Black Holes

The active galaxies appear to require a compact energy source of enormous strength. The most plausible candidate is a rotating, supermassive black hole of order a billion solar masses at their center. Until recently there has been strong circumstantial evidence to support such a mechanism. In the past few years evidence of much more direct nature has emerged.

Evidence for a Supermassive Black Hole in M87

The left portion of the following Hubble Space Telescope photograph shows the center of the giant elliptical galaxy M87, which is the 87th entry in the famous Messier Catalog. This galaxy is believed to contain a supermassive black hole of several billion solar masses at its center. The observations indicate that approximately 3 billion solar masses are concentrated in a region at the galactic core that is only about the size of the Solar System. The diagonal line across the right image is a jet of high-speed electrons approximately 6500 light years long that is probably being ejected from the galactic nucleus by the black hole located there.

The right side of the figure illustrates schematically Doppler shift measurements made on the central region of M87 that suggest rapid rotation of the matter near the center. The measurement was made by studying how the light from the disk is red shifted and blue shifted by the Doppler effect, using the Faint Object Spectrograph aboard the Hubble Space Telescope. Part of the swirling disk spins in Earth's direction and the other side spins away from Earth, thus causing opposite Doppler shifts. The gas on one side of the disk is moving away from Earth at a speed of about 550 kilometers per second (red shift). The gas on the other side of the disk is approaching the Earth at the same speed (blue shift).

This high velocity suggests a huge gravitational field at the center of M87, far larger than could be accounted for by the visible stars there. This is what would be expected for matter swirling around the supermassive black hole, with part of it falling forever into the black hole and part of it being ejected in the high-speed jet seen coming from the nucleus of M87. Here is a simulation of what gas swirling around a supermassive black hole in the core of a spiral galaxy might look like (Source).

Evidence for a Supermassive Black Hole in NGC4261

The following image shows a composite of ground based optical and radio telescope images of the galaxy NGC 4261, and a high resolution Hubble Space Telescope image of the core of this galaxy.

Click here for a more detailed description.

NGC 4261 has enormous jets shooting from its core and very strong radio frequency emission. It is thought that the jets are powered by a gargantuan black hole of perhaps a billion solar masses, and that the ring in the Hubble image is an accretion disk feeding the black hole. The black hole itself presumably lies inside the bright spot at the center. Even a billion solar mass black hole would be too small to see in this image, for as we see in the following table, it would only be the size of the solar system.

Radius for Black Hole of a Given Mass
Object Mass Black Hole Radius
Earth 5.98 x 1027 g 0.9 cm
Sun 1.989 x 1033 g 2.9 km
5 Solar Mass Star 9.945 x 1033 g 15 km
Galactic Core 109 Solar Masses 3 x 109 km

Velocities near the Center of M84

The following figure illustrates observations taken with the Hubble Space Telescope Imaging Spectrograph (Ref and Ref) of a region across the center of the galaxy M84, which is in the Virgo Cluster about 50 million light years away.

The right portion of the figure is a radial velocity distribution across the slice illustrated in the left portion of the figure, determined by looking at the Doppler shift of light coming from this region. As one approaches the nucleus (moving downward in the right image) there is a sudden blue shift, indicating rapid motion of the gas near the nucleus toward us. The Doppler shift indicates that the velocity toward us reaches as high as 400 km/s at a distance only 26 light years from the center. Then suddenly the sign of the radial velocity reverses and a redshift indicating similar velocities away from us is observed, with this red shift decreasing rapidly as one moves away from the center (toward the bottom of the right diagram).

The most obvious interpretation of these data is that there is a large rotating disk around the nucleus of M84 that we are seeing in cross section. Above the nucleus in this image the disk matter is moving toward us at 400 km/s, and below the nucleus the disk matter is moving away at similar velocities. The only simple explanation is that this is an accretion disk feeding a supermassive black hole in the center of M84, since no other explanation could easily account for gas velocities of these magnitudes near the center. Thus, we may take the "S" shape of such velocity distributions for galactic cores as telltale signs for an accretion disk around a supermassive black hole.

This black hole interpretation is strengthened by previously known information about this galaxy: its nucleus very active and emits jets of particles that are very strong radio sources. The observed radial velocities near the center suggest a mass of about 300 million solar masses for the black hole.

A Black Hole in the Sombrero Galaxy?

The image on the left is of the Sombrero Galaxy (M104). This galaxy is a strong X-Ray emitter, and unusually high velocities are observed for stars near its center; this raises speculation that it may have a black hole of approximately 1 billion solar masses at its core. Credit: T. Boroson (NOAO /USGP), W. Keel (UA), KPNO

Origin of Supermassive Black Holes

It is now believed that many galaxies have supermassive black holes at their centers, and that whether such galaxies are active galaxies is a question of whether mass is being fed into these black holes. The simplest ideas for the origin of such supermassive black holes are that they are conglomerations of many star-size black holes that were formed during the history of a galaxy, or perhaps that galaxies formed around large black holes that then grew by accreting matter.

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