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The superclouds of the local Milky Way

Lilly A. Kormann, João Alves, Michelangelo Pantaleoni González, Cameren Swiggum, Torsten A. Enßlin, Gordian Edenhofer

TL;DR

This work identifies seven large, elongated 'superclouds' in the local Milky Way by leveraging Gaia-based 3D dust maps, revealing a quasi-parallel network with typical pitch angles around $\sim$ $33.5^{\circ}$ and masses $\sim$ $10^5$–$10^6\,M_\odot$ that frequently host star-forming regions. Using the HOP structure finder on a high-resolution dust-derived hydrogen density map, the authors extract 40 smaller HOP clouds, compute their masses, shapes via PCA, and orientations, and then assemble seven coherent superclouds from these components. A notable result is the consistent, near-pressure-equilibrium behavior: while masses and lengths vary by factors of a few, volume densities differ by only $\sim$10%, suggesting self-regulation by the surrounding ISM. Furthermore, most superclouds exhibit undulations in the Z/Y$'$ plane, akin to the Radcliffe Wave, indicating that vertical oscillations may be a common feature rather than unique to a single structure. Collectively, these findings imply that large-scale Galactic dynamics shape local ISM conditions and the environments where GMCs and star formation arise, beyond conventional spiral-arm triggers.

Abstract

Recent 3D dust maps of the local Milky Way are revolutionizing our understanding of the Sun's Galactic neighborhood, providing much needed insight into the large-scale organization of the interstellar medium. Focusing on the largest scales in $\textit{Gaia}$-based 3D dust maps, we find a pattern of seven highly elongated, mostly parallel structures in the local $\sim 5\,\mathrm{kpc}^2$, five of which were previously unknown. These structures show pitch angles of $33.5 \pm 4.0 ^\circ$ and masses ranging from $10^5$ to $10^6$ $\mathrm{M}_\odot$. We refer to these structures as superclouds. Nearly all known star-forming regions in the solar neighborhood lie within the superclouds, primarily along their central axes, supporting the idea that they act as gas reservoirs for the formation of giant molecular clouds. All but one of the seven superclouds show an underlying undulation, indicating that this is not a property unique to the Radcliffe Wave. We find that while the superclouds have linear masses that vary by about a factor of 4, their volume densities only vary by about 10$\%$. This suggests that superclouds self-regulate their physical sizes and internal structure to maintain pressure equilibrium with their environment. These findings establish a new framework for understanding how large-scale Galactic structures shape the conditions for star formation in the solar vicinity, and likely in galaxies like the Milky Way.

The superclouds of the local Milky Way

TL;DR

This work identifies seven large, elongated 'superclouds' in the local Milky Way by leveraging Gaia-based 3D dust maps, revealing a quasi-parallel network with typical pitch angles around and masses – that frequently host star-forming regions. Using the HOP structure finder on a high-resolution dust-derived hydrogen density map, the authors extract 40 smaller HOP clouds, compute their masses, shapes via PCA, and orientations, and then assemble seven coherent superclouds from these components. A notable result is the consistent, near-pressure-equilibrium behavior: while masses and lengths vary by factors of a few, volume densities differ by only 10%, suggesting self-regulation by the surrounding ISM. Furthermore, most superclouds exhibit undulations in the Z/Y plane, akin to the Radcliffe Wave, indicating that vertical oscillations may be a common feature rather than unique to a single structure. Collectively, these findings imply that large-scale Galactic dynamics shape local ISM conditions and the environments where GMCs and star formation arise, beyond conventional spiral-arm triggers.

Abstract

Recent 3D dust maps of the local Milky Way are revolutionizing our understanding of the Sun's Galactic neighborhood, providing much needed insight into the large-scale organization of the interstellar medium. Focusing on the largest scales in -based 3D dust maps, we find a pattern of seven highly elongated, mostly parallel structures in the local , five of which were previously unknown. These structures show pitch angles of and masses ranging from to . We refer to these structures as superclouds. Nearly all known star-forming regions in the solar neighborhood lie within the superclouds, primarily along their central axes, supporting the idea that they act as gas reservoirs for the formation of giant molecular clouds. All but one of the seven superclouds show an underlying undulation, indicating that this is not a property unique to the Radcliffe Wave. We find that while the superclouds have linear masses that vary by about a factor of 4, their volume densities only vary by about 10. This suggests that superclouds self-regulate their physical sizes and internal structure to maintain pressure equilibrium with their environment. These findings establish a new framework for understanding how large-scale Galactic structures shape the conditions for star formation in the solar vicinity, and likely in galaxies like the Milky Way.

Paper Structure

This paper contains 27 sections, 3 equations, 9 figures, 4 tables.

Table of Contents

  1. Introduction
  2. Data
  3. Methods
  4. HOP structure finder
  5. Segmentation stages
  6. Parameter selection
  7. Structure properties
  8. Mass calculation
  9. Shape definition
  10. Orientation calculation
  11. Undulation fitting
  12. Results
  13. HOP cloud catalog
  14. Supercloud catalog
  15. Supercloud identification
  16. ...and 12 more sections

Figures (9)

  • Figure 1: Face-on view of the 40 HOP clouds (left) and their corresponding orientation (right). Each cloud is assigned a distinct color, and the length of the line corresponds to the calculated length of the cloud. The Sun is represented by the yellow dot in the center; the Galactic Center is located to the right. An interactive 3D version of the 40 HOP clouds is available https://lillykormann.github.io/superclouds_figures/hop_clouds/hop_clouds_surface.html.
  • Figure 2: Orientation distribution of the 40 HOP clouds. Colors indicate the same clouds as in Fig. \ref{['fig:structures_general']}, the distance from the center reflects the cloud mass, and the solid line represents the mean orientation. Top: Face-on view from the North Galactic Pole, with an average angle of $\alpha = 22.6 \pm 25.5 \degr$. Bottom: Edge-on view from the Galactic Center, with an average angle of $\beta = 92.3 \pm 23.4 \degr$.
  • Figure 3: Face-on view of the HOP clouds color-coded by their mean altitude in Z. The Sun is represented by the yellow dot in the center, and the Galactic Center is located to the right.
  • Figure 4: Face-on view of the seven superclouds defined in the local Milky Way (left) and their corresponding orientation (right). Each cloud is assigned a distinct color. The length of the line corresponds to the calculated length of the cloud. The Sun is represented by the yellow dot in the center, and the Galactic Center is located to the right. An interactive 3D version of the seven superclouds is available https://lillykormann.github.io/superclouds_figures/superclouds/superclouds_surface.html.
  • Figure 5: 2D projection in the Z/Y$^\prime$ plane of the selected superclouds with a sinusoidal fit. For the fit, the clouds are rotated counterclockwise by their face-on angle ($\alpha$) in the X/Y plane. The first three clouds are fit with a damped sinusoidal, the fourth cloud with a simple sinus function. All fit parameters are listed in Table \ref{['tab:fit_parameters']}.
  • ...and 4 more figures