(Note: I traded spots with Uwe Glahn, so I'm posting this week instead of him)
Data
The Cartwheel is nestled in a compact group designated as SCG 0035-3357:
1. Cartwheel Galaxy = ESO 350-40 = AM 0035-335 = MCG-06-02-022a = VV 784 = PGC 2248
Ring Galaxy in Sculptor
RA: 00 37 41
Dec: -33 42 59
Size: 1.32' × 1'
Mag (B): 19.26 (I don't believe this)
Redshift: ~0.03 (~400 Myr light-travel time)
2. PGC 2249 = [H95] G1
Galaxy in Sculptor
RA: 00 37 44
Dec: -33 42 07
Size: 0.24' × 0.16'
Mag (B): 16.66
3. PGC 2252 = [H95] G2
Galaxy in Sculptor
RA: 00 37 45
Dec: -33 42 20
Size: 0.32' × 0.26'
Mag (B): 15.26
4. 2MASX J00374627-3339515 = [H95] G3
Galaxy in Sculptor
RA: 00 37 46
Dec: -33 39 52
Size: 0.34' x 0.16'
Mag (B): 16.20
History and Science
This week's OOTW needs no introduction to almost any amateur astronomer. Moreover, it made waves on social media recently for being one of the targets of the JWST, and here on Reddit is a nice animation comparing it to the previous HST image. Indeed, it is one of the finest ring galaxies in the sky, accessible in moderately large amateur telescopes. But what is its story?
Cartwheel_JWST.jpg
Cartwheel Galaxy as seen by the James Webb Space Telescope8. Red is from MIRI and shows dust / molecules; NIRCam data in blue, yellow and orange shows young stars.
Cartwheel_HST.jpg
Cartwheel Galaxy as seen by the Hubble Space Telescope.
Cartwheel_DSS2.png
DSS2 Color Image, roughly oriented south-up, with Higdon's designations for the companions
The Cartwheel was discovered by the infamous Fritz Zwicky in 1941 using the 46 cm (18") Schmidt telescope on Mt. Palomar in California, despite its -34° declination. He then thought it to be "one of the most complicated structures awaiting its explanation on the basis of stellar dynamics". However in 1976—1978, Roger Lynds of KPNO and Alar Toomre of MIT provided what Toomre called an "almost embarrassingly simple" explanation1,2. Using simulations, they proposed that rings form as the result of a ripple induced by the passage of a companion galaxy almost through the center of a disk galaxy. They provide an analogy1: Imagine a stray star passing exactly along the axis normal to earth's orbit through the solar system, somewhere close to the sun. The gravity of the stray star would attract the inner planets and bring them closer to the sun temporarily, but after the star recedes away, the centrifugal force will throw the planets back out, to a larger orbit than their initial orbit. The same sort of rebound effect in a disk galaxy could cause such a ring. The ripple in density of material would then trigger star-formation in the gas, with the expanding ring comprised of massive blue stars and H-alpha emission, and with evolving stars in its wake5. Indeed, Higdon4 confirmed this massive star formation, and also found that the galaxy emits 10x more H-alpha compared to the strongest H-alpha emitting normal galaxy in Kennicutt's 1983 list, and that all of it comes from the ring. (I wonder what happens if one puts an OIII filter on the ring, since the H-beta will be red-shifted to OIII wavelength).
Now which galaxy of the group listed above is the collider? This is not clear yet, especially since none of the 3 galaxies seem to be massive enough for the proposed mechanism of ring formation6. Perhaps they lost mass after collision? There is some suspicion that the bullet is [H95] G3, based on a large plume of neutral hydrogen gas (HI) that connects it to the Cartwheel, discovered by J. Higdon using the VLA3. The designations G1 -- G3 for the companions are due to Higdon. I have pasted below his figure of the radio plume contours superposed on the optical image:
Cartwheel_G3_Plume.png
What did the progenitor of the Cartwheel look like? Multiple studies5,6 suggest that it looked like Malin 1 — a spiral galaxy with a large disk that had not undergone star-formation, until the collision occurred. This is supported by the metal-poor nature of the outer regions of the Cartwheel. Higdon5 estimates that the collision occurred ~300 Myr ago, whereas the JWST press release8 states 440 Myr.
