Two blazing quasars caught waltzing into a merger

Astronomers, using the Atacama Large Millimeter/submillimeter Array (ALMA), have confirmed the existence of a close quasar pair housed in a pair of merging galaxies seen when the universe was less than a billion years old, at a redshift of 5.7. The system, designated J2037–4537, is one of only two confirmed quasar pairs at redshift greater than 5 ever found. A paper outlining this work was submitted to the preprint server arXiv on April 7.

Quasars are among the most luminous objects in the universe. They belong to the family of active galactic nuclei (AGN)—galaxies that are powered by supermassive black holes at their centers. However, these black holes do not consume material all the time. They are triggered into their active state by various mechanisms. Astronomers think that a galaxy merger could be one of them.

When galaxies crash into each other over millions of years, their gas gets funneled toward the center, feeding the black holes and igniting them as quasars. However, finding two quasars blazing simultaneously in a single merging system is extraordinarily rare, especially in the first billion years of the universe.

Impostor or real?

J2037–4537 was reported as a candidate for a close quasar pair in 2021. It consists of two objects at the same redshift of z = 5.7. However, astronomers could not rule out an alternative interpretation that suggested the system could be a result of gravitational lensing, which meant that two objects could be separate images of a single quasar whose light has been bent and magnified by the gravity of a foreground galaxy.

In a new study, a team led by Minghao Yue of the University of Arizona, using high-resolution observations from ALMA, examined the system very carefully to confirm its nature.

When the team mapped the [CII] (ionized carbon) emission lines—tracers of cold, star-forming gas in galaxies—across J2037–4537, they found that this emission extended between the two objects. This bridge likely formed when they merged, as their mutual gravitational pull dragged streams of material out of each other, forming a tidal bridge between them.

Since a gravitationally lensed single quasar would not produce such a structure between two images, this result ruled out the gravitational lensing scenario.

“The dust continuum and [CII] line emissions clearly reveal the tidal bridge between the two quasars,” the team wrote in their paper. With this confirmation, J2037–4537 has now become one of only two confirmed quasar pairs at z > 5 ever found.

Cosmic ripples

Astronomers also found that both quasar-host galaxies are massive and star-forming factories. Each had a dynamical mass of at least 10 billion solar masses and a star formation rate exceeding 500 solar masses per year.

The team notes that the star formation rate estimates carry systematic uncertainties because they rely on assumed dust temperature and emissivity, which future multi-band observations will need to re-estimate.

Currently, the two supermassive black holes are separated by thousands of light-years and are not yet a binary system. The paper estimates it will take approximately 2.1 billion years for J2037–4537 to transition from a quasar pair into a gravitationally bound binary SMBH. That places the binary formation at around redshift of 2.

The eventual merger of two quasars would emit low-frequency gravitational waves that Pulsar Timing Arrays (PTAs) are sensitive to. This is particularly interesting because recent PTA experiments have detected a gravitational wave background stronger than what galaxy evolution models predicted. Systems like J2037–4537, if more common than previously assumed, could help explain that excess.

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