In the constellation Virgo
8.5 million times the mass of the Sun
Diameter about three-quarters of the distance from the Sun to Mercury
The supermassive black holes in the hearts of many galaxies form what are known as active galactic nuclei (AGN). The accretion disks around these black holes are especially hot and bright, so they produce copious amounts of energy at many different wavelengths.
Some AGN, however, are muffled. In these cases, the galaxy's nucleus is surrounded by a broad, thick doughnut of gas, known as a torus, that is far outside the accretion disk. If this disk is aligned edge-on as seen from Earth it absorbs much of the AGN's energy, so the galaxy looks less "active" than it should.
An example is NGC 4388, a spiral galaxy that is a member of the Virgo Cluster, a collection of hundreds of galaxies centered in the constellation Virgo. Observations by space-based X-ray and gamma-ray telescopes confirm that a thick torus encircles the galaxy's nucleus, absorbing light from the black hole's accretion disk.
Water molecules in a disk that is between the accretion disk and the torus are zapped by energy from nearby stars, boosting their energy level and causing them to emit microwaves. High concentrations of water form microwave hotspots known as masers. If Earth lies along the path of a maser's beam, radio telescopes sensitive to microwaves can detect them.
Precise tracking of these masers reveals their motion around the center of the galaxy. By applying the laws of orbital motion, astronomers can determine the precise mass of the central object.
Using measurements made from 2005 to 2009, a team of astronomers measured the masses of the supermassive black holes in seven galaxies, including NGC 4388.
The measurements indicate that the black hole is about 8.5 million times as massive as the Sun, which is twice as massive as the black hole at the center of the Milky Way. However, the research team notes that it has tracked only five masers so far, so its conclusions "should be used with some caution until better data are obtained."
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This document was last modified: March 14, 2012.