Explosive stars shaped the cores of early galaxies

By observing galaxies that formed 3 billion years after the Big Bang, astronomers have introduced a new evolution story for galaxies.


The proposed evolution of a lenticular galaxy shows the galaxy first dominated by its disk (left), followed by active star formation (centre), and eventually creating lenticular/elliptical galaxies (right). Image credit: NOAJ

Using three of the world’s best telescopes, astronomers have discovered that active star formation upswells galaxies. After observing several galaxies 11 billion light years away, the team of astronomers found that star formation was at its most intense in the core. This indicates that a galaxies shape can alter internally, and without the need for interaction with other galaxies.

“Massive elliptical galaxies are believed to be formed from collisions of disk galaxies,” said Kenichi Tadaki, a postdoctoral researcher at the National Astronomical Observatory of Japan (NAOJ). “But, it is uncertain whether all the elliptical galaxies have experienced galaxy collision. There may be an alternative path.”

In 1926, the American astrophysicist Edwin Hubble created the morphological classification scheme for galaxies. Since then, astronomers have dedicated plenty of time into understanding the origins of these galactic shapes. This particular study focused on distant elliptical and lenticular galaxies that lie roughly 11 billion light years from us. Because light takes time to reach us from such objects, we’re actually observing these galaxies as they were 11 billion years ago. This makes it the ideal epoch to observe the formation of galaxies. This is because these galaxies formed roughly 3 billion years after the Big Bang, which corresponds to the time where the foundations of galaxies formed and there weren’t any galaxy mergers.

ALMA created submillimetre observation (left), and HST created optical and infrared observations (centre and right respectively). Image Credit: ALMA (ESO/NAOJ/NRAO), NASA/ESA Hubble Space Telescope, Tadaki et al.

The team of astronomers first used NAOJ’s 8.2-metre Subaru Telescope in Hawaii, United States, and found 25 galaxies at this epoch. Having identified these targets, follow-up observations were made with the Hubble Space Telescope (HST) and the Atacama Large Millimetre/submillimetre Array (ALMA). The HST has the light-gathering abilities to decipher a galaxy’s ‘current’ shape – in terms of when the light was emitted – in optical and infrared light. ALMA then specialised in observing the cold clouds of gas and dust – the fundamental material of stars – within each of the galaxies.

By combining the observations from the HST and ALMA, astronomers were able to illustrate the metamorphosis of each galaxy. The HST images demonstrated that the disk dominates the galaxy, whilst ALMA shows that the star forming materials are concentrated in the core. By having the majority of the gas and dust at the core, it is most likely that star formation is very active there. Such a large number of stars in the centre of each galaxy has led astronomers to believe that galaxies will be dominated by their stellar bulge, and then eventually take the form of an elliptical or lenticular galaxy due to this upswelling.

“Here, we obtained firm evidence that dense galactic cores can be formed without galaxy collisions. They can also be formed by intense star formation in the heart of the galaxy.” said Tadaki.

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