July 23, 2013
NGC 3393, a spiral galaxy in Hydra
Image Credit: NASA/STScI
NGC 3393 is a barred spiral galaxy of some 9,800 light-years across, located about 160 million light-years away from Earth in the constellation of Hydra (Water snake). It is receding from us at approximately 3750 kilometers per second.
Remarkable is that the galaxy has two actively growing supermassive black holes – heavily obscured by dust and gas – separated by only 490 light years, which are likely the remnant of a merger of two galaxies of unequal mass a billion or more years ago.
Most likely NGC 3393 collided with a much smaller galaxy and swallowed it. The merger didn’t disrupt the galaxy’s spiral structure, as a collision between two large galaxies would, nor did it trigger an outburst of starbirth. It did, however, leave the galaxy with two supermassive black holes.
These are the nearest known pair of supermassive black holes. It is also the first time a pair of black holes has been found in a spiral galaxy like our Milky Way.
The black holes are generating X-ray emission as gas falls towards the black holes and becomes hotter. The obscured regions around both black holes block the copious amounts of optical and ultraviolet light produced by infalling material.
Dubbed “minor mergers”, such collisions of one larger and another smaller galaxy may, in fact, be the most common way for black hole pairs to form. However, it is difficult to find good candidates for minor mergers because the merged galaxy looks like an ordinary spiral galaxy. NGC 3393 is a good candidate.
If this was a minor merger, the black hole in the smaller galaxy should have had a smaller mass than the other black hole before their host galaxies started to collide. Good estimates of the masses of both black holes are not yet available to test this idea, although the observations do show that both black holes are more massive than about a million Suns.
Over time, the two supermassive black holes in NGC 3393 will move closer together, orbiting faster as they do so. Eventually, they will merge to produce a single black hole.
This optical image shows the prominent S-shaped inner region of NGC 3393, and was created with data obtained on 28 Feb. 2004 & 12 March 2011 with the Hubble Space Telescope.