Object of the Week April 6, 2015 – Copeland’s Septet (Hickson 57, Arp 320)

[Uwe Glahn]

Object of the Week April 6, 2015 – Copeland’s Septet (Hickson 57, Arp 320)

Galaxy Group (Septet)
Leo
RA (centre): 11h 37.9m
DEC: +22° 00.0′

NGC 3745 (HCG 57g)
16.2bmag

NGC 3746 (UGC 6957, HCG 57b)
15.0bmag

NGC 3748 (HCG 57e)
15.8bmag

NGC 3750 (HCG 57c, VV 282c)
14.9bmag

NGC 3751 (UGC 6601, HCG 57f)
15.0bmag

NGC 3753 (UGC 6602, HCG 57a, VV 282a)
15.5bmag

NGC 3754 (HCG 57d, VV 282b)
15.0bmag

PGC 36010 (HCG 57h)
17.0bmag

2MASX J11373896+2202269 (PGC 169872)
16.5bmag

Ralph Copeland, from January 1871 assistant of Lawrence Parsons at Birr Castle discovered during two nights in spring 1874 seven galaxies within 7' with the large 72" Leviathan reflector – largest telescope of the time. Faintest galaxy was NGC 3745 with 16.2 bmag. This brightness is interesting to make a statement about the capability of the big Speculum mirror. He misses the near and only a few tenth magnitudes fainter galaxy PGC 36010 which has a brightness of 16.5 bmag. The big problem of the mirror was the loss of reflectivity during an observing session before the change of the second, new polished mirror.

Unfortunately Dreyer made a relation fault with an from Copeland named reference star. This is because the Septet was catalogued as “nonexistent” in the RNGC.

Later on several famous catalogues pick up this group like Vorontsov-Velyaminov (galaxy trio NGC 3750/3753/3754), Hickson (eight galaxies – all NGC + PGC) and Arp. The name “Copeland’s Septet” has been introduced with the RC2 from deVaucouleurs/Corwin in 1976.

The measured redshifts of all nine (seven NGC + two other companions) named galaxies indicates a gravitational bound unit. Deep photographs also show strong signs of interacting effects. Especially the brightest member NGC 3753 shows tidal arms.

The group should be visible from 8” aperture or maybe smaller? Observers with 10” telescopes reports about separation of some galaxies. With 16.5bmag or 17.0bmag for the 2MASX galaxy it should be possible to see all nine galaxies with 16”-18” aperture. A quick and dirty look with 27” shows all nine galaxies as direct vision objects but did not show the tidal arms of NGC 3753. Bigger could show structures within the galaxies and some more background galaxies beyond 18bmag in the direct neighborhood.

labeled DSS 10'x10'
1587

200" plate of Arp Atlas
1589

16", 257x, NELM 6m3
1588

"GIVE IT A GO AND LET US KNOW"

[Howard B]

Hi Uwe,

When I checked my notes I found one observation that stated the night was so lousy that I needed to come back when conditions were better, and another observation saying I saw 7 galaxies but no details, no sketch. So this is a good reminder for me to get as decent observation of Copeland's Septet!

[Steve Gottlieb]

An interesting side note on the discovery of Copeland's Septet ---

Copeland found the group while searching in vain for Heinrich d'Arrest's GC 2464 = NGC 3760, which he assumed was in the general location. But d'Arrest had made a 1-hour transcription error in the RA, so his object was not to be found and the Septet happened to be just west of d'Arrest's erroneous position! Once 1-hour of time is subtracted from d'Arrest's position for NGC 3760 it matches NGC 3310. So Copeland's find was purely serendipitous!

[Dragan]

Steve,

So NGC 3301 & 3760 are one in the same, right? Megastar depicts just that.

[reiner]

Hi Uwe,

I had three nights in row with (almost :-) ) only galaxies. When scanning the Leo Cluster, I also visited Copelands group with my 22". As usual, it took some time to get a rought picture of the group and to separate the individual galaxies. I could see seven of Hicksons eight members and split them as individual galaxies. I could not see member h (PGC 36010), but did not spend too much time on it (it was only half an hour to moon rise). I don't remember, whether I looked at the offset galaxy PGC 169872.

Btw: I had my 7" tablet with me. Still trying to get used to it :-)

[Steve Gottlieb]

Yep, NGC 3760 is a later duplicate observation of NGC 3301, which was discovered by William Herschel. The story of Copeland's discovery based on d'Arrest's mistake was first published in 1894 (http://www.ngcicproject.org/Historical_Record/Pothier/1894AN136-3241-11_Kobold.pdf). You may need to brush up on your German ;-)

The same night, d'Arrest also "discovered" NGC 3575 --- but he made the same 1 hour error recording the position. It turns out this was a duplicate observation of NGC 3162, again discovered 80 years earlier by Herschel. d'Arrest was a very meticulous observer who made thousands of very accurate positional measurements, but everyone makes at least a few mistakes!

[Uwe Glahn]

Thanks for the addition Steve. A not so easy double fault in this group - first the 1h fault of d'Arrest and than the 27' relation fault of Dreyer. Additional to that the first RNGC did not correct the found faults and set all galaxies as "nonexistent". Sometimes history can be more interesting as the observation itself.

@Rainer
"only galaxies"
Unbelievable. Be honest - you forgot your nebula filters? I also missed "h" with my 16" in the past. Seems to be a difficult object for the 20" class.

I gave the group also another go with my small 8" ("eight" no typo). Sky was not the best; i had some trouble with dusty transparency but the high was ok for the group. Members a/c/d were visible already with 80x as one plob. Higher powers shows a/d (no separation) and b without problems steadily with averted vision. c was visible for several seconds with averted vision and f and e plobs in and out of view. I could not see g and h.

[Marko]

Great pick. This is one of my favorite Hicksons due to so many members that I'm able to see 8 in my 18" given excellent skies.

I too marked the H member as the most difficult of the group and required patience and moments of good seeing.
The observation was in a Sqm 21.67 sky with excellent transparency at 338x on 2/19/2009
The other tricky member was the separation of N3754 from N3753 from my notes.

I have re-visited it and not seen the H member which is tiny and so I feel the excellent conditions was the only reason H was detected.

[Clear Skies]

Observed this group at midnight last night, 14" SCT - 237x / 21', SQM 21.63

1599

At first glance A & D appeared as a single patch, averted vision and patience revealed separate galaxies with D - NGC3754 perpendicular to clearly elongated A - NGC3753.
C - NGC3750, E - NGC3748 & F - NGC3751 revealed a nucleus. Only H - PGC36010 proved elusive.

[reiner]

I could not see member h (PGC 36010), but did not spend too much time on it (it was only half an hour to moon rise). I don't remember, whether I looked at the offset galaxy PGC 169872.

So ... I had another look at it the night before and now I spend more time to pin down H, which proved not to be too difficult. I could hold it almost steadily with AV with my 22" at 500x.

[Robin]

With my 12 inch Dobsonian at 168x - 275x magnification, I only observed 5 of these galaxies. Please find attached my sketch.

Clear skies

Robin