Whale Galaxy with Hockeystick

NGC 4631
NGC 4631: Looks like a wide-angle panorama of our own Milky Way. Image taken in 2011 with the Hubble Space Telescope [198]

Walgalaxie NGC 4631 + NGC 4627

The galaxy NGC 4631 has been nicknamed the «Whale Galaxy» because of its slightly asymmetrical shape. In fact, in Figure 2, it looks like a humpback whale swimming upwards. One believes to recognize the flipper (pectoral fin) and fluke (tail fin). The intergalactic mammal was first sighted by William Herschel on March 20, 1787. It is a spiral galaxy that we are looking at closely on the side edge. Distances vary from 18 to 30 million light years. Assuming a distance of 25 million light years, the diameter of the galaxy is about 120'000 light years. The galaxy is in gravitational interaction with neighboring dwarf elliptical galaxy NGC 4627 and galaxy NGC 4656, less than 500'000 light-years away, which can be seen almost half a degree to the southeast. [196]

NGC 4631 + NGC 4627
NGC 4631 + NGC 4627: Whale Galaxy NGC 4631 with companion NGC 4627; 500 mm Cassegrain 5800 mm f/11.4; SBIG STL11K; 70+10+10+10 min LRGB; Bernese Highlands; © 2005 Radek Chromik
NGC 4656/7 + NGC 4631/27
NGC 4656/7 + NGC 4631/27: Hockeystick NGC 4656/7 (left) and Whale Galaxy NGC 4631 (right); 10" Meade Schmidt-Newton, Canon EOS 20Da; 100 min, ISO 1600; Oberes Schlierental, Obwalden; © 2007 Eduard von Bergen

The small galaxy NGC 4627 was discovered on March 20, 1787 by the German-British astronomer William Herschel. It is a dwarf elliptical galaxy and a companion of NGC 4631. Both galaxies show mutual interaction and are listed in the Arp catalog [199] under the number (Arp 281) as double galaxies with inflow and attraction. It can be seen at the top of NGC 4631 in Fig. 3. [196]

Hockeystick Galaxy NGC 4656/7

NGC 4656/7
NGC 4656/7: Hockeystick Galaxy; 500 mm Cassegrain 3625 mm f/7.2; SBIG STL11K; 390+40+40+40 min LRGB; Bernese Highlands; © 2015 Radek Chromik

On March 20, 1787, William Herschel came across this object for the first time. John Louis Emil Dreyer saw it as two separate nebulae and gave them the numbers 4656 and 4657 in his «New General Catalogue». Because of its asymmetric shape, this galaxy has also been nicknamed the Hockey Stick or Crowbar. In the «Catalogue of Principal Galaxies» (PGC) [144] the two are listed as one and the same galaxy (PGC 42863).

According to LEDA [134], the galaxy is moving away from us at a speed of 646±4 km/s. The calculated distances are in the range of 15 to 30 million light years.

NGC 4657 is a bright area of intense star formation with luminous stars and nebulae within galaxy NGC 4656, presumably formed by past gravitational interaction with galaxy NGC 4631, less than half a million light-years away. At a distance of approximately 25 to 30 million light years, the angular diameter of about 2 x 1 arc minutes corresponds to a true size of 7000-8000 light years. In Fig. 4, another faint structure can be seen to the north of the galaxy, which appears to be associated with NGC 4656. [196]

«Catalogue of Principal Galaxies (PGC)», Paturel et al. 1989 [144]
NameRA [hms]Dec [dms]mTypeDim [']Btot [mag]HRV [km/s]PA [°]
NGC 462712 41 59.6+32 34 29E M2.2 x 1.713.076510
NGC 463112 42 07.6+32 32 28SB M15.2 x 2.89.760986
NGC 4656, NGC 465712 43 58.6+32 10 35IB M15.3 x 2.410.764133

Finder Chart

The galaxy group is located in the constellation Canes Venatici near the border of Coma Berenices. The 4.9 mag star 37 Comae Berenices serves as a guide.

Chart NGC 4631
Chart created using SkySafari 6 Pro and STScI Digitized Sky Survey. [149, 160]

Visual Observation

NGC 4627, NGC 4631, NGC 4656, NGC 4657
NGC 4627, NGC 4631, NGC 4656, NGC 4657: Pencil drawing; 14" PWO-Dobson f/4.6, TV-Nagler 13 mm, 123x, 0.67°, D: 6.2, L: unsteady; Langis 1440 m; © 23. 3. 2003, 23:30 Eduard von Bergen

350 mm aperture: In a large field eyepiece, the four galaxies provide an impressive view of the depths of space and their diversity. On the one hand, there is a cannibalizing and strongly graphically influencing pair of galaxies. Next is a bright cigar-shaped galaxy with what appears to be a small, rounded companion. The last-mentioned, small galaxy is about three times deeper in space. It is worth looking at all four galaxies with one look in a large field eyepiece. The details of the 13.3 mag, 9.8 mag and approx. 10.7 mag bright galaxies are clearly visible in a medium magnification. — 14" PWO-Dobson, F:4.6 / TV-Nagler 31mm, 52x, 1.6° und TV-Nagler 13mm, 123x, 0.67°, Eduard von Bergen

400 mm aperture: With a 21 mm Tele Vue Ethos eyepiece (85x) both big galaxies fit into the same field of view and are pretty impressive thanks to their different shapes. The galaxies were near zenith at that time of the night and showed high contrast and structure. The small companion NGC 4627 of the Whale Galaxy NGC 4631 is easily visible. So also is the odd shape of the Hockey Stick Galaxy. With higher magnification structure and dust lanes of NGC 4631 become visible. — Taurus T400 f/4.5 Dobsonian, Bernd Nies, Glaubenberg Langis, February 28, 2022

References

134Lyon-Meudon Extragalactic Database (LEDA); leda.univ-lyon1.fr
144Catalogue of Principal Galaxies (PGC); Paturel G., Fouque P., Bottinelli L., Gouguenheim L.; Astron. Astrophys. Suppl. Ser. 80, 299 (1989); cdsarc.unistra.fr/viz-bin/cat/VII/119 (2021-02-18)
149SkySafari 6 Pro, Simulation Curriculum; skysafariastronomy.com
160The STScI Digitized Sky Survey; archive.stsci.edu/cgi-bin/dss_form
196Celestial Atlas by Curtney Seligman; cseligman.com/text/atlas.htm (2020-12-28)
198The belly of the cosmic whale; esahubble.org/images/potw1146a (2020-12-28)
199Atlas Of Peculiar Galaxies, Halton Arp; ned.ipac.caltech.edu/level5/Arp/Arp_contents.html (2020-12-28)