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ASTRO: Arp 55 The Grasshopper



 
 
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Old January 29th 12, 10:20 AM posted to alt.binaries.pictures.astro
Rick Johnson[_2_]
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Default ASTRO: Arp 55 The Grasshopper

Arp 55/UGC 04881 is a pair of interacting galaxies in Lynx about 540
million light-years distant. Arp put it in his category for spirals
with small, high surface brightness companions on their arms. The
companion apparently is the galaxy seen against the very west end of the
long arm. That galaxy is SDSS J091553.63+441937.4 but NED has no
redshift data on it. So no way to know if this is just a line of sight
galaxy or truly related. Or wasn't until Hubble imaged it. The HST
image clearly indicates the companion is far beyond the colliding
spirals as the level of resolution is very different. Arp apparently
didn't care that there was no such data. It was appearance he was
concerned with. Later astronomers could figure it out. It also appears
he didn't realize the "spiral galaxy" is really two interacting
galaxies. Or maybe he did but just didn't care. In any case he never
mentions this.

His comment indicates he was likely more interested in the four
brightenings in the arm while it was going nearly straight south. It
reads: "Arm has four separate condensations in line." I can only see 3
in his image as reproduced. Four are clear in my image however. NED
considers the top one to be a separate galaxy rather than the small
bright star cluster it most likely is. Adding to the confusion it
appears there's a blue star in our galaxy right on top of the star cloud
in Arp 55. To telescopes under our atmosphere the combination looks a
lot fuzzier than it does to the Hubble Space Telescope which clearly
resolves the star and star cloud as two separate things separated by a
half billion light-years. I don't think Arp understood how common line
of sight coincidences like this happen. It also skewed his ideas on
quasars. The HST image of this pair can be found at:
http://dic.academic.ru/pictures/wiki...8-04-24%29.jpg

The long tail and its condensations appear to be due to tidal effects
from the collision of these two galaxies. Arp gives no indication he
realized this at the time of the atlas. I see NED has a catalog entry
referring to this pair as the "grasshopper" galaxy. Looks more like a
shrimp to me. NED makes no attempt to classify the fainter
southwestern member and calls the northeastern member with the long tail
simply as "spiral" Both are listed as having HII emission which is
common with interacting galaxies.

There are several galaxies in the image with the same redshift as Arp
55. It apparently anchors a small galaxy group. One of them, HS
0912+4433, overlaps a second galaxy to the northeast, SDSS
J091546.16+442126.7. It has no redshift data. It is much bluer in my
image. No way to tell if they are at the same approximate distance or
not. They don't appear to be interacting. HS 0912+4433 is classed at
NED as a blue condensed galaxy. The neighbor seems bluer and more
condensed if they are at about the same distance.

Another, smaller group of galaxies have a redshift that puts them 450
million light-years distant.

There's a galaxy cluster at 5 billion light-years that has the same
position (within the 1.5" error circle of the cluster's center) of a
faint galaxy. Several others are seen right around it. The cluster is
WHL J091538.9+441107. NED shows it having 14 members though no diameter
is given. One of the members does have a red shift distance and it too
is 5.0 million light years.

To the west and a bit north of Arp 55 is a disk galaxy at 2.4 billion
light-years. For it to show that size at that distance it would have to
be about 150,000 light-years across. Probably bigger than our galaxy
which far above average in size for a spiral galaxy.

I've been asked recently about how big some of these galaxies really
are. This is something I give you all enough information to calculate.
All my full size images are at 1" of arc per pixel unless I state
otherwise (rare), the enlargements are given and are usually 0.67" per
pixel. Just count the pixels along a major axis and you have its
diameter in seconds of arc. The tangent of this multiplied by the
distance gives its size. Yes I know this isn't exactly right. Radius
should be used not diameter. The difference in this case is 0.00016
light-years! So do it the faster way not the way your trigonometry
teacher would have you do it.

Asteroid (91131) 1998 HX144 to the west of Arp 55 is at an estimated
magnitude of 18.3 per the Minor Planet Center. Doesn't seem that bright
in my image. The image was taken over two nights. The asteroid was
only in the first night's luminance frames. The trail represents 40
minutes of the 80 minutes I spent on the luminance data.

This one got lost on my hard drive. I happened on it when doing some
housecleaning a while back then forgot about it yet again. I finally
remembered I found some lost image data. I had to do another hunt to
refind it as I couldn't find any reference as to what it was I'd found
months earlier. Either this is it or there's another "lost" one yet to
turn up. This one was taken clear back on January 16, 2010! Somehow I
never logged it as taken though I see now it was marked off as "Done" on
the Arp list. Now it really is "done!"

Arp's image with the 200" Palomar Hale telescope:
http://ned.ipac.caltech.edu/level5/A...big_arp55.jpeg

14" LX200R @ f/10, L=8x10' RGB=2x10'x3, STL-11000XM, Paramount ME
--
Prefix is correct. Domain is arvig dot net

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