It has long been thought that supermassive black holes in the centre of galaxies and their surrounding stellar bulges grow hand in hand. However, two rebellious galaxies have been found ignoring this rule. 

NGC 4342 and NGC 4291 have black hole masses that are larger than normal, and could have prevented stars from forming in the galaxies. Image: X-ray: NASA/CXC/SAO/A.Bogdan et al; Infrared: 2MASS/UMass/IPAC-Caltech/NASA/NSF

While the central black hole of a galaxy can be billions of times the mass of our Sun, it is still usually only 0.2 per cent of the mass of the surrounding region of densely packed stars known as the bulge. Two galaxies, NGC 4342 and NGC 4291, were discovered to be breaking this relationship as their central black holes that are up to 35 times more massive than expected.

The co-evolution of black hole and bulge mass suggests that the stellar mass should be 10 to 40 times greater than it currently is. So where did all the stars go?

One theory is that tidal stripping could have reduced the number of stars in the galaxies. Tidal stripping occurs when a neighbouring galaxy rips stars away from another as it passes by, vastly reducing the number of stars in the bulge. Dark matter in the galaxies is more loosely bound than stars, and would also have been stripped away by the close encounter.

“Therefore, we expect dark matter halos around all undisturbed galaxies, but we do not expect to see extended dark matter halos around galaxies whose stellar population was tidally stripped,” explains Akos Bogdan from the Harvard-Smithsonian Center for Astrophysics.

Using NASA’s Chandra X-ray Observatory, Bogdan and his team were able to use hot X-ray gas to measure the total amount of matter in the two galaxies. “The total gravitating mass can be estimated assuming that the hot X-ray gas is in hydrostatic equilibrium, which means that the gravity compression of all the matter (including the stellar mass and the dark matter mass) is balanced by the gas pressure,” Bogdan tells Skymania News. The results showed that both galaxies have significant dark matter halos, which rules out the tidal stripping theory as the cause of the unusual mass ratios.

Rather than the stars being lost, it seems more likely that they never formed in the first place. During the early lives of the galaxies, a large concentration of gas would provide plenty of fuel to sustain rapid growth of the central black holes. Eventually, outbursts from the black hole’s accretion disc would prevent the gas from cooling, and thus prevent star formation.

The new research implies that the black hole growth rate in these galaxies is more likely to be tied to the surrounding dark matter halo, rather than to the stellar bulge.