Hubble spots rare double quasar that existed when universe was 3 billion years old

Confirming the double-quasars 

A quasar is a luminous object found in the center of a galaxy that is the result of large amounts of radiation. It is believed that they contain active supermassive black holes, which fuel quasars. The powerful jets of energy released by black holes provide the light for quasars. This light is visible from billions of light-years away. Looking at quasars is thus equivalent to looking at a black hole for astronomers.

It is unusual to come across double-quasar candidates. Initially, the team assumed the data was from a single quasar that had been distorted by the gravitational lens, making it appear to be two.

The team confirmed that these two are indeed supermassive black holes, thanks to Hubble's high resolution. These two quasars were confirmed by follow-up observations from ESA's Gaia, which precisely measured the positions, distances, and motions of these celestial objects. 

“We're starting to unveil this tip of the iceberg of the early binary quasar population. This is the uniqueness of this study. It is actually telling us that this population exists, and now we have a method to identify double quasars that are separated by less than the size of a single galaxy,” said Xin Liu of the University of Illinois at Urbana-Champaign.

Hubble looks back in time, and this double quasar pair no longer exists. The distance between the quasars was less than the size of a single galaxy. After the explosive merger of their host galaxies, they would have merged to form a supermassive black hole at its center.

Along with Hubble’s observation, astronomers used W.M. Keck Observatory in Hawaii, the International Gemini Observatory in Hawaii, NSF's Karl G. Jansky Very Large Array in New Mexico, Chandra X-ray Observatory, and ESA Gaia space observatory. 

The study has been published in the journal Nature.

Study abstract:

Galaxy mergers produce pairs of supermassive black holes (SMBHs), which may be witnessed as dual quasars if both SMBHs are rapidly accreting. The kiloparsec (kpc)-scale separation represents a physical regime sufficiently close for merger-induced effects to be important1yet wide enough to be directly resolvable with the facilities currently available. Whereas many kpc-scale, dual active galactic nuclei—the low-luminosity counterparts of quasars—have been observed in low-redshift mergers, no unambiguous dual quasar is known at cosmic noon (z ≈ 2), the peak of global star formation and quasar activity. Here we report multiwavelength observations of Sloan Digital Sky Survey (SDSS) J0749 + 2255 as a kpc-scale, dual-quasar system hosted by a galaxy merger at cosmic noon (z = 2.17). We discover extended host galaxies associated with the much brighter compact quasar nuclei (separated by 0.46″ or 3.8 kpc) and low-surface-brightness tidal features as evidence for galactic interactions. Unlike its low-redshift and low-luminosity counterparts, SDSS J0749 + 2255 is hosted by massive compact disk-dominated galaxies. The apparent lack of stellar bulges and the fact that SDSS J0749 + 2255 already follows the local SMBH mass–host stellar mass relation, suggest that at least some SMBHs may have formed before their host stellar bulges. While still at kpc-scale separations where the host-galaxy gravitational potential dominates, the two SMBHs may evolve into a gravitationally bound binary system in around 0.22 Gyr.