Molecular Gas Morphological Analogues for the Milky Way

TL;DR

The paper investigates whether the Milky Way's pronounced dip in the radial distribution of molecular gas is typical by comparing it to external galaxies from the PHANGS-ALMA survey using CO radial profiles and a molecular-morphology framework. It harmonizes Milky Way CO data with PHANGS by converting CO transitions and normalizing by the effective radius, identifying external analogues with long bars and grand-design spirals (Stuber YClxxGnR) and revealing a correlation between bar length and dip extent. The Milky Way's stellar mass and star formation rate place it near the high end of the PHANGS sample, and its dip-like morphology aligns with the external analogues, supporting a bar-driven mechanism for gas clearing and star formation patterns. Overall, the work provides a new external benchmark for Milky Way molecular-gas morphology and identifies concrete Milky Way analogues for future detailed comparisons of barred-disk gas dynamics.

Abstract

Complete catalogs of molecular clouds in the Milky Way allow analysis of the molecular medium and the star formation properties of the Milky Way that closely follows the method used for nearby galaxies. We explore whether the big dip in the radial distribution of molecular gas in the Milky Way is peculiar and find several other galaxies with similar patterns, all with similar morphological classifications of YClxxGnR, indicating a clearly defined, long bar leading to a grand-design spiral. This category is fairly rare among galaxies in the PHANGS sample, but all galaxies with this classification have some evidence for dips in the radial distribution of CO emission. The lengths of the bars correlate with the extents of the dips. The Milky Way and the other galaxies with dips have similar stellar masses and star formation rates, both lying near the high ends of the distributions for all PHANGS galaxies.

Figures (5)

• Figure 1: The average CO $J = 1\rightarrow0$ intensity in radial bins is plotted for the Milky Way and for five PHANGS galaxies, converted from CO $J = 2\rightarrow1$2022AJ....164...43S using a variable $R_{\rm 21}$2025arXiv251005214S. The radii are normalized to the effective radius of each galaxy, and the horizontal lines at the bottom left show the normalized bar length with the same color coding.
• Figure 2: The average CO $J = 1\rightarrow0$ intensity in radial bins is plotted for the Milky Way and for three PHANGS galaxies with similar classifications but less similar dips. The normalizations are the same as in Figure \ref{['bigdipgals']}.
• Figure 3: Images of the external galaxies in Table 1 with deep dips in the radial profile of $\langle\hbox{$I_{\rm CO}$}\rangle$. The top images show the integrated intensity maps of CO $J = 2\rightarrow1$ at their native resolution from the PHANGS-ALMA public data release. These so-called "broad-mom-0" maps are obtained from integrating along the spectral dimension and masking out noise as described in leroy_2021_pipeline. The images are deprojected and derotated using inclination and position angles from 2021ApJS..257...43L. The bottom images display the 3.6 light as imaged by Spitzer as part of the Spitzer Survey of Stellar Structure in Galaxies Sheth_2010PASP..122.1397S and are deprojected and derotated as well. We indicate the galaxy effective radius as a white circle ($R_\mathrm{e}$, Table \ref{['tab:bigdips']}).
• Figure 4: Images of the external galaxies in Table 1 with more shallow dips. The top images are in CO $J = 2\rightarrow1$, while the bottom images are in 3.6 light as imaged by Spitzer. Same as Figure \ref{['fig:images']}.
• Figure 5: Top panel: Global SFR as function of stellar mass for the PHANGS-ALMA sample (grey points), the Milky Way (blue hexagon), the PHANGS galaxies with big dips (cyan circles), and those with shallow dips (pink squares). The star-forming main sequence according to 2019ApJS..244...24L is added for reference (solid line, extrapolated to larger and smaller stellar masses with a dotted line). The values are listed in Table \ref{['tab:bigdips']}. Error bars 2021ApJS..257...43L are shown in the bottom right corner.