UH Hilo’s Griffiths leads team that solves gravitational lens phenomenon

Date: Thursday, October 7, 2021
Contact: Alyson Kakugawa-Leong, (808) 932-7669

For Immediate Release

A University of Hawaiʻi at Hilo physics professor led a team of scientists that recently solved the mystery of the light from a single galaxy being split into multiple distorted images, something that had puzzled astronomers for many years.

Dr. Richard Griffiths, affiliate professor of physics and astronomy at UH Hilo, and professor emeritus at Carnegie Mellon University, and his team discovered that the dark matter within the galaxy cluster is smoothly distributed on the scale of a few thousand light-years.

“This discovery, called ‘Hamilton’s Object,’ is important because astronomers still don't know what dark matter is, nearly a century after its discovery,” Griffiths noted. “The search for the nature of dark matter is one of the biggest problems in all of physics.

“This is one of only very few known galaxies where there is a chance that a star within the galaxy passes across a 'critical' line which is set by the gravitational pull of the cluster and the distances to the galaxy and the cluster,” he added. “When a star crosses this line, its brightness can be magnified by a few thousand times so it becomes clearly visible for weeks or months while the star is crossing the line.

“As this is happening, the brightness of the star will fluctuate because the dark matter in the cluster is clumped and the line is actually wavy. This would give a direct measure of the clumpiness of the dark matter.”

The team discovered that the immense gravity of an intervening, foreground cluster of galaxies was warping space, magnifying, brightening, and stretching the image of a distant galaxy behind it, a phenomenon called gravitational lensing. The gravitational pull of the cluster of galaxies is dominated by the dark matter within it.

“We know it's some form of matter, but we have no idea what the constituent particle is,” he added. “The significance of the limits of size on the clumping or smoothness is that it gives us some clues as to what the particle might be.”

Griffiths explained that in 2013, a small group of astronomers found a peculiar image seen with the Hubble Space Telescope, which had a doubled nucleus flanked by several parallel streaks, each of which was also doubled.

“The object was found in a Hubble image of the area near a supermassive black hole that was emitting x-rays, but the strange image had nothing to do with the supermassive black hole,” Griffiths said. “It was a completely serendipitous discovery and a big surprise to the Hubble observers.”

The image remained a puzzle to the discovery team until Tim Hamilton of Shawnee State University showed it years later to Griffiths, who recognized it as a “gravitational lens,” a galaxy whose image had been doubled and stretched because the light from it had passed through the center of a large cluster of galaxies.

“There should have been a third image, much less distorted, that showed the true shape of the distant, background galaxy,” Griffiths said. “By studying carefully the shapes and distortions in the lensed images, I was able to roughly reconstruct the shape and size of the anticipated third image, and then succeeded in finding it in the Hubble image, not far from the lensed images.”

Griffiths involved UH Hilo Physics and Astronomy students Mitchell Rudisel and Po-Chieh Huang to assist with data analysis, observations, and obtaining spectroscopic measurements using telescopes from Gemini and W.M. Keck observatories to show that the dark matter in the cluster of galaxies, which has the gravitational pull to double and stretch the background galaxy image, was smoothly distributed through the center of the cluster.

The analysis by the UH Hilo team, assisted by Dr. Jenny Wagner at the University of Heidelberg in Germany, showed that the dark matter may consist of ultra-light particles rather than the heavy particles favored by most physicists.

“Only the Hubble Space Telescope is capable of finding gravitational lenses like this one,” Griffiths said. “And at the time of the original discovery, there were no similar objects that had been found, so this was one of the first.

“There were no publications to guide us in the interpretation of what we had found, and world experts on gravitational lenses had never seen anything like this,” he added.

The team's paper (https://academic.oup.com/mnras/article-abstract/506/2/1595/6276726?redirectedFrom=fulltext) appears in the September 2021 issue of The Monthly Notices of the Royal Astronomical Society (https://academic.oup.com/mnras/issue/506/2).


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