Small Galaxies Reionize (Astronomy)

A study conducted by a team of astrophysicists from the University of Minnesota (USA) shows that high-energy light from small galaxies may have played a key role in the evolution of the Universe. We talk about it with Claudia Scarlata, professor at the University of Minnesota and co-author of the study

The first billion years of the universe’s history can be described as a succession of periods of dark and periods of light: in particular, there was a phase in which it was opaque to radiation, followed by an age in which it is become more and more transparent. It is the so-called epoch of reionization : the period in which the primordial gas that pervaded the universe in bands – essentially made up of hydrogen atoms – begins to pass from the neutral state to the ionized plasma state. It is the beginning of the end of the previous epoch of darkness, the dark age , in fact, and the prelude to the phase of light in which we still find ourselves.

Schematic view of the main stages of the Big Bang. Credits: National Astronomical Observatory of Japan (Naoj)

During the era of reionization, the mass of neutral hydrogen has gradually decreased, thus allowing photons to move more and more freely, to the point where almost all cosmic gas is ionized and the universe is completely transparent to light. of distant stars and galaxies. How this happened is not entirely clear, however. It is not clear, above all, the origin of the high-energy photons that converted the opaque and electrically neutral gas into ionized plasma. Indirect evidence and theoretical studies suggest that these are the photons emitted by the stars in formation within the first generations of galaxies, which in the meantime had formed from the gas densification due to gravity. In this hypothesis, however, there is a problem: the galaxies in turn contain clouds of hydrogen which absorb light, just like clouds in the earth’s atmosphere absorb sunlight on a cloudy day. This should have prevented the photons from escaping – and thus from ionizing the surrounding intergalactic gas.

One possibility to solve this problem is to hypothesize the existence of a population of primordial galaxies without hydrogen clouds inside them. To date, however, no evidence of similar galaxies has ever been found. Until today, in fact. Using data from the Gemini South telescope , a team of astrophysicists from the University of Minnesota seems to have now managed to find one.

Its name is Pox 186 : a small dwarf galaxy in the Virgo constellation , about 68 million light years from Earth, where the hydrogen clouds appear to have been completely removed. It could therefore be a galaxy similar to those present at the time of re-ionization, whose high-energy light, not absorbed by any internal gas, would have reionized the intergalactic gas.

To find out more Media Inaf reached out to one of the authors of the publication that reports the details of the discovery, the astrophysicist Claudia Scarlata. Born in Cagliari, after graduating in astronomy and a doctorate at the University of Padua, she worked for several years as a postdoctoral scholar at Eth in Zurich and at Caltech. In 2011 she became a lecturer in the School of Physics and Astronomy of the University of Minnesota, where she is also the director of the doctoral course. In 2019 he received the George W. Taylor Award for Distinguished Research from the same University .

Professor Scarlata, how did the idea behind the work that led to this discovery come about?

“One of the main problems of modern astrophysics is understanding how most of the hydrogen in the universe went from completely neutral to ionized (a transformation that is called reionization). To ionize the universe, photons more energetic than 13.6eV produced by very young stars need to be able to reach the hydrogen of the intergalactic medium, without being absorbed by the gas and dust – typically associated with younger stars. The problem is to identify the galaxies from which these photons “flee”. Pox 186 is a very small galaxy in the local universe, known for having no neutral hydrogen (or having so little of it that it cannot be measured) and for the fact that it is actively forming new stars.

 What survey technique did you use?

« Full-field spectroscopy , using the Gemini 8-meter telescope together with the Gmos spectrograph . This observational technique allows to obtain spectra in every position within the galaxy and therefore to have a much more detailed idea (compared to simple images) of the physical conditions of the gas and of the stars ».

Claudia Scarlata, professor at the University of Minnesota, co-author of the study published in The Astrophysical Journal

Why did  you focus on this galaxy?

“Pox 186 is a special galaxy for various reasons. First of all, it has some physical characteristics that make it very similar to the galaxies we think were present in the early universe, during reionization. Pox 186, however, is very close to us (in what we call the local universe) and this allows us to perform measurements that are not yet realistically possible in distant galaxies (we will have to wait until Jwst). In essence, Pox 186 is a perfect laboratory to test various hypotheses on the physical processes that can affect reionization. Also, as I said earlier, Pox 186 is famous for not having neutral hydrogen, or having so little of it that it cannot be measured. This fact is important because typically the formation of new stars is associated with the presence of neutral gas and a lot of dust ».

What are the possible physical mechanisms that in your opinion have caused this galaxy to empty its gas content, bringing it into a state you call blow-away ?

“Here I would like to quote the words of my student, Nathan Eggen , first author of the study. “One can imagine that the energy released in a star formation episode pushes gas from a galaxy like a balloon is inflating. However, if the star formation is too intense, there is the possibility that the surface of the balloon will pierce, allowing a portion of this energy to escape. In the case of Pox 186, the star formation was so intense that it destroyed the balloon completely, and thus the gas was removed completely, blown-away . The energy for this process is therefore associated with the star formation process. In particular, the energy associated with the explosion of young and massive stars such as supernovae and with istellar winds ».

What conclusions can be drawn from this study?

 What we have found is that in Pox186 some of the gas is moving at such a high speed that the gravitational pull of the galaxy is not enough to hold it back. This study confirms that it is possible to completely remove the gas from small galaxies, through “purely internal” mechanisms, that is, associated only with the galaxy itself and not with interactions with the environment. This kind of feedbackit is predicted by virtually all modern numerical simulations, but had not yet been confirmed experimentally. Consequently, our study suggests that very young galaxies in the distant universe may have gone through a phase where the lack of neutral gas has allowed ionizing radiation to escape from these objects, facilitating reionization. ‘

Problem of reionization solved?

«Solved no. But certainly our work allows us to better understand what kind of galaxies have contributed considerably to the process ».

How could confirmation of the involvement of this type of galaxies in the reionization of the universe be obtained ?

“From Earth, until the next generation of telescopes (like the European Extremely Large Telescope) arrives, it will be quite difficult to greatly increase what we already know about the first galaxies. Let’s say we are reaching the limits of what can be done with the technology we have. Fortunately, at the end of the year (in October) the James Webb Space Telescope will be launched , which will revolutionize our knowledge of galaxies in the distant universe ».

Do you already have a “sequel” to this study in mind?

“We are getting images and spectra of Pox 186 with the Hubble Space Telescope. The Hubble telescope allows us to observe in ultraviolet, where most of the energy produced in young stars is found. These data will allow us to better understand what is happening in nearby galaxies and therefore to interpret the observations of distant galaxies that Jwst will soon obtain ».

Featured image: The study by a team of astronomers led by the University of Minnesota suggests that high-energy light from small galaxies, such as the Galaxy Pox 186 pictured here, may have played a key role in the reionization and evolution of the universe. Credits: Podevin, Jf, 2006


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