Free Astronomy Magazine

47 ASTRO PUBLISHING E arliest galaxy candidates and the history of the Universe. [Harikane et al., NASA, ESA, and P. Oesch (Yale University)] To understand how and when galaxies formed in the early Universe, as- tronomers look for distant galaxies. Because of the fi- nite speed of light, it takes time for the light from dis- tant objects to reach Earth. The light we see from an object 1 billion light-years away left that object 1 billion years ago and had to travel for 1 billion years to reach us. Thus studying distant galaxies lets us look back in time. The current record holder for the most distant galaxy is GN-z11, a galaxy 13.4 billion light- years away discov- ered by the Hubble Space Telescope. However, this dis- tance is about the limit of Hubble’s detection capabilities. HD1, a candidate object for the ear- liest/most-distant galaxy, was dis- covered from more than 1,200 hours of observation data taken by the Subaru Telescope, VISTA Tele- scope, UK Infrared Telescope, and Spitzer Space Telescope. “It was tough work to find HD1 out of more than 700,000 objects,” says Yuichi Harikane, who discovered HD1. “HD1’s red color matched the expected characteristics of a galaxy 13.5 billion light-years away surpris- ingly well, giving me some goose- bumps when I found it.” The team conducted follow-up ob- servations using the Atacama Large Millimeter/submillimeter Array (ALMA) to confirm HD1’s distance. Akio Inoue, a professor at Waseda University, who led the ALMA ob- servations, says, “We found a weak signal at the frequency where an oxygen emission line was expected. The significance of the signal is 99.99%. If this signal is real, this is evidence that HD1 exists 13.5 billion light-years away, but we cannot be sure without a significance of 99.999% or more.” HD1 is very bright, suggesting that bright objects already existed in the Universe only 300 million years after the Big Bang. HD1 is hardly explained with cur- rent theoretical models of galaxy formation. Observational informa- tion on HD1 is limited, and its phys- ical properties remain a mystery. It is thought to be a very active star- forming galaxy, but it might be an active black hole. Either possibility makes it a fascinating object. In recognition of its astronomical im- portance, HD1 was selected as a tar- get for the Cycle 1 observations by the James Webb Space Telescope, launched last year. Yuichi Harikane, who is leading these observations, says, “If the spectroscopic observa- tion confirms its exact distance, HD1 will be the most distant galaxy ever recorded, 100 million light-years further away than GN-z11. We are looking forward to seeing the Uni- verse with the James Webb Space Telescope.” !