Evidence in Other Galaxies

The image on the right 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.

Radio Galaxy in 3C295
The adjacent left image shows a Chandra X-ray Telescope observation of a cluster of galaxies called 3C295. This cluster is in the constellation Boötes, at a distance of about 5 billion light years (with a redshift of 0.461, this is one of the most distant galaxy clusters ever observed in X-rays). The most intense X-ray emission is indicated by white and blue, and strong but less intense emission by red.

The cluster is dominated by a giant elliptical radio galaxy. Its nucleus coincides with the bright spot at the center of the X-ray image. On either side of the central bright spot are two other X-ray lobes (one brighter than the other). These X-ray lobes correspond to the same positions as lobes of radio frequency emission that are observed from the galaxy (click on the "Show Labels" button to annotate the figure and to show the radio lobes and their correlation with the X-ray lobes).

Hot X-ray Gas Trapped in 3C295

The diffuse red glow in the X-ray image of 3C295 is from a cloud of hot gas two million light years in diameter at a temperature of about 50 million K that is trapped in the cluster of galaxies. There are about 100 galaxies embedded in this gas. Except for the giant elliptical, they don't emit X-rays strongly so they don't show up in the X-ray image but they are seen at optical wavelengths. The gas is estimated to have the mass of about a thousand galaxies. However, this would not be enough mass to bind the cluster. It appears that at least five times more mass is present as dark matter than can be seen in the visible galaxies and the X-ray gas of the cluster.

The likely interpretation is that the X-ray and radio frequency lobes have been generated by jet outflow from a central supermassive black hole in the giant elliptical galaxy. Study of the radio frequency lobes indicates that this outflow appears to have been triggered about a million years ago, presumably by accretion onto the central black hole.

Water Masers in NGC 4258
The galaxy NGC 4258 (also catalogued as the Messier object M106) is an Sb spiral visible through a small telescope, but its nucleus is moderately active and it is also classified as a Seyfert 2 AGN. It lies in the constellation Canes Venatici (very near the Big Dipper) at a distance of around 20 Mpc. This makes it one of the nearest AGNs. This galaxy is remarkable because a set of masers has been observed in the central region of the galaxy. Masers are the analog of a visible-light laser, but for light in the microwave region of the spectrum. The maser emission in NGC 4258 is due to clouds of heated water vapor, so these are termed water masers. (They are also called megamasers because of their enormous power, which allows them to be seen at megaparsec distances.) Such naturally occurring masers are important in astronomy because they produce sharp spectral lines, allowing their Doppler shift to be measured very precisely.