Altieri’s Lens: A Rare Einstein Ring Discovered by Euclid

In this study, O’Riordan et al. unveil the discovery of a complete Einstein ring, dubbed “Altieri’s Lens,” around the elliptical galaxy NGC 6505. This extraordinary phenomenon was identified during the Euclid space telescope's early observations, marking the first strong gravitational lens detection in the NGC galaxy series. The Einstein ring occurs when light from a distant source galaxy is bent into a near-perfect circle by the gravitational pull of a foreground lens galaxy, demonstrating the effects of general relativity.

Imaging and Observations

The Einstein ring was first spotted during a Euclid contamination scan and later confirmed through deep imaging using Euclid's Visible Camera (VIS) and Near-Infrared Spectrometer and Photometer (NISP). Supplementary data from ground-based telescopes and the Keck Cosmic Web Imager (KCWI) provided additional insights. The lensing galaxy, located at a redshift of  z = 0.042, bends light from a background galaxy at z = 0.406. Detailed analysis of the light profile reveals intricate angular structures within the Einstein ring.

Lens Galaxy Properties

The research team measured the lens galaxy's velocity dispersion, a crucial parameter for understanding its mass distribution. The Einstein radius, indicating the region where gravitational lensing is most effective, spans 2.5 arcseconds corresponding to 2.1 kiloparsecs at the lens galaxy’s distance. Surprisingly, only 11% of the mass within this radius is attributed to dark matter, with the rest dominated by stars.

Significance of the Discovery

Low-redshift lenses like NGC 6505 are exceedingly rare due to the limited volume of the nearby universe. Euclid's vast survey capabilities are projected to uncover up to 20 similar low-redshift lenses, though few will match the brightness of Altieri’s Lens. The findings challenge conventional theories about galaxy composition, as the galaxy's stellar population suggests a heavier-than-usual initial mass function (IMF) compared to the standard Chabrier IMF.

Implications and Future Work

This discovery paves the way for more precise measurements of dark matter, stellar populations, and galaxy evolution at low redshifts. Ongoing analysis will refine models of the lensing galaxy’s mass distribution and probe the interplay between stars and dark matter. The Euclid mission, expected to identify over 100,000 new gravitational lenses, holds promise for further groundbreaking discoveries in cosmology and astrophysics.

This remarkable detection not only highlights the capabilities of modern astronomical surveys but also serves as a powerful example of the serendipity often involved in scientific breakthroughs. Altieri’s Lens provides a unique laboratory to study the central regions of galaxies and test theories of cosmic structure formation.

Source: O’Riordan