Object of the Week, June 8, 2025 - NGC 5917 (Arp 254), the Hooked Galaxy

Object of the Week, June 8, 2025 - NGC 5917 (Arp 254), the Hooked Galaxy
=interacting galaxy pair in Libra

NGC 5917
Type: Sb pec
RA: 15 21 33
Dec: -07 22 38
Mag: 13.2 vmag (14.0bmag)
Size: 1.6' x 0.9'

MCG-01-39-003
Type: S0 pec
RA: 15 21 33
Dec: -07 26 52
Mag: 14.7bmag
Size: ~4.8' x 0.3'

Today we go to the northern edge of Libra, just around 2° north of ß Libra named "Zubeneschamili" and visit an spectacular interacting galaxy pair which found an entry in the Atlas of Peculiar Galaxies as Arp 254.

Not knowing about the interacting violence between both galaxies, John Herschel discovered the northern galaxy in 1835 with his 18.25" Speculum Reflector at Feldhausen, near Cape Town. The NGC later listed the galaxy as NGC 5917. Herschel missed the southern companion, which is somewhat fainter.

Causing the former collision, NGC 5917 shows a strange morphology without an defined core or spiral arms, but strong HII regions and OB associations like an irregular galaxy. The MCG shows a long tidal tail towards the NGC with a broader hook and also a shorter and fainter countertail. Both galaxies are measured at around 85 million light years distance. They are the only two galaxies in the field which are gravitational bound. Others are much more distant. In the past, some rare supernova type Ia and type II were detected and researched to better understand the star formation and the number of supernova in interacting galaxies and of course to calibrate the Hubble constant.

In the telescope, both should be visible in small to mid size telescopes. NGC 5917 shows only hints of structure, even in larger telescopes. The special of this pair, the tidal tail is one of the brighter ones in the sky. With 27-inch under rural skies, the tail was visible as an easy detail, especially a slightly longish and bend knot in the half distance to NGC 5917. Also the first part of the countertail was visible without any difficulties.

picture: Capella Team (Pöpsel/Sackenheim/Binnewies), RGB, 24" Hypergraph
Attachment 5725

sketch: Uwe Glahn, 27", 293x, NELM 6m5+, Seeing III
Attachment 5726

But now it is your turn...
GIVE IT A GO AND LET US KNOW!

Back in 2012, I used NGC 5917 as a guide to find Comet 96P/Machholz (the comet was surprisingly bright, especially the nucleus. It had a small tail). Although I did not observe 5917, this galaxy is still on my to-do list. There is a nice trio of galaxies 6 degrees due south, NGC 5915, 5916, 5916A, seen twice with and 18" & 20" which may be a bit 'brighter' that 5917.

With a 16" I observed these galaxies from La Palma and my notes read:

NGC 5917 is round and can be seen with DV, but not terribly bright. It does appear to show some central brightening. With AV, MCG-1-39-3 can be seen close to it and with effort the extensions in both directions can be seen as well.

On Uwe's fine sketch I notice that I did not see the long tails, but only the small extensions.

Here's my sketch of the area:

Attachment 5727

The only observation I have is from May 2016 through Jimi Lowrey's 48-inch. The fine instrument not only rendered the tail, but also showed it get thicker towards its end around 15h 21m 33s -07° 24' 37" flanking a dim star. Thanks for putting this on the radar, Uwe. I may try give it a go in my 28-inch.