A new JWST study has found evidence of two galaxies colliding 750 million years after the Big Bang.

This image features the ZS7 galaxy system, showing a large field of hundreds of galaxies on the black background of space.
Inside this image is the galaxy system known as ZS7, possibly an ongoing merger of two galaxies and their massive black holes that occurred when the universe was only 750 million years old.
ESA/Webb, NASA, CSA, J. Dunlop, D. Magee, P. G. Pérez-González, H. Übler, R. Maiolino, et. al

The James Webb Space Telescope (JWST) has inspired a steady stream of exciting new research on black holes in the early universe. Curiously, a large number of feeding supermassive black holes appear to be “over-massive.” In the local universe, when you compare the mass of a giant central black hole to the whole stellar mass of its host galaxy, the proportion is typically around one to 1,000. But the early universe is turning out to look quite different, with many of the supermassive black holes observed measuring at 1% or even 10% of the stellar mass.

A portion of these black holes detected by Webb appear to be pairs, separated by anywhere from 2,000 to 15,000 light-years. Many of these might be in the process of merging. Repeated mergers of black holes over cosmic time is thought to be one of the two main routes that allow them to grow so quickly in mass; the other is by the accretion of gas. Which route dominates in the early universe, and how long this process actually takes is an open question.

Now, a new study in the Monthly Notices of the Royal Astronomical Society, led by Hannah Übler (University of Cambridge, UK), reports JWST’s detection of two galaxies, which seem to be in the process of merging some 750 million years after the Big Bang. This system is the most distant observation of the phenomenon so far, and provides direct evidence that merging is a route for black hole growth in the early universe.

Three panels are shown showing an increasingly small area of the PRIMER galaxy field. The first image shows a large field of galaxies on the black background of space. The second image shows a smaller region from this field, revealing the galaxies in closer detail, appearing in a variety of shapes and colours. The final image shows the ZS7 galaxy system, revealing the ionised hydrogen emission in orange and the doubly ionised oxygen emission in dark red.
JWST's near-infrared camera captured ZS7, and the telescope's spectrograph turned up evidence of the ongoing merger. Two black holes captured in this image emit both doubly ionized oxygen (OIII, dark red) and ionized hydrogen (Hβ, orange). However, in only one of those sources is the Hβ emission line broad, indicating the presence of gas clouds whipping around an unobscured black hole; this emission was used to calculate the black hole's mass. The other black hole is obscured by dust and gas, so its mass can't be determined in the same way.
ESA/Webb, NASA, CSA, J. Dunlop, D. Magee, P. G. Pérez-González, H. Übler, R. Maiolino, et. al

Übler presented her team’s results at a recent conference in Cork, Ireland: “Massive Black Holes in the First Billion Years.” They used JWST’s onboard near-infrared spectrograph to study a previously discovered galaxy hosting a feeding supermassive black hole, taking a spectrum at each pixel of the galaxy’s image. The result revealed that high-density gas clouds are indeed whipping around a massive black hole with the mass of 50 million Suns. But that’s not all.

The team found this supermassive black hole is offset by about 2,000 light-years from the center of the ZS7 galaxy, which itself hosts an obscured black hole of uncertain mass. “That [offset], along with additional imaging data, indicates a merger of two systems with massive black holes, at a very early time,” Übler says.

Übler posits that the black holes' proximity and their small relative velocities suggest the merger is likely ongoing. She suggests the two “will most likely merge within the next few hundred million years.”

Fabio Pacucci (Center for Astrophysics, Harvard & Smithsonian), who was not involved with this study, says that it’s quite difficult to tell whether or not galaxies are actually merging.

“This is an exciting discovery that tells us that black hole pairs may be common in the high-redshift universe,” he says. But Pacucci also takes seriously another scenario that the team offers as an alternative: It could be the galaxies and their black holes in fact already merged. That coalescence might have kicked off the second black hole, causing it to recoil away from the galaxy’s center.

In either case, the study provides an important data point in a growing body of research suggesting that black holes in the early universe are merging and getting more massive faster than predicted. But that’s not the only weird thing.

“Not only are we detecting more actively accreting massive black holes than we previously expected,” Übler says, “but many of the sources don't show any strong X-ray emissions, which was previously taken to be one of the main indicators of black hole activity in both the nearby and early universe.”

This is one of many topics recently discussed at the Ireland conference, where both Übler, Pacucci, and dozens of other astronomers discussed current and near-future observations by JWST and other upcoming missions. Scientists are particularly excited about the European Space Agency’s Laser Interferometer Space Antenna (LISA), a space-based gravitational wave detector which is scheduled to launch in the mid-2030s.

Even if it were currently operational, LISA would not be able to detect gravitational waves produced by the merger of the two galaxies in Übler’s study, as that is not expected to occur for another hundred million years. Studies like this will instead help to form the modeling framework for the mission, providing vital statistical constraints on masses of black holes that might occur at different times in cosmic history.


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