New research shows that a section of the Outer Milky Way is denser and less organized than previously thought.

When you go out at night in a rural area with dark sky, you can look up and see a band of stars crossing the sky. This group is our Milky Way galaxy, which we see from the side since we are inside. If we could travel faster than light and climb above the plane of our galaxy, we would see a flat disc with spiral arms wrapped around the core. But what exactly would these spiral arms have? Stuck here without a bird’s eye view, we have to apply other methods to measure the shape of the galaxy.

Moving away from Earth’s location, astronomers built a model of the nearby spiral arm, known as the Perseus arm. Previous work has suggested that Perseus’ arm has a narrow and distinct shape. However, new research shows that at least part of Perseus’ arm may be illusory, without any well-defined structure. The illusion is the result of complexities first predicted by W. Burton in 1971.

On a map of the Milky Way, the nearby spiral arm just beyond the Sun is known as the Perseus arm. Astronomers created this map by measuring the locations of natural radio sources called masers (pink dots in the indents on the right) and dust clouds (blue dots). At the top right, a shaded area shows the previously believed shape of Perseus’ arm, bounded by a combination of masers and clouds of dust. New measurements (middle right) show that some of these dust clouds are much closer to or farther from the Sun than originally thought. As a result, Perseus’ arm can be much denser and less well defined (bottom right). Credit: Joshua Peek (STScI), Robert L. Hurt (Caltech, IPAC), Leah Hustak (STScI)

Our milky way can be softer, less steep

Our Milky Way has long been known to be a spiral, fried egg-shaped galaxy with a bulbous central bulge and a thin, flat disc of stars. For decades, astronomers have struggled to map the Milky Way’s disk and its associated spiral arms. As the old saying goes, you can’t see the forest for the trees, and if you’re in the middle of the forest, how can you map its groves without a bird’s-eye view?

Previous work has suggested that the Milky Way is what is called a “grand design” spiral, with long, narrow, and well-defined spiral arms. However, new research reveals that at least part of the Outer Milky Way (beyond the location of the Sun) is much more lumpy and chaotic.

“We have long had a picture of the galaxy in our minds, based on a combination of measurements and inferences,” said Josh Peek of the Space Telescope Science Institute (STScI) in Baltimore, Maryland. “This work calls this image into question. We see no evidence that the items we have connected are actually connected. “

Distances are essential

When mapping our galaxy, the biggest challenge is finding the distance to a given star, star cluster, or gas cluster. The gold standard involves using parallax measurements from natural radio sources called masers, some of which are found in regions of high mass star formation. However, this technique inevitably leaves gaps.

To fill these gaps, astronomers are shifting from examining regions of star formation to gas clouds, and more specifically the motions of those gas clouds. In an ideal situation, the movement of the line of sight that we measure for a gas cloud is directly related to its distance due to the overall rotation of the Milky Way. As a result, by measuring the velocities of gases, we can determine the distances and therefore the underlying structure of the galaxy.

The question then becomes, what about a non-ideal situation? While the motion of a given gas cloud may be dominated by its rotation around the galactic center, it undoubtedly has additional, more random motions as well. Can these additional movements disrupt our maps?

Chunky and lumpy

To investigate this matter, Peek and his colleagues examined not the gas, but the dust. In general, within our galaxy, gas and dust are closely associated, so if you can map one, you also map the other.

3D dust maps can be created by examining the colors of large collections of stars scattered across the sky. The more dust there is between the star and our telescope, the redder the star will appear from its natural color.

Image from the Hubble Space Telescope of the neighboring galaxy Messier 83 (M83). Credit: NASA, ESA and Z. Levay (STScI / AURA) Acknowledgments: NASA, ESA and R. Khan (GSFC and ORAU)

Peek and his team examined a region of space known as the Spiral Arm of Perseus, which lies beyond our Sun in the Milky Way’s disk. They compared the distances measured via the reddening of the dust to those determined by the speed relation. They discovered that many clouds are not, in fact, within the distance of Perseus’ arm, but rather extend over a distance of about 10,000 light years.

“After all, we don’t have long, lean spiral arms, at least in this part of the galaxy. There are lumps and lumps that look like nothing, ”Peek explained. “It’s a good possibility that the Milky Way’s outer disk resembles the neighboring galaxy Messier 83, with shorter, chopped arm pieces.”

Revise our menu

While this latest research has focused on the Outer Milky Way, Hubble Fellow Catherine Zucker, a member of Peek’s team at STScI, plans to expand this work to the Inner Milky Way. The region within the orbit of the Sun is where the spiral arms that most actively form the stars reside.

Zucker plans to create 3D dust maps using existing large-scale infrared surveys to measure the redness of about 1 to 2 billion stars. By linking these new dust maps to existing gas velocity readings, astronomers can refine our map of the Inner Milky Way as they have already done with the Outer Galaxy.

“Previous 3D dust mapping efforts relied heavily on data at wavelengths visible to the human eye. No one has used deep infrared data to create a 3D dust map, ”Zucker said. “We can see that this region, like the arm of Perseus, is more chaotic and less well defined.”

Even more information could come from the upcoming Nancy Grace Roman Space Telescope and the Vera Rubin Observatory. The Roman Space Telescope will have the capacity to map the entire galactic plane in a few hundred hours. In addition, its infrared measurements will cut dust.

“We could see clearly across the galaxy for the first time. If an investigation like this is selected for Roman, it would be mind-boggling, ”said Peek.

Rubin, on the other hand, will be able to make in-depth observations of faint objects at a variety of optical wavelengths. By combining Roman’s infrared view of the sky with Rubin’s deep multi-wavelength optical data, we can finally map our own cosmic “forest”.

This work is accepted for publication in The Journal of Astrophysics.

The Space Telescope Science Institute pushes the boundaries of space astronomy by hosting the Science Operations Center of the The Hubble Space Telescope, scientific operations and mission centers for the James Webb Space Telescope, and the Scientific Operations Center of the Nancy Grace Roman Space Telescope. STScI also houses the Barbara A. Mikulski Archives for Space Telescopes (MAST) which is a ">Nasa-project funded to support and provide the astronomical community with a variety of astronomical data archives, and is the data repository for Hubble, Webb, Roman, Kepler, K2, TEST missions and more. STScI is operated for NASA by the Association of Universities for Astronomical Research in Washington, DC