J1721+8842: The first Einstein zig-zag lens (2411.04177v1)

Abstract: We report the discovery of the first example of an Einstein zig-zag lens, an extremely rare lensing configuration. In this system, J1721+8842, six images of the same background quasar are formed by two intervening galaxies, one at redshift $z_1 = 0.184$ and a second one at $z_2 = 1.885$. Two out of the six multiple images are deflected in opposite directions as they pass the first lens galaxy on one side, and the second on the other side -- the optical paths forming zig-zags between the two deflectors. In this letter, we demonstrate that J1721+8842, previously thought to be a lensed dual quasar, is in fact a compound lens with the more distant lens galaxy also being distorted as an arc by the foreground galaxy. Evidence supporting this unusual lensing scenario includes: 1- identical light curves in all six lensed quasar images obtained from two years of monitoring at the Nordic Optical Telescope; 2- detection of the additional deflector at redshift $z_2 = 1.885$ in JWST/NIRSpec IFU data; and 3- a multiple-plane lens model reproducing the observed image positions. This unique configuration offers the opportunity to combine two major lensing cosmological probes: time-delay cosmography and dual source-plane lensing since J1721+8842 features multiple lensed sources forming two distinct Einstein radii of different sizes, one of which being a variable quasar. We expect tight constraints on the Hubble constant and the equation of state of dark energy by combining these two probes on the same system. The $z_2 = 1.885$ deflector, a quiescent galaxy, is also the highest-redshift strong galaxy-scale lens with a spectroscopic redshift measurement.

• The paper identifies a unique gravitational lensing pattern—the Einstein zig-zag lens—produced by a quasar lensed by two galaxies.
• It employs a robust multi-plane lens model and identical quasar light curves across six images over two years to validate the lensing configuration.
• The study paves the way for refined cosmological measurements and deeper insights into dark matter halos and dark energy dynamics.

The Discovery of J1721$+$8842: An Einstein Zig-Zag Lens

The paper "J1721$+$8842: The first Einstein zig-zag lens" presents the identification and analysis of a novel gravitational lensing configuration termed an Einstein zig-zag lens. This paper, reporting on the object J1721$+$8842, provides evidence of this unique configuration, comprised of an alignment involving a background quasar and two intervening galaxies acting as compound lenses.

Overview of the Findings

The analysis of J1721$+$8842 reveals a complex gravitational lensing scenario. The system was originally believed to consist of a dual lensed quasar. However, the research conducted by the authors demonstrates that J1721$+$8842 is a more intricate system, one where a single background quasar, positioned at a redshift of approximately 2.38, is multiply imaged by two galaxy lenses at redshifts $z_1 = 0.184$ and $z_2 = 1.885$. This results in an image configuration where the optical paths are deflected in opposite directions, producing a zig-zag pattern.

The evidence supporting this interpretation includes:

1. Identical Light Curves: Light curves from all six images of the quasar overlap perfectly over a two-year observation period from the Nordic Optical Telescope. This excludes the possibility of independent variability if the images were from different quasars.
2. Detection at $z_2$ with JWST/NIRSpec: The team's observation using JWST/NIRSpec identified an intermediate deflector at $z_2 = 1.885$, which contributes to the zig-zag deflection of quasar light, upending the dual quasar assumption.
3. Multiple-Plane Lens Model: The construction of a robust multiple-plane lens model accountable for the image positions corroborates the presence of compound lensing. This model integrates both the lensing effects of the two galaxies and reproduces the observed image configurations with high precision.

Implications for Cosmology and Lensing

The implications of this finding are significant as J1721$+$8842 provides a unique opportunity to apply distinct cosmological probes. Specifically, it can serve as a unique testbed for time-delay cosmography, offering insights into the structure and dynamics of dark matter halos, as well as constraints on cosmological parameters such as the Hubble constant ($H_0$) and the dark energy equation of state parameter ($w$).

The system also qualifies as a double source-plane lensing system, enabling the paper of cosmology through distance ratio constraints. Such compound systems are exceptionally rare, but can play a pivotal role in lessening the impact of mass-sheet degeneracies that often complicate single-plane analyses.

Conclusion and Prospects

The paper on J1721$+$8842 opens new pathways in gravitational lensing studies by confirming the viability of these complex lensing structures. The presence of this rare Einstein zig-zag configuration not only provides a natural laboratory for testing lensing theories but also enhances our capability to infer more precise cosmological measurements. The prospect of discovering more such configurations with upcoming surveys like LSST and Euclid holds promise for significantly advancing our understanding of cosmic structure and expansion.

Future analyses focusing on J1721$+$8842 will likely explore using this system for stringent constraints on cosmological models, further exploring its dual role as a compound lens for its potential in elucidating the nature of dark energy and matter distribution across the universe.