Sukanya Chakrabarti, assistant professor at Rochester Institute of Technology in New York, uses waves in the galactic disk to map the interior structure and mass of galaxies

An international team of scientists led by an Indian-American professor has discovered a new method that may help in detecting dwarf galaxies dominated by dark matter and explain ripples in the outer disk of the galaxy.

Just as seismologists analyse waves to infer properties about the Earth’s interior, Sukanya Chakrabarti, assistant professor at Rochester Institute of Technology here, uses waves in the galactic disk to map the interior structure and mass of galaxies. This new method to characterise dark matter marks the first real application of the field of galactoseismology.

Findings presented to journal

Ms. Chakrabarti presented the findings at a press conference hosted by the American Astronomical Society meeting in Kissimmee on January 7. Her findings have been submitted to The Astrophysical Journal Letters.

Her team used spectroscopic observations to calculate the speed of the three Cepheid variables — stars used as yardsticks to measure distance in galaxies— in the Norma constellation.

Variables 3 lakh light years away

Ms. Chakrabarti’s 2015 study used Cepheid variables to mark the location of a dark-matter dominated dwarf galaxy approximately 3,00,000 light years 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 an average speed of 4,50,000 miles per hour; while the radial velocity of stars in the stellar disk of the Milky Way is about 13,000 miles per hour, Ms. Chakrabarti said in a statement on the university website.

“The radial velocity of the Cepheid variables is the last piece of evidence that we’ve been looking for,” Ms. Chakrabarti said. “You can immediately conclude that they are not part of our Galaxy.”

Invisible particles

Invisible particles known as dark matter make up 85 per cent of the mass of the universe. The mysterious matter represents a fundamental problem in astronomy because it is not understood, Ms. Chakrabarti has said.

Her method for locating satellite galaxies dominated by dark matter taps principles used in seismology to explore the interior of the galaxy.

Significant progress

“We have made significant progress into this new field of galacto-seismology 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,” Ms. Chakrabarti said.

She added: “The original prediction was based on observed waves in the outer gas disk of our galaxy which led to a specific prediction for how massive this dark matter dominated dwarf galaxy would have to be to produce these waves. It is very similar to seismology in a sense because we’re trying to infer things about the interior of galaxies and how much dark matter there is and how much there has to be to produce these disturbances.”

Questioning the standard paradigm

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,” Ms. Chakrabarti said. “It raises the question, should not 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.

Spectroscopic observations used in the study were made at the Gemini Observatory and on the Magellan telescopes, as well as on the WiFeS spectrograph.