Hubble Images a Galaxy Warped Into Arcs as NASA Preps for Roman Telescope

NASA’s Hubble Space Telescope has captured a distant galaxy that is magnified and stretched into a pair of arcs by the gravity of a closer galaxy, the agency announced. The image is one of many gravitational lens observations Hubble is collecting to prepare for science with the new Nancy Grace Roman Space Telescope.

The picture is dominated by a large barred spiral galaxy, but NASA says the real prize is the lens in the lower-right corner, where a precise chance alignment lets one galaxy act like a giant magnifying glass for another.

How a galaxy becomes a lens

According to NASA, gravitational lensing happens when the mass of a foreground object, such as a galaxy, warps spacetime. Light from something behind it bends, distorts and brightens as it passes around the foreground object. The effect only appears when the two galaxies line up closely enough.

In this image, the foreground lens is an elliptical galaxy about 4.5 billion light-years away. NASA notes that the light now reaching Hubble began its journey when the solar system was still forming. The background galaxy is about eight billion light-years away. It shows up as one large, obvious arc to the upper left of the foreground galaxy and a much smaller, flipped copy of the same galaxy to the lower right. NASA calls this an arc-counterarc pair.

Nearly everything else in the frame is also a galaxy, apart from a few foreground stars. The barred spiral, named for arms that extend from a bar of stars in its center, is about the size of the Milky Way, which astronomers think is a barred spiral too. A tiny spiral to its left may be a satellite galaxy, similar to the Large and Small Magellanic Clouds that orbit the Milky Way.

Why it matters: weighing dark matter

The heavier the foreground galaxy, the stronger the lens and the more it bends light. By measuring how much the background galaxy is distorted, researchers can work out how much mass the foreground galaxy holds, NASA explains.

That matters for the study of dark matter, the invisible substance that makes up most of the mass in the universe. The visible matter in the lens galaxy is not enough to account for the amount of distortion seen, so astronomers can use the difference to calculate both the galaxy’s total mass and how much dark matter it contains.

Two telescopes, different strengths

Hubble sees the cosmos mainly in visible and ultraviolet light, with some infrared coverage. Roman sees mostly infrared wavelengths and some visible light. NASA says Hubble delivers high-resolution views of individual objects, while Roman has a much wider field of view suited to panoramas.

That difference shapes the plan. Hubble can examine these lenses only one at a time, while Roman’s broad view of the sky will turn up a far larger sample of them.

What happens next

Astronomers are using Hubble’s sharp images to build a computer model that can better analyze the distortions caused by dark matter. NASA says that tool will help them handle Roman’s large images once they arrive. Hubble will also follow up on interesting lenses that Roman detects, so the two observatories are expected to work together to build a more complete picture of the cosmos.

Around Prague, based on reporting by NASA