NGC 4486b

Astronomers study the supermassive black holes at the centers of galaxies to learn not just about the black holes, but about the galaxies that surround them. The combination reveals important insights into galaxy evolution.

In the case of NGC 4486b, one insight is that the galaxy likely has been altered by the gravitational effects of a much larger nearby galaxy, M87.

In the 1990s, astronomers used the Canada-France-Hawaii Telescope to measure the motions of stars in the galaxy's center. These observations revealed the presence of a large concentration of mass that was most easily explained by a supermassive black hole. Later observations with Hubble Space Telescope strengthened the finding.

The central black hole is probably around 500 million times the mass of the Sun, which is roughly one-tenth of the mass of the entire galaxy. It appears to be surrounded by a disk of stars, which show up as two bright blobs of light flanking the black hole.

But observations of many galaxies have shown that central black holes typically account for only a fraction of one percent of the mass of a galaxy's central "bulge" of stars. NGC 4486b is a dwarf elliptical galaxy, which is essentially nothing but a bulge. So its central black hole is roughly 100 times more massive than the correlation suggests it should be.

One possible explanation for the discrepancy is interactions with M87, a giant elliptical galaxy that is quite close. Observations suggest that NGC 4486b orbits this larger galaxy, which contains one of the most massive black holes yet discovered.

Gravitational interactions between the two may have stripped away much of the original mass of NGC 4486b or forced some of into the black hole, leaving only a kernel of the original galaxy. So the supermassive black hole at the galaxy's center reveals something of the galaxy's disrupted history.

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This document was last modified: March 5, 2013.