Hopkins Astronomers Find Evidence of Black Hole in Andromeda
Since Andromeda is a common spiral galaxy similar to the Milky Way, the findings suggest that black holes may not be as rare as scientists had thought.
The observations were made with Hubble's Wide Field/Planetary Camera in June 1991 and with HST's Faint Object Spectrograph in June 1993, well before the December 1993 mission to repair the HST's image-blurring optical flaw, said Hopkins astrophysicist Yichuan Pei, lead author of a scientific paper on the research. The findings were released May 31, during a meeting of the American Astronomical Society in Minneapolis. Scientists used the spectrograph to study the spectra of light coming from two star clusters in the center of Andromeda, which is one of the closest galaxies to the Milky Way, at 2.4 million light years away.
Astronomers refer to the two star clusters as Andromeda's "double-peaked nucleus," since it displays two peaks of visible light.
It appears that a black hole with a mass of a few million suns may reside inside the fainter of the two star clusters, Dr. Pei said. Spectrographic analysis shows that stars near the "faint peak" of star light are traveling up to 380 kilometers per second, or 850,000 miles per hour.
Such a high velocity suggests that the stars are moving near something with huge mass and, therefore, exerting powerful gravitational attraction.
The other, brighter star cluster might be the "captured" nucleus of a dwarf galaxy, but scientists said more research is needed to confirm that possibility.
Dwarf galaxies come in several varieties, as do larger galaxies, but their common feature is that they are much smaller, typically a thousandth to a millionth as large as galaxies like the Milky Way. Our nearest galactic neighbors are dwarf galaxies called the Small Magellanic Cloud and Large Magellanic Cloud, which are companions of the Milky Way and are visible in the southern hemisphere.
Andromeda is the closest spiral galaxy to the Milky Way, and it, too, has similar dwarf companions.
The research, while not providing absolute direct proof for a black hole in Andromeda, has yielded the strongest evidence yet, said Hopkins astronomer Gerard Kriss, who was involved in the research. Other astronomers, using ground-based telescopes, have made earlier suggestions that there might be a black hole in the nucleus of Andromeda.
The space telescope gathers better evidence because it gets above Earth's atmosphere. Images taken from telescopes on the ground are smeared by turbulence in the atmosphere.
Because Andromeda is a common spiral galaxy, the findings have broad implications.
"It suggests that black holes may be a common feature in galactic nuclei," Dr. Kriss said. Astronomers have found evidence of a black hole in the center of the Milky Way, but the evidence is far less conclusive than the Andromeda findings.
The repaired HST's sharper focus could allow astronomers to resolve the question of whether a black hole is located in Andromeda.
"Making the observations now would make a tremendous difference," Dr. Kriss noted. The spherical aberration in HST's primary mirror had prevented scientists from making precise spectroscopic analyses of starlight closest to the apparent black hole, so it was difficult to determine the speeds of those stars. For example, a team of astronomers from Johns Hopkins and other institutions announced on May 25 that they had used the repaired HST to find conclusive evidence for the existence of a supermassive black hole in the constellation Virgo, 50 million light years from Earth.
Other Hopkins astronomers involved in the Andromeda research were Holland Ford and Zlatan Tsvetanov and graduate student Laura Ferrarese.
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