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Object of the Week June 9, 2019 – Arp 49
ARP 49 NGC 5665 VV 412 UGC 9352 MCG +01-37-024 PGC 51953
Class: Spiral Galaxy
Type: SAB(rs)c pec IV-V
Constellation: Boötes
RA: 14h 32m 26.0s
DEC: +08° 04’ 48”
Mag: 12.6v
Surface Brightness: 12.8
Size: 1.9’ x 1.2’
The constellation Boötes is far off the Milky Way and like most off-Milky Way constellations it is well populated with faint external galaxies. None of its galaxies are exceptionally bright, but many are part of interesting galaxy pairs or galaxy groups. Our Object of the Week, ARP 49, is one of those rather non-descript galaxy pairs.
NGC 5665 (H II-27, h1833) was discovered by William Herschel on January 30, 1784 during sweep 134. He recorded his observation as simply “a nebular following the large star under Bootis not marked in H.” His sister Caroline Herschel added the comment in her sweep copy that “the nebula is pB and not very small; is not cometic; nor is it equally bright throughout, so that it is probably resolvable.” Subsequently John Herschel recorded 2 observations of NGC 5665 (sweep 153) and called it “pB; R; gbM; 30.”
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Nearly two centuries later NGC 5665 became the cornerstone of a group of 30 peculiar galaxies (Arp 49-78) that Halton Arp categorized as “Spiral Galaxy with small High Surface Brightness companion on arm.” The galaxy pair consists of the spiral galaxy NGC 5665 and the small high surface brightness companion is frequently referred to as NGC 5665A. Arp noted that this companion caused the “Appearance of wake from stellar object in E arm.” However more recent data suggests something otherwise.
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Arp Catalog Image
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As shown by the box superimposed on the closeup of NGC 5665 above, the compact (0.08 by 0.07 arcmin) "companion" is almost certainly merely a brighter part of the larger galaxy. Its recessional velocity is within the error bar for the recessional velocity of NGC 5665, and it lies along one of the spiral arms where hot bright clusters of stars might be expected to lie. In fact there is a large number of such regions in NGC 5665, which suggest an episode of "starburst" formation, presumably due to an interaction with some other galaxy about half a billion years ago. The recessional velocity for the "object" suggests a distance of about 110 million light years, essentially the same as for NGC 5665. Given that and its apparent size, it is less than 2 thousand light years across, which is far more likely to make it a stellar birthplace within NGC 5665 than a separate galaxy.1
So is this companion known as NGC 5665A an interacting galaxy or is it a star-forming region?
Unfortunately, I do not have a successful observation of Arp 49 logged to share. However, before I send you off to give it a go I’ll share a few observations from experienced observers.
1. George Kepple and Glen Sanner (The Night Sky Observer’s Guide Volume 2 Spring and Summer): “12/14” Scopes 125x: NGC 5665 has a fairly faint halo elongated 1.75’ x 0.75’ NNW-SSE and broadly brighter toward a faint stellar nucleus.”
2. Christian Luginbuhl and Brian Skiff (Observing Handbook and Catalogue of Deep-Sky Objects): “In 25 cm this galaxy appears less well concentrated then nearby NGC 5600, cf. The halo is about 1’ diameter with a circular outline. 30 cm shows the halo up to 2’ x 1’.5, elongated N-S. The core is 1’ across with a mottled inner region containing a faint nucleus.”
3. Steve Gottlieb, Deep Sky Forum contributor and Sky & Telescope Contributing Editor: “ 17.5": moderately bright, moderately large, elongated 4:3 NW-SE, 1.6'x1.2', broad concentration to a very ill-defined core. A mag 11 star lies 5.3' ESE of center and a brighter mag 10 star 8.8' ENE.”
4. David Tosteson (The Arp Atlas of Peculiar Galaxies: A Chronicle and Observer’s Guide by Jeff Kanipe and Dennis Webb): “25-in f/5 Newtonian, 350X: Round 1.5’ diameter, mottled appearance, core off center to the NE. Mag 11 star 6’ E. “
Kanipe and Webb present this observation challenge: Observe the full counter clockwise path of arm from S to S and the stellar object in the E arm (NGC 5665A).
Now it is your turn.
Give it a go and let us know!
1Celestial Atlas
