Object of the Week June 23, 2013 -- HCG 84

HCG 84
Ursa Minor


Position (brightest member)
RA 16 44 23.0
DEC +77 51’ 31”

Size: 12'
Mag: 15.4-17.2(V)

Type: Galaxy Group
HCG 84 is a (surprise!) compact galaxy group in Ursa Minor, well located for easy field finding and well placed for at least some part of every single night, especially for northern latitudes. July evenings are the best times to observe this group as it reaches its highest point late in the evening.

HCG 84, the third HCG to feature in the OOTW series, consists of six components, ranging in magnitude from 15.4 to 17.2. Like all Hickson compact groups, the components are quite small. Thus, they benefit from high magnification and steady seeing conditions. The location of HCG 84, at declination of almost 78 degrees north, makes this group ideal for those who lack an equatorial platform as its movement in the eyepiece is quite slow. The six components are:
HCG 84A = CGCG 355-20A, mag 15.4, size 0.7’x0.4’
HCG 84B = PGC 58873, mag 16.5, size 0.6’x0.2’
HCG 84C = PGC 58884, mag 16.2, size 0.4’x0.2’
HCG 84D = PGC 58861, mag 17.2, size 0.3’x0.2’
HCG 84E = PGC 58881, mag 17.2, size 0.2’x0.1’
HCG 84F = PGC 58856, mag 17.2, size 0.2’x0.2’
(All data taken from Alvin Huey’s Hickson Group Observer’s Guide).

Attachment 733

I observed HCG 84 on two consecutive evenings, June 9th and June 10th 2013. As per my usual practice, whenever I observe a particular target for the first time, I avoid to read anything about its appearance beforehand, so as not to bias my observation.

On June 9th, transparency was so and so (SQM-L 20.95). I located the field quite easily, but I was unable to see anything beyond field stars in my 18” f4.5 dob.
On June 10th, from the same exact location, transparency improved considerably (SQM-L 21.15). Using my hooded vest, at 294X I initially could detect “something” with averted vision. Over time, two distinct fuzzy patches materialized, that I could hold steady all the time with averted vision, and could occasionally see them with direct vision. The southernmost one seemed more distinct and slightly elliptical, while the northernmost one seemed fainter and somewhat more elongated. The two faint patches appeared to be oriented nearly perpendicular to each other. Also, the northernmost fuzzy patch seemed to have a slight division in the middle. I could hold this “rift” feature steady only at times. After some more time observing, the “rift” in the northern fuzziness seemed to become steadily visible, giving this a “double galaxy” appearance. In addition, further to the south of the brighter patch, I could detect a very faint star that however much I tried I could not focus crisply enough. I tried to push my magnification to 588X, but the seeing was not steady enough and I could detect with certainty only the central fuzziness, the northernmost galaxy “pair” seemingly disappearing.

Attachment 732

After returning home and checking my drawing with DSS images and Huey’s labeled picture in the book, as well as with Megastar, I could identify the brightest patch as component “A”, CGCG 355-20A. The northernmost patch corresponded with component “B”, PGC 58873, and what’s furthermore, both photos show the presence of a superimposed star on the west edge of this galaxy – possibly the reason for the “double galaxy” image I perceived. To my surprise, Alvin does not indicate the “double galaxy” appearance in the drawing in his guide, nor is it mentioned in Reiner Vogel’s Hickson guide. Finally, the faint star that I could not focus crisply corresponds nicely with component “C”, PGC 58884. My only concern is that there is a faint star very near to the galaxy, so I suspect what I saw was the combined light from both PGC 58884 and the star. I did not detect anything that I placed on my sketch that could correspond with the other members.

So, HCG 84 can give us a nice set of challenges: What is the smallest telescope through which this group can be detected? It was hard in my 18” on a very good night, but I think that it might be detected with a 15” mirror or even less. What are the prerequisites for observing the “rift” created by the superimposed star in PGC 58873? Magnification, steadiness of seeing, or transparency? Finally, what is the minimum aperture and magnification needed for observing all six members of the group?
I know for sure that I will soon be returning to HCG 84. And don’t forget,

“Give it a go and let us know!”

Good luck and great viewing!

Hi Rolandos,

it's been some time since I observed this group the last time, it was at least three or four years back with my 22".

