In the constellation Hydra
31 million times the mass of the Sun
Diameter larger than the orbit of Mercury
A billion years ago, the spiral galaxy NGC 3393 grew a little bit bigger. It collided with a much smaller galaxy and quickly incorporated the smaller family of stars into its own body. The merger didn't disrupt the galaxy's beautiful 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 that are just a few hundred light-years apart.
Observations by the space-based Chandra X-Ray Observatory revealed the twin black holes in observations conducted in 2004 and 2011, and astronomers who study NGC 3393 say the merger of two galaxies of vastly different sizes is the most likely explanation.
Other astronomers had already discovered one of the black holes by measuring intense beams of radio waves from a disk of material that encircles the black hole.
Disks of gas and dust around black holes can contain water molecules. Energy from nearby stars, or from the disk itself, can boost the energy level of the water molecules, causing them to emit beams of microwaves known as masers. If Earth lies along such a beam's path, radio telescopes sensitive to microwaves can detect them.
Precise tracking of such a maser reveals its motion around the center of the galaxy. By applying the laws of orbital motion, astronomers can determine the precise mass of the central object. If the object is massive enough and confined to a small enough volume, it can only be a black hole.
In 2004, a team of astronomers observed NGC 3393 with a combination of the Very Large Baseline Array, which consists of 10 radio dishes spread from Hawaii to the Virgin Islands; the 27 dishes of the Very Large Array in New Mexico; and the giant Green Bank Telescope in West Virginia.
The telescopes detected a disk at the center of the galaxy, aligned edge-on to Earth, spanning about one light-year. By tracking the motions of a water maser at the edge of the disk, the astronomers measured the mass inside the disk at about 31 million times the mass of the Sun. Since some of that mass is contained in the disk itself, the central black hole would be somewhat less massive than that. The disk encircles only one of the two black holes detected by Chandra (and is designated "southwest" for its location in NGC 3393).
The Chandra observations found that the two black holes are of roughly equal mass, although the X-ray mass estimates are much less accurate than those obtained through the maser observations.
NGC 3393 is classified as an active galaxy because it produces fairly large amounts of radio waves, probably from jets of particles expelled into space by magnetic fields around the black holes. Both radio and X-ray observations show a flow of material outward from the galaxy's center, supporting the idea of black hole-powered jets.
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.
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This document was last modified: March 14, 2012.