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ASTRO: Hickson 68 reprocessed



 
 
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Old June 11th 12, 08:46 AM posted to alt.binaries.pictures.astro
Rick Johnson[_2_]
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Default ASTRO: Hickson 68 reprocessed

I took this data in 2008. Recently a Canadian amateur took a very good
wide field image of this area and noticed an odd "curtain" (his term)
plume to the south of the two central galaxies. Most plumes fade away,
this one ended suddenly. He wondered if I'd imaged it. Looking at my
2008 image I saw only a faint hint of the plume. While his image used 6
hours 10 minutes of data and mine only 2 hours I wondered why I didn't
see it. Then I looked at the subs. Two were buried deep in a bright
cloud haze. These completely wiped out the plume. So I reprocessed it
without those frames using 1 hour 40 minutes of data and the plume
appeared just as he had picked it up. My frames still have a nasty haze
from clouds but until I retake it this will have to do. Also it was
taken with my old filters that had nasty halos. With a very bright K5
star right on top of the galaxies those halos did a number on the image.
I dealt with them somewhat but they won. I doubt they will be much of
an issue with my new filters. Yet another reason to reshoot it. His
image can be seen at:
http://www.nightoverontario.com/Astr...x56K&lb=1&s=X3

The original post had little research on the field. I did a lot looking
for anything covering this odd plume but came up pretty empty. Did
learn a lot about the group however. Here's the report I came up with.

Hickson 68 is a galaxy group far larger than apparently Hickson
realized. He made the catalog from visual inspection of the Palomar
Observatory Survey Plates of the 1950's creating a catalog known as the
Hickson Compact Group catalog. Well HCG 68 is not compact. He
apparently was only considering the core region. The group is really
far larger than my image. It is organized with elliptical galaxies in
the center and spiral galaxies mostly on the edges many of which are
beyond my FOV. The large spiral NGC 5390 is an example as is UGC 8841
in the lower left corner of my image. The group is located about 115
million light-years away in the constellation of Canes Venatici.

The center is home to the overlapping galaxies NGC 5353 and NGC 5354
which inhabit a common halo. NGC 5353 is a S0 galaxy rather than
elliptical that harbors a rather active AGN indicating this might not be
a serene old group like it first appears. NGC 5354 to its north is
Classes SA0 with strong spectral lines and is a LINER galaxy. Yet more
indication this is an active group still in its formative stage. Then
there's NGC 5350 a rather classic looking barred spiral. NED classes it
SB(r)b while the NGC project says SBb-c. It is a starburst galaxy with
intense star formation going on in its core that is hidden from our view
by warm dust. Yet another indication of activity.

Other NGC members of the group as NGC 5355, an E3 galaxy by NED and S0?
by the NGC Project. NGC 5358 is classed SO/a by both. The other large
member of the group in my image is UGC 08841 which is a very nice two
armed barred spiral classed by NED as SBb.

NGC 5353 and NGC 5354 appear to share a common halo. It has a very
weird sharp cut off at its southern end and appears slightly brighter
right before it comes to that sudden end. This is often seen when a
dark nebula abuts a bright nebula forming a shock front. I scoured the
literature but found nothing addressing this odd feature. Closest I
came was an article talking about how this is a still collapsing system
with lots of activity to come over the next few billion years.
http://iopscience.iop.org/1538-3881/...135_4_1488.pdf

This image is a complete reprocess from the very beginning of data taken
back in 2008. When I processed it I included both color and luminance
frames taken though clouds which cast a strong gradient across the
image, While it was removed much of the halo, including the sharply
defined southern end were gone. I through out all these frames and
ended up with a much better image that now shows the halo's sudden
southern end. Since the color data is weak I am a bit leery of it in
faint regions like this halo. If it is right (a very big IF) then the
plume has an odd red color as it comes to a halt. This is very weird if
true. Is it really gas and dust ejected from the two massive S0
galaxies rather than stars? When galaxies of this size interact most
dust and gas is ejected. Is that the source of the southern plume? I'm
likely far off base here but what the heck, wild speculation is fun. If
anyone finds anything to cut though my wild guesses please let me know
and I'll pass it on.

This image was taken with my old, halo prone filters. The bright K5
star really made a mess with my old filters. I left a lot of it in as I
found no way to take it out without harming the faint halo around the
core galaxies. But compared to the Sloan image of this area I did a
much better job with it than they did. I need to reshoot this with more
time with the new filters that don't have the halo issue.

The image was taken over 4 nights (one night, while used in the first
image was rejected for this one). Many other frames from the other
three nights were rejected as well. The result was I am down to my
normal 40 minutes of L and 20 for each color. Not what is needed for
such a faint feature as the plume unfortunately. Nor was the night very
good. My limiting magnitude is about 21.8 compared to my normal 22.5.
That would indicate poor transparency even for the three nights I got
usable data. Yet another reason to reshoot it. For now this reprocess
is a great improvement over the original so I'll go with it.

14" LX200R @ f/10, L=4x10' RGB=2x10'x3, STL-11000XM, Paramount ME

Rick

--
Prefix is correct. Domain is arvig dot net

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