The Cartwheel Galaxy, also known as ESO 350-40, is a troubled-looking piece of cosmic real estate. If you look at it now, especially in JWST’s latest view, you’d never know it was once a magnificent spiral galaxy. That was before he was involved in a head-on collision with a passenger. The encounter happened somewhere around 200-300 million years ago. Essentially, the smaller galaxy “looked with its eyes” at the Wheel, right through its heart. A shock wave went through the system, changing everything. The consequence is what we see in this latest image from JWST.
Exploring the Wheel
When galaxies collide, interesting things happen. The gravity of two such massive objects colliding (or even passing close together) distorts the shape of each galaxy. Shock waves pass through the participants, triggering bursts of star formation. In extreme circumstances, as we see here, the result is a rare ring galaxy.
The Cartheel galaxy may seem amazingly strange (and it is). But it’s also a great example of a galactic wreckage that will eventually mend itself. In a few million years, this scene may look strikingly different. This is one of the reasons astronomers are so interested in it. It’s not often they see the evolution of a clash like this. The last view of it is worth digging into just to look at the incredible detail provided by JWST. There is not only the main Wheel, but also other companion galaxies. (A galaxy that passed through the main is not in this view.) More on all of these in a minute.
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The apparent debris from the collision consists of two glowing rings, an inner and an outer. The inner ring hosts a bright core that is home to a supermassive black hole. This is surrounded by a smaller ring of gas and hot dust. Then there is the outer ring. It actually expanded so much after the collision that it is larger than our own Milky Way galaxy. It buzzes with star-forming regions driven by shock waves from the collision and the ring’s expansion into surrounding regions of gas and dust.
Connecting the two main rings is a set of ghostly looking spokes radiating from the core. These are probably the ancient spiral arms from the original galaxy undergoing a reforming process. They are also alive with star birth nurseries. Bluish regions are young stars formed as a result of the collision.
JWST senses dust
Now there’s a lot about the Wheel that you don’t see if you look at it in visible light. But swing an infrared-sensitive telescope at it and you’ll find that the Wheel is a very dusty place. Clouds of dust obscure the view of some parts of the galaxy. However, all is not lost. Some wavelengths of infrared rays penetrate dust. In addition, the dust emits heat that is “visible” in the infrared. This is where JWST’s Near Infrared Camera (NIRCam) comes to the rescue. In a sense, he can “see” right through the dust. It can also detect clouds of dust scattered around the Wheel. For example, this is how we can see so many star-forming regions in great detail.
The mid-infrared light captured by MIRI reveals chemical details about the dusty regions in the Wheel and the young stars it contains. The young stars, many of which are present in the lower right corner of the outer ring, energize the surrounding hydrocarbon dust, causing it to glow orange. On the other hand, the well-defined dust between the core and the outer ring forms the “spokes” that inspired the galaxy’s name. This dust powers the supermassive black hole at the center of the galaxy.
To get into the details of the dust that permeates this galaxy, JWST’s Mid-Infrared Instrument (MIRI) essentially did a little chemical analysis. It turns out that the areas in the spokes are rich in hydrocarbons and other compounds, as well as silicon dust. MIRI also produces information on the distances of background galaxies. The nearest galaxies are in blue and the most distant ones in green and red. The different colors are due to bright emissions from dust that is redshifted by the expansion of the universe.
Placing the scene on the cart wheel
The wheel does not travel alone. It is part of a wheel group of four galaxies located about 500 million light-years away. The companion galaxies are much smaller and are all physically connected to the Wheel. There is a small, blue Magellanic Cloud-type galaxy called G1. Nearby is a small yellow compact spiral called G2. These two smaller ones show a lot of star formation. The lower galaxy also glows due to the presence of a supermassive black hole. The fourth member (not visible in the JWST image) is a distant spiral called G3. There is a tidal tail extending from it back to the Wheel. It could be the galaxy that went through the Wheel and caused all the chaos.
When astronomers collected all the imaging data from JWST, they saw that this scene was just a snapshot in time. The wheel changes, expands and reforms. What will happen to him when he changes? The wheel-like structure is likely to break up as the gas and dust fall back to the center. If the other companion galaxies do not intervene, then perhaps in a few hundred million years the Cartwheel will again be a beautiful spiral.
Checking Cartwheel before JWST
This isn’t the first time a space observatory has looked at the Cartwheel cluster. The Hubble Space Telescope imaged it in 1995 using the newly installed Wide-Field Planetary Camera 2 (WFPC2) instrument. For the first time, such a high-resolution image was taken of the collision site. XMM-Newton also studied this scene, and it was detected and mapped in radio frequencies by the Paul Wild Observatory at Narrabri in Australia.
The wheel is part of a group of four galaxies, as shown in this collage of images from ground-based optical and radio telescopes and the Hubble Space Telescope’s Wide Field and Planetary Camera 2. Kurt Strack and Philip Appleton (Iowa State University), Kirk Bourne (Hughes STX Corporation) and Ray Lucas (Space Telescope Science Institute), Jim Higdon (Australian National Telescope Centre’s Paul Wild Observatory), Victor Blanco (Cerro Tololo)
In 2006, NASA revealed a multiwavelength image of the Cartwheel Group. It was created using data from the Hubble Space Telescope, the Galaxy Evolution Explorer (in ultraviolet light), the Spitzer Space Telescope (in infrared light) and the Chandra X-ray Observatory. In particular, the Chandra data reveal a number of bright X-ray sources in the Wheel, possibly indicating the presence of stellar black holes. This is not surprising in a region that is alive with the formation of massive stars that die quickly as supernovae.
Cartwheel Galaxy in Visible, X-Ray, and Ultraviolet Light. Courtesy of NASA/ESA/STSci, Chandra X-ray Observatory, GALEX.
Cartwheel is likely to be enlisted for more research with both ground-based and ground-based observatories, and JWST in particular. It is a dynamic scene of galaxy evolution that astronomers can use to learn more about the physics and astrophysics of galactic mergers and encounters.
For more information
Region Galaxy Wheel
Hubble’s wheel
Webb captures stellar gymnastics in the Cartheel galaxy
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