What shines brighter than a quasar? Two quasars!
Follow-up studies of a closely interacting pair of active galaxies from the early days of the cosmos confirm the rare case of a double-quasar - and exclude an optical illusion by a gravitational lens.
Among around 1000 individual quasars, there is on average only one double quasar. At the end of their development, the two spirally approach and merge with each other. They leave an even more massive, single black hole (illustration).
INTERNATIONAL GEMINI OBSERVATORY/NOIRLAB/NSF/AURA/J. DA SILVA/ M. ZAMANI / ILLUSTRATION OF DOUBLE QUASARS IN MERGING GALAXIES / CC BY 4.0 (EXCERPT)
This is a translation of an article originally published in Spektrum, featuring U of A astronomer Minghao Yue.
With the Atacama Large Millimeter Array (ALMA), a team led by astronomer Minghao Yue from the University of Arizona succeeded in confirming an extraordinary double system: instead of two stars, there are two closely adjacent quasars, the extremely luminous cores of active galaxies. The latter are about to merge with each other and date from a time when the universe was just over a billion years old.
The system with the designation J2037-4537 was already identified in 2021 and is located in the southern constellation Indians at a cosmological red shift of z = 5.7. But at the time of discovery, it was initially unclear whether the pair was actually two separate objects or a single source double-mapped by a gravitational lens. Since quasars are extremely rare in this early cosmic era - their greatest frequency is observed at the time of the »cosmic noon« at z = 4 – it required a clear confirmation.
In the ALMA data, the team was able to prove an extensive infrared emission in the interstellar medium between the two objects. Due to the high red shift, the emitted light is shifted into the millimeter and submillimeter waves where ALMA works. This showed a tidal bridge between the two host galaxies, typical for an incipient fusion. In this case, matter - i.e. gas, dust and stars - is torn out of the participating galaxies by the tidal forces, which then distribute between them. The two quasars are separated by less than 30,000 light years. The arc-shaped structures typical of a gravitational lens, however, are missing.
The observations also allow a more accurate characterization of the host galaxies: The team estimates a component at at least 22 billion solar masses, while its partner galaxy comes to around 46 billion solar masses. Although both are much less massive and also more compact than our Milky Way, they are well developed for this early era of the universe. Their high star formation rates of more than 500 solar masses per year also speak for this - a multiple of what stars form in our galaxy over the same period of time.
The group assumes that the two central black holes, with masses of about 280 and 400 million solar masses, will form a gravitationally bound double system within the next two billion years.
With the confirmation of the double nature of J2037-4537, the team also found that the proportion of such pairs among the quasars at higher red shifts is at least 1.2 percent significantly higher than at the time of the »cosmic noon«, where this is only about 0.1 percent. Thus, they could even represent a relevant source for the cosmic gravitational wave background.