Object of the Week May 24, 2020 – Arp 90

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.”

Attachment 3891
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.

Attachment 3893

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.

Attachment 3892
Image by dcrowson

Now it is your turn. Give it a go and let us know!

One observation of Apr 90, the early morning of 15 April 2013 in the French Alps:

12" SCT @ 179x / 27'

NGC5929 and NGC5930 are both visible, separately.
NGC5929 is the SW galaxy: Smaller than NGC5930, round and faint with a round, brighter core. Using AV the faint nucleus is visible.
NGC5930 is the NE galaxy: Larger than NGC5929, slightly elongated NW to SE with a round, brighter core and an outer halo. Using AV the nucleus is visible which is fainter than the nucleus of NGC5929.
To the WNW is a mag. 13 star with another mag. 13 star to its west, to the SE is a mag. 12.5 star, slightly farther than the first mag. 13 star.

Here it is in CSOG's Arp edition:

Attachment 3894Attachment 3895Attachment 3896

"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. "

Perhaps, though Herschel often called nebulae (typically galaxies) "easily resolvable" or "resolvable" and it wasn't meant in the modern sense of resolving into two components. One of his main goals was to determine if a nebula showed signs of resolving into individual stars, indicating it was actually a distant cluster and not a "luminous fluid" (true nebulosity). So, in this context, he used "resolvable" in descriptions to mean it appeared mottled or had an uneven texture and might consist of stars.

Nice pair or better trio.

Just had a look at my notes and I wrote about UGC 9857. "steadily visible with averted vision, 1:2 central region with low surface brightness, faint bend to the N and even fainter bend to the E"

sketch: 27", 293x, NELM 6m5+, Seeing III
Attachment 3897

Hi Mark,
I observed this trio in relatively dark skies in West Virginia back on 7-24-1998 with a 24" f/4.5 at 350x as part of the AL ARP observing program. Elevation was 2,600ft. I noted the NELM to be 6.0 that evening.

NGC 5929: Small rather bright oval extended 1 1/4 to 1 with a much brighter core. Dim 14th star is 3min W. PA = 30. Contact companion Ngc 5930 is just E. PGC 55095 (UGC 5897) is about 5min NE. ARP 90

NGC 5930: Medium sized bright oval extended 2 to 1 with a much brighter core. Has a star in the center. Diffuses out away from the center. PA = 165. Contact companion 5929 is just W of the core. PGC 55095 (UGC 5897) is about 4min NE. ARP 90

PGC 55095 (UGC 5897): Very faint medium sized oval elongated 3 to 1 with no brighter core region detected. Diffuse looking slash. Averted helps to hold it steady. 13th mag star is 5min NE. Ngc 5930 is 4min SW and 5929 is 5min SW. PA = 120.

Matt Orsie
Hedgesville, WV

Like Victor, I have seen it before with a 12" SCT, resolving the companion. Here is a new sketch with 20" (570x, sky 21.73 mag/arcsec2, seeing 1 arcsec). The details in the main galaxy are tough! The shape of the UGC is obvious though, which may have something to do with how blue it is photographically, in comparison.

Attachment 3910