An international collaboration, including astronomers from Australia, Germany, the Netherlands and the UK, using the Planetary Nebulae Spectrograph (PN.S) mounted on the William Herschel Telescope (WHT) has found that stars and gas dominate gravity in the inner parts of the disc galaxy NGC 6946, while dark matter dominates in its outer parts.

The rotation curves of spirals are the primary indicator of the presence of vast amounts of dark matter in galaxies. Astronomers can measure the total gravity as it provides the centripetal force that balances the rotation of the stars and gas in their almost circular orbits around the centre of a galaxy. By measuring the rotational velocity out to large radii, they can measure gravity from the galaxy's total mass. Typically, a large fraction of galactic mass is in some form of dark matter.

One of the big uncertainties is how much gravity originates in dark matter and how much in visible stars at different distances from the centre. In doing so astronomers can map the detailed distribution of the dark matter, almost as if this invisible matter can be seen directly.

As the stars circle the centre of a disc galaxy, they also oscillate up and down vertically in the plane of the disc. These motions are like that of a spring: the star is kicked out of the plane by interacting with clouds of gas molecules and the spiral arms. Instead of flying away into space, the stars are pulled back towards the disc by the gravity of the rest of the stars in the disc. Measuring these vertical motions in face-on disc galaxies like NGC 6946, allows the measurement of the mass density of the stars and gas alone.

One complication comes from the different generations of stars that make up the disc. The younger stars, recently born from gas, move around in a very thin layer in almost circular orbits. The older stars acquire more vertical motion because as they age, they are kicked here and there by inhomogeneities in the disc. It is this old population that allows measurement of the total surface densities in the galactic disc.

Hence measurements of the velocities of the older stars is needed, as they oscillate up and down the disc. The next crucial step is to measure these oscillatory motions over a large radial interval in the disc – only in this way can astronomers measure the mass density of the stars in the disc reliably.

But what stars to observe? Astronomers have long known about planetary nebulae. These are the late phases of Sun-like stars. They are blowing off an envelope of gas which gives off a green glow of an aquamarine hue from its oxygen atoms. The motions of these stars along the line-of sight are relatively easy to measure from the Doppler shift of the bright oxygen spectroscopic line.

Furthermore, as the disc becomes fainter at larger distances from its centre, the planetary nebulae become easier to detect and can be used as beacons to trace the stellar motions out to large distances from the galaxy centres. "For this observational challenge, the accuracy of the PN.S equipped with the Halpha arm is key to ensure the effective identification of such older stars in discs", said Magda Arnaboldi, the principal investigator of the PN.S.

An international team of astronomers led by Aniyan, Ponomareva, Freeman and Arnaboldi measured the stellar motions in the disc galaxy NGC 6946 using the spectra of the total star light in the bright central regions, and hundreds of individual planetary nebulae as dynamical tracers in the outer parts. They were able to map the motions perpendicular to the disc over a large radial interval in the disc, equivalent to more than four disc-scale lengths.