The edge of the Milky Way is not smooth but has ripples, and these ripples have perplexed astronomers since their discovery decades ago. But now we know exactly what caused them.
An international team of astronomers discovered that a small and quick galaxy whizzed past the Milky Way a few hundred million years ago. The scientists were able to measure the speed of some of the stars belonging to the dwarf galaxy, which allowed them to work out what actually happened when the fly-by occurred.
“It’s a bit like throwing a stone into a pond and making ripples,” Sukanya Chakrabarti, who presented the findings at the 227th meeting of the American Astronomical Society in Florida, said in a statement. “Of course we aren’t talking about a pond, but our galaxy, which is tens of thousands of light-years across, and made of stars and gas, but the result is the same – ripples!”
Her work is part of a new discipline called galactoseismology, and just like geologists use earthquakes to study the interior of our planet, ripples and interactions allow astronomers to estimate the distribution of matter in a galaxy. “This is really the first non-theoretical application of this field, where we can infer things about the unseen composition of galaxies from analyzing galaxic-quakes,” said Chakrabarti.
The stars observed are called Cepheid variables, which are one type of standard candle, objects whose distance can be calculated based on their luminosity.
“We have a pretty good idea of the distance to these stars because the intrinsic brightness of Cepheid variable stars depends on their period of pulsation, which we can measure,” said Chakrabarti.
“What I wanted to know was how fast this speeding bullet was going when it passed by our galaxy – with that information we can begin to understand the dynamics, and ultimately how much unseen dark matter is there.”
Cepheid Variable RS Puppis as imaged by Hubble (HST).
Dark matter is the dominant type of matter in the universe (making up 84 percent of all matter), but it does not interact with light so we cannot see it. Its effects can be seen on large scales, for example keeping the spiral shape in galaxies or grouping clusters of galaxies in large filaments.
We are yet to directly observe dark matter, but Chakrabarti and her team are looking for more Cepheid variable stars in the halo of the Milky Way to estimate how much dark matter there is in our galaxy.
“There could be a population of yet undiscovered Cepheid variables that formed from a gas-rich dwarf galaxy falling into our galaxy’s halo,” said Chakrabarti. “With the capabilities of today’s telescopes and instruments we should be able to sample enough of the Milky Way’s halo to make reasonable estimates on dark matter content – one of the greatest mysteries in astronomy today!”