One Small Order of Ring Galaxies, Please

"One Ring to rule them all." Join me as we explore a unique class of galaxies forged in the chaos of collision. 

As Orion tilts to the west, the approach of spring offers a wonderful diversity of galaxies for telescopic viewing. Spirals. Ellipticals. Irregulars. Here and there among them are representatives of a rare breed — the ring galaxies. Shaped like onion rings, their formation is rooted in catastrophe.

Symmetry from chaos

Two of the most beautiful and famous ring galaxies: Hoag's Object in Serpens (left) and the Cartwheel Galaxy in Sculptor. Next to the Cartwheel are two of the three galaxies suspected to have passed head-on through it approximately 100 million years ago.

Ring galaxies form when a small neighbor galaxy plunges through the disk of a larger spiral galaxy. The collision rarely results in any stars smacking into one another, but the gravitational shock acts like a tsunami that alters the orbits of stars and gas clouds, pushing them outward in excess of 150,000 mph (241,000 km/h). When the gas clouds collide, they compress and transform into a expanding ring of new star formation — think of an enormous wave crashing ashore in a frothy chaos of white foam.

Simulation of the collision between an elliptical and spiral galaxy to form the recently discovered "Auriga's Wheel" ring galaxy.

A typical ring galaxy still hangs on to its dense nucleus, but the once-majestic spiral arms have been recast as a striking stellar halo sparkling with young, massive blue stars. In Hubble Space Telescope images, ring galaxies appear temptingly bright. Unfortunately, most are faint and rather small, but not so much so that we can't track down at least a few with telescopes with 10-inches (25 cm) of aperture or more.

Intruder makes a mess of things

The peculiar Arp 143 pair featuring the bright, compact galaxy NGC 2444 (top) and an early-formation ring galaxy NGC 2445, are located about 180 million light-years from Earth. North is up.
Stan Moore

We'll check out three of them, Arp 143,  Arp 141, and NGC 2793, all well-placed for viewing on March evenings. It's no surprise that two of them made it into astronomer Halton Arp's fascinating Atlas of Peculiar Galaxies, which includes numerous interacting pairs.

Our first stop will be Arp 143 in Lynx located high in the southern sky above Castor and Pollux at nightfall. Known also as NGC 2445 and NGC 2444, each is listed at magnitude 14, but they're clearly brighter than this. I would estimate 13. Through my 15-inch (37-cm) Obsession scope, both are immediately visible at low power as two separate objects.

Names and location, please

Each HII region within NGC 2445 has its own designation. Use this photo to help you spot them. 2445 measures ~1.5′ across; 2444 is just ~45″. Locator map at right with stars to magnitude +8 with Arp 143's celestial coordinates given. North is up. Click for a large version. 
DSS-II, Stellarium

The intruder, NGC 2444, is compact with a bright nucleus; its ring galaxy neighbor is one of the oddest galaxies I've ever set eyes upon. The nucleus appears is offset well to the north end of galactic center. Upping the magnification to 257x I could discern four glowing knots — HII star-formation regions — within the galaxy's amorphous, hazy outline. The knot south of the nucleus was easy enough, but the other three required averted vision and concentration to tease out.

At first glance the disturbed galaxy's clumpy appearance resembles a raisin pudding far more than a wheel. Where's the ring, you ask? Give it some time, say astronomers who've studied it closely. What we have here is a nascent ring on the cusp of a massive wave of star formation. We're witnessing the chaos of collision by neighboring NGC 2444, the distorted shape of which tells us it did not escape unscathed from its dashing pass.

