In the constellation Ursa Major
9 million times the mass of the Sun
Diameter about one-half the size of the orbit of Mercury
Early black hole studies revealed a possible relationship between the mass of a supermassive black hole in the center of a galaxy and the mass of the "bulge" of stars around it. Later studies showed an even tighter relationship between the black hole and the range of orbital speeds for stars within the bulge. These relationships are crucial for understanding the link between the formation of central black holes and their surrounding galaxies.
To better understand this link, astronomers are using Hubble Space Telescope and ground-based telescopes to measure the masses of central black holes and the velocities of the stars around them in many more galaxies.
A study released in 2009, for example, targeted five galaxies, including NGC 3945, a large lens-shaped galaxy that is relatively close by, at a distance of about 65 million light-years. The galaxy's center also shows a broad disk and two "bars" of stars.
The study team included Karl Gebhardt, one of the astronomers who discovered the relationship between the mass of a central black hole and the mass of the surrounding stellar bulge.
The astronomers used two Hubble instruments, the Wide Field Planetary Camera 2 (WFPC2), which takes sharp images of the galaxies, particularly their central regions; and the Space Telescope Imaging Spectrograph, which takes spectra at several locations across a galaxy. For NGC 3945, they supplemented the Hubble data with observations from 1.3-meter (52-inch) and 2.4-meter (90-inch) telescope at the MDM Observatory at Kitt Peak, Arizona.
The Hubble spectra reveal the orbital speeds of stars at different regions within the galaxy. Stars that are near a supermassive black hole move much more quickly. By measuring the speeds of stars close to the galactic center, astronomers can determine the black hole's mass. Measuring speeds across a larger section of the galaxy reveals how the orbital speed changes at different distances from the black hole.
The observations reveal that the stars at the center of NGC 3945 show little change in orbital speed. That indicates that the galaxy has a small supermassive black hole in its core -- about 9 million times the mass of the Sun, or just twice the mass of the black hole at the center of the Milky Way, which is one of the smallest known supermassive black holes. The observations even allow the possibility that the galaxy contains no central black hole at all.
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This document was last modified: March 14, 2012.
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