During the first 500 million years of cosmic history, the first stars and galaxies formed, seeding the Universe with heavy elements and eventually reionizing the intergalactic medium. Previous observations with the NASA/ESA/CSA James Webb Space Telescope uncovered a surprisingly high abundance of candidates for early star-forming galaxies, with redshifts as large as z≈16 (only 250 million years after the Big Bang). Using new spectroscopic data from Webb, astronomers with the Cosmic Evolution Early Release Science (CEERS) survey have confirmed redshifts for two very luminous galaxies with z>11, but also demonstrated that another candidate with suggested z≈16 instead has z=4.9.
This image shows the Maisie’s galaxy. Image credit: NASA / STScI / CEERS / TACC / University of Texas at Austin / S. Finkelstein / M. Bagley.
“Objects in space don’t come printed with a time stamp,” said University of Texas at Austin astronomer Steven Finkelstein and his colleagues.
“To infer when the light we observe left an object, astronomers measure its redshift, the amount that its color has been shifted due to its motion away from us.”
“Because we live in an expanding Universe, the farther back in time we look, the higher an object’s redshift.”
“The original estimates of redshifts (and hence times after the Big Bang) were based on photometry, the brightness of light in images using a small number of wide frequency filters.”
“Those estimates were made using data collected by CEERS during its originally allotted time for the telescope’s first observing season.”
To get a more accurate estimate, the CEERS team applied for follow-up spectroscopic measurements with Webb’s Near InfraRed Spectrograph (NIRSpec) instrument, which splits an object’s light into many different narrow frequencies to more accurately identify its chemical makeup, heat output, intrinsic brightness and relative motion.
They looked at several high-redshift galaxies, including the Maisie’s galaxy, a candidate for the most distant known galaxy.
According to the latest spectroscopic data from Webb, the Maisie’s galaxy is at a redshift of z=11.4.
The astronomers also found that CEERS-93316, a galaxy initially estimated to have been observed at z≈16, has a more modest redshift of z=4.9 (1 billion years after the Big Bang).
It turns out that hot gas in CEERS-93316 was emitting so much light in a few narrow frequency bands associated with oxygen and hydrogen that it made the galaxy appear much bluer than it really was.
That blue cast mimicked the signature astronomers expected to see in very early galaxies.
This is due to a quirk of the photometric method that happens only for objects with redshifts of about 4.9.
“This was a kind of weird case. Of the many tens of high-redshift candidates that have been observed spectroscopically, this is the only instance of the true redshift being much less than our initial guess,” Dr. Finkelstein said.
“Not only does this galaxy appear unnaturally blue, it also is much brighter than our current models predict for galaxies that formed so early in the Universe.”
“It would have been really challenging to explain how the Universe could create such a massive galaxy so soon.”
“So, I think this was probably always the most likely outcome, because it was so extreme, so bright, at such an apparent high redshift.”
The team’s results were published August 14, 2023 in the journal Nature.
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P.A. Haro et al. Confirmation and refutation of very luminous galaxies in the early Universe. Nature, published August 14, 2023; doi: 10.1038/s41586-023-06521-7
