The Bullseye galaxy: a ring galaxy with a resonant twist

Across the Universe, galaxies come in four major types.

At a level of 36x zoom, Euclid’s first mosaic contains the distant but abundant galaxy cluster Abell 3381, which features a line of bright galaxies similar to Markarian’s chain in the Virgo cluster. Spirals and ellipticals are the most common type of galaxy, with many others that are interacting forming transient, irregular, peculiar shapes. However, rarely galaxies will also appear with a ring-like configuration, such as the one at the top, about 25% of the way from the right image edge.

Spiral and ellipticals describe most common, normal galaxies.

The lensed galaxies shown at the center of this image appear as though they’re being stretched and pulled apart, similar to the chromosome pairs that align and separate during cellular mitosis. However, this is only an optical illusion caused by the gravitational lensing effects of the massive foreground galaxy cluster: El Gordo. These are simply background galaxies whose light is distorted by the gravitational lens. Both spirals and ellipticals are richly represented within (and behind) most galaxy clusters.

Many small or interacting galaxies become irregularly shaped.

The low-mass, dusty, irregular galaxy NGC 3077 is actively forming new stars, has a very blue center, and has a hydrogen gas bridge connecting it to the nearby, more massive M81. As one of 34 galaxies in the M81 Group, it’s an example of the most common type of galaxy in the Universe: much smaller and lower in mass, but far more numerous, than galaxies like our Milky Way. The young stars within it have formed from gas reservoirs still present within this galaxy, indicating an “alive” galaxy.

But rarely, a ring shape emerges.

This X-ray/optical composite image shows the ring galaxy AM 0644-741 along with a wide-field view of its surroundings. Below and to the left of this ring galaxy is a gas-poor ellipsoidal galaxy that may have punched through the ringed galaxy a few hundred million years earlier. The subsequent formation and evolution of a ring of new stars would be expected from the propagation of gas away from the center, like ripples in a pond.

Only 1-in-10,000 galaxies are rings: defined by circular collections of stars beyond the main galactic body.

An example of a very rare ring galaxy, NGC 1291, showcases an outer galaxy that’s rich in gas and forming new stars surrounding an old, quiet center that is virtually gas-free and has scant evidence of new star formation. Both gas-rich and gas-poor galaxies are found throughout the Universe, and Spitzer’s infrared eyes are ultra-sensitive to them.

Rings appear in many stages of formation, with one leading theory for their creation.

This Hubble Space Telescope image of Mayall’s object, also known as Arp 148, shows two galaxies in the process of collision. As one galaxy punches through the center of the other, stars form in both galaxies, but the one that got “punched” is having its gas propagate outward in waves, triggering new star formation on its way toward creating an overall ring-like shape. While they interact and merge, galaxies can take on many fascinating and peculiar shapes.

When a fast-moving galaxy “punches through” another’s center, ripples of matter propagate outward.

Known colloquially as a “perfect 10,” Arp 147 features two interacting galaxies where each one features a ring, almost certainly as the aftermath of a center-on-center collision between the two precursors. The dusty reddish knot at the lower left of the blue ring probably marks the location of the original nucleus of the galaxy that was hit.

They trigger new star-formation, while removing gas from the galaxy’s center.

The Hubble vs. JWST views of the Cartwheel galaxy (and its surroundings) showcase a spectacular difference: the 1995 vs. 2022 images reveal how foreground objects, like interloping stars from within our own galaxy, have moved relative to the background features in these external galaxies over the past 27 years. In addition the JWST data reveals features that Hubble could never see. The bright, star-forming galaxy to the left of the Cartwheel is thought to have punched through it, creating the Cartwheel’s ring.

At late times, a stable, settled-down ring structure emerges.

Discovered in 1950, this galaxy is known as Hoag’s object, and is the first known instance of a ring galaxy. It’s highly unusual, with an elliptical, very red center, a large gap between the nucleus and the outer ring, and the blue, young stars in the outer ring have no identifiable trigger leading to their formation. 75 years after its discovery, we’ve put together a general picture of how ring galaxies form, but Hoag’s object still poses unanswered questions, as it is a late-stage ring whose creator can no longer be identified.

But the newly discovered Bullseye galaxy, LEDA 1313424, represents a “ring” that’s still forming.

This Hubble image of the Bullseye galaxy, LEDA 1313424, showcases many of its low surface-brightness structures: red inner rings, where gas has been evacuated, blue, star-rich outermore rings, and a large, faded, still-expanding ring. Overall, the galaxy, because of its distended structure, spans 250,000 light-years: more than double the diameter of our Milky Way.

A small, dwarf galaxy “punched through” the larger galaxy’s center, creating gaseous ripples.

This close-up view of the blue, small, irregular dwarf galaxy that “punched through” the main galaxy is shown, along with the nearest outer spiral arm from the main Bullseye galaxy. Spectroscopic data, including data acquired with the Keck Cosmic Web Imager, confirms that this dwarf galaxy created the radiating ripples in the Bullseye.

Many (at least nine) independent rings have been serially identified, radiating outward.

This version of the Hubble image for the Bullseye galaxy, known as an unsharp-mask version of the image, shows the inner arc-minute of the galaxy, with eight visually identifiable rings traced out with the red ellipses shown here. A ninth ring, found with the aid of computational resources, was later identified as well.

The small galaxy is blue, indicating newly-forming stars from the collision.

This blue-colored patch of stars may seem unimportant, but it’s this irregular dwarf galaxy that recently “punched through” the center of the Bullseye galaxy, creating its large array of distended spiral arms and the series of ellipsoidal rings identified within it. Galaxy collisions, and their resultant ripples, are thought to be the impetus for the creation of ring galaxies throughout the cosmos.

The main galaxy, too, is littered with streams of young blue stars.

This close-up view of the Bullseye galaxy’s main, central components clearly shows the concentric rings, but also showcases the color difference between the inner, now-gas-poor region, where young blue stars are rare, and the outer, gas-rich arms that are littered with new, blue stars.

The central pile-up of rings can be teased apart owing to Hubble’s incredible optical views.

According to the theory for how rings should form, the ratio of successive rings should be predictable, where choosing “ring 3” as the outermost narrow star-forming ring best fits the data (in black). The predictions, as shown, match the data points very well, providing our first confirmation for the resonant ring theory that should lead to the creation of ring galaxies in general.

These rings obey a resonant pattern: confirming a longstanding “resonant ring” prediction.

In addition to the Bullseye galaxy and the dwarf galaxy that punched through it, there’s also a patch of faint, blue emission seen on the opposite side of the Bullseye galaxy. Follow-up observations from the Keck telescope confirmed that this emission is at the same redshift as the dwarf and main galaxies, showing that it’s part of the same smashed-up system.

With NASA’s Nancy Roman telescope coming, the Bullseye galaxy should soon be joined by many similar discoveries.

This full-color image of the Bullseye galaxy has nine rings to it: six more than any other known galaxy. The blue dwarf galaxy that smashed through the center of the Bullseye is shown at left, while extended emission, and grand, distended spiral arms, can also be seen on the galactic outskirts.

Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words.