The contradiction of a low-mass massive black hole

The black holes we’ve observed in the universe typically fall into two categories: small star-sized black holes, and gargantuan black holes lurking at the centers of galaxies. Now, a new black-hole discovery sheds some light on the gray area between these extremes.

Stellar-mass black holes of up to 100 solar masses are scattered by the millions throughout galaxies. At the opposite end of the spectrum, most galaxies are thought to contain just one massive black hole: a black hole of millions to tens of billions of solar masses that lies in the galaxy’s core.

Intriguingly, the mass of these central black holes seems to be inherently tied to that of their host. An empirical relationship known as the M-σ relation shows a correlation between a central black hole’s mass and the spread of star velocities in its host galaxy’s bulge, which acts as a proxy for the bulge mass. The M-σ relation and other, similar relationships show that black holes seem to grow in tandem with their host galaxies throughout the universe.

If the M-σ relation holds across a broad range of masses, then we would expect to find smaller massive black holes at the hearts of especially low-mass galaxies. So far, evidence for these low-mass central black holes has been scarce. But a new study led by Ingyin Zaw (New York University Abu Dhabi, UAE) has now delivered a low-mass massive black hole for us to contemplate.

Zaw and collaborators used the Very Long Baseline Array to obtain radio observations of the low-mass galaxy IC 750.

At the galaxy’s heart, the authors found emission from water masers, clumps of water molecules that emit light naturally in a process similar to laser emission. Light from the masers shows that they are orbiting in a disk around a compact central mass — a massive black hole — and Zaw and collaborators used their motion to measure the mass enclosed in their orbit, providing an upper limit on the black hole’s mass.

The authors then reduced and analyzed publicly available multiwavelength data to understand the location of the black hole and measure the properties of its host galaxy.

The result? IC 750’s central massive black hole is a definite lightweight, with an upper limit of 140,000 solar masses — and it may actually be less than a third of that weight. Not only is this remarkably small for a central massive black hole, it’s also unusually light even relative to the mass of its host galaxy: IC 750’s black hole lies two orders of magnitude below where it should sit on the M-σ relation!
What’s going on with this unusual object? There are two possible explanations: either there’s more scatter at the low-mass end of the M-σ relation, or the scaling relationship is simply different for low-mass galaxies. The latter option is supported by some simulations that suggest that black holes don’t grow efficiently in low-mass galaxies.

Source: “The Contradiction of a Low-Mass Massive Black Hole” AASNOVA, 22 July 2020

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