OR: In Search of Interacting Galaxies -- 6 June 2016
by Steve Gottlieb

It’s been over a year since I met up with observing buddies Mark Wagner and Mark Johnston at Willow Springs, one of our favorite observing spots. We were also joined by Mark McCarthy, making calling out anyone else’s name in the dark quite amusing!

The 3000-ft elevation observing site is in the southern section of the Diablo range, probably best known for Mt Hamilton, Mt. Diablo and San Benito Mountain. My drive from Berkeley opened up south of San Jose on Highway 101, turning into farmland and ranches after the Gilroy turnoff to Hollister. Once leaving Hollister on Highway 25 (which leads to Pinnacles National Monument), it’s all wide open rolling hills through the tiny town of Tres Pinos to Paicines. The last stretch is down Panoche Road, which eventually narrows into a winding canyon road towards Willow Springs.

At the turnoff to the Willow Springs area, I met the three Marks who had arrived a bit earlier. I was also greeted by the sight of crimson-tinged smoke clouds to the west, which looked quite ominous. I later learned, this was from the Coleman Fire, which started earlier in the day in southern Monterey County, just north of Fort Hunter Liggett. Although we were concerned about the possibility of smoke and fire, we decided to drive up to the observing site. Fortunately, the smoke seemed to stay to the west of us, perhaps due to a change in the direction of the winds, and conditions were quite dark all night (SQM readings of 21.7+).

Observing with my 24-inch f/3.7 Starstructure, I continued to track down little-known interacting galaxies as well as post-merger pairs with double nuclei (see report from Memorial Day). Starting at the end of astronomical twilight (now nearly 10:30) until roughly 3:30 AM, I took notes on over 50 objects. Near the end of the evening, Mark McCarthy and I shared views of the Hercules Galaxy Cluster, which I wrote about in the Going Deep column for the July issue of Sky & Tel. Another great evening under the stars. Next up is the Golden State Star Party in a couple of weeks.

— Steve Gottlieb   


Arp 194 = VV 126 = UGC 6945
11 57 55.3 +36° 23’ 20"
V = 14.2; Size 1.2'x0.8'; Surf Br = 14.0; PA = 117°

Arp 194 consists of UGC 6945a = VV 126b and UGC 6945b = VV 126a. These form a 40” pair, apparently connected by a bridge. At 260x; UGC 6945a, the brighter northwest component, appeared faint, small, roundish, 20". UGC 6945b, the southeastern component, is faint, extremely small, round, 6" diameter. The two glows are separated by 40" and the halos were cleanly separated.

Arp placed this system in his category Galaxies (not classifiable as S or E) with material ejected from nuclei. The SDSS image reveals VV 126b (the top galaxy in the image) consists of two colliding galaxies in the process of merging. A tidal tail, consisting of blue complexes of super star clusters, heads towards VV 126a. I wasn't able to use higher power in soft seeing to resolve the double nuclei of VV 126b. The redshift-based distance for the pair is ~480 million light years. Check out the HST image at http://hubblesite.org/newscenter/archive/releases/2009/18/image/a/

A 2003 paper titled "Evidence of tidal stripping of gas and cross-fueling" described the system as following:
"The northern component (A194N) is a distorted spiral or ring galaxy likely disrupted by a collision or close encounter with a southern galaxy (A194S). There is evidence that a third galaxy with similar recession velocity is projected on A194N, but its role is likely secondary. A194S is connected to A194N by a string of emission knots, which motivates our interpretation that the former was the intruder. Three of the knots are easily discernible in B, R, and H images and are assumed to trace the path of the intruder following the encounter, which we estimate occurred a few times 100 million yrs ago. Both A194S and A194N are experiencing strong bursts of star formation."

NGC 4054 = VV 136
12 03 12.4 +57° 53’ 36"
V = 14.2; Size 0.6'x0.4'; PA = 90°

At 322x the western and largest component (VV 136a) of the triple system NGC 4054 appeared faint, small, slightly elongated 20"x15", low surface brightness. The southeast component (VV 136b) is smaller but significantly higher surface brightness and was noted as fairly faint, very small, elongated 12"x9" E-W. The centers of these small galaxies are separated by just 15". VV 136c, the northeast component, was not seen.

The redshifts of these three galaxies are nearly identical, so it apparently forms a physical (interacting?) triplet. Surprisingly, there is very little known about this system as it has not been studied.

