Object of the Week, August 21, 2022 - ACO 3744 - CAPRICORN

Object of the Week, August 21, 2022 - ACO 3744 - CAPRICORN
R.A.: 21h07m12.0s
Dec.: -25°28'00"

The Abell galaxy cluster ACO 3744 is in southern Capricorn, near the bottom apex of the huge Bikini Bottom asterism. The 3 major components of the cluster are NGC 7016, NGC 7017, and NGC 7018. A slightly dimmer galaxy is nearby, PGC 66149 (MCG -4-49-16), and this completes the main group. There are lots of other galaxies swarming in the background but most are beyond the reach of my telescope.

Attachment 4834

Attachment 4835

I’ve observed this group a couple of times over the years but I never really spent a lot of time studying it, I was only interested at the time in counting as many of the mostly dim cluster members as I could. The last time I looked, a few years ago, the seeing and transparency were particularly good in my local desert observing spot and I pumped up the power a bit to see what I could see. Whoa, NGC 7018 is quite elongated and suddenly becomes an obvious pair, with one of them being slightly brighter than the other! But wait, there’s more! NGC 7017 is also elongated… but I never could split them up.

So, off to Google I go, to learn more about this cluster. It turns out that NGC 7016 and NGC 7018, both elliptical galaxies, are also bright radio galaxies near the center of the cluster, as described in this paper…

https://research-information.bris.ac...34/1312.5311v1

… and the radio images are quite impressive!

The next time I observe this galaxy cluster I will be better prepared in understanding just what I am seeing!

As always, give it a go and let us know!

I had a go at this group last night with my 20" from Rio Hurtado in Chile, just a few km due south of the mountain where Gemini South and the construction site for Rubin Observatory are. The cluster passed at an altitude of 85º though I observed it when it was a bit lower in the sky. And it turns out I came poorly prepared because I didn't understand that both NGC 7017 and NGC 7018 are double galaxies! Now that I have another look at the images you included it is obvious of course. So I can only report that I observed several galaxies in and near the group but not that I saw that those two are double. I guess it gives me something to go back for and regrettable that will not be tonight. Maybe next time I'm in Chile though I don't know yet when that will be.

At 320x in ACO 3744 seen:
NGC 7016 - Large and round
NGC 7017 - Pretty large and round
NGC 7018 - Oval and large
MCG-4-49-16 - Oval, large and faint
LEDA 93984 - Very faint
PGC 778278 - Exceedingly faint
PGC 93986 - Oval and faint
PGC 85139 - Very faint and round

I've observed this cluster 4 or 5 times over the years and have notes on 10 members. This is a mixture of observations with my 18" and 24".

ESO 529-020 = AM 2102-252 = PGC 66082
21 05 43.9 -25 14 16
Size 0.6'x0.3'; PA = 31°
24" (7/10/18): at 225x and 375x; extremely faint, small, slightly elongated, 0.4'x0.3', irregular. The interacting companion off the southeast side was not seen. This member of AGC 3744 (z = .040) is situated on the northwest edge of the cluster ~25' NW of center of the cluster.

LEDA 85125
21 07 06.2 -25 35 10
Size 0.6'x0.25'; PA = 170°
24" (9/26/19): at 375x; faint, small, elongated 3:2 N-S, 18"x12", low even surface brightness. This member of AGC 3744 (z = .036) is situated just 3' SE of mag 8.2 HD 200884, which interfered with the view, and 7' S of NGC 7018, the brightest galaxy in the cluster.

NGC 7016
21 07 16.3 -25 28 08
V = 13.9; Size 0.7'x0.7'; Surf Br = 13.5
18" (8/25/06): faint, small, round, 25" diameter, weak even concentration to a very small brighter nucleus that is occasionally stellar. Located 3' SW of NGC 7108 in the core of AGC 3744.

NGC 7017
21 07 20.6 -25 29 16
V = 14.4; Size 0.6'x0.4'; Surf Br = 12.7; PA = 90°
18" (8/25/06): faint, small, elongated 4:3 ~E-W, brighter core. Similar brightness to nearby NGC 7016 but contains a more condensed core. Located 3.7' SSW of NGC 7018 in the core of the cluster.

NGC 7018
21 07 25.4 -25 25 44
V = 13.5; Size 0.9'x0.6'; Surf Br = 12.7; PA = 86°
18" (8/25/06): brightest in the AGC 3744 cluster. At 220x appeared fairly faint, fairly small, elongated 3:2 ~E-W, 45"x30", gradually brighter core and slightly brighter along the major axis. A mag 12.5 star lies 1' N. Located 10' NE of an 8th magnitude star and 25' S of 4.5-magnitiude 24 Capricorni.

LEDA 93986
21 07 31.5 -25 31 53
Size 0.7'x0.25'; PA = 143°
24" (9/26/19): at 375x; very faint, very small, round, 12", only occasionally popped. This member of AGC 3744 (z = .038) is located 3.6' SE of NGC 7017 and 8.7' ENE of mag 8.2 HD 200884.

LEDA 85142
21 07 32.3 -25 38 35
Size 0.6'x0.45'; PA = 52°
24" (9/26/19): at 375x; between faint and fairly faint, small, round, ~18" diameter, could hold continuously with averted vision. This member of AGC 3744 is located on the S side of the cluster, 10' SE of mag 8.2 HD 200884 and 13' S of NGC 7018 (brightest member). ESO 530-001 lies 5' E.

MCG -04-49-016
21 07 36.7 -25 25 05
Size 0.6'x0.4'
18" (8/25/06): very faint, extremely small, round, 15" diameter. A mag 15 star lies 30" SW. Located 2.6' ENE of NGC 7018 and faintest of 4 in the core of AGC 3744 (z = .038).

ESO 530-001
21 07 55.7 -25 39 07
Size 0.6'x0.4'; PA = 177°
18" (8/25/06): extremely faint and small, round, only 5" diameter. One of the faintest members of the cluster (z = .039) and only visible intermittently.

ESO 530-002
21 08 23.1 -25 23 25
Size 0.9'x0.3'; PA = 37°
18" (8/25/06): very faint, small, very elongated 3:1 SW-NE, 20"x6". Situated between two mag 12.5 stars 1.4' NW and 1.4' SE with the orientation roughly perpendicular to the line connecting the stars. Furthest east of 6 galaxies viewed this night in AGC 3744 (z = .039).

I didn't resolve the NGC 7017 and NGC 7018 merged pair in my 18". But they were split in my 24" at 375x with NGC 7018 easier to resolve. In good seeing conditions, higher power would be useful.

NGC 7017 appeared fairly faint, small, slightly elongated E-W, 25"x20". Occasionally a very faint "knot" was attached at the east end, ~6" diameter. This is a merged double system [12" between centers] with the eastern "knot" listed in NED as NGC 7017 NED2.

NGC 7018 appeared fairly faint, elongated 5:3 ~E-W. This double system often resolved into two tangent components; the eastern member (2MASX J21072565-2525430) is slightly larger and brighter (18" diameter) and the western component (2MASX J21072463-2525569) is very compact (12" diameter).

Steve

I am back in Chile and am observing this galaxy cluster again. Both NGC 7017 and NGC 7018 very clearly are double at 320x and even more so at 427x. NGC 7018 appears as an oval glow with two nuclei. NGC 7017 looks like a small oval glow with a slightly elongated oval glow glued to it to the west. The nucleus of the western component lies off-centre right next to the one of the eastern component and fainter.