Joel Bregman of the University of Michigan.
Results from NASA's Chandra X-ray Observatory and the Magellan telescopes suggest that a dense stellar remnant has been ripped apart by a black hole a thousand times as massive as the Sun in NGC 1399, an elliptical galaxy about 65 million light years from Earth.
The Chandra observations showed that this object is a so-called ultraluminous X-ray source (ULX). ULXs emit more X-rays than stars, but less than quasars. Their exact nature has remained a mystery, but one suggestion is that some ULXs are black holes with masses between about a hundred and a thousands times that of the Sun.
The intensity of the X-ray emission places the source in the "ultraluminous X-ray source" or ULX category, meaning that it is more luminous than any known stellar X-ray source, but less luminous than the bright X-ray sources (active galactic nuclei) associated with supermassive black holes in the nuclei of galaxies. The nature of ULXs is a mystery, but one suggestion is that some ULXs are black holes with masses between about a hundred and several thousand times that of the Sun, a range intermediate between stellar-mass black holes and supermassive black holes located in the nuclei of galaxies.
This ULX is in a globular cluster, a very old and crowded conglomeration of stars. Astronomers have suspected that globular clusters could contain intermediate-mass black holes, but conclusive evidence for this has been elusive.
"Astronomers have made cases for stars being torn apart by supermassive black holes in the centers of galaxies before, but this is the first good evidence for such an event in a globular cluster," said Jimmy Irwin of the University of Alabama who led the study.
Irwin and his colleagues obtained optical spectra of the object using the Magellan I and II telescopes in Las Campanas, Chile. These data reveal emission from gas rich in oxygen and nitrogen but no hydrogen, a rare set of signals from globular clusters. The physical conditions deduced from the spectra suggest that the gas is orbiting a black hole of at least 1,000 solar masses. The abundant amount of oxygen and absence of hydrogen indicate that the destroyed star was a white dwarf, the end phase of a solar-type star that has burned its hydrogen leaving a high concentration of oxygen. The nitrogen seen in the optical spectrum remains an enigma.
Theoretical work suggests that the tidal disruption-induced X-ray emission could stay bright for more than a century, but it should fade with time. So far, the team has observed there has been a 35 percent in X-ray emission from 2000 to 2009.
Casey Kazan via JPL/NASA