The Cosmic Smile of the Cheshire Cat

In the center of this image, taken with the NASA/ESA Hubble Space Telescope, is the galaxy cluster SDSS J1038+4849 that looks like it is smiling.
NASA/ESA/Acknowledgement: Judy Schmidt

Astronomers have long marveled at the strange and beautiful phenomena that unfold across the cosmos, and the “Cheshire Cat” group of galaxies is no exception. Named for its resemblance to the iconic grinning feline from Alice in Wonderland, this galactic ensemble showcases the bizarre and wondrous effects of gravitational lensing — a striking consequence of Einstein’s general theory of relativity. In this system, the light from distant background galaxies is stretched and curved into arcs and rings by the immense gravity of a foreground galaxy cluster, most of whose mass is composed of invisible dark matter detectable only through its gravitational effects.

The grinning face is anchored by two brilliant “eyes” — the massive elliptical galaxies SDSS J103843.58+484917.7 and SDSS J103842.68+484920.2 — located at redshifts z = 0.426 and z = 0.433, respectively. Between them lies a smaller “nose” galaxy, and surrounding them are multiple arc-like features: distorted images of background galaxies, some at redshifts as high as z = 2.78 — representing objects seen as they were billions of years ago. These arcs are magnified by the gravitational lensing effect, allowing astronomers to peer further into the universe than otherwise possible. These features were captured in breathtaking optical detail by NASA’s Hubble Space Telescope. Complementary X-ray data from NASA’s Chandra X-ray Observatory reveal glowing, million-degree gas and an active supermassive black hole within one of the “eyes” — signs of a high-speed galactic collision underway.

This composite image shows a galaxy group nicknamed the ‘Cheshire Cat’ for its resemblance to a grinning feline face. Optical data from the Hubble Space Telescope reveal the ‘eyes’ and ‘smile,’ while X-ray observations from NASA’s Chandra X-ray Observatory (shown in purple) highlight hot gas heated by merging galaxies.

What makes the Cheshire Cat group especially compelling is that it may represent a fossil group progenitor — a rare, transitional phase in galaxy evolution where smaller galaxy groups merge into one massive elliptical galaxy surrounded by fainter companions (Irwin et al., 2015). While fossil groups are believed to be a common outcome of galaxy mergers, catching one in the act is unusual. The Cheshire Cat offers a unique opportunity to study this process in real time, shedding light on the dynamic forces that shape galaxy clusters over cosmic time.

The precise alignment of galaxies in this system also produces a rare and beautiful phenomenon: an Einstein Ring — a nearly perfect circle of light formed when the background galaxy, lensing mass, and observer are aligned just right. These naturally occurring lenses serve as powerful tools in astronomy, enabling the study of galaxies otherwise too faint or distant to observe. In the case of the Cheshire Cat, Einstein’s century-old theory continues to help us see further, deeper, and with more wonder than ever before.

Catalog: GLC-12
Paper: The Cheshire Cat Gravitational Lens: The Formation of a Massive Fossil Group

Confirmation of Three Gravitationally Lensed Quasars

Researchers confirmed the existence of three new gravitationally lensed quasars using data from the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP). These lensed quasars are interesting phenomena where the immense gravitational pull of a foreground galaxy bends and magnifies the light of the more distant quasar, creating multiple images of it. This discovery includes systems with separations ranging from 0.85 to 2.26 arcseconds, and redshifts indicating significant distances. The identification and analysis of these systems were enhanced by advanced imaging techniques and follow-up spectroscopy, which not only confirmed the lensing effect but also provided detailed characterizations of the galaxies and quasars involved. These findings add valuable examples to the catalog of known lensed quasars, which are key tools for probing dark matter, studying quasar properties, and refining our measurements of the universe’s expansion rate. This study involves combining deep, wide-field imaging with follow-up observations to confirm they are in fact gravitational lenses.

Reference paper: https://doi.org/10.1093/mnras/stab145