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Black Holes Encyclopedia

NGC 3998

Printable version




In the constellation Ursa Major


45 million light-years (13.7 megaparsecs)


270 million to 800 million times the mass of the Sun


Diameter roughly the diameter of Jupiter’s orbit to the diameter of Uranus’ orbit around the Sun

Discovery Methods


NGC 3998

Using a laser to help compensate for the blurring effects of Earth's atmosphere, astronomers have obtained precise measurements of the motions of stars in the core of NGC 3998, allowing them to place the mass of the galaxy's central black hole at roughly 800 million times the mass of the Sun. However, their estimate is about four times greater than one made a few years earlier using the motions of gas instead of stars.

NGC 3998 is classified as a lenticular galaxy, with a massive central "bulge" of stars surrounded by a tapering disk, like a lens.

In 2012, a team reported observations of the central part of the bulge made with adaptive optics on the Keck II Telescope in Hawaii. The adaptive optics system uses a laser to "excite" molecules high in the atmosphere, causing them to glow. This creates an artificial star. Light from these molecules is distorted as it passes through the atmosphere, blurring the view. However, small mirrors in the adaptive optics system move hundreds of times per second to compensate for that blurring effect, keeping the view of the laser guide star -- and the true stars and galaxies beyond it -- in sharp focus. The astronomers also looked at spectroscopic observations made by the Keck I Telescope and the orbiting Hubble Space Telescope.

With this system, the astronomers found a large difference in the velocities of stars close to the galaxy's center and those that are farther from the center. Comparing the difference in velocity with measurements of the total amount of starlight from the galaxy's center allowed the astronomers to calculate the black hole's mass.

An earlier study, however, which measured the motions of gas instead of stars, found a mass of roughly 220 million times the Sun's mass.

Comparing the results from these two techniques should help astronomers determine the ratio of the mass of the black hole to the mass of the surrounding bulge of stars and gas. There appears to be a relationship between the two numbers, suggesting that there is a link between the formation of supermassive black holes and their surrounding galaxies. A precise measure of that relationship would help astronomers select among competing theories of how that link works. However, it has been difficult to get agreement between the gas and star measurements, leaving the debate unsettled.

Observations with radio telescopes reveal a possible "jet" of superhot gas squirting away from NGC 3998's central black hole. The jet would come from a wide disk of gas that is spiraling into the black hole. The disk is quite bright, making it hard to see the galaxy's most central regions in detail, even with adaptive optics. A wider disk of gas and dust that is slowly circling the black hole may span up to 45 light-years.


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This document was last modified: November 28, 2012.


NGC 3998
Ground-Based Photo

NGC 3998
Ground-Based Photo


No animations available for this black hole.