Making Supermassive Black Holes Spin

Where does the angular momentum come from that causes supermassive black holes (SMBHs) to spin on their axes and launch powerful jets? A new study of nearby SMBHs may help to answer this question.

High-mass SMBHs are thought to form when two galaxies collide and the SMBHs at their centers merge. [NASA/Hubble Heritage Team (STScI)]

High-mass SMBHs are thought to form when two galaxies collide and the SMBHs at their centers merge. [NASA/Hubble Heritage Team (STScI)]

High- vs. Low-Mass Monsters

Observational evidence suggests a dichotomy between low-mass SMBHs (those with 106-7 M) and high-mass ones (those with 108-10 M). High-mass SMBHs are thought to form via the merger of two smaller black holes, and the final black hole is likely spun up by the rotational dynamics of the merger. But what spins up low-mass SMBHs, which are thought to build up very gradually via accretion?

A team of scientists led by Jing Wang (National Astronomical Observatories, Chinese Academy of Sciences) have attempted to address this puzzle by examining the properties of the galaxies hosting low-mass SMBHs.

A Sample of Neighboring SMBHs

Wang and collaborators began by constructing a sample of radio-selected nearby Seyfert 2 galaxies: those galaxies in which the stellar population and morphology of the host galaxy are visible to us, instead of being overwhelmed by continuum emission from the galaxy’s active nucleus.

sersic index

An example of a galaxy with a concentrated, classical bulge (M87; top) and a one with a disk-like pseudo bulge (Triangulum Galaxy; bottom). The authors find that for galaxies hosting low-mass SMBHs, those with more disk-like bulges appear to have more powerful radio jets. [Top: NASA/Hubble Heritage Team (STScI), Bottom: Hewholooks]

From this sample, the authors then selected 31 galaxies that have low-mass SMBHs at their centers, as measured using the surrounding stellar dynamics. Wang and collaborators cataloged radio information revealing properties of the powerful jets launched by the SMBHs, and they analyzed the host galaxies’ properties by modeling their brightness profiles.

Spin-Up From Accreting Gas

By examining this sample, the authors discovered an intriguing relationship: the radio power of jets launched by an SMBH appears to be dependent upon its host galaxy’s bulge surface brightness. Specifically, Wang and collaborators found that more powerful radio emission comes from SMBHs associated with less-concentrated bulges, i.e. those that are more disk-like.

The authors’ findings allow them to rule out many common explanations for the radio-loudness of such galaxies with small SMBH masses. Instead, they argue that the tendency for galaxies with more disk-like bulges to host SMBHs with more powerful jets is evidence that low-mass SMBHs are spun up by the accretion of surrounding gas.

In this scenario, the angular momentum of gas with significant disk-like rotational dynamics provides the spin to the SMBH, and this rotational energy can then be extracted to launch the powerful jets. If this explanation is correct, it strengthens the dichotomy between low-mass and high-mass SMBHs, supporting the idea that the two categories of black holes are indeed formed and spun up via completely different mechanisms.


J. Wang et al 2016 ApJL 833 L2. doi:10.3847/2041-8205/833/1/L2

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