Two articles, one published in the journal “Astronomy & Astrophysics” and one in the journal “Monthly Notices of the Royal Astronomical Society”, report various aspects of the discovery of a galaxy protocluster in which there was an extraordinary rate of star formation when the universe was about 3 billion years old. A team of researchers led by Mari Polletta of the Italian National Institute of Astrophysics, Milan, found traces of the protocluster G237.01+42.50, or simply G237, in the data collected by the Planck Surveyor space probe and then used various telescopes to observe the galaxies inside it. The results were collected by a team led by Yusei Koyama of the National Astronomical Observatory, Japan.
The study of galaxy protoclusters is important to astronomers because they formed when the universe was young and over time evolved into the current clusters, the most massive structures in existence. Studying protoclusters means studying the history of the universe to understand why it has become as it is today.
The Planck Surveyor space probe conducted a mission between 2009 and 2013 to map the cosmic microwave background radiation. This map has been useful not only to study the newborn universe but also to look for traces of cosmic structures that formed over time. That’s because their emissions changed some characteristics of the cosmic microwave background radiation. Among them are the traces that led to the discovery of the G237 protocluster.
The researchers used observations conducted with the LUCI spectrograph mounted on the Large Binocular Telescope (LBT) and archival data from ESA’s Herschel and NASA’s Spitzer space telescopes to discover 31 galaxies part of the protocluster G237. They’re concentrated in two subgroups which over time probably formed two distinct galaxy clusters. The Subaru telescope made it possible to identify 32 more galaxies and to observe 6 of the ones already discovered. The observations revealed that star formation rates within those galaxies are extremely high with the birth of stars for at least 2200 solar masses per year. That’s a much higher rate than that predicted by cosmological models.
The image (ESA/Herschel and XMM-Newton; NASA/Spitzer; NAOJ/Subaru; Large Binocular Telescope; ESO/VISTA. Polletta, M. et al. 2021; Koyama, Y. et al. 2021) shows the region of sky containing the galaxy protocluster G237. On the left side, a combination of observations from the space telescopes: Herschel in red for submillimeter wavelengths, Spitzer in green for infrared, and XMM-Newton in blue for X-rays. The inset on the right side shows the region observed with the Subaru telescope. The 31 galaxies identified with the Large Binocular Telescope are indicated by blue circles, the 38 observed with Subaru are indicated by yellow diamonds.
The study of the protocluster G237 showed that in the central regions there’s a higher density of galaxies, which tend to be more massive, to form more stars, and to host more supermassive black holes in the growth phase. It’s the kind of information that can help astronomers understand the processes that lead to the formation of the massive galaxies observed at the center of galaxy clusters.
Mari Polletta explained that she and her colleagues are analyzing other Planck Surveyor data on G237 and other protoclusters to study the gas that forms new stars and feeds supermassive black holes. The aim is to understand their origin and explain the extraordinary rate of activity within those galaxies.
Galaxy protoclusters are very far away, so their images are very faint and the new telescopes that will come into service in the next few years will allow better observations to obtain more information on the history of the most massive structures in the universe.