Arp 97 = VV 13 = UGC 7085
12 05 45.5 +31° 04’ 08"
V = 14.1; Size 2.6'x1.0'

VV 13B is the slightly brighter of an interacting pair (Arp 97) with VV 13A 1.2' due south. A tidal bridge (not seen) stretches from the core of VV 13A to VV 13B. At 260x, VV 13B appeared faint, small, round, 15" diameter. VV 13A appeared very faint, small, round, 12" diameter. Often the beginning of the two faint spiral arms was visible (mostly on the southwest side), giving a SW-NE elongation and increasing the dimensions to ~20"x12". MCG +05-29-012 lies 1.3' SE. At V = 16.4 this was a challenging object, but seen as an extremely faint glow of ~10”.

The redshift-based distance of this trio is roughly 325 million l.y. Although the system is known as Arp 97, it was first catalogued by Russian astronomer Boris Vorontsov-Velyaminov in his 1959 “Atlas and Catalogue of Interacting Galaxies”. I wrote about several galaxies in the VV catalogue in the September 2014 issue of Sky and Telescope. The article is titled “Seeking Interacting Galaxies”.

Arp 260 = VV 128 = UGC 7230
12 13 37.7 +16° 07’ 11"
V = 13.9; Size 1.0'x0.4'; Surf Br = 13.6; PA = 54°

I reobserved this pair of interacting galaxies (also viewed the previous Monday night) to confirm the sighting of VV 128b. At 322x, VV 128b appeared extremely faint, very small, no structure, but it was confirmed. Situated just off the northeast end of VV 128a, 35" from the center of the main galaxy. CGCG 09-101, located 7.7' WSW of VV 128, was also picked up as a fairly faint, small, round glow, 18" diameter, slightly brighter nucleus.

The previous Monday night (May 30), VV 128a was logged as faint, fairly small, elongated ~2:1 SSW-NNE, 0.8'x0.4', irregular surface brightness, slightly brighter core. Occasionally a 16th mag star was seen at the northeast end (0.4' from center). A mag 15.3 star is 1' SW. CGCG 098-102, located 12.7' NW, appeared fairly faint, round, 20" diameter, easily visible continuously.

The redshift-based distance for this pair is also ~325 million light years. You’ll find this blue pair (indicating ongoing massive star formation) in Coma Berenices, a little over a degree north of M98.

NGC 4410
12 26 28.9 +09° 01’ 11"
V = 12.8; Size 1.3'x0.8'; PA = 110°

As this trio is connected by tidal bridges, it’s surprising it was missed by both Arp and Vorontsov-Velyaminov. NGC 4410 is a merged, interacting pair at 20" separation within a common halo. NGC 4410B, the brighter eastern component, appeared moderately bright, small, round, 20"-25" diameter (the halos overlap), very small bright core. NGC 4410A, the western galaxy, appeared fairly faint, small, round, 15" diameter, weak concentration, lower surface brightness than NGC 4410B.

IC 790 = NGC 4410C, located 1.8' ENE, appeared fairly faint, small, elongated 3:2 E-W, 24"x16", very small brighter nucleus. A very diffuse tidal plume (not seen) connects IC 790 with NGC 4410A/B. PGC 40736, 2.3' ENE of IC 790 (and also connected by a tidal plume), is faint, small, elongated 2:1 WNW-ESE, 24"x12”.

A 2002 study states "The NGC 4410 group of galaxies provides us with a rare opportunity to study a nearby example of a radio galaxy (NGC 4410A) embedded in an extended X-ray source, with evidence for star formation that can be readily spatially distinguished from regions dominated by the active galactic nucleus and shocks.”

IC 3481/3481A/3483 = Arp 175 = VV 43 = Zwicky's Triplet
12 32 52.2 +11° 24’ 15"
V = 13.6; Size 0.9'x0.8'

IC 3481 appeared fairly faint to moderately bright, small, round, fairly high surface brightness, 20" diameter, fairly bright stellar nucleus. Based on my size estimate, I only noticed the bright core region. IC 3481 is the first of three in a linear trio with IC 3481A 1.4' SE and IC 3483 5.5' SE. IC 3481A appeared faint, small, round, 12" diameter, low surface brightness. IC 3483 is fairly faint, elongated 5:2 N-S, 36"x15", low surface brightness. Situated just southwest of a mag 10 star.

On deep images, IC 3481 and 3481A are connected by a tidal plume and a huge arcing tail from IC 3481A reaches about 2/3 of the way to IC 3483. This apparent interaction was discovered by Caltech astronomer Fritz Zwicky before 1952 on an early Palomar 48-inch Schmidt plate. He assumed all three galaxies had experienced a close encounter, disrupted one another, and gas and stars had been ejected into space. But IC 3483 has a very low recessional velocity, so only IC 3481 and IC 3481 form an interacting pair.

More on Zwicky’s discoveries can be found in his 1956 paper “Multiple Galaxies” at http://articles.adsabs.harvard.edu/full/1956ErNW...29..344Z. The nickname “Zwicky’s Triplet” is also applied to other connected systems described by Zwicky.

