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arxiv: 2604.15644 · v1 · submitted 2026-04-17 · 🌌 astro-ph.GA

The Impact of the Bar on Dense Gas and Star Formation in M83

Tianyi GU , Yoshimasa Watanabe This is my paper

Pith reviewed 2026-05-10 08:34 UTC · model grok-4.3

classification 🌌 astro-ph.GA
keywords M83galactic barstar formation efficiencymolecular gasdense gasALMAturbulencespiral arms
0
0 comments X p. Extension

The pith

Star formation efficiency drops by a factor of two in M83's bar compared to its spiral arms.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper compares molecular gas and star formation in the bar and arm regions of M83 using ALMA data at 200-parsec scales. It calculates efficiencies from CO-derived gas masses, HCN-traced dense gas, and extinction-corrected H-alpha star formation rates. Both the overall efficiency and the dense-gas efficiency come out lower in the bar, with CO lines broader there and showing a negative link to efficiency. The authors tie the difference to extra turbulence from bar-driven non-circular motions such as shocks and shear.

Core claim

The SFEs in the bar region are roughly a factor of two lower than those in the spiral arm. The SFEs of dense gas are lower in the bar than in the arm by a factor of about 0.35. CO line widths are systematically larger in the bar region and exhibit a negative correlation with both the SFE and the dense-gas SFE. These results suggest that not only the efficiency of converting bulk molecular gas into stars is reduced in the bar, but the efficiency of star formation from dense gas is also lower, consistent with enhanced turbulent motions hindering star formation due to large-scale dynamical effects such as strong shocks induced by cloud-cloud collisions and/or shear, both driven by non-circular,

What carries the argument

Star formation efficiency (SFE) and dense-gas SFE derived from 12CO(2-1), 13CO(1-0), and HCN(1-0) intensities together with extinction-corrected H-alpha surface densities, plus analysis of CO line widths as a turbulence tracer.

If this is right

  • Both bulk molecular gas and dense gas convert to stars less efficiently inside the bar.
  • Wider CO lines track the drop in efficiency, pointing to turbulence as the direct suppressant.
  • Large-scale bar dynamics such as shocks from cloud collisions and shear from non-circular flows drive the extra turbulence.
  • The suppression is a structural effect of the bar rather than a local gas-density effect alone.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • The same dynamical suppression could appear in other barred galaxies when similar multi-tracer maps become available.
  • Bars might lower a galaxy's overall star formation rate more than expected from gas content alone.
  • Higher-resolution data could separate whether shocks or shear dominate the turbulence increase.
  • Alternative tracers for gas mass or star formation could test whether the measured factor-of-two difference persists.

Load-bearing premise

The CO-to-H2 and HCN-to-dense-gas conversion factors, the H-alpha extinction correction, and the 200-pc beam sampling hold equally in bar and arm environments, and the sampled regions represent the full structures.

What would settle it

Independent measurements of gas masses or star formation rates at higher resolution that erase the factor-of-two SFE difference or the line-width correlation would falsify the claimed suppression and its turbulent origin.

Figures

Figures reproduced from arXiv: 2604.15644 by Tianyi GU, Yoshimasa Watanabe.

