Object of the Week May 24, 2020 – Arp 90
NGC 5929, VV823, UGC 9851, PGC 55076, MCG+07-32-006
Constellation: Boötes
Type: Sab pec
Class: Seyfert 2 galaxy
RA: 15h 26m 6.2s
DEC: +41° 40' 12.6"
Magnitude: 13.6v
Size: 1.0’ x 0.9’
NGC 5930, VV 823, UGC 9852, PGC 55080, MCG+07-32-007
Constellation: Boötes
Type: SAB(RS)b pec (weakly-barred spiral galaxy with a poorly defined nuclear ring structure)
Class: Starburst galaxy
RA: 15h 26m 8.1s
DEC: +41° 40' 31.7"
Magnitude: 12.2v
Size: 1.8’ x 0.7’
Tucked away in Boötes approximately 133 light-years away you’ll find two rather non-descript galaxies, NGC 5929 and NGC 5930, that just happen to be interacting with each other. Halton Arp cataloged this pair as Arp 90 and classifying them as “Spirals with large high surface brightness (HSB) companions on arms.”
arp90.gif
Arp 90 from Arp Atlas of Peculiar Galaxies
NGC 5930 is an oblique barred spiral galaxy with a bright core and a bright compressed S shaped spiral pattern emerging from the bar. Its spiral pattern fades rapidly to a broader, smooth spiral pattern and the arm on the western side appears to be in direct contact with NGC 5929. Visually NGC 5929 and NGC 5930 form an interesting pair, and at medium magnification they definitely appear to be in contact. NGC 5930 is the larger and brighter galaxy. It is a roundish, mottled patch of light with a much brighter core. Its extremities are moderately well defined. NGC 5929 is located immediately SW. What is curious is that William Herschel did not note the galaxy (5929) as a separate object. However in his March 18, 1787 entry for NGC 5930, Herschel called it: “Pretty bright, pretty large, irregularly extended. Easily resolvable.” This may indicate that he saw the two components but thought they were one object.
Arp 90 begleiter.jpg
Under reasonably dark skies Arp 90 should be visible in an 8” telescope. With certainty, larger aperture and higher powers will provide more detailed reviews. Here a few observations that should help set expectations when you seek out Arp 90.
Steve Coe, using a 13" f/5.6, notes: "Pretty faint, small, round and little brighter in the middle at 165X. These two galaxies (NGC 5929 and NGC 5930) form a contact pair that is elongated 2 X 1 in PA 45, somewhat like a figure 8."
George Kepple and Glenn Sanner, using 12”/14” telescopes at 125x, note: “NGC 5929 and NGC 5930 are a NE-SW contact pair which appear as a hazy figure S. Both galaxies are fairly faint, small, and round with little central brightening. NGC 5930, the northwestern nodule, is the larger of the two.” Using 20”/22” telescopes at 175x, they note: “This is a classic example of interacting galaxies connected by a thin nebulous bridge. NGC 5930 is the larger and brighter of the pair, appearing elongated 1’ x 0.5’ NNW-SSE with an oval core that contains a stellar nucleus. NGC 5929, only half the size of its companion, is an amorphous glow about 30” diameter with a very faint stellar nucleus.”
As a secondary challenge, look for UGC 9857 (mag 15.8) approximately 5’ NE of NGC 5930. UGC 9857 is an unusual Barred Magellanic type (SBm/IBm) galaxy. Perhaps it interacted with NGC 5930 sometime long ago in the past.
Arp 90 wf dcrowson.jpg
Image by dcrowson
Now it is your turn. Give it a go and let us know!
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