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Object of the Week May 4 2014- NGC 4631 and NGC 4627, The Whale and Pup galaxies
NGC 4631 and NGC 4627 (= Arp 281)
Interacting Galaxy Pair
Canes Venatici
R.A. 12h 42.1'
Dec +32 32'
Magnitude NGC 4631 9.2
Magnitude NGC 4627 12.4
Most galaxies, when seen through telescopes, appear as fuzzy circles, ellipses, or spindles. It is very rare to have one that actually looks like an animal, but NGC 4631 in Canes Venatici does indeed look like its namesake: The “Whale galaxy”. In addition, it is accompanied by a smaller satellite galaxy that indeed looks like it is the whale’s “Pup”. And what’s more, this celestial Whale is indeed an amazing galaxy “animal”: Superbly detailed for almost any telescope, with many challenges for the keen observer, and a nice astrophysical twist to boot!
NGC 4631 is a late type barred spiral (SBd) galaxy, with a diameter of more than 90,000 light years and a mass in excess of 50 billion suns. This galaxy is the largest in the NGC 4631 galaxy group, which includes the aforementioned NGC 4627, as well as NGC 4656/4657, the well known “Hockey Stick Galaxy” (OOTW July 21, 2013 http://www.deepskyforum.com/showthre...e-Hockey-Stick). The “Whale” lies around 22 million light years away and we see it inclined 5 degrees from edge-on. The “Pup”, NGC 4627, is a dwarf elliptical, located about 3’ north of NGC 4631. Very deep photographs show a bridge of light connecting the two galaxies, while radio astronomy has shown that there are two expanding supershells of hydrogen gas at the limps of NGC 4631. The “Whale” shape of NGC 4631 seems to be the result of tidal interactions between it and NGC 4627, but also between it and NGC 4656/4657, which is located about half a degree to the southeast in the sky (and about 300,000 light years away in space). In a 1978 article, F. Combs has outlined the possible tidal interactions between the two neighboring galaxies (http://adsabs.harvard.edu/full/1978A%26A....65...47C), while a 1975 paper by A.J.B. Winter (http://adsabs.harvard.edu/full/1975MNRAS.172....1W) outlines the presence of a neutral hydrogen cloud between NGC 4631 and NGC 4656/4657. Thus, there is little doubt that the abnormalities observed in both galaxies can be attributed to the interactions between the two systems. The patterns of light and dust in the main bulge of NGC 4631 indicate massive and intensive star formation , which has been likely initiated by the tidal influence of NGC 4656/4657 (and to a much lesser degree by 4627). The warped edges of the spiral arms of NGC 4656/4657 also confirm these tidal interactions. In addition, there is evidence of tidal plumes to the north and south of NGC4627, which lies at an actual distance of 13,000 light years from NGC4631.
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Photo taken at Mosfiloti, Cyprus using a Takahashi FS102 refractor
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Photo by Jean Baptiste - Faure (http://jean-baptiste-faure.blogspot....-ngc_1775.html)
Enough astrophysics, what about observing the extragalactic "Whale" and its pup? Well, they are very easy to locate about halfway between Cor Caroli (Alpha CVn) and the Coma Berenices cluster, with a very easy star hop. NGC 4631, at magnitude 9.2 is readily visible in practically any telescope, even a 60mm refractor, as a spindle of light. There are many reports of NGC 4631 actually being seen in binoculars. I have attempted this galaxy in handheld 9X63 and 15X70 binoculars but I was unable to see it, although it was readily visible as a tiny spindle in my tripod-mounted Takahashi 22X60 binos. I believe that the galaxy should be visible in 10x50s if they are tripod mounted, and should be visible in image-stabilized instruments.
With telescopes, the view changes from impressive to magnificent to fantastic as the aperture increases! With my 4” apo, I can easily see NGC 4631 as a long spindle of light with a distinctly brighter center and some mottling around the nucleus. The mottling became more evident at high powers. NGC 4627 can be readily seen as well, despite it being magnitude 12.4, as a tiny puff of roundish light almost touching the northern edge of 4631. Of course, the best view is at medium powers which gives a field wide enough to include the Hockey Stick (NGC 4656/4657) in addition to the Whale and Pup, without sacrificing detail on either galaxy.
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Under dark skies, my 6” achromat provides significantly more detail on the center areas of NGC 4631. Hints of dark lanes became apparent with averted vision, especially at higher powers, while the “whale” shape becomes obvious, with the western edge longer and sharper, and the eastern side broader and somewhat fainter. The shape of NGC 4627 also becomes more defined as an elliptical, especially with higher magnification, and the “gap” between 4631 and 4627 is evident.
Using an 18” dob the whole image is again transformed. A very bright core is located almost due south of NGC 4627 (which itself becomes more detailed) with a 12th magnitude field star in between. To the east and (especially) to the west of the central nuclear brightening, dark lanes, almost vertical to the plane of the spiral arms, can be seen dissecting the galaxy, while another bright patch can be seen just next to the westernmost dark lane. The whole galaxy looks like it is “boiling”, thanks to many brightenings (presumably star clouds), adding a strange 3-D effect to the extragalactic whale. I tried very hard but I failed to detect the (very faint) bridge of light connecting the Whale and Pup like an enormous umbilical cord. Stephen O Meara in his Caldwell observing guide mentions that he and Larry Mitchell did observe this light bridge with a 36” dob, so I guess it should be quite visible in Jimi Lowrey’s 48”! You should keep in mind that although NGC 4631 is quite bright and can withstand light pollution quite well, you really need dark skies to squeeze out all the “internal” details of this celestial whale.
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So, next time you are under dark skies, turn your scope to NGC 4631, and… “GIVE IT A GO AND LET US KNOW!”
