Arp 296 and Arp 299: An Extragalactic Mess in the Big Dipper
Arp 296 (data for PGC 35345):
PGC 35345: RA 11h 28.83' Dec +58 34.3', mag 15.7
Arp 299 (center of VV18)
VV18: RA 11h 28.52' Dec +58 33.8', mag 11.8
During the northern hemisphere’s spring evenings, the familiar “Big Dipper” asterism dominates the zenith. And is here that we encounter today’s OOTW, with several galaxies representing two entries in Halton Arp’s famous catalogue, but which at the same time lie in a confusing galaxy identification mess, AND do show a somewhat messy appearance in the eyepiece!
But let’s tackle these issues one by one. In his “Atlas of Peculiar Galaxies”, Arp has assigned number 296 to a “spiral connecting to another galaxy with long filaments”, while Arp’s 299 was assigned to a “Double Galaxy” next to it.
arp296small.jpgarp299small.jpg
Original Arp Atlas of Peculiar Galaxies photos from http://ned.ipac.caltech.edu/
Unfortunately, Arp in the original paper assigned no galaxies to the “Double Galaxy” but assigned NGC 3690 and IC 694 to Arp 296, the galaxies connected by long filaments. Very soon both professional and amateur astronomers realized that there was something amiss. Now we do know that Arp 296 in reality consists of 16th magnitude PGC 35345 and 18th magnitude SDSS J112850.64+583336.7. Things though were not as clear regarding Arp 299. There is no doubt that Arp 299 consists of NGC 3690 and IC694. But which is which? In addition, several sources associate Arp 299 with THREE galaxies, and in the Vorontsov-Velyaminov (VV) catalog, Arp 299 has three constituent galaxies (VV18 a, b, c). Running some search in the internet showed that there is still a lot of confusion about which galaxy is which.
The VV catalog splits NGC 3690 into components A and B, while VV18c is the third galaxy IC 694. Our own Steve Gottlieb, in his NGC 3690 entry in the NGC/IC project (http://www.ngcicproject.org/) agrees with this interpretation and this is the line I will follow.
Modern deep photos of the region do show well (much better than in the original 200-inch Palomar Observatory photos of the Arp Atlas) the turbulent chaotic mess of the region. The region as imaged by the Hubble Space Telescope is really stunning:
heic0810as1.jpg
http://www.spacetelescope.org/static...heic0810as.jpg, while this photo by Astroimaging Southwest presents nicely the positions of the Arp 296 and Arp 299 components: http://518241.xobor.com/g9p54-Arp-and.html. The Deep Sky Survey photo has overexposed the region of NGC 3690 but Arp 96’s components are well visible.
DSS296.jpg
In the astrophysical context, the Arp 299 components (which are approximately 134 million light years away) seem to have recently gone through major interactions, resulting in major bursts of star formation, while a rather large number of supernovae have been seen in the system (a total of eight over the last thirty years). A January 8, 2015 JPL release suggests that two black holes exist in the heart of the two galaxies comprising NGC 3690, one of which is a real monster (http://www.jpl.nasa.gov/news/news.php?feature=4435). Very little data though exists regarding the astrophysics of the galaxies comprising Arp 296.
So, now that we have found some reasonable order in the naming chaos of Arp 296 and Arp 299, what can be seen visually?
The two Arp systems can be readily located within the ”dipper” part of Ursa Major (see attached map).
A299 finder.jpg
When I located the correct field with my 18” I was surprised on how obvious the central part of the Arp 299 was, even at low power. I have observed it twice; the first time the seeing was so and so, but on the second occasion the seeing was relatively steady. At 275X I was able to readily detect the central glow of the Arp 299 system, which appeared to have an upside-down heart shape when viewed with north up. A much brighter double nucleus appeared near the center of the galaxy conglomerate, this central part being somewhat rombus-shaped. I did not see the 16th IC 694 nucleus which should be slightly NW of the 3690 nucleus. I must admit though that during my observations I was under the impression that IC 694 was actually the western lobe, so I did not specifically look for it.
Arp 296 was much harder. Nothing was detected at first, but then, at 275X, I could locate a very faint round patch of even surface brightness that continuously crossed the boundary between direct and averted vision. Needless to say, only PGC 35345 was detected. The 18th magnitude galaxy was invisible and I suspect it will need more than 25 inches to be detected. As for the filament connecting the two galaxies, it will probably require Jimi’s 48 inches or even more to be detected!.
A299W1a.jpgA299B1a.jpg
I did not have the chance to observe these galaxies with a smaller scope. Arp 299 should definitely be visible in much smaller scopes. I am sure it will be visible in 8 inches and under good conditions in smaller scopes. I plan to revisit them with my 6” during the next new moon period.
So, as always, “GIVE IT A GO AND LET US KNOW”
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