Object of the Week January 5, 2020 – Arp 82
NGC 2535, Arp 82A, UGC 4262, PGC 22957, MCG+04-20-004, VV 9a
Constellation: Cancer
Class: Spiral
Type: Sc
RA: 08h 11m 13.5s
DEC: +25° 12' 25"
Mag(V): 12.6
Mag(B): 13.3
Size: 2.1’ x 1.1’
NGC 2536, Arp 82B, PGC 22958, MCG+04-20-005, VV 9b
Constellation: Cancer
Class: Barred Spiral
Type: SBc
RA: 08h 11m 15.9s
DEC: +25° 10' 46"
Mag(V): 13.9
Mag(B): 14.7
Size: 45"x 26"
NGC 2535 and 2536 are a pair of interacting galaxies comprising the peculiar galaxy listed as Arp 82 in Halton Arp’s “Atlas of Peculiar Galaxies” (Astrophysical Journal Supplement, vol. 14, 1966). Both of these galaxies were discovered on the night of January 22, 1877 by Jean Marie Édouard Stephan, using the 80 cm (31.5 inch) Foucault reflector at the Marseille (France) Observatory. Historical identification compiled by John Dreyer in his “The New General Catalogue of Nebulae and Clusters of Stars” notes NGC 2535 as "extremely faint, very small, round" and NGC 2536 as "very faint, very small, round".
The Arp classification of this pair is “spiral galaxy with large high surface brightness companion on arm.” The most well-known galaxy in this classification is the Whirlpool galaxy (M51; Arp 85), which is composed of a spiral galaxy NGC 5194 that is interacting with a smaller elliptical galaxy NGC 5195. The interaction has distorted the shape of both galaxies; the spiral arm pattern has been enhanced in the larger spiral galaxy, and a bridge of stars and gas has formed between the two galaxies. Many of the other galaxies in this category are also connected by bridges.
big_arp82.jpeg
Halton Arp's image of NGC 2535 & 2536 (Palomar 200”)
Arp 82 is an interacting pair of galaxies with a strong bridge and a long tail. NGC 2535 is the big galaxy and NGC 2536 is its smaller companion. Arp remarks that the "arm opposite companion extremely long." The disk of the main galaxy looks like an eye, with a bright "pupil" in the center and oval-shaped "eyelids." This ocular structure is indicative of a recent grazing encounter. Dramatic "beads on a string" features are visible as chains of evenly spaced star-formation complexes along the eyelids. These are presumably the result of large-scale gaseous shocks from a grazing encounter. The colors of this galaxy indicate that the observed stars are young to intermediate in age, around 2 million to 2 billion years old, much less than the age of the universe (13.7 billion years).
ngc2535wide.jpg
Image courtesy of Professor Courtney Seligman
Why didn't Arp 82 form many stars earlier, like most galaxies of this mass range1? Scientifically, it is an oddball and provides a relatively nearby lab for studying the age of intermediate-mass galaxies. In more popular terms, think of this as an example of arrested development. For some reason, it took a kick-in-the-pants to get the stars forming recently, whereas most other galaxies of this mass range formed their stars much earlier (between 4 and 8 billion years ago).
So what are we going to see? Amateur observing reports on Arp 82 are few and far between. Here is one by Bob Hill, Amarillo (TX) Astronomy Club. “In the 16" [f/4.5] at 233x NGC 2535 is a large faint object with a brighter central body that is about 1'x.7' elongated in an E-W orientation. There are very faint arms extending about 1' to the N and S from this core area. There is no central brightening of the nucleus. 2536 is at the end of the S arm, a small bright oval about .5'x.3' aligned NE-SW with a very bright non-stellar core.”
Christian B. Luginbuhl and Brian A. Skiff in their Observing Handbook and Catalogue of Deep-Sky Objects provide these observation notes. “Of this pair, only eg 2535 is visible in 15 cm. It appears as small spot of low surface brightness with three mag. 11.5-12.5 stars nearline in contact in its NW side. In 30 cm the halo is about 30” across and shows some weak broad concentration. The interacting companion, eg 2536, is visible as a tiny circular spot 1.8’ SSE. The overall size is only 15”, but it shows relatively good concentration to a faint substellar nucleus.”
Now it is your turn. Give it a go and let us know!
1 Arp 82=VV 009 is the system of M51-type. It consists of a luminous spiral galaxy with the well-defined two-armed spiral structure (NGC 2535), one arm of which connects or overlaps with the close satellite (NGC 2536). In addition, NGC 2535 possesses a long tidal tail as the continuation of the north spiral arm at the opposite side from the satellite. The systemic velocity of the main galaxy is close to 4000 km s−1 , so we adopt the distance 54 Mpc for this system. Both galaxies NGC 2535/36 in the system Arp 82 are embedded in the large H I envelope evidently associated with the main galaxy. The extended gaseous disc is too large to be the result of the observed interaction, and most probably it existed before the galaxies became close (Kaufman et al. 1997). For the adopted distance a total mass of H I in the galaxy pair is 2.3 × 1010 MO (in agreement with single dish observations) where only a small portion of H I (5×108 MO) belongs to the satellite (Kaufman et al. 1997), however the latter overlaps with the gaseous tidal bridge extending from NGC 2535, which makes the H I mass estimation model dependent.
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