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A research team using data from the Dark Energy Spectroscopic Instrument in Arizona has identified seven new quasar systems that appear to act as gravitational lenses. They reported the results this year after training a neural network to scan more than eight hundred thousand quasar spectra.
The work matters because these rare alignments let astronomers measure the masses of quasar host galaxies, which are normally hidden behind the quasar’s strong light. The group built its model by mixing real quasar data with simulated background galaxies, and the method found far more strong candidates than earlier surveys.
Quasars shine when supermassive black holes consume nearby gas and dust. Their light usually hides the galaxies that surround them. In rare cases, a quasar sits in the near-perfect position to bend the light of a more distant galaxy behind it. The closer galaxy’s gravity pulls the background light into our line of sight. This forms what astronomers call a lens, but the bright quasar often hides the pattern in normal images.
These systems are rare because the alignment must be very close to perfect. Large surveys such as the Sloan Digital Sky Survey contain hundreds of thousands of quasars, yet only a small number show clear signs of lensing. Before this study, researchers had confirmed only three systems where the quasar host served as the lens. Several more were possible but needed stronger evidence.
The DESI project collects millions of spectra with a wide-field telescope at Kitt Peak in Arizona. Its early data release included spectra from more than eight hundred thousand quasars. Instead of relying on imaging, the team focused on the spectra. To train the network, they created mock lenses by adding distant galaxy signals to real quasar spectra. This approach gave the model enough examples to learn what a lensed system should look like.
The network searched for extra emission lines at higher redshifts than the quasar’s own features. These lines come from the background galaxy. Ground-based telescopes blend light from close objects into one spectrum, so the background signal can ride along with the quasar’s light. The model looked for these faint features that would normally be lost in the noise.
The system returned seven strong candidates on the first pass. All showed an oxygen line from a more distant galaxy. Four candidates included extra lines from hydrogen and additional oxygen signals. The process also found the one known system in DESI’s footprint, confirming that the method works. This result more than doubled the number of high-quality candidates in the survey area.
These quasars sit at redshifts up to about 1.8, meaning their light began its journey billions of years ago. Some of the background galaxies lie even farther away. In a few cases, the farther sources may sit at redshift 3 or beyond, which places their light in the early universe.
The interest in these lenses goes beyond the alignment. When a quasar host galaxy bends the light of a background galaxy, astronomers can measure the Einstein radius, which relates directly to mass. It gives a clear reading of the host galaxy’s mass, including dark matter, even when the quasar normally blocks the view.
Measuring these masses helps test how galaxies and their central black holes grew together. In the nearby universe, the size of a galaxy and the mass of its black hole follow a close trend. At higher redshifts, quasars are so bright that standard methods cannot separate their host galaxies. Lensing offers a way around that problem.
The new candidates also help scientists study feeding habits of black holes in the early universe. When a quasar lenses an even more distant galaxy, it gives a view of objects that formed when the universe was only a few billion years old. These systems show how matter moved between galaxies at that time.
DESI will expand its survey in future releases. With more spectra and improved training sets, researchers expect many more detections. Space telescopes such as Hubble and the James Webb Space Telescope (JWST) can then take sharp images of the best systems. Those images can confirm the lensing pattern and map the mass in the quasar host galaxy.
The latest study shows how machine learning can raise the discovery rate from rare finds to a steady flow. Seven new strong candidates from one search mark a clear shift in how quickly these systems can be found. As the dataset grows, the list of quasar lenses is expected to grow with it.
Source: Quasars acting as Strong Lenses Found in DESI DR1