Gamma-ray bursts: from dying stars to first galaxies
On 4 October 2017, the Swift satellite detected a gamma-ray burst (GRB), an echo of a dying star. The burst was followed by a long-lived emission, called afterglow, which was quickly observed with the Very Large Telescope in Chile. The results were delightful, as the burst offered a rare opportunity to contribute to the study of the Universe’s rise from the Dark age.
Phases of hydrogen
The Universe is not just planets and stars and galaxies. Most of the matter in the Universe—the regular one building everything we see, including us—floats in space between galaxies. This intergalactic medium spans vast volumes of space, yet it is so tenuous, we cannot see it directly. However, astronomers use clever tricks to study it in detail.
The intergalactic medium is mostly composed of hydrogen. While traces of helium and a touch of heavier elements also populate it, hydrogen is the boss. In this post, I focus only on hydrogen, an element with a fascinating cosmological history.
Fortunately for us, the matter started to clump into clouds of gas. The clouds collapsed under their gravity and formed first stars and first galaxies. Stars radiate light; not only one frequency of light but a spectrum. The high energy part of the spectrum can remove electrons from hydrogen atoms—we say that it ionizes them. The first galaxies thus ionized the surrounding hydrogen gas and created ionized bubbles. As the galaxies grew, and new ones formed, the bubbles grew larger and merged. About one billion years after the Big Bang, most of the hydrogen in the Universe was ionized. It was the dawn of the Epoch of reionization.
Galaxies, yes, but how?
We seem to know the general train of events, but the devil is in the details. How exactly did the Universe transition into the ionized state? Was the reionization fast or gradual, precipitated by a few big galaxies or many small ones? Knowing what kind of galaxies were responsible for reionization would help. The problem is that these galaxies lie very far away, are faint and difficult to observe.
Astronomers approached the problem by searching for nearby galaxies emitting ionizing light. The idea is that a sample of such galaxies can tell us what kind of galaxies produce ionizing radiation in general, and then to search for equivalent galaxies lying beyond the Epoch of reionization.
Astronomers indeed found tens of galaxies emitting ionizing radiation in recent years. Not a big number, considering how many galaxies they investigated. Most of the galaxies either don’t have young massive stars that would produce ionizing radiation or contain a lot of dust and gas that absorbs all ionizing radiation before it could escape the galaxy. Only galaxies with an observed ionizing radiation represent those that play a role in the reionization.
The galaxies with detected ionizing radiation are typically small and compact, forming many stars (see image). These are the galaxies we should search for in the period of reionization. You know what is also small and compact and actively forms stars? Host galaxies of gamma-ray bursts. Could that seemingly random fact help address the problem?
Leaving gas and dust behind
Gamma-ray bursts mark the deaths of massive stars. The short burst of gamma-rays, lasting from a second to a few minutes, is typically followed by an afterglow. The afterglow can be observed for days across the whole electromagnetic spectrum. Both the burst and the afterglow are incredibly energetic and luminous; gamma-ray bursts occurring as far away as the Epoch of reionization, and beyond, were found.
Being produced by massive stars, gamma-ray bursts explode in or near regions where stars form. The afterglow light that we observe has to pass clouds of dust and gas before escaping from the host galaxy (see image). The clouds leave an imprint on the afterglow spectrum, which we can observe and thus learn of the properties of the medium in the host galaxy.
In the study lead by Jean-Baptiste Vielfaure astronomers investigated afterglow spectra of three GRBs: GRB191004B and two others. These three GRBs are special because their afterglow spectra show high-energy light that can ionize hydrogen. That means that the galaxy has at least some regions through which the ionizing light can escape. Their host galaxies can thus be added to the larger sample of other galaxies leaking the ionizing radiation.
Why is this such a nice result? It provides an alternative way to find and characterize galaxies with ionizing light. What is more, the host galaxies of gamma-ray bursts are typically very faint; searching for the ionizing light directly by observing these galaxies would be extremely difficult. The three host galaxies seem to differ in several aspects from the other leaking galaxies. Galaxies like the three host galaxies are normally disregarded in a direct search for ionizing radiation. The new study shows that without considering such galaxies, the conclusions give an incomplete picture.
Preparing for the future
Many current and future experiments aim to study the period of reionization. The upcoming space telescope James Webb Space Telescope and the next generation of thirty-meter telescopes like the Extremely Large Telescope will observe distant galaxies lying beyond the Epoch of reionization. Thanks to their superb capabilities, we will get a better understanding of the first galaxies in the Universe and their role in the process of reionization.
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