The Astrophysical Journal Letters” reported the observation of interstellar dust in the galaxy A2744_YD4, the most distant observed with the ALMA radio telescope. The galaxy MACS0416_Y1 has a similar age but the studies conducted provided unlikely results because the color of the light detected using the Hubble and Spitzer space telescopes and ESO’s VLT between ultraviolets and far infrareds suggested that the stars within it are about 4 million years old and at the same time the amount of dust was too high to have formed in a time that’s very short from the astronomical point of view.

Ken Mawatari of the University of Tokyo, another member of the team that conducted this research, called this situation a dust budget crisis commenting on the fact that several ideas were proposed to overcome it. The current star formation models didn’t seem adequate to that kind of situation and in the end a new one was created which isn’t really revolutionary, in the sense that it explains the characteristics of the galaxy MACS0416_Y1 without the need for extreme hypotheses that would diverge greatly from our knowledge of star life in today’s universe.

The model created by the researchers includes two periods of intense star formation in the galaxy MACS0416_Y1: the first started about 300 million years after the Big Bang and lasted about 100 million years followed by a quiet period broken by the second period of intense star formation around 600 million years after the Big Bang. In essence, with ALMA we can observe the beginning of the second period with the dust generated by stars born in the first period, a combination that gave an estimate of the age of the stars apparently incompatible with the amount of dust.

Avishai Dekel of the Hebrew University proposed some time ago the theory called cold flow accretion in which a galaxy grows collecting gas around it during its formation but this can happen in two phases separated by a long period of stasis. This possibility is still under study so it’s still too early to understand if a process of that type could have taken place in the galaxy A2744_YD4. The study of primordial galaxies is useful for research on the first stars but also for those on the evolution of galaxies in general. The humble interstellar dust plays a crucial role in the formation of stars and planets so the studies about it will continue, also with the new generation instruments that will be activated in the coming years.

Artist’s concept of the galaxy MACS0416_Y1 (Image National Astronomical Observatory of Japan)
Artist’s concept of the galaxy MACS0416_Y1 (Image National Astronomical Observatory of Japan)

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