A double quasar spiraling toward a great merger has been discovered lighting up the "cosmic dawn," just 900 million years after the Big Bang.
They are the first quasar pair spotted that far back in cosmic time.
Quasars are rapidly growing supermassive black holes in the cores of hyperactive galaxies. Torrents of gas are thrust down the black holes' throats and get hung up in the bottleneck of an accretion disk, which is a dense ring of ultrahot gas that is queuing up to fall into the black hole. Not all of it does fall in; magnetic fields wrapped up in the rotating accretion disk are able to whip up lots of charged particles and beam them back into deep space in the form of two jets racing away at almost the speed of light. The jets and accretion disk combined make the quasar appear highly luminous, even across billions of light-years.
Since each large galaxy has a monstrous black hole as its dark heat, when galaxies collide and merge, eventually so too do their supermassive black holes. Back during the cosmic dawn — which describes the first billion years of cosmic history, when stars and galaxies first appeared on the scene — the expanding universe was smaller than it is today, and therefore galaxies were closer together and merged more often. Yet while over 330 lone quasars have been spotted so far in the universe's first billion years, the expected abundant population of double quasars has been notable by their absence — until now.
The newly discovered double quasar, J121503.42–014858.7 and J121503.55–014859.3 — referred to as C1 and C2 by their discoverers — was spotted using the Subaru Telescope on Hawaii's Mauna Kea by a team led by Yoshiki Matsuoka of Ehime University in Japan.
The astronomers followed up spectroscopically using the Faint Object Camera and Spectrograph (FOCAS) on Subaru and the Gemini Near-Infrared Spectrograph (GNIRS) on the Gemini North telescope, which also sits atop Mauna Kea.
"What we learned from the GNIRS observations was that the quasars are too faint to detect in near-infrared, even with one of the largest telescopes on the ground," said Matsuoka in a statement.
Having traveled for 12.9 billion years, the light of the quasars has been redshifted and stretched to longer wavelengths by cosmic expansion, so light that began as X-rays or ultraviolet ends up near the red and infrared end of the electromagnetic spectrum. The light from the quasars should be detectable in near-infrared, but the fact that they are faint at that wavelength means that a good chunk of their light is actually at other wavelengths produced by the enhanced star formation in the galaxies that host the quasars.
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Increased star formation, which for C1 and C2 is estimated to be between 100 and 550 solar masses per year (compared to one to 10 solar masses per year in our Milky Way galaxy), is a common symptom of galaxy mergers, because raw molecular hydrogen gas gets stirred up by the interaction and triggered into forming new stars.
The two black holes have also moved to within 40,000 light-years (12,000 parsecs) of one another. While this is still a large distance, observations with the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile have found a bridge of gas spanning this distance between C1 and C2. Already the two black holes are connected, and that link will only grow stronger as they continue to close in on each other.
The existence of C1 and C2 is further evidence that galaxies and their black holes grew rapidly, and to immense size and mass, during the era of cosmic dawn, challenging our models of how they should form. The black holes each have a mass of about 100 million times the mass of our sun, which is huge; Sagittarius A*, the black hole at the center of our Milky Way galaxy, is tiny in comparison, with a mass of just 4.1 million solar masses. Plus, the host galaxies of C1 and C2 have overall masses in the region of 90 billion and 50 billion solar masses, respectively, which, while substantially less than the Milky Way, is nevertheless gargantuan for the time.
As such, the discovery of this double quasar and their host galaxies provides a vital data point for better understanding the early universe and especially the epoch of reionization, when most of the gas in the universe was ionized by radiation from the first stars, galaxies and quasars, ending the cosmic dark ages. One of the great puzzles of cosmology is which one of those three things contributed the most to reionization.
"The statistical properties of quasars in the epoch of reionization tell us many things, such as the progress and origin of the reionization, the formation of supermassive black holes during cosmic dawn, and the earliest evolution of the quasar host galaxies," said Matsuoka.
We are seeing these two quasars as they were about 12.9 billion years ago. What became of them since? Simulations indicate that eventually the two black holes will merge in a burst of gravitational waves. This will make the combined quasar even more luminous and boost the star-formation rate in the merged galaxy to above 1,000 solar masses per year, creating one of the most extreme galaxies in the universe. Ultimately, it may become one of the giant elliptical galaxies at the heart of a massive galaxy cluster, such as M87 in the Virgo Cluster.
The findings were published on 5 April in The Astrophysical Journal Letters, with a companion paper discussing the ALMA measurements.
