Object of the Week, March 20, 2022: NGC 3646, a Ring Galaxy in Leo

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].

Attachment 4634
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:
Attachment 4635


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.
Attachment 4636

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"

Two observations in my log, most recently in april 2015 under dark Alpine skies. 14" SCT @ 168x / 29':

Quite a faint NE-SW elongated glow with a bright nucleus that is visible without AV. With AV subtly brighter in a very small core that is slightly elongated ENE to WSW and that appears to be just SE of the middle. On the northern side the halo of the galaxy is slightly larger but fainter. With AV faint structure is visible and a darker region can be discerned NW of the middle. A faint but beautiful galaxy with details.
To the NW is a mag. 13 star, 1/4 FoV to the ENE is the galaxy NGC3649.

Rated it 7/10.

In 2009 I observed it under Dutch skies in a 12" SCT, but then and there it was no more than a NE-SW elongated patch that made for a tough observation without use of averted vision.

Attachment 4637 Attachment 4638 Attachment 4639

Click here to download the observing guide.

I've also observed it a couple of times in Jimi's scope (May '12 and Apr '13) --

This showpiece spiral appeared bright, large, oval ~2:1 SW-NE, sharply concentrated with a very bright core. The visual treat was a prominent lens or eye-shaped ring surrounding the core! The ring was slightly brighter in an arc along the north side. At the west edge of this arc was a very faint quasi-stellar knot (SDSS J112141.34+201039.0, V = 17.2). Another section of the ring that stands out is along the southwest edge, with a brighter linear piece about 40" long that has several SDSS designations. The interior of the ring is fairly dark near this section as well as other sections, providing a good contrast between the core and ring. A very small, weak brightening was also noted on the northeast end of the ring.

Sad to say I have seen this twice but have only noted its obliqueness & low surface brightness. The last time was a year ago under 20.6 skies at 337x and a 22". Worth another shot at it if the clouds will ever disperse.

Spectacular observing notes through the 48-inch Akarsh.

My sketch through the 16-inch was less detailed but fascinating enough. I have to revisit it with the 27-inch.

sketch: 16", 257x, NELM 6m5+, Seeing II
Attachment 4646
home

Uwe, your 16” sketch is absolutely fantastic for a 16”, as has come to be expected from you. Looking forward to your 27” observations.

I wanted to add a note. I re-observed this object through my 18” from Bortle 3 skies near Big Pine, California. This was at 5000ft elevation, but there was significant light pollution in one direction from the town of Big Pine including a set of very bright lights that illuminated the landscape and cast diffuse shadows despite being miles away. The other side of the sky was very dark, corresponding to the region of the famous Grand View campground observing site frequented by Steve Gottlieb and others. There were passing clouds of varying altitude all night. Seeing was not very good.

Surprisingly, despite the below average seeing I was able to get a great view of this galaxy. I’m yet to see it under truly good conditions, I suspect this is a showpiece hiding quietly in Jimi’s rings list. Anyway here are my notes:
Attachment 4653

Hi everyone,

2 weeks ago I observed NGC 3646 with my 20" Dobsonian and 256x and saw a stellar nucleus surrounded by what looked like an inner ring structure and an outer ring.

Clear skies

Robin