NGC 3646 = UGC 6376 = PGC 34836 = MCG +03-29-037
Ring Galaxy in Leo
RA: 11:21:43
Dec: +20:10:10
Size: 3.9' x 2.2'
Mag (V): 11.13

There aren't many bright ring galaxies around where the ring structure yields in a moderate-sized Dobsonian; NGC 3646 is one of the few I know of. I came across this object on Jimi Lowrey's ring galaxy list.

Background:

NGC 3646 was discovered by William Herschel in 1784. Since then, it was re-observed by John Herschel in 1827 who considered it "considerably faint" [1]. Distance estimates based on the Tully-Fisher method place the galaxy at about 60Mpc from us.

In 1961, the Burbidges and K H Prendergast studied this galaxy in a paper entitled "Motions in NGC 3646, A strange spiral galaxy". They note that it is unusually large measuring more than 50 kpc. Using the 82 inch telescope at McDonald Observatory that is well-known in this group, they obtained spectroscopic measurements of the inner and outer parts of the galaxy, to study the rotation curve of the galaxy. They observed that the rotation curves of the inner region and the outer ring / arms are notably different, leading them to conclude that NGC 3646 is not a normal galaxy in equilibrium, but rather the ring is in a state of rapid dynamical evolution -- it could be that the ring is a separate irregular companion system forming in the vicinity of the core, or it could be a structure that has become unstable after having been originally gravitationally bound to the main galaxy. They suggest that perhaps the ring does not even lie in the same plane as the more regular inner structure [2].

Burbidge_NGC3646.png
Image from the 82" (now called the Otto Struve telescope) at McDonald Observatory [3]

NED has several notes on this galaxy, one coming from the Carnegie Atlas of Galaxies: They point out that the morphology is indeed peculiar, as if it were the result of an encounter. Given that nearby NGC 3649 does not appear distorted, they rule out the possibility of interaction with NGC 3649 as the cause of the peculiarities of NGC 3646. They estimate the outer ring of NGC 3646 to be 107kpc in diameter, abnormally large (I wonder how they got to this estimate). They go on to postulate: "The fact that the inner image of NGC 3646 has a normal morphology and is of normal size suggests that the ring is a result of a dynamical process such as the dropping of one galaxy through another, as postulated by Theys and Spiegel (1976, 1977) and by Lynds and Toomre (1976) in other ring galaxies." Another note in NED, from a 2002 IR + optical morphological survey, opines that the spiral pattern is very odd, and notes "The referee suggests, and we agree, that this may be an example of a galaxy with visible leading and trailing arms."

Late amateur astrophotographer Rick Johnson produced a beautiful image of this galaxy:
NGC3646L4X10RGB2X10X3CROP150.JPG


Visual observation:

I have only had one opportunity to observe this galaxy with my 18" f/4.5 dob, and that was earlier this year at Lake Sonoma on Jan 25 2022, just as the moon was rising. I was pleasantly surprised by the amount of the ring I was able to see despite the sky brightness (~21.3 MPSAS zenith), poor seeing, dew/moisture, lunar interference, high airmass (I observed it well before transit), and observing through the substantial light dome of Santa Rosa, CA. Using a 10mm Delos, my notes read "The brightest and easiest part of the ring is the SW side. Occasionally, the ring flashes into view in the NW too. On one occasion the entire western ring flashed in. Nothing much was seen on the eastern side." I was hoping to get another look at this object in February, but I did not have good conditions. I would love to try this galaxy from a darker site.

However, I was looking up my logs and realized I had already seen it through the Leviathan of Fort Davis. This is from the night of April 28th 2019, through Jimi's 48". Bob Douglas from Mill Valley, CA was also present. Below is the relevant snippet from my logbook.
NGC3646_48inch_Log.jpg

I would love to hear how much of the ring of this peculiar galaxy you caught in your telescope. Being on the eastern side of Leo, this galaxy will be well-placed for a couple more months now, so why not
GIVE IT A GO AND LET US KNOW!

References
[1] Courtney Seligman on NGC 3646
[2] Burbidge E. M. "The Measurement of Rotation in Spiral, Irregular, and S0 Galaxies"
[3] Burbidge, E. M., Burbidge, G. R., & Hoyle, F. "Condensations in the Intergalactic Medium"