RESEARCH
Achievements
Spectroscopic confirmation of the most distant galaxy at redshift 10.957
Time:2020-12-15ClickTimes:

An  international team of astronomers, led by Professor Linhua Jiang at the  Kavli Institute for Astronomy and Astrophysics at Peking University,  obtained near-infrared spectra and successfully measured the redshift of  a very faint galaxy 13.4 billion light-years away, the most distant  astrophysical object known to date. Meanwhile, the team also detected a  burst signal with a duration of minutes from the galaxy, which can be  explained as an ultraviolet flash associated with a gamma-ray burst  (GRB). These results are important to understand the formation of stars  and galaxies in the very early Universe (Figure 1). Two papers based on  the results were recently published in Nature Astronomy.

Figure 1. An artist’s conception of the most distant galaxy with a Gamma-ray burst (credit: Jingchuan Yu).


Based  on current theory and cosmological simulations, the Universe  experienced an era of so-called “cosmic dark age” shortly after the Big  Bang. At the end of the dark age, cosmic large-scale structures started  to form, resulting in the birth of the first-generation stars and  galaxies. The first stars formed about 150 million years after the Big  Bang, and the first galaxies formed roughly 100 million years later.  This is also the epoch when the seeds of the first supermassive black  holes started to form and grow. These astrophysical objects gradually  re-ionized the Universe. This “cosmic reionization” lasted several  hundred million years and represents one of the most important phase  transitions of the Universe. One of the main scientific goals of the  next generation telescopes is to understand cosmic reionization and  detect light from the first objects. But for current astronomical  facilities, it is very difficult to detect optical/infrared spectra from  such distant galaxies.

Using  one of the largest optical/infrared telescopes, the 10m Keck I  telescope on Mauna Kea, Hawaii, Jiang’s team carried out deep  spectroscopic observations of the galaxy “GN-z11” in the near-infrared.  Based on existing data from the Hubble Space Telescope, GN-z11 is  generally believed to be a very distant galaxy at a redshift around 11,  or at least greater than 10. But its accurate redshift has remained  unclear. From the high-quality spectra obtained from Keck, the team  detected three emission lines from GN-z11 and determined a redshift  10.957, meaning that GN-z11 is a galaxy with a distance of 13.4 billion  light-years (Figure 2). This is the most distant astrophysical object  known to date. The three emission lines are produced by gas with doubly  ionized carbon and oxygen, suggesting a high metal abundance (elements  other than hydrogen and helium) in this galaxy. It further means that  GN-z11 is not one of the first-generation galaxies, which would contain  almost no metals.

Figure  2. Keck near-infrared spectra of GN-z11. Panels (a) and (c) show the 2D  and 1D spectra, respectively; panels (b) and (d) show the smoothed  version of the 2D and 1D spectra. The doubly ionized carbon emission  lines at rest-frame ~191 nm have shifted to about 2283 nm in the  observed frame, indicating a redshift 10.957.


During  their spectroscopic observations on Keck, the team members also  serendipitously caught a bright burst from GN-z11. This burst  (hereinafter GN-z11-flash) showed up as a compact continuum spectrum in  one image with a duration shorter than 3 minutes (Figure 3). The  spectrum exhibits prominent telluric absorption features, indicating  that GN-z11-flash arose from above the atmosphere. The team members  performed a comprehensive analysis of the origin of GN-z11-flash, and  ruled out the possibility that the flash was from any known sources such  as man-made satellites or moving objects in the Solar system. They  concluded that GN-z11-flash was likely produced by a GRB in GN-z11, as  the spectrum, brightness, and duration of the flash are consistent with  such an interpretation. Although the chance probability of being a GRB  is very low, it is still two orders of magnitude higher than the  probability of being any other known sources. This result may imply that  GRBs can be produced as early as 400 million years after the Big Bang.

Figure  3. Spectra of the burst GN-z11-flash. Panel (a) shows its 2D spectrum,  panel (b) the 1D spectrum (blue) extracted from (a), and panel (c) the  fully calibrated spectrum (blue).


The  above results suggest that the largest telescopes currently available  are capable of detecting near-infrared spectra from some of the  brightest galaxies at high redshift. Future infrared facilities will be  able to find the progenitors of GN-z11-like galaxies and even the first  galaxies. The next generation GRB detectors will directly detect GRBs  from the very distant Universe.

This  work was supported by the National Science Foundation of China, the  National Key R&D Program of China, and the CAS South America Center  for Astronomy.



Links to the two papers:

1. Evidence for GN-z11 as a luminous galaxy at redshift 10.957(DOI: 10.1038/s41550-020-01275-y)

2. A possible bright ultraviolet flash from a galaxy at redshift ≈ 11 (DOI: 10.1038/s41550-020-01266-z)