July 23, 2001 NEW BRUNSWICK/PISCATAWAY, N.J. – Rutgers astronomers have made a provocative discovery -- the first galaxy without a supermassive black hole (SBH) at its center or the smallest black hole ever detected in the center of a galaxy. They used the Hubble Space Telescope to observe the galaxy M33, one of the nearest neighbors to the Milky Way galaxy at a relatively short distance of 3 million light-years. By using Hubble's high-resolution instruments, the investigators were able to see details at a scale that is at least 10 times finer than was ever possible in the past from the ground.
A paper describing this research, "No Supermassive Black Hole in M33?" by Professor David Merritt, Assistant Professor Laura Ferrarese and Assistant Research Professor Charles Joseph, all of Rutgers' department of physics and astronomy, is being published online by the journal Science, as part of the Science Express web site, on July 19. (See http://www.sciencexpress.org)
Merritt, who leads the Supermassive Black Hole Research Group at Rutgers, is a theorist who has worked extensively on the interaction of black holes with galaxies; Ferrarese discovered two of the first supermassive black holes external to the Milky Way; and Joseph is a key member of Hubble's Instrument Development Team for the Space Telescope Imaging Spectrograph, the instrument used for the observations on which the current research was based.
A black hole is a region of space where the force of gravity is so powerful that nothing, not even light, can escape its pull. Supermassive black holes have been found in the center of every galaxy where astronomers have looked. From a few million to a few billion times the size of our sun (or solar masses), SBHs may have formed from giant gas clouds or from the collapse of clusters of immense numbers of stars shortly after the Big Bang when the universe began. They are a key to understanding the early evolution of the universe because their mass is a measure of the total mass and energy accumulated while the galaxies were being born and provide insights into the processes that operated.
Our own Milky Way galaxy contains one of the smallest of these SBHs, with a mass of about three million times the mass of the sun. Astronomers have debated whether this is a lower limit or whether some galaxies might have smaller SBHs, perhaps with masses of a few thousand solar masses.
"Detecting black holes of only a few thousand solar masses is observationally challenging, but it is critical for establishing how supermassive black holes relate to their host galaxies, and which mechanisms influence the formation and evolution of both," said Merritt.
Based on their observations and calculations, the team concluded that only a black hole smaller than 3,000 solar masses could be present at the center of M33. If this galaxy contains a SBH, it would be roughly a thousand times smaller than the SBH in the Milky Way and by far the smallest SBH known.
A year ago, Ferrarese and Merritt established a correlation between the mass of a black hole and the speed at which the stars in its galaxy are moving ("A Fundamental Relation Between Supermassive Black Holes and Their Host Galaxies," The Astrophysical Journal, 539, L9-L12 (2000)). This "M-sigma" relation is the most fundamental connection yet discovered between SBHs and their host galaxies and it has been hailed as one of the most important discoveries to come out of the Hubble Space Telescope.
In the current investigation, Merritt, Ferrarese and Joseph employed this correlation to predict that any black hole in M33 must be relatively small. Using the HST, they were then able to verify that the upper limit to its mass is only about 3000 solar masses.
"For the first time, we are able to put a constraint on a black hole which is a thousand times or three orders of magnitude less massive than the least massive black hole that has been detected," said Ferrarese.
Even with the relatively close proximity of M33, limits of detection preclude more precise measurement of this black hole's mass. "However, the upper limit on this mass (3,000 solar masses) is still consistent with the relation that we discovered a year ago and this implies that small black holes, if they exist, formed in much the same way as the very massive ones," she said.
The Rutgers team is hoping to be awarded additional observing time on the Hubble Space Telescope in order to definitively determine whether or not a black hole is present at the center of M33. If there is one, it will be the smallest black hole to be detected in the center of a galaxy. If there is no black hole here, the team will have found the first galaxy without a central black hole. Either way, this is an important discovery.
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