April 23, 2014
NGC 4631, a barred spiral galaxy in Canes Venatici
NGC 4631 (also known as the Whale Galaxy) is an edge-on, barred spiral galaxy of about 140,000 light-years across that lies some 30 million light-years away from Earth in the small northern constellation of Canes Venatici (the Hunting Dogs), while it is receding from us at approximately 606 kilometers per second. This galaxy’s slightly distorted wedge shape gives it the appearance of a whale, hence its nickname.
This galaxy has a nearby companion, named NGC 4627, a dwarf elliptical galaxy which lies just above it. Together also called Arp 281, this interacting galaxy pair was listed in the Atlas of Peculiar Galaxies as an example of a “double galaxy”. Arp’s Atlas of Peculiar Galaxies is a catalog of 338 peculiar galaxies produced by Halton Arp in 1966.
NGC 4631 is also a gravitationally bound companion of NGC 4656 (a barred spiral galaxy of some 75 thousand light-years across and about 29 million light years away from Earth), which lies about half a degree to the southeast, and could be within half a million light-years of its neighbor. Their interaction is probably responsible for the curved end of the galaxy (which is listed as NGC 4657), and its nickname, the Hockey Stick.
NGC 4631,NGC 4627,NGC 4656 and NGC 4657 are all four part of the NGC 4631 Group of galaxies. Estimates of the number of galaxies in this group range from 5 to 27, and all studies identify very different member galaxies for this group.
The Whale Galaxy is a bright, large and extended galaxy with a very bright nucleus. It contains a central starburst, which is a region of intense star formation. The strong star formation is evident in the emission from ionized hydrogen and interstellar dust heated by the stars formed in the starburst.
The most massive stars that form in star formation regions only live for a short period of time, after which they explode as supernovae. So many supernovae have exploded in the center of NGC 4631 that they are blowing gas out of the plane of the galaxy. This superwind can be seen in X-rays and in spectral line emission. The gas from this superwind has produced a giant, diffuse corona of hot, X-ray emitting gas around the whole galaxy.
Because this nearby galaxy is seen edge-on from Earth, professional astronomers observe this galaxy to better understand the gas and stars located outside the plane of the galaxy. It can be seen with a small telescope. North is up in this image.