Editor's note: In her article, "Goldilocks Black Holes," Jenny E. Greene discusses the search for black holes with masses ranging from roughly 1,000 suns to a million suns—middleweights on the cosmic scale. These intermediate-mass holes may provide clues about the origins of galaxies and the supermassive black holes (millions to billions of suns in mass) found at galaxy centers. Astronomers have now found hundreds of middleweight holes, in particular by analyzing data from the Sloan Digital Sky Survey, which has been recording light from galaxies across a large area of the sky for over a decade now. In many ways, however, this census of middleweight holes is incomplete. Below, Greene discusses additional studies that will fill in the gaps and perhaps push astrophysicists' understanding of middleweights in new directions in the future.

The searches for middleweight holes are still in their infancy, and our picture of these objects may change with larger, more complete samples of galaxies. For instance, the Sloan survey (http://www.sdss.org/) has been invaluable for conducting a census of middleweights across a major sample of galaxies in the sky. Yet it is limited to galaxies bright enough for suitable observations with its dedicated telescope. Edward C. Moran of Wesleyan University and his collaborators are extending the census by examining all galaxies in the northern sky within 200 million light-years and down to very faint limits. They are obtaining a spectrum for each galaxy and analyzing it for signs of active black holes. They have already found nearly 20 new middleweight black hole candidates in small galaxies.

Middleweights could also reveal themselves in rare events, as when one rips apart and swallows a star that passes too close. Optical surveys looking at the same part of the sky a few times every week may spot the characteristic flares these cataclysms produce. Such a flare would last a few months and would usually be brightest in ultraviolet light, but small holes could produce flares brightest in the optical. In March the orbiting gamma-ray telescope Swift detected an outburst that was likely from a star in a distant galaxy being ripped apart by a black hole with a mass no greater than about 10 million suns.

Improving the census of middleweight holes requires good mass measurements. The surest way to measure a black hole's mass (and, in fact, to confirm that the black hole is there at all) is by detecting signs of the hole's gravity in the motions of nearby stars and gas. Unfortunately, for middleweight holes these effects are too small for astronomers to detect in any but the closest galaxies. Last year Anil C. Seth, now at the University of Utah in Salt Lake City, his co-workers mapped star motions in the nearby galaxy NGC 404 and concluded it contained a hole of a few hundred thousand solar masses at its core. Just 10 million light years away, NGC 404 is the closest galaxy of its kind (a type with a very small bulge and thus a prime candidate for a middleweight).

The next generation of ground-based optical telescopes, due to begin operation later this decade, will bring hundreds more galaxies within range for such studies. These telescopes (the Giant Magellan Telescope and the Thirty Meter Telescope) will be 20 to 30 meters across, more than double today's largest, making them capable of detecting middleweight holes in galaxies as far away as the Virgo cluster, 50 million light-years away.

More middleweights may also turn up in unexpected ways. Amy Reines, now at the National Radio Astronomy Observatory in Charlottesville, Virginia, was looking at radio wave and infrared images in 2010 to study how stars form in stellar population explosions in very small galaxies. One day she noticed something funny at the center of one of her galaxies, away from the clusters of newly forming stars. Comparison with an old x-ray image revealed that her object was most likely a black hole, of about 100,000 to 10 million solar masses. Even if this hole turns out to be larger than a true middleweight, its unique location—in a tiny galaxy of newly forming stars—is intriguingly similar to the conditions the universe's first holes probably encountered. Follow-up observations are under way to confirm that the object actually is a black hole and learn more about it. Meanwhile Reines is looking for more examples in similar galaxies. Future searches using new radio telescopes with higher sensitivity may also uncover previously overlooked black holes in the cores of nearby galaxies when combined with x-ray data.


Editor's note: In her article, "Goldilocks Black Holes," Jenny E. Greene discusses black holes with masses ranging from roughly 1,000 suns to a million suns—middleweights on the cosmic scale. Greene's article focuses on the search for middleweights at the centers of galaxies. Here she discusses the possibilities for finding middleweights in other places.

Supermassive holes, with masses equal to millions or billions of suns, are found in the centers of galaxies. And that is where most researchers look for the middleweights as well—the holes that are smaller than about a million suns in mass because their original seed holes, for whatever reason, never grew to supermassive size. Yet galactic nuclei could be the wrong place to look for these middleweights.

For instance, if the original seed holes formed in the centers of dense star clusters (as I discuss in the January 2012 feature), maybe that is where astronomers should be looking for failed supermassive black holes. Indeed, astronomers have reported detecting the gravitational effects of middleweight black holes in two massive star clusters, one in a cluster called Omega Centauri in our own Milky Way and another in our neighbor, the Andromeda galaxy. So far, however, these observations remain controversial.

Even if all seed holes formed in the centers of proto-galaxies, many of them may no longer live in a galactic nucleus. When a little galaxy falls into a bigger one, the black hole at the center of the little galaxy may end up orbiting in the outer regions of the big galaxy. Black holes also may be ejected from their galaxy centers when black holes merge. Such ejections may occur when three (or more) holes get close and one gets slung out, leaving the other two to merge. Alternatively, a simple two-hole merger may emit a directed burst of gravitational waves, causing the merged hole to recoil out of the galactic center.

Astronomers have found objects far from galactic nuclei that emit surprising quantities of x-rays, which may be middleweight black holes. If these ultraluminous x-ray sources are radiating equally in all directions, then they are making too many x-rays to come from a mere stellar mass black hole. This fact, combined with other properties of the x-rays, has led some astronomers to propose that black holes comprising 1,000 to 10,000 solar masses are powering these sources.

Now that orbiting x-ray telescopes such as Chandra have been flying for more than a decade, astronomers have a large database of x-ray sources to consider. In one survey, researchers are doing follow-up optical and (eventually) radio observations of x-ray sources not in galactic nuclei to determine the nature of these sources. Another survey, started at the University of Texas at Austin, specifically looks for signs of accreting black holes in the outskirts of big elliptical galaxies. This survey will use instrumentation with a very large field of view, so that a single observation can look for interesting black hole candidates out way beyond the edges of these galaxies down to very sensitive limits. These surveys will significantly strengthen astronomers' census of failed supermassive black holes.