ASTRO: IC 749 and IC 750

IC 749 and 750 are a rather close pair of galaxies. NGC 751 and 752 are
a pair of galaxies possibly ten times further away. All are quite
interesting. I'll start with the close pair.

Redshift puts them some 40 to 50 million light-years distant though
other distance measurements put them 70 to 150 million light-years
distant. Most sources are preferring the redshift distances. Though I
looked at an NIC camera image of IC 749 which showed few stars. That
would indicate the more distant estimates. However other sources say
the stars start to resolve at magnitude 22 which would argue for the
nearer distance. The galaxy is very blue so is full of young super hot
stars. Most of which haven't as yet turned to red giants. This could
mean there aren't many IR bright stars in the first place. One note at
NED indicates these are definitely interacting. Others make no mention
of interaction. Another question which is it issue. Visual light
doesn't seem to show any interaction. Assuming a distance of 45 million
light-years the projected separation of these two (assuming the same
distance to both) would be only 26 thousand light-years. I'd expect
rather strong interaction if this were true. Since none is seen I doubt
they are as close as they appear. At the distance of 45 million
light-years these are rather small galaxies with both being about 27,000
light-years across. If the larger distances are assumed then IC 749 is
almost 90,000 light-years across and IC 750 41,000 light-years across.

The color contrast of the two is striking. IC 750 hasn't had much star
formation in billions of years judging by its mostly red color but is a
very dusty galaxy indicating it could create quite a few stars under the
right conditions and turn blue if it has an equivalent amount of
hydrogen. Only its outer disk has a blue tinge to it. To me it shows a
lot of detail for a red spiral. Could the red color be due to dust
reddening rather than stellar aging? Maybe some of both? I found
nothing on its stellar spectra which would help answer this question.
For now we can just enjoy this highly contrasting pair of galaxies.

IC 751 and 752 lie about 425 million light-years distant by redshift.
Again we have a face on blue spiral and a somewhat red highly tilted
galaxy. Both have interesting but hard to explain internal structure.
IC 751 may be rather red but it has a Seyfert 2 core so is very active.
IC 752 has a very weird spiral structure with one well defined arm and
the other virtually invisible with lots of blue star masses. If the
distance is assumed to be 425 million light-years then IC 751 is 130,000
light-years across and IC 752 just under 90,000 light-years across.
That makes them a lot larger than the first pair. Note the somewhat
detached arc of stars at the southern end of IC 751. It almost looks
like the tidal plume of a galaxy torn apart as it was devoured by IC
751. Could one of those bright blobs be the remains of this doomed dwarf
galaxy? I'd love to see both of these at HST resolution.

Another interesting galaxy is KUG 1155+428. A nice two arm spiral with
a bright knot at the end of the shorter arm. Is this a star cloud or a
separate galaxy. NED offers no enlightenment. The Sloan survey
apparently considers it part of the galaxy.

The annotated image shows the classifications of the major galaxies and
distances to all that NED had data for. Distances in parentheses are
the median value of non redshift distance estimates at NED.

While seeing was better than average for this image I again lost a lot
due to clouds. Though three of the 4 luminance subs were taken under
very good skies the 4th was poor. I included it, though I doubt it
helped or hurt the final image. With basically three subs my noise
level is higher than normal but good seeing seems to have compensated
nicely. Color was a disaster. The color taken after the luminance was
worthless. A second night and a third night and a fourth night also
failed to get color data. Finally a month later I got my usual 2 color
frames. Unfortunately something went wrong, clouds apparently, and the
second round of color was nearly blank! Only the brightest stars showed
with nasty cloud type fog around them. Normally the cloud sensor would
have shut me down so I suspect a cloud bank sat over this field but was
small so didn't trip the cloud sensor. The result is I got only one
usable 10 minute frame for each color and didn't know it until I went to
process the image. But again, good seeing and good transparency for
that first round seems to have saved the image. Many of this years
images with two 10 minute color frames were far worse in signal to noise
ratio than these single 10 minute frames. I was amazed how well this
turned out for only 30 minutes of color data. Maybe this is all I need
on good nights? I like 2 as that allows me to remove satellite trails
without cloning them out. Fortunately no satellites made an appearance
for the color frames. Two did for the luminance frames. One I left in
as it is very hard to see. Where the other was removed there's only two
good stacked luminance images plus the very poor one. Even that didn't
seem to increase the noise level in the area of the trail when it was
removed.

14" LX200R @ f/10, L=4x10' (effectively only 3x10' and 1x2') RGB=1x10',
STL-11000XM, Paramount ME

Rick
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