An episode of ancient star formation

The Hubble Space Telescope‘s images of the galaxy GN-z11 show the oldest starlight ever seen; the faint glow observed originated only 400 million years after the Big Bang. Now an international team led by Takuya Hashimoto (Osaka Sangyo University, Japan) and Nicolas Laporte (University College London) has reported empirical evidence for star formation at an even earlier time. Observations of the galaxy MACS1149-JD1, they say, imply that most of that galaxy’s stars were created when the universe was only 250 million years old.

The researchers’ argument combines their measurement of a far-IR oxygen spectral line, taken at the Atacama Large Millimeter/Submillimeter Array (ALMA), with a broad spectral distribution obtained with the Infrared Array Camera (IRAC) aboard the Spitzer Space Telescope. ALMA’s precise measurement of the oxygen line established that MACS1149-JD1 has a redshift of 9.1096—that is, ALMA saw the galaxy as it was 550 million years after the Big Bang. That determination enabled the researchers to reinterpret a key feature in the IRAC spectrum: a rise in intensity beginning at an observed wavelength of about 4 microns. Previous work had suggested that specific spectral lines could account for the rise, but those explanations require a redshift that is incompatible with ALMA’s determination.

Instead, say Hashimoto, Laporte, and colleagues, the IRAC spectrum must be explained in terms of the star formation history of MACS1149-JD1. According to their modeling, two periods of star formation are needed to explain both the ALMA-observed oxygen intensity and the IRAC spectrum. The bulk of the galaxy’s stars ignited 250 million years after the Big Bang, but a second episode of star creation occurred about 300 million years later.

The galaxy MACS1149-JD1 is the most distant one whose redshift has been determined from a spectral-line measurement. Some 50 other galaxies have redshifts greater than 9 that were inferred from the galaxies’ broadband spectra, a less accurate method. The James Webb Space Telescope, scheduled for launch in 2020, will be able to obtain spectral-line redshifts for those galaxies and more. With its help, astronomers can collect a statistically meaningful sample to comb for clues about star formation in the universe’s infancy. (T. Hashimoto et al., Nature 557, 392, 2018 .)