Astronomers have found a new way of accurately mapping the outer gas disk of the Milky Way using the positions of young stars.
In the process, they’ve also discovered that our galaxy’s structure is more complex than everyone thought, complete with tufty-looking “flocculent” gas clouds.
University of Alabama’s Sukanya Chakrabarti and MSU astronomer Peter Craig came up with the process of mapping hydrogen gas in the Milky Way’s outer disk by determining distances to very young stars in it.
They used data from the European Space Agency’s GAIA satellite, which measured brightnesses, positions, motions, and distances to nearly two billion Milky Way stars.
That last characteristic—distance—is particularly important in charting the gas distribution.
“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 said. “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.”
Mapping the distances to regions rich in hydrogen (HI) gas is important to understand the structure and dynamics of the Milky Way.
Understanding where and how much of that gas exists in the Galaxy impacts our understanding of star formation, the prevalence of x-ray binaries, which tend to be found along the spiral arms, and helps astronomers create accurate models of diffuse gamma-ray emissions related to interactions in the interstellar medium.
Distance is a Challenge
Determining the whereabouts of objects in the Galaxy—and the Galaxy’s shape itself—depends on measuring their positions and velocities. The Milky Way, with all its parts, is a giant rotating system.
Its stars and nebulae all orbit the center at various distances and also have their own individual motions. Getting all the distances right is a challenge. One of the characteristics of objects in space that astronomers measure is their radial velocity. That’s an object’s speed as it moves away from or toward us.
They get that measurement by taking a Doppler shift of the light coming from it. From there, astronomers calculate a rotation curve, which is the orbital speed of the object taking into account its distance from the center of the Galaxy.
This method is good for some objects in the Milky Way, but not as useful for others, according to Chakrabarti.
“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.”
Using that process to calculate distances to gas clouds is an inexact science, and Chakrabarti and Craig set out to find a better way to map those features. “The structure in stars is a lot smoother than in the gas,” she explained. “The gas is very fleecy looking, and it actually looks a lot more disturbed.”
Kinematic measurements can produce inaccurate measurements, and that accuracy depends on what assumptions astronomers make for the overall rotation curve of the disk. There are several places in the Milky Way where models of the velocity of clouds are inexact.
Regions of the disk that deviate from the assumed velocity model, such as near-streaming motions along spiral arms,” Chakrabarti said, “will lead to systematically inaccurate distance estimates and produce misleading features in the resultant maps of the Milky Way’s gas distribution.
Pattern-Matching to Create New Maps
So, how to get around the inherent inaccuracies? Look at similar areas in other galaxies and figure out if their patterns of star and gas-cloud distributions look familiar.
Chakrabarti and Craig noticed that the spiral structure in the gas clouds of nearby galaxies is very much like the spiral structure in young stars (less than 400 million years old).
“That’s not very surprising,” Chakrabarti said, “because young stars are born from the gas that 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 gas.”
Their pattern-matching approach paired young stars that have known locations with nearby clumps of gas. That gives a new map that doesn’t depend on the challenges posed by kinematic mapping. To get accurate distances, the team chose nearby Cepheid variable stars. These stars pulsate on a regular schedule—that is, they vary in their brightnesses over a few days to a few months.
Astronomers use that pulsation and the luminosity of the stars to calculate very accurate distances. In fact, a Cepheid variable in the Andromeda Galaxy was what alerted astronomer Edwin Hubble to the fact that Andromeda was not in our own Galaxy, but was, in fact, its own very distant galaxy.
Based on Cepheid variable work by astronomer Henrietta Leavitt, he was able to calculate the distance to Andromeda quite accurately. Ever since then, astronomers have used Cepheids in the Milky Way and other galaxies to come up with distances.
The new way of mapping by using patterns and Cepheid distance markers gave Chakrabarti and Craig a better and more accurate way to measure distances to the clouds in the outer disk of the Milky Way. It also showed that those clouds are fluffier and more tufty-looking than previously known.
“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,” Craig pointed out. “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.
This technique of pattern-matching paired with more accurate stellar distances should really improve astronomers’ understanding of the prevalence and shapes of the clouds in the hydrogen disk.
In addition, the team points out in their paper that more accurate distances should help supplement three-dimensional dust maps of the entire Galaxy. Previously they’d only been done in the close neighborhood of the Sun. In addition, improved mapping can help astronomers understand any disturbances in the disk, including interactions with nearby dwarf galaxies and the presence of dark matter.
Written by Carolyn Collins Petersen/Universe Today.
