An international research team led by Willem Adrianus Baan of the Xinjiang Astronomical Observatory (XAO) of the Chinese Academy of Sciences (CAS) discovered evidence for periodic disc instability in the disc of the H2O megamaser galaxy NGC 4258.
On June 30, their findings were published in Nature Astronomy.
NGC 4258, also known as Messier 106, is a nearby galaxy with abundant H2O MegaMaser emission. This emission comes from the fast-rotating disc that surrounds the active galactic nucleus, but the physical conditions that cause it are unknown.
The researchers conducted Space Very Long Baseline Interferometry (SVLBI) experiments in an elongated Earth orbit with the Russian-built RadioAstron Observatory and large ground telescopes in Green Bank, US, and Effelsberg, GER. They discovered that the H2O MegaMaser emission was caused by a series of evenly spaced clouds within the gas disc surrounding the nucleus of NGC 4258.
These SVLBI experiments were carried out with an Earth-space connecting baseline of up to 19.5 Earth diameters and an observational angular resolution of 11 micro-arcseconds (3 x 10-9 of a degree), corresponding to a footprint of only 62 AU at the galaxy itself. The observed molecular emission regions were discovered to be orbiting inside the galaxy’s rotating thin disc at a radius of only about 0.126 parsec (0.38 lightyears) from the galaxy’s black hole nucleus.
The H2O MegaMaser emission in these regions is caused by maser-amplification by excited/pumped water molecules as the multiple clouds drift in front of the radio continuum in NGC 4258’s nucleus. These emission regions’ formation, regular velocity separation, and time-dependent emission all appear to be consistent with the occurrence of a periodic magneto-rotational instability.
This type of shear instability is caused by differential rotation in the disc and was previously thought to control radial momentum transfer and viscosity within the accretion disc.
These SVLBI observations provided the first detailed look inside a thin Keplerian accretion disc surrounding a hot galactic nucleus.