Sunday, February 24, 2013

Capturing an Ultra-Massive Black Hole

Ultra-massive black holes (UMBH) are known to exist in the cores of giant elliptical galaxies. These black holes have masses exceeding 10 billion times the mass of our Sun. Located approximately 220 million light years from the Milky Way, a galaxy known as NGC 1277 is known to harbour an unusually massive black hole with 17 billion times the mass of our Sun. This is odd because the galaxy NGC 1277 is far too small to be hosting a black hole with such an enormous mass. Typical supermassive black holes do not exceed 0.1 percent of the mass of their host galaxies but the mass of the black hole in NGC 1277 is equivalent to 14 percent of the total stellar mass of the galaxy. NGC 1277 is a modest sized lenticular galaxy that is a member of the Perseus Cluster of galaxies.

Figure: The Perseus Cluster of galaxies.

One possible scenario for the origin of the UMBH in the heart of NGC 1277 is that it did not form at where it currently resides. Instead, such an UMBH is expected to form during the merger of two giant elliptical galaxies, with each galaxy having its own supermassive black hole. Following the merger of the two galaxies, the final in-spiral and merger of the two supermassive black holes can generate sufficient gravitational radiation recoil to eject the resultant UMBH from the post-merger galaxy.

Not far from NGC 1277 is a giant elliptical galaxy known as NGC 1275. This galaxy is the dominant central galaxy of the Perseus Cluster of galaxies and it is likely to be the post-merger galaxy from which the UMBH was ejected. The ejected UMBH probably wandered in the core of the Perseus Cluster of galaxies before it was eventually captured by NGC 1277 during a close encounter. In order for the UMBH to be ejected from NGC 1275, the gravitational radiation recoil following the merger of the two supermassive black holes must be large enough to provide the necessary “natal kick” to escape the galaxy. Assuming the escape velocity from the nucleus of NGC 1275 is 1250 km/s, a “natal kick” of 1800 km/s will give the ejected UMBH a terminal velocity of 1300 km/s.

As the UMBH orbits within the core of the Perseus Cluster of galaxies, it collided with NGC 1277 and became gravitationally captured by the galaxy. Once captured into orbit around NGC 1277, dynamical friction with stars and matter in the galaxy causes the UMBH to gradually in-spiral towards the centre of the galaxy within a timescale of a few hundred million years. If the capture happened more than a billion years ago, that should give NGC 1277 ample time to settle down and regain its current symmetrical appearance. Furthermore, the capture of an UMBH is likely to impart a substantial amount of angular momentum to NGC 1277 and contribute significantly to the current rotation of the galaxy.

The presence of an UMBH in a modest sized galaxy like NGC 1277 shows that migration of black holes between galaxies is possible. In this case, a very massive black hole was ejected from its galaxy (NGC 1275) and got captured by a much smaller galaxy (NGC 1277). In fact, the present day appearance of NCG 1275 is consistent with such an event since the black hole currently residing in its core is too small for a galaxy of its size. The black hole in NGC 1275 is half a billion times the mass of our Sun and it is likely to be a moderate mass black hole that was regenerated after the ejection of the UMBH.

1. van den Bosch and Remco C. E. et al. (29 Nov 2012), “An over-massive black hole in the compact lenticular galaxy NGC 1277”, Nature 491 (7426): 729-731.
2. G. A. Shields and E. W. Bonning (18 Feb 2013), “A Captured Runaway Black Hole in NGC 1277?”, arXiv:1302.4458 [astro-ph.CO]