dynamics of gas or stars in the vicinity of the hole. The masses of the black holes across this sample range from millions to billions of solar masses and show remarkable correlations with both the stellar velocity dispersion and luminosity of the host elliptical galaxy or spiral bulge — the bigger the galaxy, the bigger the black hole (Figure 2). The most luminous galaxies in the local universe are Brightest Cluster Galaxies (BCGs), giant ellipticals lying near the centers of galaxy clusters. M87 resides in the Virgo cluster, the nearest prominent galaxy cluster to us at a distance of 17 Megaparsecs (Mpc; about 55 million light-years (ly)). To hunt for black holes bigger than M87, we would need to venture beyond the Virgo cluster. Making direct observations of a black hole’s presence in BCGs is extremely difficult because these massive galaxies are rare, and their distances are typically several times greater than M87. Like most present-day elliptical galaxies, BCGs and their black holes are “red and dead,” with only small traces of gas left over from earlier periods of star formation. Without a gas reservoir feeding the black holes, they become dormant and do not give out X-ray or optical line emissions that are characteristic of active galaxies such as quasars. Yet a quiet black hole can still reveal its presence by the effect of its gravitational pull on the orbits of nearby stars. At distances of tens to hundreds of light-years from the black hole, stars are rarely consumed but are instead accelerated to large orbital speeds. The difficult task for astronomers is to disentangle the motions of stars near the black hole from other stars orbiting beyond the black hole’s region of influence. Doing so requires spectroscopic observations with superb angular resolution, in order to clearly map the galaxy’s center. Only a few of the world’s largest telescopes, including Gemini North and South, can resolve the center of a BCG and also collect enough starlight to permit meaningful observations. After much effort, our team –– four University of California, Berkeley, scientists (the two authors, Shelley Wright, and James Graham (also University of Toronto)), Karl Gebhardt and Jeremy Murphy (University of Texas, Austin), Tod Lauer (National Optical Astronomy Observatory), and Doug Rich-
stone (University of Michigan) –– achieved a breakthrough this year, discovering two black holes with unprecedented masses in the local universe. We’ve measured a 9.7-billion solar-mass black hole in the galaxy NGC 3842, the BCG of Abell cluster 1367, and another that could be as large as 21 billion solar masses in NGC 4889, the BCG of the Coma cluster (Abell 1656; Figure 3). Both galaxies are at approximately 100 Mpc (~ 330 million ly) from our Galaxy, six times further than the previous record holder M87.
Figure 2. In the local universe, black hole masses correlate with host galaxies’ stellar velocity dispersions (left) and luminosities (right). This figure illustrates the galaxies with direct dynamical measurements of black hole masses based on masers (triangles), stars (stars), or gas (circles), from publications through August 2011. Figure courtesy of Nature.
Figure 3.
Our measurements are based on analysis of the stellar motions near the centers of the two galaxies, using data from GMOS on Gemini North and instruments at Keck Observatory in Hawai‘i and McDonald Observatory in Texas. We obtained the most essential data for measuring the black holes in NGC 3842 and NGC 4889 with the integral-field unit (IFU) on GMOS (Figure 4). The GMOS IFU uses an array of tightly packed hexagonal lenslets to divide the focal plane, such that a single exposure records hundreds of independent spectra across a two-di-
December2011
NGC 4889 is the most luminous galaxy in the Coma galaxy cluster, which contains over a thousand galaxies. This galaxy cluster is one of the most massive gravitationally-bound structures in the nearby universe, and now includes at least one black hole with approximately 21 billion solar masses. This Hubble Space Telescope panorama of the Coma cluster has been marked with a box enclosing NGC 4889. Image credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA).
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