The oldest galaxy ever: stars or a black hole ?

After spotting the most distant individual star, astronomers break the record for the most distant galaxy !

Observing for 1200 hours with the Subaru Telescope, Visible and Infrared Survey Telescope for Astronomy, UK Infrared Telescope, and Spitzer Space Telescope, the researchers found themselves with 700 000 objects in their images. It’s HD1’s deep red colour that made it stand out; in fact, its redshift corresponds to that of a galaxy 13.5 billion light-years away, 100 million light-years more than the previous record-holder GNz11. Follow-up observations with the Atacama Large Millimetre/submillimetre Array (ALMA) confirmed this incredible distance.

This of course also makes HD1 the oldest galaxy we know of – remember the saying that seeing far is seeing back in time. The scientists are still speculating as to what HD1 could contain. As the lead author of the study puts it, “It’s like guessing the nationality of a ship from the flag it flies, while being faraway ashore, with the vessel in the middle of a gale and dense fog. One can maybe see some colours and shapes of the flag, but not in their entirety. It’s ultimately a long game of analysis and exclusion of implausible scenarios.”

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This game of analysis has started, and what was found is that HD1 is extremely bright in ultraviolet light, implying that there must be some highly energetic processes going on. A first theory for HD1’s composition, then, is that HD1 is a typical starburst galaxy, one that forms stars at a high rate. However, when applying the standard model for starburst galaxies to HD1 the astronomers found that it must be forming 100 stars per year, 10 times more than what they expected. We have to remember though, that this galaxy existed only a few hundred million years after the Big Bang: therefore, the stars it formed are more likely to be Population III stars. Those are more massive, more luminous and hotter than current stars – and they emit more UV light, too, explaining the high luminosity at these wavelengths.

The other plausible scenario is that HD1 contains a supermassive black hole. It would again be the oldest one we know of, and would have had to grow at an incredible rate from a massive “seed”.  

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Speaking of supermassive black holes, using data from the Hubble Space Telescope another team of astronomers identified a growing black hole when the Universe was just 750 million years old. Located in a well-explored region of the night sky, the Great Observatories Origins Deep Survey-North (GOODS-North) field, GNz7q’s UV and infrared emissions are most consistent with a black hole accreting matter inside a dusty, star-forming galaxy. This observation thus provides the first clear link between young star-forming galaxies and the first supermassive black holes, which until now had only been modelled with computer simulations.

Both HD1 and GNz7q will be targets for the James Webb Space Telescope: in the first case, to triple-check the distance as well as verifying its nature, and for GNz7q, to fully characterise it and try to find more such intermediate objects.

Cover Image: HD1, Harikane et al.
Image Credits:
1 - Subaru Telescope, NAOJ
2 - VISTA, ESO
3 - UKIRT, UKIRT/Mauna Kea Observatory/University of Hawaii
4 - Spitzer, NASA/JPL-Caltech
5 - Timeline, Harikane et al., NASA, EST and P. Oesch/Yale
6 - GNz7q, NASA/ESA/G. Illingworth (UC Santa Cruz)/P. Oesch (UC Santa Cruz, Yale)/R. Bouwens (LEI)/I. Labbe (LEI)/Cosmic Dawn Center/Niels Bohr Institute/University of Copenhagen

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