Using a deep residual neural network and data from the Dark Energy Spectroscopic Instrument (DESI) Legacy Imaging Surveys, astronomers have discovered 1,200 new strong gravitational lensing systems, approximately doubling the number of known lenses.
An example of a gravitational lens found in the DESI Legacy Surveys data. There are four sets of lensed images in DESI-090.9854-35.9683, corresponding to four distinct background galaxies — from the outermost giant red arc to the innermost bright blue arc, arranged in four concentric circles. All of them are gravitationally lensed by the orange galaxy at the very center. Image credit: NOIRLab.
Gravitational lenses are a manifestation of gravity’s ability to bend light, which was predicted by Einstein’s general theory of relativity in 1915.
They allow astronomers to explore the most profound questions of our Universe, including the nature of dark matter and the value of the Hubble constant, which defines the expansion of the Universe.
They consist of two objects: one that is further away and supplies the light, and the other, the lensing mass or gravitational lens, which sits between Earth and the distant light source, and whose gravity deflects the light.
When the observer, the lens, and the distant light source are precisely aligned, the observer sees a so-called Einstein ring, a perfect circle of light that is the projected and greatly magnified image of the distant light source.
Only 1 in 10,000 massive galaxies are expected to show evidence of strong gravitational lensing, and locating them is not easy.
“A massive galaxy warps the spacetime around it, but usually you don’t notice this effect,” said Dr. Xiaosheng Huang, an astronomer at the University of San Francisco.
“Only when a galaxy is hidden directly behind a giant galaxy is a lens possible to see.”
“When we started this project in 2018, there were only about 300 confirmed strong lenses.”
Sixteen of the 1,210 new lensing candidates found in the DESI Legacy Surveys data. Image credit: Huang et al., arXiv: 2005.04730.
Dr. Huang and colleagues carried out a search for strong gravitational lensing systems in the DESI Legacy Surveys data by using a deep residual neural network.
“As a co-leader in the DESI Legacy Surveys I realized this would be the perfect dataset to search for gravitational lenses,” said co-author Dr. David Schlegel, a researcher at Lawrence Berkeley National Laboratory.
To analyze the data, the team used National Energy Research Scientific Computer Center’s (NERSC) supercomputer at Berkeley Lab.
“The DESI Legacy Imaging Surveys were absolutely crucial to this study; not just the telescopes, instruments, and facilities but also data reduction and source extraction,” Dr. Huang said.
“The combination of the breadth and depth of the observations is unparalleled.”
The results will be published in the Astrophysical Journal.
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X. Huang et al. 2021. Discovering New Strong Gravitational Lenses in the DESI Legacy Imaging Surveys. ApJ, in press; arXiv: 2005.04730
