The findings are being presented at the 246th meeting of the American Astronomical Society in Anchorage, Alaska, on June 8-12, in conjunction with Michigan State University (MSU). The study employs a technique the researchers call "pattern matching," using Cepheid variable stars to effectively anchor the distances of the gas on the outer edges of the galaxy's spiral arms for the first time.
"The structure in stars is a lot smoother than in the gas. The gas is very fleecy looking, and it actually looks a lot more disturbed," Chakrabarti says. "This is an area I worked on for many years, starting when I was a Ph.D. student with my very first work on spiral structure, particularly with a focus on the gas. Later, as a postdoctoral fellow, I did computer simulations to understand the disturbances in the traditional atomic hydrogen map that was produced by researchers at Berkeley."
Building a hydrogen map for the Milky Way has long been crucial to understanding the galaxy's structure, dynamics, star formation and evolution, as well as refining the observations of distant objects. The new study is a collaboration with Chakrabarti's former doctoral student, Dr. Peter Craig, a research associate in the MSU Department of Physics and Astronomy and lead author of the paper. The study incorporates data from the European Space Agency (ESA) Gaia mission.
"Distance is one of the most fundamental things you can measure in the universe, and one of the most foundational things we can work on in astronomy," Chakrabarti explains. "Unless you know distances, you can't map anything. The Berkeley map was based on the traditional method using so-called 'kinematic distances' that assume a model for the velocity fields of the galaxy."
Kinematic distances rely on measuring the velocity of objects and comparing it to a model of the galaxy's rotation, allowing astronomers to associate velocities with distances.
"The assignment of kinematic distances uses an assumed rotation curve to convert velocity information into a distance estimate," Chakrabarti notes. "For stars, there are some really good ways of doing this. But for the gas, there isn't anything."
"Regions of the disk that deviate from the assumed velocity model, such as near streaming motions along spiral arms, will lead to systematically inaccurate distance estimates and produce misleading features in the resultant maps of the Milky Way's gas distribution. There can be deviations from the assumed velocity model also towards the galactic bulge and bar. So, when you have a real galaxy you will see these deviations, and if your distance method relies on the simple velocity model, of course you're going to get some inaccurate distance estimates," says Chakrabarti.
To address these challenges, the new study uses "pattern matching," the idea of pairing young stars that have known locations with nearby clumps of gas to provide a new map that for the first time does not depend on kinematic distances.
"When you look at galaxies, you see the spiral structure in the gas is very similar to the spiral structure in the young stars," Chakrabarti says. "That's not very surprising, because young stars are born from the gas which collapses and forms these stars, and then they move from their birth site. With young stars, the patterns of the spiral structure are still very similar to that of a gas."
For the study, Chakrabarti and her colleagues chose Cepheid variable stars. Cepheids are pulsating stars that vary in brightness regularly, with periods ranging from a few days to several months. These stars are extremely luminous and can be seen at great distances, making them useful for determining the distances to galaxies.
"The three basic quantities that go into the gas data are longitude, latitude and velocity," Chakrabarti says. "We plot these for the gas and for the young stars, and you see they're very similar. Cepheid variable stars have very precise distances because [American astronomer] Henrietta Leavitt worked out this fantastic relationship many years ago. So, if you have Cepheids that are close to a clump of gas that doesn't have distances, we give the distances of the young stars to the gas that doesn't have it. This is the idea of pattern matching."
"Our new maps nicely demonstrate that the spiral structure in the gas disk of the Milky Way is highly flocculent, and that the overall structure of the disk is complex," Dr. Craig adds. "The maps generated using our new technique can capture features in the gas that might be missed when assuming a smooth rotation model for the Milky Way. Pattern matching maps of the hydrogen disk will only get better with time, as accurate distance measurements become available for an ever-increasing sample of young stars in the galaxy. These new, more accurate, maps of the galactic hydrogen disk may allow for better characterization of the dark matter in the galaxy."
Analysis of simulated spiral galaxies indicates that pattern matching distances are 24% more accurate than kinematic distances for gas, which promises important implications for understanding large-scale disturbances in the gas disk and how they are impacted by interactions between the Milky Way and dwarf galaxies.
In all, the research employed over 37,000 stars distributed throughout the Milky Way disk with ages under 210 million years to create the new map. "In the future, this novel distance method can be improved with more complete stellar samples," the study concludes.
Research Report:A map of the outer gas disk of the Galaxy with direct distances from young stars
Related Links
University of Alabama in Huntsville
Stellar Chemistry, The Universe And All Within It
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