Dancing galaxies make a monster at the cosmic dawn

A team of researchers including the Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU, WPI) has observed a pair of galaxies merging 12.8 billion years ago, forming one of the brightest objects in the early Universe, reports a new study in The Astrophysical Journal.

Observations to date have discovered a number of supermassive black holes with masses exceeding a billion times that of the Sun in the early Universe, when the Universe was less than a billion years old. These objects, known as high-luminosity quasars, shine brighter than an entire galaxy when they consume huge amounts of interstellar matter. It is also known that galaxies hosting these quasars often experience a burst of star formation, producing hundreds to thousands of times the mass of our Sun in new stars every year. But what triggers and sustains such rapid growth of supermassive black holes and starburst activity?

A leading hypothesis is the mergers of gas-rich galaxies. It is thought that when gas-rich galaxies merge, some of the gas is compressed to form a large number of stars, while some of it flows into the center of the galaxy, fueling the growth of the central black hole. To better understand galaxy/black hole formation in the early Universe, we need detailed investigations of the probable ancestors of high-luminosity quasars, that is to say galaxies and black holes in the pre-merger stage. However, research on such ancestors has long been stagnant because they are not yet bright, high-luminosity quasars (before merging), making them extremely faint and difficult to detect.

To overcome this challenge, a research team led by Associate Professor Yoshiki Matsuoka of Ehime University, and including Kavli IPMU Professor John Silverman and Project Researcher Masafusa Onoue, analyzed large-scale survey data taken with the Subaru Telescope's Hyper Suprime-Cam, which boasts an extremely wide field of view. This survey, utilizing the Subaru Telescope's high light-gathering power, is significantly more sensitive than other large-scale surveys, enabling us to detect fainter objects. As a result, the team discovered a system where two very faint quasars (about 10 to 100 times fainter than high-luminosity quasars of the same era) were found side by side (link to Subaru Telescope release, https://subarutelescope.org/en/results/2024/06/17/3412.html). Located approximately 12.8 billion light-years away, corresponding to the "Cosmic Dawn" era when the Universe was only 900 million years old, this is the most distant record of such "pair quasars." Due to their faintness, these objects were thought to be in the pre-merger stage before the rapid growth of the supermassive black holes. However, observations with the Subaru Telescope could only provide information about the central supermassive black holes, leaving unanswered the question of, whether the host galaxies were indeed destined to merge and eventually grow into high-luminosity quasars. 

As a next step, a research team led by Associate Professor Takuma Izumi of the National Astronomical Observatory of Japan conducted observations of the host galaxies of these pair quasars using the ALMA radio telescope. The results, obtained during Director's Discretionary Time, were astonishing. The distribution of interstellar matter observed (two host galaxies and the structure bridging them: Figure 1) and the nature of their motion clearly indicated that these galaxies were interacting. They are undoubtedly on the path to merging into a single galaxy in the near future. Furthermore, calculations from the observational data revealed that the total gas mass in these galaxies (around 100 billion times the mass of the Sun) is comparable to or even greater than the gas masses of the host galaxies of most high-luminosity quasars, whose nuclei are extraordinarily bright. With this huge amount of material, post-merger explosive star formation and fueling of the supermassive black holes should be easily triggered and sustained. Therefore, these findings represent a significant achievement in identifying the ancestors of high-luminosity quasars—the brightest celestial objects in the early Universe—and starburst galaxies, from multiple perspectives including galaxy structure, motion, and the amount of interstellar matter.

Commenting on this discovery, Associate Professor Takuma Izumi, who led the research, expresses his excitement: "When we first observed the interaction between these two galaxies, it was like watching a dance, with the black holes at their centers having started their growth. It was truly beautiful." He also looked forward to future research, saying, "With the combined power of the Subaru Telescope and ALMA, we have begun to unveil the nature of the central engines (supermassive blackholes), as well as the gas in the host galaxies. However, the properties of the stars in the host galaxies remain unknown. By using the James Webb Space Telescope, which is currently operational, we will be able to learn about the stellar properties of these objects. As these are the long-saught ancestors of high-luminosity quasars, which should serve as a precious cosmic laboratory, I hope to deepen our understanding of their nature and evolution through various observations in the future."