Images Credit X-ray: NASA/CXC/Swinburne Univ. of Technology/A. Graham et al.; Optical: NASA/ESA/STScI)
Using the NASA X-Ray observatory Chandra and the infrared (IR) channel of Hubble’s Wide Field Camera 3 (WFC3), a team of astronomers detected a high-energy X-ray source inside a source stretched out towards the center of the spiral galaxy NGC 4424. After exploring several possibilities, this team concluded that the observations give evidence of an intermediate-mass black hole that is infalling towards the center of the larger galaxy NGC 4424.
How did the group reach this conclusion? The HST IR observations revealed an elongated stellar cluster shape pointing toward the center of NGC 4424, while the X-ray observations revealed a point source coincident with the nucleus of the cluster. The lack of material pointing outwards goes against a theory suggesting that this object was the result of material ejected by a kickback from a gravitational interaction event with NGC 4424. The astronomers also explored the possibility that this region is an X-ray binary, but the observations were not consistent with this theory. With this in mind and after obtaining the total mass of the stars in the stellar cluster, the group concluded that this cluster must harbor a medium-mass black hole in its center. This black hole is at the center of the smaller interacting galaxy, and the cluster is the remnant of its nuclear stellar cluster.
Observations of the stellar mass and velocity dispersion in the center of NGC 4424 predict a quiescent supermassive black hole with a mass of more than 100 thousand solar masses, while the observations in the stretched region correspond to a black hole of about 70,000 solar masses. Without any other perturbations, the merging black hole will take more than 1.4 Gyrs to reach the center of NGC 4424, but this value is quite uncertain.
For quite some time now, observations support the theory that galaxies merge to form larger galaxies. With the telescope Hubble, we also learned that most galaxies have black holes in their centers. The idea that the black hole at the center of a merging galaxy is now infalling
towards the center of the new host to form a supermassive black hole, those that we usually see in the late-type galaxies are plausible.