Object of the Week, Dec 3, 2023 – Arp 258 and Hickson 18 = VV 143 in Aries

R.A.: 02h39m06.1s Dec.: +18°23'02"

Size: 1.7'x 0.8', Magnitude: 15.50 B

(I’m filling in this week for Dragan as he is busy drinking margaritas in Mexico)

I will tell you right off that these guys are stinkers, even though they appear in both the Arp and Hickson catalogues… but Arp 258 is not quite the same as Hickson 18!

Look at this labeled chart, borrowed from Alvin Huey’s “Hickson Group Observer’s Guide”…

Attachment 5295

… and note that the galaxy labeled “A” (MCG+03-07-038, PGC 10046 and others), is a part of Hickson 18 but apparently is not part of Arp 258, which is the faint train wreck heading off towards the northwest centered only about 1.5 arc-minutes away. These (3) galaxies have designations of UGC 2140 A, B and C from south to north.

Here is what Paul Hickson had to say about this group… “Group 18, also known as Arp 258 or VV143, consists of a background galaxy (A) and a line of three irregular objects apparently in contact. At present [1994] no redshift is available for the brightest of these. These three objects may simply be a concentration of star and gas within a singular irregular galaxy, or they may be the result of an interaction between two or more low-mass galaxies”.

We know now that the 3-smudges Arp 258 trio has a redshift indicating a distance of about 175 million light years whereas galaxy A is a whopping 445 million light years in the background, so it is clearly just a line-of-sight coincidence.

This is the best photo I could find of the area, from the late astrophotographer Rick Johnson (RIP Rick), here…

Attachment 5296

… and you can see that there is a lot going on here and it is hard to discern much in the way of details, and that there is a lot of fluff in the area… just not in my eyepiece!

Read what Rick has to say about these galaxies here at his webpage, it is really an interesting read… and he was even able to identify several asteroids in his astrophoto!

Halton Arp puts this group of galaxies in his “Irregular clumps” category and it easy to why! In my 25” f/5 Obsession I could easily see the “A” galaxy, PGC 10046, but the other guys were just a dim smudge in the eyepiece and there is no way that I could discern individual objects. This is not an object for the faint of heart!

As always, give it a go and let us know

H-alpha (ionized gas) and H I kinematics for the HCG B/C/D group point to a giant, highly irregular galaxy with several knots, at least in the 2000 paper "Kinematics and morphology of ionized gas in Hickson Compact Group 18"

Through my 18", HCG 18A was very faint, but not too difficult with averted, perhaps 30"x20" NW-SE. The combined glow of connected HCG 18B, C and D was an extremely faint, elongated haze, though it was larger than HCG 18A just 1.5' SE. No resolution of the components B-C-D in this scope.

Through Jimi's 48", it was a different story, of course. HCG 18A appeared moderately bright and large, very elongated 7:2 WNW-ESE, ~40"x12", with a small brighter core.

HCG 18D is the northwest component of UGC 2140. It appeared as a fairly faint, very small knot, just 6"-8". This knot is embedded in a low surface brightness glow extending to the southeast that contains 18C and 18B.

HCG 18C is the central component. Its nucleus appeared faint, also very small, ~9"x6" and surrounded by low surface extensions to the northwest and southeast, merging into HCG 18B on the southeast end.

HCG 18B is fairly faint, very elongated 3:1 or 4:1 NNW-SSE, perhaps 45"x12". This portion of UGC 2140 is a similar in size to HCG 18A but with a fairly uniform surface brightness. A mag 15.5-16 star lies 0.6' E. I can see very faint haze bulging out just east of HCG 18B.

Hickson 18 was an averted vision grouping with my 20" back in 2001. Similarly, averted vision with Hickson 20 in Aries also.

I've logged two observations of this interesting group.

20", 500x, NELM 7m0+, Seeing III
central region visible as a 4:1 elongated body; a visible as a elongated galaxy with averted vision; b, c and d not resolvable as single clumps within the main body

27", 586x, NELM 7m0+, Seeing I
very irregular and peculiar group; main body visible as a 5:1 NW-SE elongated bar; a in the SE rollover of the bar with a 1:2 elongation and concentration; b still visible with direct vision as a elongated detail at the SE tip of the bar; c and d clear visible as small knots at the NW tip of the bar with averted vision

sketch:
Attachment 5297
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