NGC4258 Massive Black Hole & Accretion Disk
|Minimum credit line: Image courtesy of NRAO/AUI
and L Greenhill (Harvard-Smithsonian Center for Astrophysics)(for details, see .|
NGC 4258 is a Seyfert galaxy in the constellation Ursa major. It is generally agreed to have at its heart a supermassive black hole some 39 million times more massive than the Sun. Gas orbiting this black hole has formed a warped disk nearly two light years in diameter. By measuring extremely small shifts in the positions of H2O masers (water vapor that amplifies microwave radio emissions) located in this disk, scientists have been able to measure the distance to this galaxy as 23.5 million light years, a measurement the scientists say is accurate to within a million light years.
The disk is viewed nearly edge-on from Earth and orbits the black hole at a speed of more than 2 million miles per hour. Using the Very Long Baseline Array, the astronomers observed the Doppler shift of the masers at the disk's sides at 4 to 8 month intervals over more than three years. This shift, or "proper motion," combined with previous observations in which astronomers had measured the speed of the orbiting disk and ascertained the mass of the black hole, has enabled the scientists to make the distance determination.
Previous observations using Cepheid variable stars and the Hubble Space Telescope had set the distance at 27 or 29 million light years. The difference in these two distance determinations will make a significant impact on future calculations of the size and age of the Universe. For more information see the NRAO Press Release Radio Astronomers Set New Standard for Accurate Cosmic Distance Measurement.
Investigator(s): J. M. Moran, L. J. Greenhill, J. R. Herrnstein.
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Date of Observation
Type of Observation
Spectral Line Observations
Center of Image
RA: 12:18:57.52, Dec: 47:18:14.20 (J2000)
Field of View
0.0500 x 0.0500 degrees
Upper Left: Ground-based Halpha+[NII] (red) and HST
continuum F547M+F791W (blue/green) images of NGC 4258 (from
Cecil, Greenhill, DePree et al. 2000). The cartoon
in the upper corner indicates the direction of rotation for the galactic
stellar disk. The Halpha+[NII] light traces stellar and nonstellar (anomalous)
spiral arms. The anomalous arms are related to outflow from the AGN. They are
sources of synchrotron emission and are prominent at radio wavelengths. Upper
Right: Compiliation of VLA A+B+C-array observations at 20cm (1.4 GHz), also
presented in Cecil et al. 2000. Middle: Data and model for accretion and
outflow structures in the central parsec. Red, blue, and green filled circles
show the distribution of water (maser) emission mapped with the VLBA (
Moran et al. 1995;
Herrnstein et al. 1997;
Moran et al. 1999). The masers may be used as point-like test particles whose
positions and line-of-sight velocities can be fit with high
accuracy to a simple geometric model of a thin, warped accretion disk
(wireframe). The central mass is estimated to be 39 million solar masses,
confined within about 0.1 pc. In this way, VLBA study of NGC4258 provided the
first conclusive evidence of a massive black hole in the nucleus of another
galaxy. The VLBA has also detected the base of the anomalous arms, in the form
of a flat spectrum radio jet. A 22 GHz continuum image is shown in falsecolor.
The black dot at the center of the image indicates the fit location of the
central black hole. Bottom: Fully resolved rotation curve of the accretion
disk. The velocities of the masing regions are shown with the best fit
rotation curve, which obeys Kepler's Law to better than 1 percent. At this
time, no other accretion disk has been studied in as great detail (via direct
imaging) on as small scale. For type-2 AGN, there is no means other than
interferometric imaging of water masers to map accretion disks at sub-parsec radii.
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