Molecular Gas Morphological Analogues for the Milky Way

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.

💡 Research Summary

The paper investigates whether the pronounced dip in the radial distribution of molecular gas observed in the Milky Way is an unusual feature or part of a broader class of galactic phenomena. Using the complete catalog of Milky Way molecular clouds derived from CO J=1→0 emission (Miville‑Deschênes et al. 2017; Elia et al. 2025), the authors construct an annular average intensity profile ⟨I_CO⟩ with 0.5 kpc bins from the Galactic centre out to 20 kpc. This profile shows a deep minimum—about an order of magnitude below the interpolated trend—between roughly 1 and 4 kpc, coincident with the region identified as the long bar in previous infrared studies.

To place this feature in an extragalactic context, the authors turn to the PHANGS‑ALMA survey, which provides high‑resolution CO J=2→1 maps for 90 nearby, massive, star‑forming galaxies. From Sun et al. (2022, 2025) they extract radial CO intensity profiles, convert them to the CO J=1→0 line using a radius‑dependent R_21 factor, and normalise radii by each galaxy’s effective (half‑light) radius R_e. Among the 26 PHANGS galaxies with CO data extending to at least 8 kpc, eight are classified under the Stub er morphological scheme as YClxxGnR: clearly visible disks (Y), clear (C) long bars (l), grand‑design spirals (G), and no central rings (nR).

Five of these eight (NGC 1300, NGC 3627, NGC 4321, NGC 4535, and NGC 4548) display a “big dip” that is strikingly similar in depth, width, and radial location to the Milky Way’s dip when plotted as ⟨I_CO⟩ versus R_gal/R_e. The extent of each dip matches the measured bar length (R_bar) for the same galaxy, establishing a clear correlation: longer bars produce wider, deeper deficits in molecular gas surface density. Three additional galaxies (NGC 1097, NGC 2903, NGC 5236) show shallower dips, while one (NGC 4548) exhibits a large but shallow dip; these variations are consistent with modest differences in bar length or bar strength.

Table 1 summarises key physical parameters—distance, stellar mass (log M_*), effective radius, bar length, star‑formation rate (SFR), and CO J=1→0 luminosity (log L_CO)—for all eight galaxies and the Milky Way. The Milky Way’s stellar mass (≈5×10^10 M_⊙) and SFR (≈1.6 M_⊙ yr⁻¹) place it near the high‑mass, high‑SFR end of the PHANGS distribution, a region also occupied by the analogue galaxies. NGC 4548 is highlighted as the closest match: its CO luminosity, SFR, and bar length (≈5.9 kpc) are nearly identical to the Milky Way’s values, making it an excellent molecular‑gas analogue.

The authors argue that the Milky Way’s molecular‑gas morphology is not “typical” of all PHANGS galaxies but is characteristic of a relatively rare subclass (~10 % of the sample) defined by the YClxxGnR morphology. This subclass is defined by the presence of a well‑defined, long bar that appears to clear out molecular gas from the inner disc, producing the observed dip, and by a grand‑design spiral pattern outside the bar. The correlation between bar length and dip extent provides independent support for earlier dynamical estimates of a Milky Way bar radius of ∼4 kpc (e.g., Blitz & Spergel 1991; Lucey et al. 2023).

The paper also contrasts its molecular‑gas‑based analogue selection with previous analogue studies based on stellar morphology or photometry (e.g., Fraser‑McKelvie et al. 2019). None of the eight YClxxGnR galaxies appear in those earlier analogue lists, largely because of differing sky coverage (PHANGS is southern‑hemisphere biased, while the SDSS‑based studies are northern) and because the earlier works did not consider molecular‑gas morphology. This underscores the added value of using CO observations to identify structural analogues.

Finally, the authors discuss methodological considerations: distances are taken from the PHANGS tables; CO‑to‑H₂ conversion factors (α_CO) are allowed to vary with metallicity; and the conversion from CO J=2→1 to J=1→0 incorporates a radius‑dependent line ratio. These steps mitigate systematic uncertainties and enable a fair comparison between the Milky Way and external galaxies.

In summary, the study demonstrates that the Milky Way’s pronounced molecular‑gas dip is a natural consequence of its long bar, a feature shared by a small but well‑defined group of nearby galaxies. By identifying NGC 4548 and several others as close molecular analogues, the work provides a new extragalactic benchmark for interpreting the Milky Way’s inner‑disk dynamics, bar‑driven gas redistribution, and star‑formation patterns.