This observation of water maser
emission from the central region of NGC 4258 using long-baseline radio interferometry
leads to the following
results.
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The masers are mapped with a precision of better than 1 milli-arcsecond and their
radial velocities
indicate bulk motion of the gas within several light years of the core at velocities
approaching
1000 km/s. In the figure above masers approaching us (blue shift) are shown in blue,
masers receding from us (red shift) are shown in red, and those with no net radial
velocity are shown in green. From this we may infer the general rotation of the gas
disk, as indicated by the arrow labeled "Keplerian motion".
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The line-of-sight velocity inferred from the Doppler shifts, and the precise positions of the
masers, allows the detailed orbital motion of gas around the nucleus to be determined with high
confidence. These measurements indicate that the masers are embedded in
a dusty, molecular
gas disk
that is revolving around the core. The disk is very thin, with a
diameter that is more than
300 times its thickness, and it has a pronounced warp, as indicated in the figure.
It is not very well understood why the disk is so thin, or why it is warped.
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The motion of the masers is Keplerian (the rotation curve follows
Kepler's laws) to better than 1
percent accuracy, meaning that the masers are in orbit around a large mass that is completely
contained within all
their orbits. (Recall the discussion of rotation curves for
spiral galaxies in
Chapter 23--in that case the motion is not like this
because there is a
dark matter
halo and increasing the orbital size increases the amount of mass enclosed within it.)
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The Keplerian motion of the masers allows the value of the central mass about which they are
revolving to be determined with high precision. The mass
obtained is approximately 35 million
times that of the Sun.
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The measured central mass and measured size of the region enclosed by the maser orbits
allows a minimum density of 100 million solar masses per cubic light year to be computed. This is 10,000 times more dense than any known star cluster.
If such a star cluster existed, the average separation between the stars would be
only about
twice the radius of the Solar System and calculations indicate that collisions of the
stars with each other would either cause the stars of the cluster to drift apart or
mutually collapse to a huge
black hole. Therefore, the only plausible explanation
for this enormous
density is a supermassive black hole.
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The nucleus of the galaxy produces radio jets that appear to come from the dynamical center of
the rotating disk and are approximately perpendicular to it.
The radio emission at 1 cm wavelength is superposed on the drawing of the warped disk.
In addition the "arms" shown in the 20 cm RF observation (top image on the right)
do not coincide with the spiral arms in the optical image (lower right image).
It is commonly believed that the "arms" in the 20 cm radio frequency map are extensions
of the sychrotron jet seen nearer the core.
The position of the
radio jets, coupled with the precise location of the center of the disk,
permits the position of
the central black hole engine to be determined within the uncertainty of the black circle shown
in the preceding figure. Note that this black circle denotes the uncertainty in location of
the black hole, not its size. From
the scale attached to the top figure, the black circle is about 0.05 ly in diameter, but a
supermassive black hole would have a radius hundreds of times smaller than this.
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These results taken together make NGC 4258 one of the strongest cases yet for the presence of
supermassive black hole engines at the cores of active galactic nuclei.
