Artist concept of a galaxy in the early universe. Image: NRAO/AUI/NSF, S. Dagnello
By pure chance, astronomers have discovered two galaxies at the edge of space and time that have remained hidden behind a thick veil of dust until now. The obscured galaxies formed more than 13 billion years ago, only about 800 million years after the birth of the universe itself, and could help scientists find other ancient objects that are clouded by dust. Scientists led by Yoshinobu Fudamoto, an astronomer at the Research Institute for Science and Engineering at Waseda University, Japan, and the National Astronomical Observatory of Japan (NAOJ), spotted eerie spectral signals from the two galaxies completely by accident while studying neighboring galaxies that shine much brighter in ultraviolet (UV) light.
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The team noted that the “serendipitous discovery of these two dusty galaxies” at the edge of the universe “shows that our current (UV-based) census of very early galaxies is still incomplete,” according to a study published on Wednesday in Nature. In November 2019, Fudamoto and his colleagues observed galaxies in this distant era of the universe using the Atacama Large Millimeter/submillimeter Array (ALMA), an extremely sensitive interferometer in Chile. ALMA can peer across huge distances and through dusty environments to see objects during an ancient era known as “cosmic dawn” or the “epoch of reionization,” when the first stars and galaxies formed. Fudamoto and his colleagues are part of an ALMA program called Reionization-Era Bright Emission Line Survey (REBELS) that has been studying 40 luminous galaxies that existed at cosmic dawn. The team was examining two target galaxies, known as REBELS-12 and REBELS-29, when they saw blurry patterns of emissions located several thousand light years away from the known brighter sources.
Follow-up observations revealed that the murky signals were, in fact, two previously unknown galaxies that had been lurking behind thick clouds of dust. The objects, which Fudamoto’s team named REBELS-12-2 and REBELS-29-2, are invisible in UV and optical light, and were only detectable by ALMA because of its sensitivity to far-infrared wavelengths. The lucky discovery suggests that as many as one in five galaxies at cosmic dawn may be hidden behind clouds of cosmic dust, which has implications for models of star and galaxy formation during this bygone epoch. Fudamoto and his colleagues suggest that “a blind, wide-area survey for such sources is required in the future,” in the study. “These surveys must observe substantially deeper than had been envisioned previously to sample the fainter dust-obscured, but otherwise ‘normal’ galaxies such as REBELS-12-2 and REBELS-29-2,” the team concluded.