October 25, 2012
Low Mass Supermassive Black Hole Found Within Spiral Galaxy
April Flowers for redOrbit.com - Your Universe Online
NASA's Chandra X-ray Observatory, along with other observatories, has identified one of the lowest mass supermassive black holes ever observed in the middle of a galaxy that one would not expect to harbor this type of beast. This suggests that this black hole, while related to its supermassive cousins, may have a different origin.
Located in the middle of the spiral galaxy NGC 4178, the black hole is shown in the image from the Sloan Digital Sky Survey. The inset in the image reveals an X-ray source at the position of the black hole, in the center of a Chandra image. Combining data from Chandra and NASA's Spitzer Space Telescope along with radio data from the National Science Foundation's Very Large Array, the study suggests that the black hole is at the extreme low-mass end of the supermassive black hole range.
The brightness and spectrum — the amount of X-rays at different wavelengths — of the X-ray source, and its brightness at infrared wavelengths, suggest that a black hole at the center of NGC 4178 is rapidly sucking in material from its surroundings. Gas and dust surrounding the black hole heavily absorb the light generated by this infalling material.
The mass of a black hole and the amount of X-rays and radio waves it generates is a known relationship used to estimate the mass of the black hole. Using this method, the team estimates the mass of this black hole at less than 200,000 times that of the sun. The authors employed other mass estimate methods to confirm this finding. At this mass, the black hole is lower than typical values for supermassive black holes which are millions to billions of times the mass of the sun.
Located about 55 million light years from Earth, NGC 4178 is a spiral galaxy that does not have a bright concentration, or bulge, of stars in its center. Four other bulgeless galaxies are thought to contain supermassive black holes, two of which have masses that may be close to that of the black hole in NGC 4178.
ESA's XMM-Newton, the most powerful X-ray telescope ever placed in orbit, made observations of an X-ray source discovered by Chandra in the center of NGC 4561 that indicate the mass of this black hole is greater than 20,000 times the mass of the sun. The mass could be substantially higher if the black hole is pulling in slowly, as this would cause it to generate less X-ray emissions. NGC 4395's black hole mass is estimated to be about 360,000 times the mass of the sun.
A large number of galaxies are consistent with a close correlation between the mass of a supermassive black hole and the mass of the bulge of its host galaxy have been previously found by astronomers. Scientists developed theoretical models to explain these results. These models invoke mergers of galaxies, and predict that galaxies without bulges are unlikely to host supermassive black holes. The findings for NGC 4178 and the other four galaxies mentioned disagree with the model's predictions. This may suggest that more than one mechanism is at work in forming supermassive black holes.
The Chandra image also revealed three other X-ray sources. If these sources are within NGC 4178, they are likely to be binary systems containing a black hole or neutron star. One of the three sources, the brightest, may be an intermediate-mass black hole with a mass of around 6,000 times that of the sun.
The findings of this study were published in The Astrophysical Journal on July 1, 2012 issue.