The symmetrical, onion-like layers of shell galaxy NGC 3923 are showcased in this galaxy-rich image taken by the US Department of Energy’s (DOE) Dark Energy Camera mounted on the National Science Foundation’s (NSF) Víctor M. Blanco 4-meter Telescope at Cerro Tololo Inter-American Observatory in Chile, a Program of NSF’s NOIRLab. A nearby, massive galaxy cluster is also captured exhibiting the phenomenon known as gravitational lensing.
Much like humans, galaxies are shaped by the environment in which they form. While no two are exactly alike, they can be divided into three main types: spiral, elliptical and irregular. Of these types, elliptical galaxies are the largest and are thought to evolve out of galactic collisions and mergers between spirals. About one-tenth of elliptical galaxies are classified as shell galaxies, characterized by the concentric shells that make up their galactic halos.
A striking example of this type of galaxy is NGC 3923, with its onion-like layers beautifully showcased in this image taken with the DOE-built Dark Energy Camera (DECam) on the Víctor M. Blanco 4-meter Telescope at Cerro Tololo Inter-American Observatory (CTIO), a Program of NSF’s NOIRLab. Located in the constellation Hydra
(the Serpent), NGC 3923 is about 70 million light-years away from Earth
and 150,000 light-years across, making it about 50% larger than our Milky Way.
As is thought to be the story of all shell galaxies, the layered
structure seen in NGC 3923 likely developed as a consequence of a merger
with another, smaller spiral galaxy in the past. As they merged, the
larger galaxy’s gravitational field slowly peeled off stars from the
smaller galaxy’s disk. Those stars began to gradually mix with the
larger galaxy’s outer halo, forming concentric bands, or shells. A
simple analogy is adding a drop of food color to a bowl of batter that
you’re slowly stirring. The drop gets stretched out in a spiral that
remains visible for a long time before completely mixing.
The shells of NGC 3923 make the galaxy quite exceptional. Not only
does it have the largest known shell of all observed shell galaxies, but
it also has the 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 outermost layers having been created first, followed by
the innermost layers 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
interestingly 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 evolutionary
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 thousands of galaxies and countless foreground 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 Theory of Relativity, which states that a massive object, such as a cluster of galaxies, can
warp spacetime. Narrow arc-like shapes located around clusters of galaxies were first found in 1989 by NOIRLab (then NOAO) astronomer Roger Lynds and Stanford colleague Vahé Petrosian using the Nicholas U. Mayall 4-meter Telescope at Kitt Peak National Observatory, 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 substance 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.
More information
[1] Strong gravitational lenses are those where the effect is easily visible in the form of arcs or Einstein Rings.
Links
- Photos of the Víctor M. Blanco 4-meter Telescope
- Videos of the Víctor M. Blanco 4-meter Telescope
- Photos of DECam
Contacts:
Josie Fenske
Communications
NSF’s NOIRLab
Email: josie.fenske@noirlab.edu
