ASTRO: Hickson 67 held a major surprise

Hickson scoured the POSS plates and cataloged small compact groups of
galaxies. One I found very interesting is #67 in the list. Interesting
because Hickson and others got it wrong it appears! Probably because
most work from the POSS plates which over exposed the core galaxy of the
group hiding its true nature.

The group consists of two major galaxies, NGC 5306/VV135 and MCG
-01-35-013. MCG -01-35-013 is a nice edge on, somewhat distorted,
spiral. The southern arm is rather odd. The dust lane ends and
suddenly the arm gets very narrow and blue. The transition is very
sudden and unusual. The galaxy has not been studied that I could find.

NGC 5306 is where things started to go wrong. It appears NGC 5306 is the
center "galaxy" with the two others, north and south of it, not
considered part of NGC 5306. The Vorontsov-Velyaminov Interacting
Galaxies catalog however does include the galaxies to the northeast and
southwest as they reside in a common halo. But all catalogs miss the
fact that the core galaxy is actually two galaxies. NED doesn't show
them separately nor does any catalog I could find. On the POSS 2 plates
they merge into one due to over exposure. Apparently no one looked
further. The VV catalog lists the pair as VV 135a as I've shown in the
annotated image. The other two being c and b of course. NED classes
NGC 5306/VV 135a as either S0 pec or E1. The NGC project says S0P? The
redshift value like their classification is apparently a mash-up of that
for both galaxies.

This field was recommended to me by Sakib Rasool so I contacted him to
ask if he wanted me to image it because he suspected that NGC 5306/VV135
was a double galaxy. Nope, it came as a surprise to him as well. He
was interested the the spiral MCG -01-35-013 as it appeared warped or
distorted on the POSS plates. He did some digging and contacted the
astronomer John Hibbard who studies interacting galaxies. Dr. Hibbard,
then did some digging and came up with one paper on it I'd not found.
http://aas.aanda.org/index.php?optio...513&Itemid=129

The paper labels it a double core galaxy; n1 and n2 which I labeled on
the annotated image. But it goes on to indicate the authors feel these
are two superimposed galaxies not as close as their angular separation
indicates. I would agree with this as while the overall halo is
oriented toward VV135c and VV135d, n2 is oriented almost at right angles
to this. Oddly it is the brighter of the two however. This would need
some explanation it would seem. This is one group that needs a lot more
study and some catalog asterisks or corrections.

VV 135b is classed as S0 pec. I don't know what causes them to tack on
the peculiar label. VV 135c is classes Scd. It sure has some odd
structure to it. Unfortunately the structure is beyond my abilities to
determine what it is. It certainly looks disturbed but somehow avoided
the pec label.

A few other galaxies are scattered across the image that are members of
the Hickson group. Others are about twice as distant. The field is
little studied with most galaxies being anonymous.

While the image is my usual size it was taken at 0.5" per pixel rather
than my normal 1" per pixel so covers only one fourth the area of my
normal images. As such most of the remaining members of HCG 67 lie
beyond the field of view. If I'd used my normal field (would have been
4 times bigger image and too big for internet) though now I wish I'd
have taken the full field and reduced it to my normal scale, I'd have
picked up almost a dozen other members of the group. It is rather
widely scattered. I was hoping to bring out a jet that one source says
is coming from the nucleus of NGC 5306 but I don't see it. How they
find such a jet yet miss it is a double galaxy I can't understand.
Since it is already at a greater image scale than my enlargements I've
not done one this time. I am including a cropped image as I get a lot
of requests for smaller images from those with pixel challenged monitors.

Dr. Hibbard's email goes on to say about Hickson 67: "It appears to be
the central concentration of a larger group with at least 15-17 members.
About 10-15% of HCGs are thought to be such creates, and thus not true
compact groups in the way Hickson meant.

"These double nucleated ellipticals are interested - they are sometimes
considered examples of galactic "cannibalization". About 25% of cD
galaxies have multiple nuclei (although I am not sure if NGC5306 is a
full-blown cD; that requires it to have an extended luminous envelop).
See e.g.
http://ned.ipac.caltech.edu/level5/M...razin2_10.html.

"Well, back to work. Thanks for the diversion and beautiful image!"

For a bit there I thought I might have found something previously
unknown. Nope, just something hidden deep in the literature. Still,
finding something this obscure is quite exciting.

14" LX200R @ f/10, L=8x10'x1 RGB=2x10', STL-11000XM, Paramount ME

Rick
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