I don't remember that night, but my notes say, that at 500x, c was quite obvious with averted vision and that I could separate c from the nearby star. Both a and b were difficult, but could be seen with certainty with indirect vision. I haven't taken notes about the superimposed star next to b, and I don't remember, whether I split galaxy and star (probably not).

This is certainly one of the more challenging Hickson groups.

Attachment 737

Click to download observing guide.

Great example, Rolandos, of what makes the Hickson groups so appealing -- second looks with perhaps better conditions/darker skies, or using a larger aperture will often catch additional members.

Here are some of my notes with different scopes, including Jimi's 48", Paul Alsing's 25" Obsession and my 18" Starmaster! This group is also another example which has a discordant redshift member. HCG 84F is twice the distance of the other 5 members (at 1.4 billion light years), though interestingly, it was more prominent than HCG 84E in the 48". Also, in my 18", HCG 84C was less obvious than the attached 15th magnitude star and in fact I missed 84C in an earlier observation (1996).

HCG 84A
48" (10/22/11): at 488x appeared moderately bright, fairly small, oval 3:2 N-S, ~30"x20", brighter core. HCG 84A (brightest member of 5 members) forms a very close pair with HCG 84E (faintest member), which is barely off the east side [13" between centers]. A mag 14.4 star lies 38" WSW. The entire sextet fits in a 2.4' circle!
18" (7/4/08): at 280x, the brightest member of HCG 84 appeared faint, small, elongated 5:3 N-S, ~25"x15", very weak concentration. Located 1.6' NE of a mag 11.7 star and 2.1' SW of a mag 11.4 star. Also nearby is a mag 14.5 star 40" WSW and a mag 15 star 40" SSE (HCG 84C is attached to this star).

HCG 84B
48" (10/22/11): fairly faint, fairly small, elongated 2:1 WNW-ESE, ~25"x12", brighter core. Located 1.3' NNW of HCG 84A and 1.2' E of a mag 11.4 star. HCG 84D lies 0.6' WSW and 84F is 1.4' SW. Second brightest in very compact group of 6 galaxies.
18" (7/4/08): at 280x appeared very faint, very small, elongated at least 2:1 WNW-ESE, ~27"x12", low even surface brightness. Located 1.3' NNW of brighter HCG 84A = CGCG 355-020. This is the second brightest of 3 viewed in HCG 84.

HCG 84C
48" (10/22/11): faint, small, irregularly round, 15"x12". A mag 15 star is at the SW edge. Located 38" SE of HCG 84A. HCG 84E lies 30" NNW (just east of 84A). A mag 12.3 star lies 1.0' due east. Third brightest of six in the group.
25" (7/4/08): at 360x appeared very faint, very small, round, 10" diameter. Attached to mag 15 star (just 15" between centers).
18" (7/4/08): the third brightest member of HCG 84 was a marginal glow at 280x, highly suspected attached to the NE of a mag 15 star. Situated just 0.6' SE of HCG 84A.

HCG 84D
48" (10/22/11): at 488x appeared faint, small, slightly elongated E-W, 9"x6". Located just 0.6' ESE of a mag 11.4 star and a similar distance WSW of HCG 84B.
25" (7/4/08): at 360x appeared extremely faint and small, round, 4" diameter. Squeezed between HCG 84B 34" E and a mag 11.4 star 38" E. Required some concentration.

HCG 84E
48" (10/22/11): very faint, extremely small, round, 6" diameter. Forms a very close double with HCG 84A, just 13" E of center! HCG 84C is just 30" SSE. Although listed with the same magnitude (B = 17.2) as 84F, this galaxy was slightly the faintest in the sextet.

HCG 84F
48" (10/22/11): HCG 84F was easily visible though very faint to faint, very small, round, 10" diameter. Situated just 39" S of a mag 11.4 star and 1.9' WNW of HCG 84A. Although assigned the lowest group letter, this galaxy was slightly easier than 84E (both are B = 17.2). The redshift of this galaxy is nearly twice the other 5 members, with the group at a distance of ~740 million ly and HCG 84F at a distance of 1.4 billion light years!

