The Era of Reionization
Just after the Big Bang, the Universe was a big ball of hot plasma, a soup of energetic particles like protons and electrons. Light was trapped within the plasma, bumping off from particle to particle. As the Universe expanded, it also cooled down. Energetic particles slowed down enough that atoms could form. The protons and electrons combined to create hydrogen and a small amount of helium and lithium. This was a dark period in the history of the Universe. There were no stars or galaxies, only pristine hydrogen gas and the afterglow of the big bang, which we can still see as the Cosmic Microwave Background radiation. After about a hundred million years, the hydrogen gas started coalesce under the its own gravitational force in many regions of the Universe. As the gas compressed, the pressure and the heat caused hydrogen fusion, the first stars were born and finally there was light again.
The first stars were giants, potentially hundreds of times more massive than the Sun. Consequently, their lifespans were brief, lasting only a few million years before exploding as supernovae. These explosions scattered heavy elements and a vast amount of energy into their surroundings, leaving behind black holes. From the remnants of these first stars, a second generation emerged, incorporating the newly forged heavy elements. The first galaxies likely housed both these first and second generation stars. The intense radiation from these galaxies re-ionized the hydrogen atoms, separating protons and electrons. By the end of the first billion years, virtually all hydrogen in the Universe was ionized, a state that persists today.
JWST image of the field of galaxies with the inset showing one of the earliest galaxies ever detected, GN-z11, just 430 million years after the Big Bang. (Credit: NASA, ESA, CSA, Brant Robertson (UC Santa Cruz), Ben Johnson (CfA), Sandro Tacchella (Cambridge), Marcia Rieke (University of Arizona), Daniel Eisenstein (CfA))
Although JWST will continue to discover many such galaxies, studying them in detail is challenging. Understanding how these galaxies evolve requires a study with telescopes across the electromagnetic spectrum. Radio and X-ray telescopes give us valuable information about the properties of galaxies. But detecting the first galaxies with these telescopes is both difficult and expensive, as they require long observation times. Instead, we can look at some local galaxies, which are similar to the first galaxies, and study them in detail.
Galactic Gastronomy
Green Peas and Blueberries are local galaxies which resemble the first galaxies. Green Peas were first discovered in the Galaxy Zoo citizen science project while classifying the galaxy images from the Sloan Digital Sky Survey. They are known for their characteristic green colour and a round shape, resembling the vegetable. They are dwarf galaxies, much smaller than our Milky Way. Galaxies like our Milky Way have about a hundred billion stars, dwarf galaxies like Green Peas have only a few billion stars. Although they are smaller than the Milky Way, they are forming dozens of new stars every year. In comparison, the star formation rate of the Milky Way is about one star per year. This vigorous star formation ionizes the oxygen gas, and produces a characteristic colour, which appears green due to the expansion of the Universe. Blueberries are somewhat smaller than Green Peas and are much closer to us. Green Peas are about 3-6 billion light years away, while the Blueberries are less than a billion light years away. They are small, compact, low-mass, low amount of heavy elements, and are vigorously forming new stars. This makes them the ideal test case to study the first galaxies.
Green Pea galaxies (Jaskott et al. 2019)
Blueberry galaxies (Kouroumpatzakis et al. 2024)
We know that the energetic radiation from the first galaxies caused the reionization of hydrogen in the early Universe. But what process emitted this strong radiation? Was it driven by the intense star formation, or was another mechanism at play? In the centres of most galaxies lie massive black holes, many million times more massive than the Sun. When these black holes are actively accreting a vast amount of gas, they emit strong radiation and can often outshine the entire galaxy! JWST observations show that many first galaxies contain growing massive black holes. Could these black holes be responsible for the reionization? To investigate this, we can look at the local counterparts like Green Peas and Blueberries for clues.
Peering through the confusion
Radio and X-ray telescopes are some of the most effective tools of finding black holes in galaxies. Accreting black holes often emit strong relativistic jets, which can be observed by radio telescopes. The innermost part of the accretion disk also emits a vast amount of X-ray radiation, which is a telltale sign of the presence of a black hole. Although it is difficult to observe the first galaxies with radio or X-ray telescopes, we can study the relatively nearby Peas and Berries.
In a galaxy, young stars, supernovae, and stellar remnants like neutron stars or stellar mass black holes produce radio and X-ray radiation, aside from the massive black holes. To identify the signature of a supermassive black hole, we look for excess radiation beyond what can be attributed to these stellar processes. We observed a number of Green Pea and Blueberry galaxies with X-ray and radio telescopes. The X-ray observations of two green peas and one blueberry galaxy showed that they are much brighter than what we expect from star formation [1, 2]. On the other hand, many other galaxies are much dimmer than we expect stellar processes! When we look at the radio observations, we see a similar picture [3]. Some dwarf galaxies are bright in radio, many others are much less luminous than expected. And some are not detected at all! Overall green pea and blueberry galaxy population exhibits lower luminosity than what we expect from star formation and the detection of strong radio and X-ray radiation is a strong indicator of an active supermassive black hole.
What could cause the lower luminosity in these galaxies? One reason for the deficiency in radiation could be due to the very recent star formation. These galaxies started forming stars only recently, and most of the star are less than a few million years old. They have not produced the X-ray emitting stellar remnants yet. The low mass and irregular magnetic fields of the dwarf galaxies can lead to radio producing electrons to escape the galaxy, which reduces the overall radio luminosity. The young age of their stellar population and the low mass of the galaxies can explain the luminosity deficit. It also highlights the need for refining the luminosity and star formation relations, particularly suitable for the low-mass galaxies. However, the detection of strong X-ray or radio emissions strongly suggests the presence of active black holes, and observations confirm that several Green Pea and Blueberry galaxies do indeed host actively growing supermassive black holes at their centres.
The presence of such massive black holes in local dwarf galaxies and also in the first galaxies detected by JWST shows us that reionization was a complex process driven by a combination of the intense star formation and massive black hole activity. Unraveling the precise contributions of each to the energy budget that transformed the early universe remains a key goal for future research, offering crucial insights into the evolution of the cosmos.
References:
[1] Adamcova et al., 2024 X-ray observations of Blueberry galaxies
[2] Svoboda et al., 2019 Green Peas in X-Rays
[3] Borkar et al., 2024 Radio properties of green pea galaxies