There is a surprising amount of order in this chaotic universe of ours, but without a doubt this new discovery is even more incredible. Astronomers have discovered that all galaxies rotate once every 1 billion years, no matter how large; that is, regardless of their size, they take about 1 billion years to complete a full rotation.
This means that the material at the outer edge of a galaxy takes this long to complete one orbit of the center of the galaxy ( in the case of the Milky Way, that center is Sagittarius A, a supermassive black hole ).
“ It doesn’t have the precision of a Swiss watch ,” Gerhardt Meurer of the University of Western Australia, a subsidiary of the International Center for Radio Astronomy Research (ICRAR) and leader of the work, explained to the Monthly Notices of the Royal Astronomical Society. “But regardless of whether a galaxy is very large or very small, if you could sit on the edge of its disk as it spins, it would take you around 1 billion years to do the whole journey.”
Interestingly, because all the stars in our galaxy orbit around the center at roughly the same speed, objects closest to the center of the galaxy do not take as long to orbit Sagittarius A.
That’s why a cosmic year, the time it takes for our solar system to complete one orbit, is only 225-250 million years.
This is where the dark matter theory comes from, because there is simply not enough observable mass within the Milky Way to account for this effect, unless we have misunderstood gravity, or there is a mass that we cannot see.
The team’s finding means that, for smaller galaxies, the material needs to move more slowly, as it has less distance to cover in the same amount of time.
The researchers measured the radial velocities of neutral hydrogen in about 130 galaxies, from small irregular dwarf galaxies to huge spiral galaxies. The range of galaxies encompassed a factor of 30 in size and speed.
These measurements allowed us to calculate the time it takes for each galaxy to complete one rotation, and the result was that all took about 1 billion years. They also found a correlation between the distribution of light and atomic hydrogen at the galactic edge , allowing the team to confirm that their observation of the 1 billion-year rotation was actually occurring at the edges of the examined galaxies.
But they found something else around the edges: older stars. According to existing models, there should be mainly young stars populating the galactic edges, but the population of older stars they found was significant, with a thin scattering of younger stars and interstellar gas.
“This is an important result because knowing where a galaxy ends up means that astronomers can limit our observations and not waste time, effort and computer processing power studying data from beyond that point,” Meurer said.
“So because of this work, we now know that galaxies rotate once every billion years, with a sharp edge that is populated with a mixture of interstellar gas, with old and young stars.”
Referencia: ‘Cosmic clocks: A Tight Radius – Velocity Relationship for HI-Selected Galaxies`, published in the Monthly Notices of the Royal Astronomical Society on March 14th, 2018. Available at http://www.icrar.org/cosmic-clocks
