ASTRO: Arp 87 -- It's a wrap

Arp 87 is a pair of galaxies in northeast Leo about 335 to 340 million
light years away. Arp put it in his category of spirals with large high
surface brightness companions on arms." His comment is more telling.
It reads "Arm appears wrapped around cylindrical companion." His was a
pure descriptive narrative. He left it to others to explain what was
going on. In this case we have a pretty good idea and its quite
interesting. This appears to be the formation of a polar ring galaxy
caught in the act of formation. The companion is well under way of
becoming a polar ring galaxy.

The two galaxies are known as NGC 3808A and B. B is the "companion."
NED classes A as SAB(rs)c: pec and B as I0? pec.

A note at NED references an article about the strange velocity
distribution of the companion galaxy's disk and forming ring is at:
http://adsabs.harvard.edu/cgi-bin/bi...6A...291...57R
A summary at NED reads: "The gas radial velocities on the major axis of
the galaxy give evidence of very peculiar kinematics: photometric and
kinematic centers differ by about 0.3 kpc. Rotational velocities after
the global maximum at r~3" (1.4 kpc) show a rapid decrease to zero
velocity (!). The rotation curve of the forming polar ring represents
almost a straight line ({sigma} = 9.3 km/s) with a gradient of 12.9
km/s/arcsec (28.7 km/s/kpc). The crude estimation of mass to
B-luminosity ratio is 3 f_sun_."

This one has attracted the interest of enough astronomers that the HST
was used to study it. The resulting image is at:
Hubble image: http://dic.academic.ru/pictures/wiki.../Arp87full.jpg
This really shows the ring being wrapped around the galaxy quite
clearly. Though to my eye it seems to change the direction of its
rotation. I can't quite wrap my mind around it.

Just beyond the "companion" is a nice near edge on spiral that shows a
lot of detail in the Hubble image. I couldn't find any red shift data
on it. In fact I couldn't find it! NED has the faint 22.3 magnitude
star just of the tip but not the galaxy itself. It's amazing what gets
left out of these databases. Dennis Webb did a lot of work identifying
galaxies in Arp fields but he couldn't find this one either. He does
though draw a diagram of how the arm wraps around the companion. I
can't quite see it however as mentioned above.

To the lower right of Arp 87 is a candidate galaxy cluster at about 1.5
billion light years. NED has no data on its size or galaxy count. Many
possible galaxies are in the area of the label however which is centered
over the location. It reads "GC 1.5". The candidate cluster is NSC
J114001+221959.

Toward the upper left corner of my image is an interesting double
galaxy. The lower one is a spiral, SDSS J114132.58+223351.3, at 1.1
billion light years. It has a possible companion on its northern arm,
SDSS J114132.48+223402.8 distance unknown. So I can't verify if they
are interacting or just line of sight.

Below and further left of these two is an obvious pair of spirals that
do both have redshift distance and they are virtually identical at 1.2
billion light years. Though I see no hint of interaction as I may be
seeing with the previous two. Between these two pairs is what may
appear to be yet another pair but right most object is just a foreground
star in our galaxy. The distance to the galaxy is unknown.

A lot of galaxies I'd like to know the distance of have yet to be
measured. In the lower left corner for instance are several galaxies of
about the same angular size and brightness. The two with known
distances are widely separated however. What about the others. Are the
blue near and redder twice as distant. Color can be a good distance
indicator but it can be a great way to go wrong with confidence as well.

Arp's image
http://nedwww.ipac.caltech.edu/level...big_arp87.jpeg

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

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