Gravitational Lensing

Astronomers Find a Rare “Einstein Cross”

Gravitational lensing is one of astronomy’s great wonders: a natural lens that magnifies the distant universe. Sometimes a lensing system takes the shape of a so-called “Einstein Cross”. Those are rare and amazingly useful ways to study objects far away in space and time.

A team of astronomers recently found a new one using the Dark Energy Spectroscopic Instrument mounted on a telescope at Kitt Peak National Observatory. This instrument is surveying the sky and has found many instances of gravitational lensing. Followup observations show the new one to be both beautiful and a scientific treasure trove of information about the early universe.

The lens system, called DESI-253.2534+26.8843, is actually a massive foreground elliptical galaxy surrounded by four blue images of a background galaxy. Team leader Aleksandar Cikota of NOIRLab, pointed out what those images that form a perfect Einstein Cross pattern reveal. “The four images that display consistent spectral features tell the astronomers that the source is a single galaxy, which allowed them to confirm the lens system,” he said. “The cross pattern tells them about the mass distribution of the lens galaxy. Elongated mass distributions result in Einstein crosses, and a spherical mass distribution would result in an Einstein ring.”

Exploring DESI-253.2534+26.8843

This latest Einstein Cross has some interesting statistics. The main galaxy doing the lensing lies about 5.998 billion light-years away. The distant galaxy that it’s lensing is more than 11.179 billion light-years away. Thus, the foreground lensing galaxy is giving an amazing look at a galaxy in the early Universe.

This is a color image of DESI-253.2534+26.8843 Einstein Cross, observed with MUSE on May 22, 2023. Other galaxies with similar redshifts are labeled G1-G7. The white square shows the area in the image below. Courtesy Aleksandar Cikota.
This cutout from the above image shows the strong gravitational lens provided by the foreground galaxy. The four blue regions are images of the more distant galaxy. They form the “Einstein Ring” effect. Aleksandar Cikota.

The astronomy team determined the distance to the more distant galaxy by doing a spectral analysis of the light in each image. Based on color information contained in the DESI Legacy Survey, the team thinks that the lensing galaxy is in a galaxy group. They found at least seven other members of the same group. They describe those galaxies as “passive”, which could mean they are all older or elliptical. However, there’s not enough information to completely describe the galaxy group. And, they don’t seem to be participating in the lensing.

As the team analyzed the system using data from the Multi-Unit Spectroscopic Explorer (MUSE) at the Very Large Telescope in Chile, it turned out the background object is very typical of a starburst galaxy. They also found traces of a faint galaxy lying in front of one of the lensed images. It’s about 4.2 billion light-years away.

Using Spectroscopy to Study the Einstein Cross

This study of DESI-253.2534+26.8843 benefits from advances in computer modeling and hardware. Cikota pointed out that it’s a good example of the capabilities of the MUSE instrument coupled with computer modeling. “It is part of a bigger project – to confirm and parametrize many gravitational lens candidates discovered in the DESI legacy survey data using neural networks,” he said in an email. “We got over 100 hours for our observing program with MUSE for the characterization of gravitational lenses (data acquisition is still ongoing), and over 30 gravitational lens systems were successfully observed.”

MUSE is a powerful spectroscopic instrument that can cover wide areas of the sky in visible light wavelengths. It dissects the light into its component wavelengths (creating spectra) and each pixel in the image from the integral field unit contains a spectrum. The team used the data from its observations in a software package called GIGA-Lens. It models gravitational lensing systems and allows astronomers a quick way to model these complex objects. It can run a model through in about a minute and a half using Nvidia GPUs.

In their paper, the astronomers point out this is the first time data from a real gravitational lensing system got modeled in this way using GIGA-Lens. They write, “This concretely demonstrates a very promising future of modeling of strong lensing systems that are expected to be discovered in the next decade (e.g., Euclid, LSST, and the Roman Space Telescope), in a fast, robust, and scalable way.”

What Makes an Einstein Cross?

When a massive galaxy sits directly “in front of” a more distant background object (such as a galaxy or a quasar) the distribution of matter around that galaxy and its gravitational effect can “bend” the light from the object as it passes by. That results in lensed images (or a ring).

Hubble image of a luminous red galaxy (LRG) gravitationally distorting the light from a much more distant blue galaxy, a technique known as gravitational lensing. The shape of the galaxy doing the lensing created an almost circular image. An oblong galaxy would create an Einstein Ring effect. Credit: NASA/ESA/STScI

The first “Einstein Cross” was a surprise. Astronomer John Huchra and his team actually discovered it in 1985. It’s called “Huchra’s Lens”. It really looked baffling to the observers, as if there were four identical quasars around the center (where there was a faint image of the quasar). To figure out exactly why this would happen, the astronomers took studied the light from each “image”. Eventually, the redshift of the light from the quasar revealed that it lay 8 billion light-years away. The lensing galaxy is only about 400 million light-years distant.

A gravitational lens caused by a galaxy in the foreground leads to an “Einstein Cross.” This was discovered in 1985 and was also called “Huchra’s Lens.” Credit: NASA/ESA/STScI

Einstein Crosses and Beyond

Why are these so rare? It turns out that gravitational lensing happens everywhere in the universe, mostly in the form of so-called “weak lensing”. Creating an Einstein Cross requires a precise alignment of the lensing body and light source and astronomers refer to this as “strong gravitational lensing”. After the discovery of Huchra’s Lens, astronomers found a few more using Hubble Space Telescope and other instruments. Then, in 2021, the Gaia satellite found a dozen more. And, astronomers predict that more will be found as more powerful instruments and techniques perform surveys like Gaia’s.

More lenses like these will extend astronomy’s view to earlier epochs. They could perform as excellent probes of the dark matter distribution in the different epochs of cosmic time. And, there are other applications to be developed.

His team’s goal, said Cikota, is to study and characterize gravitational lens systems as a cosmological tool. “We are preparing for the time domain era (after Vera Rubin Observatory starts operation),” he wrote. “One of our goals is to do a targeted search for supernovae in hundreds of gravitational lens systems, which will allow us to directly measure Hubble’s constant by observing the time delay of supernova light curves between the lensed images of a supernova.”

For More Information

DESI-253.2534+26.8843: A New Einstein Cross Spectroscopically Confirmed with VLT/MUSE and Modeled with GIGA-Lens
DESI Project
GIGA-Lens: Fast Bayesian Inference for Strong Gravitational Lens Modeling
MUSE Instrument

NOIRLab

Carolyn Collins Petersen

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