A night of interacting pairings ...

Steve Gottlieb

of the Arp variety, of course!

As Darrell mentioned, there was about a dozen observers at Lake Sonoma last night -- one of the largest groups I've seen there in quite awhile. I was set up at the north end of the lot near Bob Douglas and Carter Scholz. On the drive up 101, I noticed how milky white the western sky appeared, so I was concerned about the transparency. And as expected there was quite a bit of extinction, particularly to the south -- the stars of Scorpius and Sagittarius barely poked through the haze and light dome on the southern horizon. But at high elevations and in the northern half of the sky the transparency was still pretty decent (mag 6.0 or so). I usually observe galaxies in my 18-inch Starmaster at 220x but by upping the magnification a bit to 270x that increased the contrast (darker background) and I had no problem picking up 15th magnitude galaxies. Other than some breeze later in the evening, it was a fun evening and I worked mostly of Mark Wagner's list for May, but focused on Arp groups -- particularly close pair and groups. Here a full recap of the evening. The links are for scanned images from Arp's original catalogue.

Steve

Arp 18 (NGC 4088) http://nedwww.ipac.caltech.edu/level5/Arp/Arp5.html
This showpiece galaxy appeared appeared very bright, large, elongated 5:2 SW-NE, 4.5'x1.8', with a bright elongated core. The galaxy is very asymmetric, with an unusually mottled or clumpy appearance and an irregular outline. At the north end is a faint extension (this is a disrupted arm on the DSS) that juts out or dangles from the NE end of the main body and hooks slightly towards a mag 13.5 star located 3.7' NE of center. A more subtle irregular extension is at the SW end. Just east of the south side of the core the surface brightness drops, either due to a dust lane or large dust patch on the NE side of the core and then brightens a bit again just following the dust patch. N4085 lies 11' SSW.

Arp 79 (NGC 5490C) http://nedwww.ipac.caltech.edu/level5/Arp/Arp20.html
Only a single galaxy here though located 4.7' NE of N5490 and 7' S of IC 983/982 = Arp 117, so there are actually two different Arps in this field. At 280x it appeared very faint, fairly small, 25" diameter, irregularly round, low even surface brightness. This galaxy is in the class of spirals with "large high surface brightness companions on arms", though the galaxy was much too faint to see this detail.

Arp 84 (NGC 5394/5395) http://nedwww.ipac.caltech.edu/level5/Arp/ Arp21.html
Another interacting double system. The brighter member, N5395, appeared fairly bright, very elongated ~3:1 N-S, 1.7'x0.5', brighter 25"x20" core, irregular appearance with a brighter knot or extension on the NW side. A very faint extension or haze is off the W side (this is an "arm" that interacts with N5394). Several faint stars are near including a mag 13.5 1.7' S of center, a mag 15 star 1.9' N of center and another mag 15 star ~1.5' WNW of center. Its companion, N5394, is just 1.8' NNW and appeared fairly faint, very small, slightly elongated, 20"x15". A mag 15 star is off the E end (the one due N of N5395). IC 4356, a much fainter galaxy, is in the field 4' NNW. Mark Wagner and I had looked at this pair from Willow Springs last weekend, but I wanted to take another look at this interesting pair.

Arp 90 (NGC 5929/5930) http://nedwww.ipac.caltech.edu/level5/Arp/ Arp23.html
The brighter member, N5930, appeared fairly bright, moderately large, elongated 2:1 NNW-SSE, 1.3'x0.6', with a small, bright round core. N5929 is attached on the SW side of the core and appeared moderately bright, small, round, 25" diameter, high surface brightness, very small bright core, faint stellar nucleus.

Arp 95 (IC 4461/IC 4462/MCG +05-34-078) http:// nedwww.ipac.caltech.edu/level5/Arp/Arp24.html
IC 4462 is the brightest member of a very close trio. At 260x it appeared faint, small, round, 25" diameter, low even surface brightness. Located 1.3' W of a mag 12.5 star. This galaxy is misidentified as IC 4461 in all modern catalogues. IC 4461 lies 45" SW of center and is a very faint round glow, ~12"-15" diameter. Finally, MCG +05-34-078 is just 32" E of center and was an extremely faint, hazy spot with averted vision. This last galaxy is misidentified as IC 4462 in modern sources.

Arp 117 (IC 982/983) http://nedwww.ipac.caltech.edu/level5/Arp/ Arp30.html
IC 983 is moderately bright but fairly small, slightly elongated NW- SE, ~25"x20", weak concentration to a very faint stellar nucleus. Located 1.5' NW of a mag 9 star which overwhelms the much larger fainter halo. IC 982 is located 2.5' SW and appeared fairly faint, fairly small, round, weak concentration, 35" diameter.

Arp 136 (NGC 5820/5821) http://nedwww.ipac.caltech.edu/level5/Arp/ Arp34.html
The brighter member here is N5820 -- a bright, moderately large glow, elongated 2:1 E-W, 1.1'x0.5', containing a small bright core. Located 8' WNW of the bright double star SHJ 191 = 6.8/7.6 at 40". It helped to keep this bright pair out of the field while observing N5821. Just 3.7' NE is N5820, a faint, moderately large glow, elongated ~2:1 NW-SE, 1.1'x0.6', low even surface brightness.

Arp 199 (NGC 5544/5545) http://nedwww.ipac.caltech.edu/level5/Arp/ Arp50.html
This is a really odd-looking interacting pair (see the Arp image). N5545 forms the eastern component of this contact pair and it appeared fairly faint, fairly small, very elongated 3:1 SW-NE, 0.9'x0.3', weak concentration. N5544 appears as a brighter knot, ~25" diameter, attached at the preceding end of N5545 with a weak even concentration.

Arp 239 (NGC 5278/5279) http://nedwww.ipac.caltech.edu/level5/Arp/ Arp60.html
This is an interacting double system consisting of N5278 and N5279. N5278 is the brighter southwestern component and appeared fairly faint, small, irregularly round, 30"x25", weak concentration with a slightly brighter core. The surface seemed slightly irregular or mottled. A mag 6.9 star lies 8' ENE, but it was easy to keep it out of the field. N5279 was barely off the east end and appeared faint, very small, round, 12" diameter, really too small for any details. but it was just resolved off the ENE end of N5278. This is an M51- type interacting pair, though there was no sign of the connecting arm.

Arp 297 (NGC 5752/5753/5754/5755) http://nedwww.ipac.caltech.edu/ level5/Arp/Arp75.html
This is a faint quartet with brightest member N5754. The group is basically split into two pairs -- N5754/5752 and N5753/5755, although some modern sources have the identifications mixed up. The first pair consists of N5754, a moderately bright glow, round, 1' diameter, with a very small bright core and faint stellar nucleus. Its companion, N5753, is just 1' W and appeared faint, small, elongated 3:2 NW-SE, 24"x18". The second pair is pretty dim -- the brighter component, N5755 is very faint, round, just 20" diameter, with a faint stellar nucleus or a faint star seemed to be superimposed. On the DSS, this is a tiny interacting double system, so perhaps I was just beginning to resolve the components. N5753 is just an extremely faint spot just under 2' NW. It was only 10" in diameter and required averted vision to identify.