How Gravitational Lensing Works
The light from quasar J0439+1634, some 12.8 billion light-years away, passes close to a faint galaxy that is about six billion light-years away. The gravity of the foreground galaxy warps the space around it, according to Einstein’s theory of general relativity, which bends the light like an optical lens and magnifies the quasar image by a factor of 50, while at the same time splitting the quasar image into three. (Image: NASA, ESA, Xiaohui Fan/UA)
With the help of NASA’s Hubble Space Telescope, astronomers have discovered the brightest object ever seen at a time when the universe was less than one billion years old. The brilliant beacon is a quasar, the core of a galaxy with a black hole ravenously eating material surrounding it.
Though the quasar is very far away – 12.8 billion light-years – astronomers can detect it because a galaxy closer to Earth acts as a lens and makes the quasar look extra bright. The gravitational field of the closer galaxy warps space itself, bending and amplifying the distant quasar’s light in an effect known as gravitational lensing.
Researchers have searched for extremely remote quasars for more than 20 years before a rare and fortuitous celestial alignment made this one visible. Discovered with the University of Arizona's MMT Telescope, located south of Tucson, the super-bright quasar could hold the record of being the brightest in the early universe for some time, making it a unique object for follow-up studies.
“We don’t expect to find many quasars brighter than that in the whole observable universe,” lead investigator Xiaohui Fan said of the quasar, which has been cataloged as J043947.08+163415.7.
Shining with light equivalent to almost 600 trillion suns, the quasar is fueled by a supermassive black hole at the heart of a young galaxy in the process of forming. An immense amount of energy is emitted as the black hole consumes material around it. The detection provides a rare opportunity to study a zoomed-in image of how such black holes accompanied star formation in the very early universe and influenced the assembly of galaxies.
"If this quasar was not lensed, its brightness would have been more than 50 times 'fainter,'" said Fan, a Regents' Professor of Astronomy in the UA's Steward Observatory. "This one is off-the-charts bright."
The first images suggesting a possible lensing structure were obtained with the Large Binocular Telescope, which is located in in southeastern Arizona and managed by the UA.
In addition to being bright in visible and infrared wavelengths, the lensed quasar is also bright in submillimeter wavelengths, observed with the James Clerk Maxwell Telescope on Mauna Kea, Hawaii. That emission is due to hot dust heated by intense star formation in the galaxy hosting the lensed quasar. The formation rate is estimated to be up to 10,000 stars per year (by comparison, our Milky Way galaxy makes one star per year).
“Clearly, this black hole is not only accreting gas, but it also has a lot of star formation around it,” said UA team member Jinyi Yang. "However, because of the boosting effect of gravitational lensing, the actual rate of star formation could be much lower than the observed brightness suggests."
The quasar existed at a transitional period in the universe’s evolution called reionization, where light from young galaxies and quasars reheated the obscuring hydrogen that cooled off not long after the big bang.
The quasar would have gone undetected if not for the power of gravitational lensing, which boosted its brightness by a factor of 50.
"Essentially, we have a very magnified view of this supermassive black hole," Fan said. "It's like a magnifying glass that gives you an image that's not just brighter, but also bigger. We now can look at the much smaller structures surrounding that black hole."
Very distant quasars are identified by their red color (due to absorption by diffuse gas in intergalactic space), although sometimes their light is “contaminated” and looks bluer because of the starlight of an intervening galaxy. As a result, they may be overlooked in quasar searches because their color is diluted to resemble that of a normal galaxy.
"We think there probably are 10 to 20 of such objects that we haven't found because they would have looked fuzzy and the colors not redshifted enough," Fan said. "This by extension means that our traditional way of finding quasars may not work anymore, and we have to find some new, big data approach to broaden our search."
Fan said his team got lucky with finding J043947.08+163415.7, because the quasar is so bright it drowns out the starlight from the especially faint foreground lensing galaxy.
“Without this high level of magnification, it would make it impossible for us to see the galaxy," said team member Feige Wang of the University of California, Santa Barbara. “We can even look for gas around the black hole and what the black hole may be influencing in the galaxy.”
The object was selected by its color by combining photometric data from the United Kingdom Infrared Telescope Hemisphere Survey, the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS1) at optical wavelengths and NASA’s Wide-field Infrared Survey Explorer archive in the mid-infrared.
Follow-up spectroscopic observations were conducted by the UA’s Multi-Mirror Telescope, the Gemini Observatory and the Keck Observatory. These observations revealed the signature of a faint foreground galaxy directly between the quasar and Earth that is magnifying the quasar image. However, because the source looks fuzzy in the ground-based observations (and so could be mistaken for only a galaxy), the researchers used Hubble’s exquisite imaging capabilities to confirm it is a lensed quasar.
“It’s a hard system to photograph because it turns out to be so compact, which requires the sharpest view from Hubble,” Fan said.
The quasar is ripe for future scrutiny. Fan’s team is analyzing a detailed 20-hour spectrum from the European Southern Observatory’s Very Large Telescope, which would show gas absorption features to identify chemical composition and temperatures of intergalactic gas in the early universe. Astronomers also will use the Atacama Large Millimeter/submillimeter Array, and eventually NASA’s James Webb Space Telescope, to look within 150 light-years of the black hole to directly detect the influence of the black hole's gravity on gas motion and star formation in its vicinity.
The team presented the discovery during a press briefing Jan. 9 at the 233rd Meeting of the American Astronomical Society in Seattle.