Galaxies are sometimes called “cities of stars”. If so, there is truly a “Darkness on The Edge Of Town”, not just because the stars become sparser at the edge but because dark matter takes over as the dominant form of mass. Near the center, however, stars and planets make up most of the galactic heart.
Ever since Vera Rubin discovered that the speed with which galaxies rotate required far more mass than could be seen, astronomers have tried to learn what makes up this so-called “dark matter”. Resolving how this mysterious substance is distributed within galaxies is part of that quest. Professor Kenneth Freeman of the Australian National University has used a novel technique to provide an answer.
From inside the Milky Way much of our own galaxy is obscured, so astronomers study similar galaxies instead. “We chose NGC 6946, which is more than 22 million light years away from Earth,” Freeman said in a statement.
The way to work out dark matter's distribution, Freeman told IFLScience, is to find the overall distribution of mass and subtract the baryonic (ordinary or non-dark) matter. The first part has been done using the speeds at which stars orbit the galactic center to calculate the total radial gravitational pull they are experiencing.
Measuring ordinary matter contribution has proven harder, but in Monthly Notices of the Royal Astronomical Society Freeman and colleagues use NGC 6946's planetary nebulae to find an answer. Despite their name, planetary nebulae have nothing to do with planets. Instead, they represent a stage in a star's death where stellar winds push its outer layers into vast diffuse formations. Although exceptionally brief by astronomical standards, there are several hundred planetary nebulae at any one time in a galaxy the size of NGC 6946.
Planetary nebulae emit a distinctive green glow that if even slightly blue or red shifted reveals movement towards or away from us at even modest speeds. When a galactic disk faces us, astronomers can measure these nebulae's movements relative to the galactic plane, rather than the usual focus on orbits in the plane. These movements tell us the local gravitational forces from the disk. Since ordinary matter is known to be compressed into the disk, compared to the near-spherical distribution of dark matter, the new information allowed Freeman to determine the amount of ordinary mass at different distances from the galactic center.
“We’ve discovered that stars and gas account for about three-quarters of the gravitational pull in the inner parts of the Milky Way and other spiral galaxies,” Freeman said. “But the dark matter totally dominates gravity in the outer parts of these galaxies.
Freeman acknowledged to IFLScience that the Milky Way's mass distribution may not match NGC 6946, despite their common spiral structure and similarity in mass. “We're doing more galaxies in the same orientation to us,” he said. “If they are all consistent we can generalize, and that seems to be what we are seeing.”
Asked whether knowing the distribution has helped in understanding what dark matter is, however, Freeman answered: “Nope. We haven't a clue.”