The Hunt for the Smallest Supermassive Black Holes

Vivienne Baldassare

We are very pleased to welcome Vivienne Baldassare as a guest blogger today. Vivienne is the first author of a paper that is the subject of our latest press release, about an exceptionally small supermassive black hole. She is a PhD candidate and National Science Foundation Graduate Research Fellow in the University of Michigan Department of Astronomy. Vivienne obtained her undergraduate degree in physics from Hunter College, where she was also a member of the Macaulay Honors College.

It is well established that most, if not all, massive galaxies host supermassive black holes at their centers − our own Milky Way houses a several million solar mass black hole in its core. Little is known, however, about the population of black holes in the centers of dwarf galaxies. I study the centers of small galaxies to see how many host supermassive black holes and if those black holes interact with their galaxies in the same way as bigger supermassive black holes.

Though dwarf galaxies are more common than large galaxies, it is more difficult to determine whether they host black holes. When galaxies are large and nearby, we can find central black holes by studying the motions of stars in the region dominated by the gravity of the black hole. Since smaller black holes influence smaller regions, this method would only work for dwarf galaxies in our cosmological back yard. Instead, we must search for active black holes in dwarf galaxies, i.e. black holes that are voraciously consuming material, releasing large amounts of light from the galaxy’s center.

In 2013, astronomers Amy Reines, Jenny Greene, and Marla Geha completed a search for active black holes in dwarf galaxies using data from the Sloan Digital Sky Survey. Out of the more than 25,000 dwarf galaxies studied, they found signs of supermassive black hole activity in roughly one percent, increasing the number of known dwarf galaxies with active black holes by a factor of ten! Some of these objects were targeted for follow-up observations with more powerful telescopes to get a better understanding of the host galaxies and their resident black holes.

One such object was RGG 118 – a dwarf, disk galaxy weighing just a few percent of the mass of the Milky Way. RGG 118 is about 340 million light years away, located near the constellation Serpens in the night sky, though it is much too faint to view with the naked eye. Reines, Greene, and Geha observed that the relative strengths of emission lines – that is, spikes of intensity at very specific wavelengths – in its visible spectrum match those of other galaxies with active black holes. This led our team – myself, Dr. Amy Reines, Prof. Elena Gallo, and Prof. Jenny Greene – to analyze follow-up observations taken with the Magellan Echellette Spectrograph on the 6.5 meter Clay Telescope at Las Campanas Observatory in Chile. In addition to confirming the emission line ratios, the new spectrum clearly showed broad emission lines: a hallmark of gas rotating around a black hole.

When gas rotates around a black hole, the motion causes the frequency of the light it emits to spread in a characteristic way. The width of this spread is related to the speed of rotation, which in turn is related to the mass of the black hole. By measuring the spread, we found that the black hole in RGG 118 weighs just 50,000 times the mass of the Sun, the smallest supermassive black hole yet reported!

Our team also observed RGG 118 with the Chandra X-ray Observatory, and found point-like X-ray emission at the nucleus of the galaxy, further confirming the presence of an accreting black hole. The X-ray luminosity indicates that RGG 118’s black hole is consuming material at one percent of its maximum rate. This accretion rate is similar to those seen for much bigger active black holes in much larger galaxies.

Despite the diminutive size of the black hole in RGG 118, it and others like it, could provide key information about black hole growth in the early universe. These present-day dwarf galaxies are our best analogues to the first galaxies and studying how their black holes grow may help us better understand how black holes developed in the several hundred million years following the Big Bang.

Next, we’ll be observing RGG 118 with the Hubble Space Telescope to study the detailed structure of the galaxy. We also hope to find and weigh black holes in more dwarf galaxies so we can continue to study the growth of central black holes on all scales!