Object of the Week Apr 8, 2012 - Arp 321

[Dragan]

AKA Hickson 40 in Hydra

RA 09 38 53.4
DEC -04 50 55


5 Components

Component A - MCG -1-25-9 PGC 27509 Mag 12.8
Component B - MCG -1-25-10 PGC 27513 Mag 14.0
Component C - MCG -1-25-8 PGC 27508 Mag 14.7
Component D - MCG -1-25-12 PGC 27516 Mag 14.1
Component E - MCG -1-25-11 PGC 27515 Mag 17.3

146 147


Located in western Hydra, I last visited this object in the winter of 2009. Once in February with the 48" and again in March with my 25" here in Illinois. While not a particularly difficult object in and of itself, I was really drawn to it due to all its variation in galaxy types. That, and the way all the galaxies appeared to be "stacked". I felt it would make a great object to test various size scopes. Whats the smallest scope you can break up this grouping and see individual galaxy shapes?

Some of my notes: (Literal transcript from my handwritten notes)

23FEB09 48" 488x 10mm ZAOII
All 5 components were clearly visible at moments of good seeing with E being the most difficult of the 5.(barely) I was able to hold E 3/4ths of the time but overall not too hard. I like the overall shape of this object. Will have to revisit with the 25.

28MAR09 25"f/5 9mm UO Ortho 350x & 7mm UO Ortho 450x T/4 S/3
With the 16mm and 9mm I was able to clearly see the overal shape of H40 and recognized the object. At 450x I could discern 5 individual glows using averted at moments of seeing. 6mm at 525x proved to be a bit too much power tonight. The 7mm gave the best view at moments and I could see the different overall shapes of 4 of the 5 galaxies. Though all I felt I was seeing was the core of E, E was a little easier than the 17.3m suggested. Could that be a mistake?

After observing this object in my scope I intended to verify component E's true visual magnitude but I never did. Does anyone know its true mag? Is Megastar correct at 17.3? Is Megastar correct about the others? E was a bit more difficult to discern but I didn't feel it was nearly 3 magnitudes dimmer than the others! (BTW, Megastar gives the magnitudes in the Blue. I just subtracted a full magnitude for this posting)

Nonetheless, I give you this weeks OOTW, so......

"GIVE IT A GO AND LET US KNOW"

GOOD LUCK AND GREAT VIEWING!

[Marko]

Here is an 18" observation from 3-27-2009 in the 18" dob with 5mm nagler and no paracorr for 338x. 2000ft elevation.l It was a very good night with SQM 21.6 and seeing 5/5 with transparency 4/5 but at that point I was not consistent on noting NELM but the sky was noted as very nice that night and about as good as it gets at 'Deep Sky Ranch', a friends ranch well south of bay area lights. I only really noted detection here so it's a bit minimal.

A Main member and brightest by good margin.
B S end of cluster. Round and tiny.
C Requires averted but will show > 60% of time.
D Not too hard to spot. 100% averted.
E Very hard. Tried 5mm and then best in 7mm barlowed 2x (3.5mm) [480x] Had to wait for times of best seeing and then it would be 20% averted only

In a friends 22" on 2-16-2010 it was a borrowed view so I bet I did not take enough time but the sky was worse with 6.5 NELM and lower seeing and transparency. For that observation I noted C was at the limits and was unable to detect E. I did not work for it like before and it was a 22" so it may have been possible, I'll never know. I think the elongated C member makes it all the more difficult. This seems a nice 'meter' for sky judgement. I like small groups with ranges in tight patterns for noting sky conditions but have not gotten into the habit of using them on a regular basis to note sky conditions. I'll tend to do that when 7331 is about but I should do it with hicksons like this one.

Thanks for bringing this one to mind Dragan

[Steve Gottlieb]

Ah, I think we're on to something here -- that extra 1/2" might have been the difference maker, Mark, as 16 years ago I failed to log HCG 40E with my old 17.5" ;-) In any case, congratulations on nabbing the faintest member from Deep Sky Ranch (I have the date as 3/28/09 -- weren't we there on the same night?)

As far as the Megastar magnitude of 17.3, that's copied (as well as all the other magnitudes in HCGs) from Hickson, Kindl and Auman's 1989 "A Photometric Catalog of Compact Groups of Galaxies", where the total blue magnitude is given as 17.32. The catalog is scanned online here (http://ads.ari.uni-heidelberg.de/cgi-bin/nph-journal_query?volume=70&plate_select=NO&page=687&plate=&cover=&journal=ApJS.). The margin of error is given as +/-0.2 to one standard deviation, so at least according to this paper, the range might be 17.1 to 17.5 (with 68% confidence).

[Dragan]

As far as the Megastar magnitude of 17.3, that's copied (as well as all the other magnitudes in HCGs) from Hickson, Kindl and Auman's 1989 "A Photometric Catalog of Compact Groups of Galaxies", where the total blue magnitude is given as 17.32. The catalog is scanned online here (http://ads.ari.uni-heidelberg.de/cgi-bin/nph-journal_query?volume=70&plate_select=NO&page=687&plate=&cover=&journal=ApJS.). The margin of error is given as +/-0.2 to one standard deviation, so at least according to this paper, the range might be 17.1 to 17.5 (with 68% confidence).

