New data from Spitzer and Hubble space telescopes show that in a few specific wavelengths of infrared light, some of the first galaxies to form in the Universe, less than 1 billion years after the Big Bang, were considerably brighter than astronomers anticipated.
This artist’s illustration shows what one of the very first galaxies in the Universe might have looked like. Image credit: James Josephides, Swinburne Astronomy Productions.
No one knows for sure when the first stars in our Universe burst to life. But evidence suggests that between about 100 million and 200 million years after the Big Bang, the Universe was filled mostly with neutral hydrogen gas that had perhaps just begun to coalesce into stars, which then began to form the first galaxies.
By about 1 billion years after the Big Bang, the Universe had become a sparkling firmament. Something else had changed, too: electrons of the omnipresent neutral hydrogen gas had been stripped away in a process known as ionization.
The Epoch of Reionization — the changeover from the Universe full of neutral hydrogen to one filled with ionized hydrogen — is well documented.
Before this Universe-wide transformation, long-wavelength forms of light, such as radio waves and visible light, traversed the Universe more or less unencumbered. But shorter wavelengths of light — including ultraviolet light, X-rays and gamma rays — were stopped short by neutral hydrogen atoms. These collisions would strip the neutral hydrogen atoms of their electrons, ionizing them.
But what could have possibly produced enough ionizing radiation to affect all the hydrogen in the Universe? Was it individual stars? Giant galaxies?
If either were the culprit, those early cosmic colonizers would have been different than most modern stars and galaxies, which typically don’t release high amounts of ionizing radiation. Then again, perhaps something else entirely caused the event, such as quasars.
This deep-field view of the sky, taken by NASA’s Spitzer Space Telescope, is dominated by galaxies — including some very faint, very distant ones — circled in red. The bottom right inset shows one of those distant galaxies, made visible thanks to a long-duration observation by Spitzer. The wide-field view also includes data from the NASA/ESA Hubble Space Telescope. Image credit: NASA / JPL-Caltech / ESA / Spitzer / P. Oesch / S. De Barros / I.Labbe.
To peer back in time to the era just before the Epoch of Reionization ended, NASA’s Spitzer Space Telescope stared at two regions of the sky for more than 200 hours each, allowing the telescope to collect light that had traveled for more than 13 billion years to reach us.
As some of the longest science observations ever carried out by Spitzer, they were part of an observing campaign called the GOODS Re-ionization Era wide-Area Treasury from Spitzer (GREATS).
Using Spitzer observations and data from the NASA/ESA Hubble Space Telescope, University of Geneva astronomer Stephane De Barros and colleagues studied 135 distant galaxies.
The researchers found that these galaxies were all particularly bright in two specific wavelengths of infrared light produced by ionizing radiation interacting with hydrogen and oxygen gases within the galaxies.
This implies that the galaxies were dominated by young, massive stars composed mostly of hydrogen and helium. They contain very small amounts of heavy elements — like nitrogen, carbon and oxygen — compared to stars found in average modern galaxies.
These stars were not the first stars to form in the Universe (those would have been composed of hydrogen and helium only) but were still members of a very early generation of stars.
The Epoch of Reionization wasn’t an instantaneous event, so while the new results are not enough to close the book on this cosmic event, they do provide new details about how the Universe evolved at this time and how the transition played out.
“These results by Spitzer are certainly another step in solving the mystery of cosmic reionization,” said team member Dr. Pascal Oesch, also from the University of Geneva.
“We now know that the physical conditions in these early galaxies were very different than in typical galaxies today.”
The findings were published in the Monthly Notices of the Royal Astronomical Society.
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S. De Barros et al. The GREATS Hβ+[O III]Luminosity Function and Galaxy Properties at z∼8: Walking the Way of JWST. MNRAS, published online April 4, 2019; doi: 10.1093/mnras/stz940
