The search for seeds of black holes

Galaxy mergers are not necessary to create big black holes, new observations suggest
March 31, 2014

The galaxy NGC 4395 is shown here in infrared light, captured by NASA’s Spitzer Space Telescope (credit: NASA/JPL-Caltech)

A new study using data from NASA’s Wide-field Infrared Survey Explorer (WISE) has turned up what might be the cosmic seeds from which a┬ásupermassive black hole will sprout.

The smallest black holes, only a few times greater in mass than our sun, form from exploding stars. The biggest, billions of times the mass of our sun, grow together with their host galaxies over time, deep in the interiors. But how this process works is an ongoing mystery.

Researchers using WISE addressed this question by looking for black holes in smaller, “dwarf” galaxies. These galaxies have not undergone much change, so they are more pristine than their heavier counterparts. In some ways, they resemble the types of galaxies that might have existed when the universe was young, and thus they offer a glimpse into the nurseries of supermassive black holes.

In this new study, using data of the entire sky taken by WISE in infrared light, up to hundreds of dwarf galaxies have been discovered in which buried black holes may be lurking. Infrared light, the kind that WISE collects, can see through dust, unlike visible light, so it’s better able to find the dusty, hidden black holes. The researchers found that the dwarf galaxies’ black holes may be about 1,000 to 10,000 times the mass of our sun — larger than expected for these small galaxies.

How supermassive black holes form

“Our findings suggest the original seeds of supermassive black holes are quite massive themselves,” said Shobita Satyapal of George Mason University, lead author of a paper published in the March issue of Astrophysical Journal.

The new observations argue against one popular theory of black hole growth, which holds that the objects bulk up in size through galaxy collisions. In that theory, when our universe was young, galaxies were more likely to crash into others and merge. It is possible the galaxies’ black holes merged too, accumulating more mass. In this scenario, supermassive black holes grow in size through a series of galaxy mergers.

The discovery of dwarf galaxy black holes that are bigger than expected suggests that galaxy mergers are not necessary to create big black holes. Dwarf galaxies don’t have a history of galactic smash-ups, and yet their black holes are already relatively big.

Instead, supermassive black holes might form very early in the history of the universe. Or, they might grow harmoniously with their host galaxies, feeding off surrounding gas.

“We still don’t know how the monstrous black holes that reside in galaxy centers formed,” said Satyapal. “But finding big black holes in tiny galaxies shows us that big black holes must somehow have been created in the early universe, before galaxies collided with other galaxies.”

Other authors of the study include N.J. Secrest, W. McAlpine and J.L. Rosenberg of George Mason University; S.L. Ellison of the University of Victoria, Canada; and J. Fischer of the Naval Research Laboratory, Washington.

Abstract of The Astrophysical Journal paper

In contrast to massive, bulge hosting galaxies, very few supermassive black holes (SMBHs) are known in either low-mass or bulgeless galaxies. Such a population could provide clues to the origins of SMBHs and to secular pathways for their growth. Using the all-sky Wide-field Infrared Survey Explorer (WISE ) survey, and bulge-to-disk decompositions from the Sloan Digital Sky Survey (SDSS) Data Release 7, we report the discovery of a population of local (z < 0.3) bulgeless disk galaxies with extremely red mid-infrared colors which are highly suggestive of a dominant active galactic nucleus (AGN), despite having no optical AGN signatures in their SDSS spectra. Using various mid-infrared selection criteria from the literature, there are between 30 and over 300 bulgeless galaxies with possible AGNs. Other known scenarios that can heat the dust to high temperatures do not appear to explain the observed colors of this sample. If these galaxies are confirmed to host AGNs, this study will provide a breakthrough in characterizing the properties of SMBHs in the low bulge mass regime and in understanding their relation with their host galaxies. Mid-infrared selection identifies AGNs that dominate their host galaxy’s emission and therefore reveal a different AGN population than that uncovered by optical studies. We find that the fraction of all galaxies identified as candidate AGNs by WISE is highest at lower stellar masses and drops dramatically in higher mass galaxies, in striking contrast to the findings from optical studies.