Webb Space Telescope’s Special Aperture Turns One Sensor Into Seven

NASA’s James Webb Space Telescope used its sophisticated aperture mechanism to capture unprecedented images of the heart of the Circinus Galaxy, providing astronomers with data unlike anything previously obtained and completely upending the understanding of super-bright infrared regions surrounding supermassive black holes.

While the James Webb Space Telescope is celebrated for its impeccable resolution across a wide wavelength range, some of its more niche, specialized imaging tools are arguably just as important for the telescope’s scientific objectives. One such tool is the Aperture Masking Interferometer on the Webb Near-Infrared Imager and Slitless Spectrograph (NIRISS).

As NASA explains, interferometers on Earth typically rely on multiple telescopes, or arrays, that combine their efforts to act as a single telescope.

“An interferometer does this by gathering and combining the light from whichever source it is pointed toward, causing the electromagnetic waves that make up light to ‘interfere’ with each other (hence, ‘interfere-ometer’) and creating interference patterns,” NASA explains. “These patterns can be analyzed by astronomers to reconstruct the size, shape, and features of distant objects with much greater detail than non-interferometric techniques.”

What makes Webb’s approach innovative and unique is that it uses its Aperture Masking Interferometer to turn one telescope into a series of smaller telescopes that can work together to form an interferometer.

‘A prototype of the NIRISS non-redundant mask, which shows the layout of the 7 hexagonal apertures (holes) in the mask with respect to the JWST primary mirror segments and secondary mirror supports. Pairs of these apertures define 21 unique (“non-redundant”) vector separations (“baselines”) that produce an interferogram on the detector. The holes transmit ~15% of the light incident on the mask. They are smaller than the re-imaged mirror segments to allow for small misalignments in the optical system.’ | Credit: Anand Sivaramakrishnan (STScI)

Webb’s special aperture has seven small hexagonal holes, each controlling the amount and direction of light reaching the telescope’s detectors and sensors. As NASA notes, this is like an aperture on a photographic lens, albeit with seven instead of the usual one.

“These holes in the mask are transformed into small collectors of light that guide the light toward the detector of the camera and create an interference pattern,” explains Joel Sanchez-Bermudez, co-author of a new research paper published today in Nature and researcher at the National University of Mexico.

“By using an advanced imaging mode of the camera, we can effectively double its resolution over a smaller area of the sky,” continues Sanchez-Bermudez. “This allows us to see images twice as sharp. Instead of Webb’s 6.5-meter diameter, it’s like we are observing this region with a 13-meter space telescope.” 

With this aperture-based innovation, new observations delivered evidence that, contrary to belief, the largest source of infrared light near the supermassive black holes at the center of nearly all large galaxies in the Universe, is the result of an inflow of material to the supermassive black hole, not an outflow.

“Supermassive black holes like those in Circinus remain active by consuming surrounding matter. Infalling gas and dust accumulates into a donut-shaped ring around the black hole, known as a torus,” NASA explains.

As supermassive black holes accumulate matter, an accretion disk forms. As astronomers explain, this is like when water forms a whirlpool as it circles a drain. In the case of the cosmic whirlpool surrounding a supermassive black hole, the material moves so fast and generates enough friction that it becomes superhot and emits light.

This matter gets so hot that ground-based telescopes have thus far been unable to resolve the region in question with sufficient detail. Then there is the matter of the dust, which is also challenging to see through.

“For decades, astronomers contended with these difficulties, designing and improving models of Circinus with as much data as they could gather,” NASA explains.

Astronomers have developed models, but since the 1990s, they have been unable to explain the “excess infrared emissions” produced by the hot dust in the cores of active galaxies. Models have been able to explain the torus or the outflows of matter, but never the excess infrared light.

‘This image from NASA’s Hubble Space Telescope shows a full view of the Circinus galaxy, a nearby spiral galaxy about 13 million light-years away. The inset highlights a Webb close-up of the galaxy’s core, where infrared observations pierce through dust to reveal hot material feeding its central supermassive black hole. Webb’s image, made using the Aperture Masking Interferometer (AMI) tool on its NIRISS (Near-Infrared Imager and Slitless Spectrograph) instrument, isolates hot dust in the immediate surroundings of the supermassive black hole, revealing that most of the infrared emission comes from a compact, dusty structure feeding the black hole rather than from outflowing material. In the Webb image, the inner face of the torus glows in infrared light, while the darker areas represent where the outer ring is blocking light.’ | Credit: Image: NASA, ESA, CSA, Enrique Lopez-Rodriguez (University of South Carolina), Deepashri Thatte (STScI); Image Processing: Alyssa Pagan (STScI); Acknowledgment: NSF’s NOIRLab, CTIO

As the lead author of the new research, Enrique Lopez-Rodriguez of the University of South Carolina, explains, Webb provided astronomers with precisely the tools they needed to finally unravel this mystery.

“It is the first time a high-contrast mode of Webb has been used to look at an extragalactic source,” adds Julien Girard, paper co-author and senior research scientist at the Space Telescope Science Institute. “We hope our work inspires other astronomers to use the Aperture Masking Interferometer mode to study faint, but relatively small, dusty structures in the vicinity of any bright object.”

As for whether Circinus’ excess emissions are similar to those in other black holes, astronomers will need to use Webb to study even more black holes in the Universe. As NASA says, there are billions of them out there.

“We need a statistical sample of black holes, perhaps a dozen or two dozen, to understand how mass in their accretion disks and their outflows relate to their power,” concludes Lopez-Rodriguez.

Image credits: NASA, ESA, CSA, Enrique Lopez-Rodriguez (University of South Carolina), Deepashri Thatte (STScI); Image Processing: Alyssa Pagan (STScI); Acknowledgment: NSF’s NOIRLab, CTIO. The referenced research paper, “JWST interferometric imaging reveals the dusty torus obscuring the supermassive black hole of Circinus galaxy,” was published in Nature. The authors are Enrique Lopez-Rodriguez, Joel Sanchez-Bermudez, Omaira González-Martín, Robert Nikutta, Ryan M. Lau, Deepashri Thatte, Ismael García-Bernete, Julien H. Girard & Matthew J. Hankins.