Hubble Images Cosmic Lenses That Could Test a Dark Matter Theory

NASA has released a gallery of gravitational lenses photographed by the Hubble Space Telescope, and the images are meant to prepare the ground for the new Nancy Grace Roman Space Telescope. Astronomers plan to use them to weigh the invisible dark matter around foreground galaxies and to test a leading dark matter theory.

Gravitational lensing, first predicted by Albert Einstein, happens when the gravity of a massive object, such as a galaxy or a cluster of galaxies, bends and magnifies light from a more distant object behind it. NASA compares the effect to a giant lens in space that lets astronomers see farther than any telescope could on its own.

A gallery of very different lenses

The lenses in the image formed when a distant background galaxy lined up with the mass of a foreground galaxy, producing warped, magnified copies of the background galaxy that show up as arcs. NASA describes several of the shapes:

Panel A shows an arc-counterarc configuration, where the background galaxy appears as a duplicated pair of arcs bent around a pair of foreground galaxies.
Panel F is an Einstein Ring, in which the alignment is so precise that the lensed galaxy almost fully encircles the foreground galaxy.
Panels I and C show quadruply lensed galaxies, where the background galaxy is replicated four times. In panel C, double knots are most likely strong star-forming regions.
Panel D has a foreground galaxy with a visible dust lane, which NASA calls rare in a lensing galaxy.

Some of the warped background galaxies look blue and some look red. According to NASA, bluish arcs point to young stars and reddish arcs to older ones, a difference Roman and Hubble will use to study how galaxies evolved early in the universe. Most foreground lenses are elliptical galaxies, which are older, denser and more compact, but a few appear elongated or show spiral arms, which lets researchers explore the different paths galaxies take as they evolve.

What the lenses could reveal about dark matter

By studying how much the background galaxies are distorted, astronomers can work out how much mass sits in the foreground galaxy, including its dark matter. They will build lens models from a large and varied population of lenses, which could prove or dispel a theory about dark matter.

Hubble's sharp imaging could also expose small imperfections in the arcs that would signal small clumps of dark matter. Finding such clumps would support the cold dark matter theory, which predicts many of them, between a million and a billion solar masses. By comparison, NASA says the dark matter halo of the Milky Way is around 1.3 trillion solar masses.

Why it matters: Roman and Hubble will work as a team

The two telescopes complement each other. Hubble views the cosmos mainly in visible and ultraviolet light, with some infrared, and gives a high-resolution look at individual objects. Roman sees mainly infrared light and some visible light across a much larger field of view, so it can capture more lenses even though Hubble gets the better view of each one.

If Roman finds a particularly interesting lens, researchers may point Hubble at it for follow-up observations. Infrared light is also less blocked by dust, so Roman could help show whether the arcs around the dusty galaxy in panel D form a more complete Einstein Ring.

Researchers also intend to study the Hubble lens images in detail to build a tool that can efficiently search for similar arcs in Roman's upcoming images. NASA says the Hubble images are expected to yield their own results on dark matter and pave the way for analysis of Roman's larger sample of lenses.

How the lenses were found

The strong lenses were first detected with the Dark Energy Spectroscopic Instrument at Kitt Peak. Hubble then observed them, and Roman will take further observations. NASA presents the work as an example of collaboration among its Great Observatories, a group that includes Hubble, the Chandra X-ray Observatory and the James Webb Space Telescope, and now Roman.

Around Prague, based on reporting by NASA

Cover photo: Daag / Wikimedia Commons (public domain)

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