Leo
Galaxy interaction tidal tail
Magnitude: faint
RA: 11 21 03
Dec: +13 34 56

NGC3628Final_northup_small_redlined.jpg
Image by Mark Hanson, https://www.hansonastronomy.com/ngc-3628-mosaic


NGC 3628 is a well know edge-on galaxy in Leo, and is the northern most member of the Leo Triplet with M65 and M66 to its south. 3628 has been covered in previous OOTW’s, but not its tidal tail, which is our focus for this week.

3628’s tail is the result of its close encounter with M66 about 800,000 years ago when they came within about 82,000 light years of each other. The complete tidal tail extends about four times further to the east than shown in my sketch, and includes the Leo-TDG, a rare Tidal Dwarf Galaxy. More about that in a moment.

I’ve seen only the brightest portion of 3628’s tidal tail once with my 28-inch scope, and that was under good, but not great observing conditions:

“An excellent view, especially considering that it’s 2/3 of the way to the western horizon. The dark lane is prominent, and long, with the tidal tail readily seen. Low contrast except for the brightest part of the (core’s) central area, which also delineates the sharpest edge of the dark lane. 253x, 21.47 SQM.”

NGC3628_sketch_2_smaller.jpg NGC3628_4_invert_FinalBrighter_small.jpg

My sketch is a good approximation of what I saw, and I also remember my surprise at how easily visible the tidal tail and other faint features of 3628 were that night. On the other hand, I tried to observe it during the March 2021 new moon period with the same scope and a 22.00 SQM sky, but saw absolutely no trace of the tidal tail! After a bit of head scratching, I noticed that it was in the outskirts of a very bright Gegenschein, but I was still disappointed that I couldn’t detect even a hint of its glow.

So, is it surprisingly easy or surprisingly difficult to see? For now, the answer seems to be it’s both, and is probably a function of how sky darkness, transparency, the Gegenschein, and the observer interact. I’ll try again next month if the weather permits.

The Leo-TDG is located toward the much fainter eastern end of 3628’s tidal tail. It seems to have actually formed out of the tidal plume itself, making it a relatively rare object. Its location is easy to find because it’s just a few arc minutes north of the 15th magnitude background galaxy, IC 2787 – it’s between the two red lines in the Mark Hanson photo above. On the same night last month that I couldn’t see the tidal tail, I could barely see IC 2787, which may mean the bright Gegenschein really was reducing contrast in this area.

Jimi has seen the Leo-TDG with his 48-inch, and it seems possible other observers at a high-altitude site might have a chance to see it as well. I plan to keep trying, and hope many of you will too.

Give it a go and let us know!