Page last updated Aug 30, 2015|
PGC 44117 (= Markarian 231)
A magnitude 13.6 spiral galaxy (type SA(rs)c? pec) in Ursa Major (RA 12 56 14.2, Dec +56 52 23)
Physical Information: Based on a recessional velocity of 12640 km/sec, a straightforward calculation indicates that PGC 44117 is 585 to 590 million light years away. However, for objects at such distances we should take into account the expansion of the Universe during the time it took their light to reach us. Doing that shows that the galaxy was about 560 million light years away at the time the light by which we see it was emitted, over 570 million years ago (the difference between the two numbers being due to the expansion of the intervening space during the light-travel time). The brighter portion of the galaxy is about 0.85 by 0.5 arcmin, corresponding to 135 to 140 thousand light years, while material scattered all around it spans about 2.25 by 1.75 arcmin, corresponding to 365 to 370 thousand light years. The galaxy has an extremely active core, containing extensive starburst regions, is classified as a Seyfert galaxy (type Sy 1) and is the closest known quasar.
System Dynamics: Ultraviolet studies of the galaxy have recently shown that its core must contain a binary black hole, consisting of a 150 million solar mass black hole in the center of a nearly empty region, orbited by a 4 million solar mass black hole every 1.2 years. The orbital motion helps sweep material out of the accretion disk of gas surrounding the core, so that UV radiation emitted by the accretion disk falls off dramatically in the nearly clear central region. Even before the discovery of the binary black hole it was presumed that PGC 44117 was a late-stage result of the merger of two galaxies, and it is now thought that the orbiting black holes represent the central components of the merged galaxies. Over time the smaller black hole will gradually spiral inwards, finally merging with its larger companion, most likely within just a few hundred thousand years. Although the galaxy merger must have taken place over a period of tens of millions of years, the quasar is thought to have been reactivated only about a million years ago.
Above, a 12 arcmin wide SDSS image centered on PGC 44117
Below, a 2.8 arcmin wide SDSS image of the galaxy and its extendeded "halo"
Below, a 2 arcmin wide HST image of the galaxy (Image Credit NASA, ESA, the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration, and A. Evans (University of Virginia, Charlottesville/NRAO/Stony Brook University))
Below, a 1 by 2 arcmin HST image of the galaxy (Image Credit as above)
The starlike image at the center of the galaxy is caused by the quasar
Below, an artist's conception of the black holes and accretion disk (Image Credit NASA/ESA & G. Bacon (STScI))