Like pulling taffy with gravity

Arp 141 or UGC 3730 is a pair of interacting galaxies where the elliptical member (top) has torn through its spiral companion leaving a distorted ring in its wake. A drawing at right conveys the impression through a 16-inch telescope. The pair is about 120 million light-years away. North is up.
DSS-II (left), Uwe Glahn

Once you've cut your teeth on Arp 143, you're ready for UGC 3730, or Arp 141, a 13th-magnitude ring galaxy in Camelopardalis located about 10° northwest of Ursa Major's bright M81-M82 galaxies. Arp 141's core is a strange beast with an obscured nucleus and tortured ring. Like Arp 141, it's faintly but immediately apparent as a fuzzy patch about 2.5′ long and extended north-south at 64x. Closer inspection at 257x shows the elliptical member as a "fuzzy star".

Hop, skip and a jump to Arp 141

Arp 141 locator map with a suggested star hopping route to find your way there. Stars to magnitude +9. North is down. Click to enlarge. 

With averted vision I could clearly see the fainter ring galaxy and a bright patch (HII region?) where they touch. The ring shape itself was visible only fleetingly. I bet an 18-incher would reveal it.

Our last stop is NGC 2793, conveniently located less than 1° west of 3rd-magnitude Alpha Lyncis. This little button of a galaxy measures just 1′ across and glows at a modest magnitude 13. As with the others, it's easy to spot but takes concentration to ferret out the details that reveal its nature. Many ring galaxies aren't that difficult to find, but seeing the contrast between the ring and the rest of the galaxy can be challenging.

Another ringer in Lynx

The minute-wide ring galaxy NGC 2793 in Lynx. A ring of new star formation appears around the galaxy's northern edge. Photo from Astronomical Journal, "Stellar disks of Collisional Ring Galaxies I. New multiband images, Radial intensity and color profiles, and confrontation with N-body simulations," (Sept 2008). North is up.

With high magnification I can see a brighter, non-stellar nucleus off to the galaxy's east side; honing in by alternating averted with direct vision, I glimpsed several tiny "sparkles" or knots of what are doubtless new star clusters within its southeastern border. Overall, the galaxy's eastern third appeared brighter and denser than the hollowed-out portion seen in the photo above.

This trio of rings is seasonal and samples only a small number of an already small class of galaxies. I encourage you to explore them as well as others. A simple click will download a .pdf with photos of 40 others in this illustrated ring galaxy catalog, enough to guarantee many nights of delicious exploration. You may also find this Color Atlas of Ring Galaxies helpful.

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Bob King

About Bob King

Amateur astronomer since childhood and long-time member of the American Association of Variable Star Observers (AAVSO), Bob King also teaches community education astronomy and writes the blog Astro Bob. The universe invites us on an adventure every single night. All we need do is look up. My book "Night Sky with the Naked Eye" was just published and is now available on Amazon and BN. It covers all the great things you can see at night with just your eyeballs. No equipment needed!

4 thoughts on “One Small Order of Ring Galaxies, Please

  1. Aqua4U

    Hi Bob! Thanks for the writing this great article… I recently finished building a f3.4, 12 1/2″ Newtonian and have only just begun using it for galaxy surveys. The above mentioned galaxies are now in my ‘to view’ list! I have been very impressed with my telescope’s ability to pick out dim objects and have recently begun surveying the Virgo galaxy cluster and the environs around Leo and Ursula Major. My goal is to become familiar enough with them to know a nova or supernova when I see one! I have ‘earmarked’ the galaxies you mentioned above and will get to them on the next clear/moonless night!

    I don’t know about other amateur astronomers, but I think locating dim galaxies is the greatest part of the fun! The viewing is cool enough, but I most enjoy FINDING them in the first place! That “AH-HA!” moment! My scope is not computerized or hooked up to GPS.. but I DO get around! HO!

    1. Bob KingBob King Post author

      Hi Aqua4U,
      Nice to see you here. Didn’t know you were a galaxy-crazed amateur. Thanks for your kind words about the article. Another group of galaxies I’ve spent lots of time hunting are those with visible spiral arms (in my 15-inch). At first I thought it might be only a couple dozen, but it turns out there are lots more. I also enjoy clusters. Be sure to check out Abell 1367 in Leo and the Coma Cluster. The latter will blow you away.

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