These images of the Milky Way show the distribution of gas, at left, compared to the distribution of stars, at right, after the dwarf satellite disrupts the galaxy. Credit: Sukanya Chakrabarti/Rochester Institute of Technology
A trio of brightly pulsating stars at the outskirts of the Milky Way is racing away from the galaxy and may confirm a method for detecting dwarf galaxies dominated by dark matter and explain ripples in the outer disk of the galaxy. This new method to characterize dark matter marks the first real application of galactoseismology. Just as seismologists analyze waves to infer properties about Earth’s interior, Sukanya Chakrabarti, assistant professor at Rochester Institute of Technology, uses waves in the galactic disk to map the interior structure and mass of galaxies.
Her team used spectroscopic observations to calculate the speed of the 3 Cepheid variables – stars used as yardsticks to measure distance in galaxies – in the Norma constellation. Chakrabarti’s 2015 study used Cepheid variables to mark the location of a dark-matter dominated dwarf galaxy ~300,000 light yrs away. In contrast, the disk of the Milky Way terminates at 48,000 light years.
The current study tracks a cluster of Cepheids that are racing away at 450,000 miles/h; while the radial velocity of stars in the stellar disk of the Milky Way is about 13,000 miles/h, Chakrabarti said. The method confirms her 2009 prediction. “The radial velocity of the Cepheid variables is the last piece of evidence that we’ve been looking for,” Chakrabarti said. “You can immediately conclude that they are not part of our Galaxy.”
Her method for locating satellite galaxies dominated by dark-matter taps principles used in seismology to explore the interior of the galaxy. “We have made significant progress into this new field of galactoseismology where by you can infer the dark matter content of dwarf galaxies, where they are, as well as properties of the interior of galaxies by looking at observable disturbances in the gas disk,” Chakrabarti said.
The study further questions the standard paradigm that old stars populate the dark matter halo and young stars form in the gas-rich stellar disks.
“Given the evidence, these are very likely young Cepheid variables,” Chakrabarti said. “It raises the question, shouldn’t we also be exploring and looking for young Cepheid variables in the halo?” There could be a population of yet undiscovered Cepheid variables that formed from a gas-rich dwarf galaxy falling into the halo, she said.
http://www.rit.edu/news/story.php?id=54309
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