Observing two areas of the sky with NASA’s Spitzer Space Telescope, the astronomers were surprised to find that some of the very first galaxies in the universe were brighter than models predict. The discovery, according to the authors of the study, could potentially unleash the driving force of the Epoch of Reionization, the main cosmic event that turned an opaque cosmos into a brilliant star landscape visible today.
We have observed some of the first galaxies that were formed less than a billion years after the Big Bang, or a little more than 13 billion years ago. The data showed that at some specific wavelengths of infrared light, galaxies are much brighter than expected. Our work is the first to confirm this phenomenon for a large sample of galaxies of that period, and shows that even ordinary ancient galaxies at these wavelengths were much brighter than those that we see today.
Today it is not precisely known when the first stars in the Universe were lit, however, some data indicate that approximately 100–200 million years after the Big Bang, the space was mostly filled with neutral hydrogen, which, perhaps, only began to unite into the first luminaries that became building blocks of the most ancient galaxies.
About 1 billion years after birth, the universe flashed. And something else has changed: the electrons of the omnipresent neutral hydrogen were removed during a process known as ionization. The epoch of reionization – the transition from the space filled with neutral hydrogen to the space with ionized hydrogen – is well documented.
Before this transformation, long-wave forms of light, such as radio waves and visible light, passed through the Universe more or less freely. But shorter wavelengths of light, including ultraviolet light, X-rays and gamma rays, were trapped by neutral hydrogen atoms. These collisions deprived them of electrons, that is, ionized.
But what could produce enough ionizing radiation to affect all the hydrogen from that era? These were individual stars? Giant galaxies? If the culprits were these objects, then those early cosmic settlers would be different from most modern stars and galaxies, which usually do not emit a large amount of ionizing radiation. On the other hand, perhaps something else could provoke this event, for example, quasars – galaxies with incredibly bright hearts, driven by supermassive black holes with a huge amount of material rotating around them.
This is one of the biggest open questions in observational cosmology. We know that it happened, but what caused it? Our results can be a great hint.
-Stefan de Barros, lead author of a study from the University of Geneva (Switzerland)
To look into the past by the time shortly before the end of the Eon of Reionization, Spitzer looked at two areas of the sky for about 200 hours, collecting light that had traveled to us for more than 13 billion years.
Using these ultra-deep observations, astronomers identified 135 distant galaxies, which turned out to be particularly bright at two specific wavelengths of infrared light produced by ionizing radiation interacting with hydrogen and oxygen in them. This means that young massive stars predominated in the population of these galaxies, consisting mainly of hydrogen and helium and containing a very small amount of heavy elements (such as nitrogen, carbon and oxygen) compared with the stars found in average modern galaxies.
We did not expect that Spitzer with a mirror that does not exceed the diameter of the hoop, will be able to get close to the origins of the Universe. But nature is full of surprises, and the unexpected brightness of early galaxies, along with the excellent telescope characteristics, makes them available to our small but powerful observatory.
-Michael Werner, a scientist at the Spitzer project at NASA’s Jet Propulsion Laboratory
The NASA James Webb space telescope, which is scheduled to launch in 2021, will explore the Universe at many wavelengths observed by Spitzer. But, considering that the diameter of the main Spitzer mirror is only 85 centimeters, and the heir of Hubble is 6.5 meters, which is about 7.5 times larger, this will allow the future telescope to study the first galaxies in much more detail. In fact, James Webb will try to detect light from the first stars and galaxies in the universe, and the data obtained show that because of their brightness, this will be easier to do than previously thought.
The Spitzer data is certainly another step towards unraveling the mystery of cosmic reionization. Now we know that the physical conditions in the early galaxies were very different from those observed in typical galaxies today, and the James Webb space telescope will have to determine the cause of this discrepancy.
-Pascal Osh, co-author of the study from the University of Geneva
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