Finally, there is the mystery of the spokes of the Cartwheel. This is one of the unique features of the Cartwheel in comparison to other ring galaxies, so perhaps Zwicky was right after all. One hypothesis is that there are also spiral ripples due to asymmetric collision. The more favored hypothesis however, is that it is an effect that results after the collision from internal gravitational instabilities7: The gas in the disk fragments under its own self-gravity seeding the growth of the spokes5.
One wonders what new light the JWST studies of the Cartwheel will shed.
In amateur land
I presume this galaxy was made incredibly popular by its Hubble picture. A funny incident, when I was in high-school at a outreach event in Bangalore, the skies were cloudy so somebody pointed the telescope at a wheel of a nearby car, and called out that they had found the "Car wheel galaxy". That may have been how I first heard of this object, after the joke was explained to me.
Anyway, I thought it was outside of the scope of visual observation, as I did (and still do) about many things. So imagine my excitement when within a couple months of landing in Texas, in my very second observing session, John Tatarchuk shows it to me with his 25". This was in Pontotoc, TX (30°N) on the night of 8th October 2010. I have a rudimentary sketch:
Cartwheel_Sketch.jpg
This experience is somewhat etched in my memory, but so that I don't mess up the details, I ferreted out a couple e-mails from which I quote: "bright part of ring touching the star was clearly visible [continuously]", "full ring visible for short periods of time". "It was a very challenging object. The moment I looked into the eyepiece, I was able to see a large oval-shaped fuzzy blob with some sort of empty spaces inside it. This was a combination of the bright half of the ring, and the other two galaxies [G1 and G2], with my brain interpolating the rest. After some staring, I was able to separate the two galaxies...", "the nucleus [of Cartwheel] would appear clearly if I directed averted vision appropriately. I could not see both the ring and the nucleus together, but I could alternate between seeing the two. For a fraction of a second, the entire ring appeared and vanished! That was amazing!"
I would love to one day see what this looks like in even larger aperture, perhaps in Jimi's 48" scope. Incidentally, the spokes of the Cartwheel are #17 on the famous AINTNO catalog, but I imagine something of them might be visible under good conditions with such large aperture.
Coming back to more simpler apertures, I have tried several times to see something with my 18". In 2015 October, also from Pontotoc, TX, I wrote "Stars there are big fuzzballs and the sky background is bright. Can see nothing more than the combined glow of the Cartwheel complex". More recently, at the 2021 Okie-Tex Star Party (37°N), I wrote "Sensed a large patch in the general area 1--2 times. Not good conditions". However, two nights later, I was able to resolve something: "Two diffuse patches ['A' and 'B'] intermittently appear, both <10% holding. 'B' is fainter. 'A' occasionally resolves into two.". Looking at the star-field and matching it to DSS2, 'B' is the main Cartwheel galaxy (ostensibly its core), and 'A' is the combined light of G1 and G2.
So there we go — at least something can be detected in an 18", even from USA latitudes. Surely, someone must have better results from the southern hemisphere. My striking mistake in all of this is that I missed G3 because I wasn't looking for it — it would be nice to see the likely intruder responsible for the Cartwheel.
Even if all one can see of this object is a fuzzy blob, the allure of this incredible object might make it worth the effort for you, as it did for me. So why not
GIVE IT A GO AND LET US KNOW!
References
[1] Lynds and Toomre 1976, "On the Interpretation of Ring Galaxies: The Binary Ring System II Hz 4"
[2] A. Toomre 1978, "Interacting Systems"
[3] J. Higdon 1996, "Wheels of Fire. II. Neutral Hydrogen in the Cartwheel Ring Galaxy"
[4] J. Higdon 1995, "Wheels of Fire. I. Massive Star Formation in the Cartwheel Ring Galaxy"
[5] C. Horellou and F. Combes 2001, "A Model for the Cartwheel Ring Galaxy"
[6] C. Horellou et. al. 1995, "The CO Emission of Ring Galaxies"
[7] Appleton and Struck-Marcell 1996, "Collisional Ring Galaxies"
[8] JWST Press Release, "Webb Captures Stellar Gymnastics in The Cartwheel Galaxy"
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