NGC 4519
12 33 30.3 +08° 39’ 16"
V = 11.8; Size 3.2'x2.5'; Surf Br = 13.9; PA = 145°

At 322x NGC 4519 appeared bright, large, slightly elongated 5:4 SW-NE, at least 1.5'x1.2’. It contains a relatively large brighter core. The halo is noticeably mottled with an uneven surface brightness (probably due to HII knots in the arms). The large spiral forms a pair with NGC 4519A 2.6' NW. This companion appeared very faint (V ~15.4), small, elongated 2:1 SW-NE, low even surface brightness. A mag 14.3 star is 35" SW.

These Virgo cluster members have a similar redshift so probably form a physical pair, but there is no obvious sign of any interaction abnormalities.

NGC 4893 = IC 4015/4016 = VV 222
12 59 59.6 +37° 11’ 36"
V = 14.6; Size 0.5'x0.4'; PA = 3°

The two close components (IC 4015 and IC 4016) of NGC 4893 were resolved at 322x. The brighter northern component (also catalogued as IC 4015) appeared faint to fairly faint, small, round, 15"-18" diameter. The southern member (IC 4016) is faint, extremely small, round, 6" diameter. The centers of the two galaxies are separated by just 19”! A mag 13.5 star is 44" E and a mag 15.0 star is just under 1' SSW. IC 4027, located 4' SE, is extremely faint, very small, round, 12"-15" diameter.

This pair resides at a redshift-based distance of nearly 500 million light years. It forms the southern vertex of an isosceles triangle with bright galaxies NGC 4868 12’ NW and NGC 4914 11’ NE in the constellation Canes Venatici.

NGC 5032
13 13 26.9 +27° 48’ 09"
V = 12.8; Size 2.1'x1.1'; Surf Br = 13.6; PA = 22°

Using 375x the large spiral appeared moderately bright and large, oval 3:2 ~N-S, 1.2'x0.8'. Contains a bright elongated core or bar and a small bright nucleus. A mag 14 star is 1.2' E and a slightly fainter star is 1.3' SW. It was easy to locate just 21' ESE of mag 4.3 Beta Comae.

NGC 5032B forms a physical pair 2.4’ to the south. The companion appeared faint, fairly small, elongated 5:3 WNW-ESE, 20"x12", very small brighter nucleus. The SDSS image doesn’t reveal any signs of interaction between the pair.

NGC 5214
13 32 48.5 +41° 52’ 19"
V = 13.6; Size 1.2'x0.9'; Surf Br = 13.5; PA = 140°

NGC 5214, a face-on spiral, is fairly faint to moderately bright, slightly elongated NW-SE, 45"x35", very small bright core. Mag 9.9 SAO 44651 lies 5' NE along and a mag 10.7 star 2.8' NW. Located 22' SE of mag 6.1 HD 117710.

NGC 5214A forms a contact pair just off the southwest edge, only 30" between centers. The companion appeared extremely faint and small, slightly elongated SW-NE, 0.2'x0.1’.

In 2014, NGC 5214 hosted the type-Ia supernova SN 2014bb. The light-travel time for this pair is ~370 million years.

NGC 5256
13 38 17.6 +48° 16’ 37"
V = 13.2; Size 1.2'x1.1'; Surf Br = 13.3

This galactic train wreck appeared fairly faint to moderately bright, fairly small, elongated 4:3 SW-NE, 40"x30", brighter ill-defined core, uneven surface brightness. Occasionally, a brighter quasi-stellar knot (nucleus of the merged companion) would pop on the northeast end of the glow! HJ 2667, a wide pair (14") of mag 11 stars, lies 5.6' WSW.

This colliding galactic pair consists of a Seyfert 2 nucleus to the southwest and a LINER nucleus (low ionization nuclear emission-line region) to the northeast. The system is also classified as a LIRG (Luminous Infrared Galaxy) and has been well studied in X-ray, radio and infrared wavelengths, with 304 references in NED (Nasa-IPAC Extragalactic Database).

NGC 5144
13 22 54.2 +70° 30’ 52"
V = 13.1; Size 1.2'x0.9'; Surf Br = 13.0; PA = 150°

This appears to be another post-merger system with two nuclei though it could be a single highly disrupted system. The general glow is moderately bright, irregularly round, fairly small, 0.7'x0.6', mottled or uneven surface brightness but no distinct core. Situated at the midpoint of a mag 11.8 star 1.9' NNW and a mag 13.7 2.0' SSW.

NGC 5144 probably has a merged companion at the south edge, catalogued as NGC 5144 NED01 as well as as LEDA 200298. It appeared as a quasi-stellar knot (less than 6" diameter) at the south edge of the halo, just 18" from the center of the main galaxy!