An international team of astronomers has found evidence for a unique gravitational lensing system, in which a massive star cluster is magnifying an extremely distant galaxy containing a jet-spewing supermassive black hole.
This illustration shows a likely configuration of a gravitational lensing system discovered by the team: the milli-lens is located in or near the intervening spiral galaxy; the lens is magnifying blobs of jet material within the active galaxy PKS1413+135, but the blobs are too small to be seen in the radio image (top left); only when the blobs move far away from the yellow core do they expand and are visible as the pink blobs in the image. Image credit: Anthony Readhead / Caltech / MOJAVE.
Many supermassive black holes at the centers of galaxies blast out jets of gas traveling near the speed of light.
The gravity of black holes pulls material toward them, but some of that material ends up ejected away from the black hole in jets. The jets are active for one to 10 million years — every few years, they spit out additional clumps of hot material.
With the new gravitational lensing system, these clumps can be seen at scales about 100 times smaller than before.
“The clumps we’re seeing are very close to the central black hole and are tiny — only a few light-days across,” said team leader Professor Anthony Readhead, Director of the Owens Valley Radio Observatory.
“We think these tiny components moving at close to the speed of light are being magnified by a gravitational lens in the foreground spiral galaxy. This provides exquisite resolution of a millionth of a second of arc, which is equivalent to viewing a grain of salt on the Moon from Earth.”
Professor Readhead and co-authors think that the gravitational lens could be the first lens of intermediate mass — which means that it is bigger than previously observed ‘micro’ lenses consisting of single stars and smaller than the well-studied massive lenses as big as galaxies.
Dubbed a ‘milli-lens,’ the lens is thought to be approximately 10,000 solar masses.
An advantage of a milli-sized lens is that it is small enough not to block the entire source, which allows the jet clumps to be magnified and viewed as they travel, one by one, behind the lens.
“This system could provide a superb cosmic laboratory for both the study of gravitational milli-lensing and the inner workings of the nuclear jet in an active galaxy,” Professor Readhead said.
In 2010, he and his colleagues noticed something unusual happening with the lensed galaxy, called J1415+1320 (PKS 1413+135).
Its radio emission had brightened, faded, and then brightened again in a very symmetrical fashion over the course of a year. The same type of event happened again in 2015.
After a careful analysis that ruled out other scenarios, the team concluded that the overall brightening of PKS 1413+135 is most likely due to two successive high-speed clumps ejected by the galaxy’s black hole a few years apart. The clumps traveled along the jet and became magnified when they passed behind the milli-lens.
“It has taken observations of a huge number of galaxies to find this one object with the symmetrical dips in brightness that point to the presence of a gravitational lens,” said co-author Dr. Timothy Pearson, also of the Owens Valley Radio Observatory.
“We are now looking hard at all our other data to try to find similar objects that can give a magnified view of galactic nuclei.”
The astronomers reported their results in a pair of papers in the Astrophysical Journal.
_____
H.K. Vedantham et al. 2017. Symmetric Achromatic Variability in Active Galaxies: A Powerful New Gravitational Lensing Probe? ApJ 845, 89; doi: 10.3847/1538-4357/aa745c
H.K. Vedantham et al. 2017. The Peculiar Light Curve of J1415+1320: A Case Study in Extreme Scattering Events. ApJ 845, 90; doi: 10.3847/1538-4357/aa7741