Ah, good information.

So let me ask this Steve. Am I correct in subtracting a full magnitude from the B magnitude in Megastar to come to an approximate visual mag? (assuming the data in MS is accurate)

Also, in the paper you referenced, is column 11 (R Mag) the equivalent to visual magnitudes? What is R Mag?

[Steve Gottlieb]

As far as I understand, a conversion from B to V depends on several factors including galaxy type and galactic extinction but subtracting 1 mag is reasonable rule of thumb. So, the magnitude listed in your original table should read 16.3 for HCG 40E (you've already subtracted 1 from the other mags). The R mag (from column 11) is through a red filter and is not equivalent to a visual mag.

[Dragan]

As far as I understand, a conversion from B to V depends on several factors including galaxy type and galactic extinction but subtracting 1 mag is reasonable rule of thumb. So, the magnitude listed in your original table should read 16.3 for HCG 40E (you've already subtracted 1 from the other mags).

I subtracted 1 full magnitude from any magnitudes listed in the B. (HCG A thru D.) HCG 40E in Megastar is listed as 17.3.... without the B. So I assumed that was in the visual. And therein lies my original doubt in regards to its magnitude. I never felt it was 17+ mag.

So maybe MS should actually say HCG 40E's magnitude is 17.3(B). Subtract 1 full magnitude like I did with the others and that would bring it more in line with my observations.

:thinking:

[Howard B]

I have one observation of Hickson 40 from about 10 years ago with my old 20 inch f/5 Obsession:

This is an excellent group that rewards patience. However, galaxy E is not to be seen at mag 16.7 – I suspect it has a very low surface brightness… 575x was a bit too much power, making galaxy C much more difficult to see. Best at 413x.”

If I can get to a dark sky this month I'll have a look with my 28 and see if I can detect the E galaxy and come up with a decent sketch.

[Steve Gottlieb]

I subtracted 1 full magnitude from any magnitudes listed in the B. (HCG A thru D.) HCG 40E in Megastar is listed as 17.3.... without the B. So I assumed that was in the visual. And therein lies my original doubt in regards to its magnitude. I never felt it was 17+ mag.

So maybe MS should actually say HCG 40E's magnitude is 17.3(B). Subtract 1 full magnitude like I did with the others and that would bring it more in line with my observations.

:thinking:

Ah, I didn't realize Megastar didn't explicitly state it was a blue magnitude like the others. That was an oversight, so the V magnitude is closer to 16.3.

[Marko]

Info in reply to Steve: I was there for Hickson 40 at DSR the night of 3-27 and also there 3-28 where all the 'Sons of Dobzilla' were present. Only Kevin and I were around 3-27. Here is my opening note on the common night of 3-28-2009:

Observers: KevinR, Steve Gottlieb, Mark Wagner, Richard N, Greg LaFlamme, Mark Johnston
Clouded over till about 10 then clearing. Great after 11pm.
21.5 4/5 see and 3/5 trans till after 12 then 21.64 5/5 see 4/5 trans est MWag OR
Projects: Hydra 1 Cluster, Ngc3158 group, Leo AGC1367

From what I can tell once it got later the 2 nights were similar in conditions and DSR in prime form.
I logged many things but I think my favorite for the night of 3-28 where we were all there was the Ngc3158 group that many of us like. Picked up 13 of them which included the dim 2 MAC objects just west of N3158 but alas, I digress from this thread and as such will shut up now.

[FaintFuzzies]

I also agree that the listed mags were a bit low, but Component E was very difficult with my 22" and saw only the core.

Here are my observation with my 22" under NELM 6.8 skies.

At 377x and 528x, this nice very compact group of five galaxies, occupy a space of only 2’ across. There were two edge-on members, one very dominant and a smaller elliptical and a very tough elliptical.

Component A (MCG-1-25-9) –This dominant elliptical is very bright and round with a gradually brighter center. Averted vision gave very little more area. Estimated size is about 45” across. A 16th magnitude star lies 45” NW of the center.

Component B (MCG-1-25-10) – A small bright round bright patch about 60” SSE of MCG-1-25-9 defines this elliptical galaxy. It has a very much brighter center in this 30” disk. It is about a magnitude fainter than MCG-1-25-9.

Component C (MCG-1-25-8) – This edge-on galaxy is significantly fainter than MCG-1-25-9 and MCG-1-25-10. It shows a 4:1 elongated streak with a PA of 135˚. The length is about 60”. It lies between MCG-1-25-9 and MCG-1-25-10 off-center to the west, directly 30” south of the former.

Component D (MCG-1-25-12) – This galaxy is an elongated bright patch about 40” NE of MCG-1-25-10. Aspect ratio is about 5:2 and PA = 45˚. Estimated size is about 20 by 8”. A 16th magnitude star lies less than 20” ESE from the center of this galaxy.

Component E (MCG-1-25-11) – This extremely faint, extremely small round galaxy is about 10” across. It lies 30” north of MCG-1-25-10. This is a 50% averted vision object.

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22" at 528x Field: 6.8'