Talk about throwing down the gauntlet! I'll now have to try this with my 18" and be quite pleased if I can come out with 3 like Steve did in 18". Given excellent transparency and seeing it could happen. Last new moon (a real seeing skunk experience) I was going to go for AGC2256 and Shakhbazian 166 which are right in that area so what the heck, maybe at GSSP this area will be quite the project.

I had to look in my Hickson observing log then cross reference with my old written notes to come up with my observation of this group. It was in the early morning hours of Aug 10, 2008. I was at the Mt Kobau star party at 6000ft. Very dry clear air with SQM 21.54 and excellent seeing. I was using my 12.5". My notes say that at 365X I was able to detect galaxies A, B and C. Looking at images now I wonder if I was seeing the star next to C. Regardless, I am confident of my observation of A and B.

I haven't been to Mt Kobau in a few years and am looking forward to getting back there later this summer and observe with old friends.

I've been quite surprised at how many of the Hicksons have been observable with my 12.5". I've logged 52 so far with it and a total of 64 between the 12.5 and my 20".

Bill

As has been mentioned before, the hicksons make for a great set of objects to use to gauge the current sky conditions. In the late summer I often use the 'fleas' of 7331 for this sort of test but hicksons are all over the sky so picking 6-8 with a wide range of magnitudes offers us a sort of sky condition 'scale' for a given instrument to better judge transparency. Seeing is easier to put a number on than transparency from my experience but perhaps I lack the 'tricks' of the deep sky experts in putting a number on transparency alone.

I am really excited about all the comments I received about HCG 84. I am really surprised that no-one has mentioned definitely "splitting" galaxy B. To be honest I expected to read that it was a definite split using some of the larger scopes used by this group (especially Jimi's 48").

I plan to return to HCG 84 next Saturday, as we have planned already a trip to our high altitude (~ 1300m) site, with its steadier, darker sites. If conditions allow observing the group, my main effort will be to confirm "C", then to confirm again my observation of "B" as a double glow, and finally, if seeing allows, I will try to detect the other three members with my 18". I'll keep you posted!

Hi Rolandos,

"Splitting" 84B as in two separate galaxies? To the best of my knowledge the feature on the WNW side of B is a faint star. Is there other data?

http://skyservice.pha.jhu.edu/DR8/Im...12&opt=&query=

Quote:

---

Originally Posted by Clear Skies

Hi Rolandos,

"Splitting" 84B as in two separate galaxies? To the best of my knowledge the feature on the WNW side of B is a faint star. Is there other data?

http://skyservice.pha.jhu.edu/DR8/Im...12&opt=&query=

---

Well, I guess I did not phrase it right! As I said in the original post, I got the impression that galaxy "B" had two bright (relative term of course!) patches within it. It gave me the impression of a "double galaxy" - but of course in reality it is the faint star and the galaxy nucleus. What I want is to try to "replicate" my original observation, though of course now I am a bit biased - I know there is a faint star there! When I did my first observation of HCG 84 I was not aware of the star superimposed on B.

I have one observation of Hickson 84 from October 2001 with my old 20 inch. Although this was before I had a SQM my notes indicate it was a very dark and transparent night on Steens Mountain, so the SQM was probably somewhere in the 21.8 to 21.9 range. Here are my notes and eyepiece sketch from that observation:

"Faint and tough to see, the E and F galaxies are invisible, perhaps because the soft seeing blurs out their magnitude 16.9 images...413x"

Attachment 741

I'm going to re-observe this group in a few nights at the GSSP with my 28" scope.

I had a couple of good looks at Hickson 84 at the GSSP a couple weeks ago, here are my notes and sketch:

"Tough object but lots of fun to observe. This is a tight galaxy group that took 695x to blow up large enough to see well, plus a good amount of observing time to see everything I can. The dark background at 695 and fov framed the group perfectly. 21.60 SQM."

"Continuing my observations from last night, I was able to definitely see two of the three 17th magnitude galaxies I missed last night. The third one is a "maybe" because I had a couple of hints of a faint spot in the right location but not enough to be sure. 408x, 695x, 21.60 SQM."

Attachment 763 Attachment 764

The biggest challenge for me was placing as many of the bright foreground stars out of the fov as possible, and I couldn't get them all out because the cluster is surrounded by them. Seeing was fairly soft during both observations and I couldn't see the E galaxy, but this will be another good one to observe again, hopefully in steadier seeing, at the OSP next month.