A massive stellar bulge in a regularly rotating galaxy 1.2 billion years after the Big Bang

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Science  12 Feb 2021:
Vol. 371, Issue 6530, pp. 713-716
DOI: 10.1126/science.abc1893

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Early assembly of a galaxy disk and bulge

Galaxy formation in the early Universe is thought to have been a chaotic process, producing disturbed and asymmetric galaxy morphologies. Over billions of years, galaxies dynamically relaxed to form stable morphological features. Lelli et al. observed a distant galaxy at a redshift when the Universe was 1.2 billion years old (see the Perspective by Wardlow). They used gas and dust emission to measure its kinematics, and then modeled the mass distribution within the galaxy. The authors found that the galaxy contains a massive stellar bulge and a regularly rotating disk, features that models predict take billions of years to form. These results indicate that galaxy evolution is a more rapid process than previously thought.

Science, this issue p. 713; see also p. 674


Cosmological models predict that galaxies forming in the early Universe experience a chaotic phase of gas accretion and star formation, followed by gas ejection due to feedback processes. Galaxy bulges may assemble later via mergers or internal evolution. Here we present submillimeter observations (with spatial resolution of 700 parsecs) of ALESS 073.1, a starburst galaxy at redshift z5 when the Universe was 1.2 billion years old. This galaxy’s cold gas forms a regularly rotating disk with negligible noncircular motions. The galaxy rotation curve requires the presence of a central bulge in addition to a star-forming disk. We conclude that massive bulges and regularly rotating disks can form more rapidly in the early Universe than predicted by models of galaxy formation.

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