Figure 1: An illustration of the Milky Way with the galactic halo and Sun’s position indicated. The Milky Way is a barred spiral galaxy measuring over 100,000 light-years across and contains a few hundred billion stars. Credit: Pearson Education Inc.
A study done by Rashkov & Madau (2013) show that there may be as many as 2000 to as few as 70 intermediate-mass black holes (IMBHs) lingering in the halo of the Milky Way. Unlike the supermassive black hole (~ 4 million solar-mass) that sits in the heart of the Milky Way, IMBHs have masses ranging from a few 100 to a few 100,000 solar-mass. These IMBHs were once surrounded by subhalos of stars and matter, which were the subgalactic building blocks of present-day massive galaxies. When these subhalos merged in the past to form the present-day Milky Way, a relic population of IMBHs is left behind in the Milky Way’s halo.
The relic population of IMBHs can be divided into two main subpopulations - “naked” IMBHs and “clothed” IMBHs. “Naked” IMBHs dominate in the inner region of the Milky Way’s halo, but become increasingly rarer at larger distances where “clothed” IMBHs dominate. This is consistant with the fact that subhalos orbiting in the denser inner regions of the Milky Way’s halo experience strong disruption which strip off all stars and matter, leaving the IMBHs exposed. In the rarefied outer region of the Milky Way’s halo, subhalos experience weaker disruption and results in “clothed” IMBHs that still hold on to surviving clouds of stars and matter around them.
Figure 2: Artist’s impression of an accretion disk around a black hole.
An IMBH lurking in the Milky Way’s halo can occasionally flare-up if it happens to pass through denser regions of the Milky Way and accrete from the interstellar medium. Such flare-ups can be observed across intergalactic distances. Another way to search for IMBHs in the Milky Way’s halo is to look for stars that may accompany an IMBH. Even a “naked” IMBH will posses some stars in a tight cluster around it. Due to its compactness, the cluster of stars around an IMBH may appear point-like, especially so for a more distant IMBH. IMBHs in the Milky Way’s halo can have tangential velocities of up to a few 100 km/s which translate to proper motions of up to a few milli-arcseconds per year. This motion is detectable using the current Hubble Space Telescope and other future space-based telescopes.
Valery Rashkov and Piero Madau (2013), “A Population of Relic Intermediate-Mass Black Holes in the Halo of the Milky Way”, arXiv:1303.3929 [astro-ph.CO]