Free Astronomy Magazine November-December 2023

26 NOVEMBER-DECEMBER 2023 ASTRO PUBLISHING T he symmetri- cal, onion-like layers of shell galaxy NGC 3923 are showcased in this galaxy-rich image, taken by the Dark Energy Camera. A nearby, massive galaxy cluster is also captured ex- hibiting the phe- nomenon known as gravitational lensing. [DESI Legacy Imaging Surveys/ LBNL/DOE & KPNO/CTIO/NOIR- Lab/NSF/AURA. Image process- ing: T.A. Rector (University of Alaska Anchor- age/NSF’s NOIR- Lab), M. Zamani, R. Colombari & D. de Martin (NSF’s NOIRLab)] largest number of shells and the largest ratio between the radii of the outermost and innermost shells. A 2016 study determined that NGC 3923 could be made up of as many as 42 distinct shells, with the outer- most layers having been created first, followed by the innermost lay- ers as the galaxies’ celestial dance slowed. Another notable characteristic of NGC 3923 is that its shells are much more subtle than those of other shell galaxies. Its shells are also in- terestingly symmetrical, while other shell galaxies are more skewed. These uncommon features are a sublime example of the unique structures that galaxies can embody depending on their specific evolu- tionary conditions. While NGC 3923 is certainly the main attraction in this expansive, 250-megapixel image, the longer one spends perusing the glittering field the more cosmic treasures can be found. Among the thou- sands of galaxies and countless fore- ground Milky Way stars speckling this image are the face-on spiral galaxies LEDA 744285 and ESO 440- 11. And near the top of the image is the extremely large gravitational lens around galaxy cluster PLCK G287.0+32.9. Discussed in scientific journals since the 1930s, gravitational lenses are predicted by Einstein’s General The- ory of Relativity, which states that a massive object, such as a cluster of galaxies, can warp spacetime. Nar- row arc-like shapes located around clusters of galaxies were first found in 1989 by NOIRLab (then NOAO) as- tronomer Roger Lynds and Stanford colleague Vahé Petrosian using the Nicholas U. Mayall 4-meter Tele- scope at Kitt Peak National Observa- tory, a Program of NSF’s NOIRLab. These extragalactic properties were interpreted as the result of strong gravitational lensing from distant galaxies in the background. Indeed, when zoomed into this image, a handful of galaxies can be seen stretched out and distorted under the gravitational influence of dark matter, the mysterious sub- stance found concentrated around clusters of galaxies. Gravitational lenses allow astronomers to explore the most profound questions of our Universe, including the nature of dark matter and the value of the Hubble constant, which defines the expansion of the Universe. !