Galactic Dust Skeletons? They Might be Closer Than You Think!

Title: The Radcliffe Wave as the gas spine of the Orion Arm

Authors: C. Swiggum , J. Alves , E. D’Onghia , R.A. Benjamin , L. Thulasidharan , C. Zucker , E. Poggio , R. Drimmel , J.S. Gallagher III , A. Goodman

First Author’s Institution: Department of Astrophysics, University of Vienna

Status: Submitted to A&A Letters [open access]

There are a lot of perks to living in the Milky Way Galaxy, but from our perspective stuck in the barred spiral galaxy’s disk, it can be difficult to make sense of the true 3-D structure of the dust, gas and stars around us. Over the last century, a lot of effort has gone into mapping out the structure of our home galaxy, both the large-scale spiral structure as well as our local solar neighborhood

With the advent of the Gaia mission, we’re living in an extremely exciting time for the study of Galactic structure. Gaia has been able to measure parallax distances for OVER A BILLION stars with completely unprecedented accuracy, including hundreds of thousands of nearby stars nearby to our Sun. This dataset is revolutionizing our understanding of the Sun’s neighborhood, as researchers map out the nearby positions and motions of stars and the stuff between them, building towards a full 3-D understanding of our place in the Galaxy. But this mapping process has not been without surprises!

Figure 1. The clouds that make up the Radcliffe Wave, shown as filled blue dots as they appear on the plane of the sky. The highly-linear, wave-like structure is completely disguised by its 2-D projection. For 3-D visualizations of the wave, check out the resources here! Image credit: figure 1 from Alves et al. 2020

A kiloparsec-scale “spine” of dense gas resembling an undulating wave was recently discovered, made up by many of the most nearby star forming regions in the solar neighborhood. This structure, called the Radcliffe Wave, was hidden in plain sight, disguised by its misleading 2-D projection on the sky (see Figure 1). But in a spiral galaxy with supposedly beautiful arching arms, what’s this linear spine of gas and dust doing in the middle of everything? Could it somehow be the skeletal structure of our Galaxy’s spiral arms? Today’s paper aims to place the Radcliffe wave in the context of the Milky Way’s Galactic spiral arm structure, solving an intriguing problem in this exciting era of mapping our home Galaxy.

Figure 2. A top-down portrayal of the distribution of young stars (in blue colorscale), with nearby clouds of dust and gas overlaid in red, and other signatures of recent star formation shown as cyan circles. The shape of the Orion arm is clearly visible in the distribution of young stars, and the line of red dots represents the Radcliffe Wave, aligned with but offset from the young stars’ spiral pattern. Image credit: adapted from figure 1 from today’s paper. 

The authors of today’s paper consider the combination of multiple powerful datasets to figure out how this spine of clouds fits into our picture of Galactic structure.  In addition to the 3-D dust map that describes the positions of dust clouds in the Radcliffe Wave, they consider the placement of young stars in the Solar neighborhood (from the paper covered in this great astrobite!), along with other signatures of recent star formation such as masers and open clusters. Using a map of the distribution of young O, B and A type stars, with positions determined by the Gaia mission’s early data release 3, shows quite clearly how recent star formation follows a spiral pattern (see Figure 2), which is thought to be caused by the compression caused by the passage of the Galaxy’s spiral arms!

The clouds associated with the Radcliffe wave certainly appear aligned with the over-abundance of young stars representing the Galaxy’s spiral structure, specifically the nearby “Orion arm”. Strangely enough, the Radcliffe wave and the Orion arm structures are offset from each other by a few hundred parsecs, perhaps suggesting that the wave will go on to form a future population of young stars as the next stage of the spiral wave’s passage through the Galactic disk. 

But why is the Radcliffe wave so linear while the Orion arm appears curved? It turns out mostly to be a matter of perspective. We’re used to thinking of spiral arms as smoothly curved structures, but the authors point out that other spiral galaxies show segmented, “linear” dust features that are at least as long as the Radcliffe wave, as highlighted in Figure 3, aligned with their arching spiral arms. While several mysteries remain with regard to how such a long, coherent structure formed, and why it has its characteristic undulating shape, the authors have shown clearly that the Radcliffe wave appears to be a spine of dense gas associated with the Milky Way’s spiral arms, an exciting step in understanding the true nature of our Galaxy’s skeleton! 

Figure 3. A face-on view of two other spiral galaxies (M74 and M83), demonstrating the presence of kilosparsec-scale linear dust features, potentially resembling the Radcliffe Wave in our Milky Way. Image credit: Figure 3 in today’s paper.

Astrobite edited by Evan Lewis

Featured image credit: edited from a background image from ESAGaiaDPAC

About H Perry Hatchfield

I'm a PhD candidate in Physics at the University of Connecticut, where I study star formation and gas structure in the Milky Way's Galactic Center. I do this using radio observations of molecular clouds as well as hydrodynamic simulations, and I'm all about trying to find ways to compare these two exciting means of exploring the universe.

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