What appeared to be a normal galaxy concealed a ring of light never seen before

It is not every day that a nearby, cataloged galaxy reveals a secret hiding in plain sight. A new study reports a complete Einstein ring, a near perfect loop of light made when a more distant galaxy’s glow is bent and magnified by a massive foreground galaxy.

The result arrives from Euclid’s early observations and centers on NGC 6505, a well known elliptical. The new analysis presents the first strong lens discovered by Euclid and pins down key numbers for both the lens and the hidden background source.

Einstein ring in a familiar galaxy

The ring encircles the core of NGC 6505 with an Einstein radius of about 2.5 arcseconds.

Spectroscopy identifies the background source at redshift z = 0.406 and the lens at z = 0.042, placing the ring at a physical scale of roughly 2.1 kiloparsecs in the lens plane, while remaining compact compared with the galaxy’s effective radius.

Euclid’s visible and near infrared images reach exceptionally high signal to noise.

That imaging, paired with follow up spectroscopy from the Keck Cosmic Web Imager (KCWI) and data from the Dark Energy Spectroscopic Instrument (DESI), allowed precise modeling of the light and mass in the lens.

What an Einstein ring tells us

Gravitational lensing occurs when mass bends the path of light, causing background objects to be displaced, stretched, or multiply imaged.

In the special case where the alignment between source, lens, and observer is close to exact, the images merge into a ring whose radius encodes the mass inside it.

These systems serve many roles. They trace how matter, including invisible dark matter, is arranged in and around galaxies, and they can probe how the universe has grown over time by connecting lens statistics to cosmic expansion and structure growth.

Strong lenses also act as natural magnifying glasses for the lensed sources. That gives astronomers a clearer look at distant galaxies than direct imaging alone can provide, especially when the lensing geometry boosts surface brightness and effective resolution.

Why this case matters

Low redshift lenses are uncommon, so a bright, complete ring around a nearby galaxy is a valuable laboratory. The small Einstein radius samples the inner region of the lens where stars dominate the mass budget.

Only a handful of comparably nearby systems have allowed similarly tight tests of stellar mass and orbital structure in the central kiloparsecs.

Classic examples include the lens 2237+0305 at z ≈ 0.039, known as the “Einstein Cross,” which has long anchored studies of galaxy center mass profiles.

NGC 6505 adds a cleaner geometry to that toolbox. The high quality Euclid imaging enables a detailed subtraction of the galaxy’s own light and a careful reconstruction of the lensed arcs.

With those pieces in place, the team could compare dynamical measurements to the mass implied by the ring and check for consistency. That cross check is essential when testing how mass is partitioned between stars and dark matter in galaxy cores.

A window on dark matter and stars

The analysis measures a central velocity dispersion of 303 ± 15 kilometers per second and combines it with the ring size to infer the enclosed mass.

Modeling points to a dark matter fraction of about 11 percent inside the Einstein radius, modest for that inner region but supported by the data.

Those constraints translate into a statement about the stellar initial mass function (IMF) in the galaxy’s center.

The team reports an IMF mismatch parameter of 1.26 with narrow uncertainties, indicating a heavier mix of low mass stars than a Chabrier IMF, but lighter than a Salpeter case in the same region.

That result aligns with emerging evidence that many massive ellipticals have bottom heavy IMFs in their inner parts, with gradients toward more Milky Way-like IMFs at larger radii.

It also highlights how nearby strong lenses sharpen those inferences by focusing on a small, well defined aperture in the galaxy center.

The geometry here even reveals subtle departures from perfect ellipticity in the total mass distribution. The preferred models include mild “boxiness,” echoing features seen in the starlight and hinting at close ties between light and mass within the ring.

What comes next for Euclid

Euclid is built to map more than a third of the sky and observe billions of galaxies out to roughly 10 billion light years, a scope that will flood lensing studies with statistics and rare gems alike.

The survey formally began routine science on February 14, 2024, and will run for about six years.

Forecasts suggest Euclid alone should uncover on the order of one hundred thousand reliable strong galaxy galaxy lenses, turning what used to be small samples into population studies.

That scale opens the door to percent level constraints on cosmological parameters when lensing is combined with other probes, as well as systematic tests of how dark energy influences growth of structure.

Finds like the NGC 6505 ring preview that future. They show Euclid can deliver exquisite detail on nearby systems while scanning the deep universe for the weak lensing signatures it was designed to measure.

The work was led by Conor M. O’Riordan of the Max Planck Institute for Astrophysics (MPA), with critical involvement from Euclid Archive Scientist Bruno Altieri at the European Space Agency and Euclid Project Scientist Valeria Pettorino at ESA. Their combined efforts turned routine instrument checks into a discovery.

The study is published in Astronomy & Astrophysics.

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