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Unexpected Hidden Galaxies Question Theories of Universe Evolution

ALMA identified 39 faint galaxies that are not seen with the Hubble Space Telescope’s most in-depth view of the Universe 10 billion light-years away. This example image shows a comparison of Hubble and ALMA observations. The squares numbered from 1 to 4 are the locations of faint galaxies unseen in the Hubble image. Credit: The University of Tokyo/CEA/NAOJ

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Astronomers used the Atacama Large Millimeter/submillimeter Array (ALMA) to identify 39 faint galaxies that are not seen with the Hubble Space Telescope’s most in-depth view of the Universe, 10 billion light-years away. They are ten times more numerous than similarly massive but optically–bright galaxies detected with Hubble. The research team assumes that these faint galaxies precede massive elliptical galaxies in the present Universe. However, no significant theories for the evolution of the Universe have predicted such an abundant population of star-forming, dark, massive galaxies. The new ALMA results throw into question our understanding of the early Universe. ...

Wang and his team targeted three ALMA windows to the deep Universe opened up by the Hubble Space Telescope (HST): the CANDELS fields. The team discovered 63 extremely red objects in the infrared images taken by NASA’s Spitzer Space Telescope: they are too red to be detected with HST. However, Spitzer’s limited spatial resolution prevented astronomers from identifying their nature.

ALMA detected submillimeter-wave emission from 39 out of the 63 extremely red objects. Thanks to its high resolution and sensitivity, ALMA confirmed that they are massive, star-forming galaxies that are producing stars 100 times more efficiently than the Milky Way. These galaxies are representative of the majority of massive galaxies in the Universe 10 billion years ago, most of which have so far been missed by previous studies. ...

• Nature 572(7768):211 (08 Aug 2019) DOI: 10.1038/s41586-019-1452-4