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ASTRO: Arp 27

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Old February 15th 11, 08:59 PM posted to alt.binaries.pictures.astro
Rick Johnson[_2_]
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Posts: 3,085
Default ASTRO: Arp 27

See I forgot the ASTRO: subject on my M81 re-redo. No granddaughter
here so haven't been turning on my filter as it hasn't been needed.
Shouldn't open my mouth I suppose. Anyway back to Arp galaxies.

Arp 27/NGC 3631 is a grand design spiral in Ursa Major. Red shift shows
it to be about 62 million light-years distant. A Tully-Fisher
measurement says 70 million light-years. Pretty good agreement as these
things go. Arp puts this one in his category of spiral galaxies with
one heavy arm. NED classes it as SA(s)c. At magnitude 10.6 this is a
good one for visual observers. Arp's comment on this one reads: "Note
straight arm, absorption tube crossing from inside to outside of south
arm." The straight arm he refers to is easy to spot. The absorption
tube is likely the dark feature from upper left to lower right crossing
the narrow arm. A narrow line of star clusters continues on what would
be the upper side if it continued further southwest. 3 clouds might be
along the lower side of it.

Often I've mentioned "So many galaxies, too few grad students." Finally
I hit one a student (undergraduate) worked on in 2007. She didn't have
time to finish her project as it was a summer stipend only but has a web
page up on what she did find. So rather than me rattle on I'll refer
you to her web page on it.

My annotated image has one galaxy listed as an AGN but Sloan lists it as
a quasar. It looks like a galaxy in my image, I see no starlike point
that would be expected from a quasar. Still I've listed it as G/Q.

I found some images of the galaxy at the Hubble Legacy Archive. I made
a mono image from the best of them. None are all that good and the
wrong bands for good color. At Hubble's limited field and higher
resolution the galaxy looks very normal. Even the "straight arm" looks
normal along the part that is within its limited field of view. Due to
the high noise level in the image I'm reproducing it at half scale.
This is sufficient to show Hubble can resolve the brightest blue stars
in this galaxy. Some of the knots in my image are made up of the light
of only two or three of them in some cases. If they were standing alone
some would be within my reach.

The Sloan survey shows several "galaxies" within the disk of Arp 27. I
didn't include these when I made the annotated image as I felt they were
just parts of the galaxy and not really separate galaxies. Some Sloan
did mark as PoG for this reason but most are shown without that label.
Those that are within the Hubble field are seen to be just PoG as I
suspected. This pretty well convinces me those outside the field are
likely not separate galaxies either. Even though the Sloan image
doesn't compare to that of the HST I've included it anyway.

I found few images of this one taken by amateurs. That surprised me as
it is within easy reach of most imagers and is a rather classic galaxy.
Just not one you hear of very often.

The image is noisier than I'd like as the night cooled more than I
expected. I was imaging at -25C when the temperature suddenly fell to
well below that. I was running unregulated for three of the 4 luminance
images. The temperature fell about 2C every 10 minutes! I was unable
to fully compensate for the dark current because of this. The software
records the temperature it was taken at but only at one point in the
exposure, the end. This tells the software the wrong temperature and it
under compensates for the dark current noise. I had to guess an average
temperature to use for each frame and edit the header to that value.
I'm sure I didn't always hit it right judging by the resulting noise
figure. Still it will have to do until I can reimage it.

Arp's image:

14" LX200R @ f/10, L=4x10'x1 RGB=2x10', STL-11000XM, Paramount ME

Correct domain name is arvig and it is net not com. Prefix is correct.
Third character is a zero rather than a capital "Oh".

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