iMAGE

The COSMOS-Web ring

The COSMOS-Web ring: in-depth characterization of an Einstein ring lensing system at z~2, Mercier et al., A&A, submitted

We recently reported the discovery and an in-depth characterization of the most distant and absolutely gorgeous Einstein ring that we found in the COSMOS-Web survey. We call this the COSMOS-Web ring. We carried out an in-depth analysis that involves photometry in over 25 bands from space and ground observatories, SED fitting, and detailed lens modeling with multiple codes.



We find this remarkable system to be composed of a massive, quiescent elliptical galaxy at z=2 that acts as a lens to a background galaxy that is star-forming and potentially dust-obscured at z~5.5. This forms a perfect ring that shows two red clumps and diffuse blue regions.
Our results show that the total mass of the lens responsible for creating the ring is consistently explained by the contribution of stars and dark matter in the halo at z=2. This discovery will allow us to study the substructure of dark matter halos at z=2, for the first time.
Another exciting aspect is that this lens opens a window into the sub-kiloparsec structure of a star-forming and potentially dust-obscured source at z~5.5. Our lens modeling shows that this source is made of at least two components, and can even be a merger.