In the contemporary Universe, massive galaxies are plentiful. But the Universe wasn’t always like this. Astronomers think that galaxies grew large through mergers, so what we see in space is the result of billions of years of galaxies merging. When galaxies merge, the merger can feed large quantities of gas into their centers, sometimes creating a quasar.
Much of this is theoretical and shrouded in mystery, but astronomers might have found evidence of a galaxy merger creating a quasar.
All galaxies contain interstellar gas, but some—typically younger ones—have a much higher concentration. When gas-rich galaxies merge, they trigger rapid star formation and feed large quantities of gas into the central black hole, which then flares brightly and appears as a luminous quasar.
A quasar is basically an extremely active black hole. It appears that all large galaxies host a supermassive black hole in their centers, and when these black holes are actively feeding, they’re called active galactic nuclei (AGN.) Quasars are the most luminous of all AGN and can outshine entire galaxies.
But quasars are mostly a thing of the past. Quasar activity seems to have peaked about 10 billion years ago, which is one reason there are still so many questions about how they form.
Astronomers have spotted two ancient, distant galaxies merging. Both have dim quasars at their centers. Could they be the progenitors of bright, massive quasars in the early Universe? One international team of researchers thinks so.
Their results are in new research published in The Astrophysical Journal titled “Merging Gas-rich Galaxies That Harbor Low-luminosity Twin Quasars at z = 6.05: A Promising Progenitor of the Most Luminous Quasars.” Takuma Izumi from the National Astronomical Observatory of Japan is the lead author.
The pair of distant, dim quasars detected with the Subaru Telescope. Image Credit: NAOJ/Izumi et al. 2024. “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.”
Quasars become extremely luminous and are more easily observed, but by that time, the merger that created them has played out. Astronomers need to see the dim ones in a pre-merger state to find answers to their questions. They want to know what processes govern merging gas-rich galaxies and how some of the gas is taken up in a burst of star formation while some of it is funnelled into the center, creating a quasar.
“While multiwavelength observations of quasars have progressed significantly in recent years, understanding of their progenitors lags behind,” the authors write in their paper.
At z = 6.05, these quasars are extraordinarily distant and ancient. The light reaching us now left these objects about 12.7 billion years ago in the Universe’s Cosmic Dawn. Due to the expansion of the Universe, the light has been travelling for about 23.5 billion light years. For many of these photons, their long journey ended when they reached the Subaru Telescope and the ALMA radio telescope.
The Subaru Telescope is an optical/infrared telescope on the summit of Maunakea, Hawaii, operated by the National Astronomical Observatory of Japan (NAOJ). It is equipped with the Hyper Suprime-Cam, a 900-megapixel digital camera with an extremely wide field of view. Together, the Subaru telescope and Hyper Suprime-Cam allow astronomers to detect very faint objects in surveys.
Subaru/Hyper Suprime-Cam discovered the pair of dim galaxies earlier this year with help from the Gemini North Telescope. Yoshiki Matsuoka, at Ehime University in Japan, was looking over images taken by the Subaru Telescope and noticed a faint patch of red. “While screening images of quasar candidates I noticed two similarly and extremely red sources next to each other,” says Matsuoka, “The discovery was purely serendipitous.”
The Subaru Telescope, with its Hyper Suprime-Cam, detected the pair of galaxies. Image Credit: The pair of quasars the Subaru detected is so dim that astronomers assumed it was a pre-merger pair. But to determine the exact nature of the objects, lead author Izumi and his colleagues turned to another powerful observatory: ALMA, the Atacama Large Millimetre/submillimetre Array. To understand what they were seeing, the researchers needed to see how the gas in the galaxies was behaving. ALMA is one of astronomers’ most powerful tools for observing gas.
Most of the gas in galaxies is hydrogen, but it can be difficult to detect. ALMA observes what’s called the CII absorption line. Since both hydrogen and CII are commonly found in gas clouds, the CII line serves as a tracer for hydrogen.
By observing the distribution and motion of hydrogen in the galaxies, the astronomers concluded that the pair is in the process of merging. Two pieces of evidence support their conclusion: the bridge of matter connecting them and the motion of the gas.
This figure from the research shows the quasar locations C2 and C1. It also shows the ‘bridge’ and ‘tail’ features, both signs that the pair of galaxies is merging. “Both the Bridge and the Tail are most likely formed by interactions of the host galaxies of C1 and C2,” the authors write. Image Credit: Izumi et al. 2024. However, establishing that the pair is merging was just the first step. The real question is if the pair of merging galaxies will produce a luminous quasar. To determine that, the researchers had to measure the amount of gas.
The panel on the left is a velocity map of the galaxies and their quasars, marked C2 and C1. The panel on the right shows the four stages of the merger, including stage IV, the observed stage. Image Credit: Izumi et al. 2024. Using ALMA, the researchers determined that the galaxies hold 100 billion solar masses of gas. That’s more gas than some of the galaxies that host the brightest quasars. This extraordinarily large amount of gas won’t be depleted quickly. It’s enough to trigger and sustain both explosive post-merger star formation and fuelling of the supermassive black hole.
“According to models of merger-driven galaxy evolution, both star formation and AGN are activated by the interaction of gas-rich galaxies,” the authors write in their research. “Thus, we expect that this pair will evolve into a luminous quasar with a high SFR of greater than 1000 solar masses yr?1, comparable to the value for optically luminous quasars observed so far at high redshifts.”
Astronomers concluded that the pair of galaxies are interacting and in the process of merging. Image Credit: ALMA/Izumi et al. 2024. “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,” said lead author Izumi.
These findings are significant because they provide astronomers with perspectives not only on quasar formation and explosive star formation but also on galaxy structure and motion.
“With the combined power of the Subaru Telescope and ALMA, we have begun to unveil the nature of the central engines (supermassive black holes), as well as the gas in the host galaxies,” Izumi said.
Finding a pair of pre-merger quasars is a milestone achievement. Quasars have puzzled astronomers since they were first detected with radio astronomy in the 1950s. At first, they didn’t know what they were, and astronomers referred to them as quasi-stellar objects (QSOs) and quasi-stellar radio sources. The name was shortened to quasar, and it stuck.
By 1960, astronomers had detected hundreds of quasars. Now we know what they are, but we have questions about how these objects come to be. This study is answering some of them, but astronomers always crave a deeper understanding of nature, and according to Izumi, the pair is ripe for further observations which should uncover some answers.
Izumi points out that the properties of the stars in both host galaxies are unknown. “Using the James Webb Space Telescope, which is currently operational, we could learn about the stellar properties of these objects. As these are the long-sought 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,” Izumi said.
