Using the Subaru Telescope and W. M. Keck and Gemini observatories, astronomers have spotted three dual quasars — merging galaxy systems that have two supermassive black holes on a collision course with each other.
SDSS J141637.44+003352.2, a dual quasar at a distance for which the light reaching us was emitted 4.6 billion years ago. The two quasars are 13,000 light-years apart on the sky, placing them near the center of a single massive galaxy that appears to be part of a group, as shown by the neighboring galaxies in the left panel. In the lower panels, optical spectroscopy has revealed broad emission lines associated with each of the two quasars, indicating that the gas is moving at thousands of kilometers per second in the vicinity of two distinct supermassive black holes. The two quasars are different colors, due to different amounts of dust in front of them. Image credit: Silverman et al, doi: 10.3847/1538-4357/aba4a3.
Quasars are one of the most luminous, energetic objects known in the Universe, powered by supermassive black holes that are millions to billions times more massive than our Sun.
As material swirls around a black hole at the center of a galaxy, it is heated to high temperatures, releasing so much light that the quasar can outshine its host galaxy.
This makes a merging pair of galaxies with quasar activity hard to detect; it is difficult to separate the light from the two quasars because they are in such close proximity to each other.
Also, observing a wide enough area of the sky to catch these rare events in sufficient numbers is a challenge.
To overcome these obstacles, Dr. John Silverman, an astronomer at the Kavli Institute for the Physics and Mathematics of the Universe, and colleagues took advantage of a sensitive wide survey of the sky using the Hyper Suprime-Cam (HSC) camera on the Subaru Telescope.
“To make our job easier, we started by looking at the 34,476 known quasars from the Sloan Digital Sky Survey with HSC imaging to identify those having two or more distinct centers,” Dr. Silverman said.
“Honestly, we didn’t start out looking for dual quasars. We were examining images of these luminous quasars to determine which type of galaxies they preferred to reside in when we started to see cases with two optical sources in their centers where we only expected one.”
The astronomers identified 421 candidates at distances less than 12.4 billion light-years.
However, there was still the chance many of these were not bona-fide dual quasars but rather chance projections such as starlight from our own Milky Way Galaxy.
Confirmation required detailed analysis of the light from the candidates to search for definitive signs of two distinct quasars.
Using Keck Observatory’s Low Resolution Imaging Spectrometer (LRIS) and Gemini Observatory’s Near-Infrared Integral Field Spectrometer, the team identified three dual quasars, two of which were previously unknown.
Each object in the pair showed the signature of gas moving at thousands of km per second under the influence of a supermassive black hole.
“In spite of their rarity, dual quasars represent an important stage in the evolution of galaxies, where the central giant is awakened, gaining mass, and potentially impacting the growth of its host galaxy,” said co-author Shenli Tang, a graduate student at the University of Tokyo.
The team’s paper was published in the Astrophysical Journal.
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John D. Silverman et al. 2020. Dual Supermassive Black Holes at Close Separation Revealed by the Hyper Suprime-Cam Subaru Strategic Program. ApJ 899, 154; doi: 10.3847/1538-4357/aba4a3