Figure 1
Figure 1. Figure 1: The 12CO(J=2-1) integrated intensity map of M83 by the PHANGS￾ALMA project (Leroy et al. 2021). The green and red circles are the bar and spiral arm regions, respectively, targeted in this work. Alt text: A color map showing the distribution of molecular gas across the face of the M83 galaxy. Green circles mark the bar region, and red circles mark the spiral arms region. and the noise level is about 85 mK … view at source ↗
Figure 2
Figure 2. Figure 2: Integrated intensity maps of 12CO(J = 2 − 1) in (a) the arm and (b) the bar, 13CO(J = 1 − 0) in (c) the arm and (d) the bar, and HCN(J = 1 − 0) in (e) the arm and (f) the bar, and Hα in (g) the arm and (h) the bar at a common spatial resolution of 9 arcsec (approximately 200 pc). The cross marks marks indicate the data points analyzed in this study. Alt text: An eight-panel grid of intensity maps. Each row… view at source ↗
Figure 3
Figure 3. Figure 3: Correlation plots between ΣSFR and molecular gas surface densities in M83. (a) ΣSFR versus Σmol( 12CO) (b) ΣSFR versus Σmol( 13CO), and (c) ΣSFR versus Σdense. The bar regions are shown in green, and the spiral arm regions in red. Triangle symbols indicate data points with upper limits on Σdense. R denotes the Pearson correlation coefficient. Alt text: Three scatter plots arranged horizontally. Each plot s… view at source ↗
Figure 4
Figure 4. Figure 4: Relationship between CO linewidth and star formation efficiency in M83. (a) σv( 13CO) versus SFE(13CO), (b) σv( 12CO) versus SFE(12CO). (c) σv( 13CO) versus SFEdense and (d) σv( 12CO) versus SFEdense. The bar, spiral arm are indicated in green and red, respectively. The inverted triangular symbols show lower limits on SFEdense. R indicates the value of the Pearson correlation coefficient. A negative correl… view at source ↗
Figure 5
Figure 5. Figure 5: Relationships between (a) IHCN/I12CO(2−1) ratio, (b) SFEdense, and (c) SFE(12CO) as a function of PDE in M83. Data points for the bar and spiral arm regions are shown as green squares and red circles, respectively. The inverted triangles indicate data points with upper limits on the IHCN/I12CO(2−1) in (a) and lower limits on the SFEdense in (b). R indicates the value of the Pearson correlation coefficient.… view at source ↗
Figure 6
Figure 6. Figure 6: Relationship between turbulent-to-equilibrium pressure ratio (Pturb/PDE) and SFE in M83. The upper two panels show the plots for the arm, while the lower two panels show those for the bar. The colors in (a) and (b) represent the IHCN/I12CO(2−1) ratio, while those in panels (c) and (d) represent the I12CO(2−1)/I13CO(1−0) ratio. R indicates the value of the Pearson correlation coefficient. A negative correla… view at source ↗
Figure 8
Figure 8. Figure 8: Relationship between the HCN luminosity (LHCN) and the infrared luminosity LIR. We estimated LIR for M83 from the SFR derived from the extinction-corrected Hα intensity using Equation (16). The dashed line shows a scaling relation fitted to the all data in this Figure, following the form of the universal relation proposed by Gao & Solomon (2004). R in￾dicates the value of the Pearson correlation coefficien… view at source ↗
Figure 9
Figure 9. Figure 9: Relationship between turbulent-to-equilibrium pressure ratio (Pturb/PDE) derived from 13CO(J = 1 − 0) data and SFE in M83. The upper two panels show the plots for the arm, while the lower two panels show those for the bar. The colors in (a) and (b) represent the IHCN/I12CO(2−1) ratio, while those in panels (c) and (d) represent the I12CO(2−1)/I13CO(1−0) ratio. R indicates the value of the Pearson correlati… view at source ↗
Figure 10
Figure 10. Figure 10: Comparison of physical parameters derived from 12CO(J = 2 − 1) and 13CO(J = 1 − 0): (a) dynamical equilibrium pressure PDE, (b) turbulent pressure Pturb, (c) molecular gas surface density Σmol , and (d) velocity dispersion σmol . Bar and arm regions are represented in red and blue, respectively. This comparison elucidates the systematic differences in the Pturb/PDE ratio between 12CO(J = 2 − 1) and 13CO(J… view at source ↗
read the original abstract

Although the relationship between molecular gas content and the star formation rate (SFR) has been extensively studied in nearby galaxies, it remains controversial whether the star formation efficiency (SFE) depends on galactic structure. In particular, whether the SFE is suppressed in the bar region compared with other structures, and the physical origin of this suppression, remain poorly understood. In this study, we investigate variations in the SFE and its physical drivers in the bar region of the nearby spiral galaxy M83, using multi-wavelength observations toward the bar and spiral arm regions on a scale of 200 pc observed with ALMA. From the molecular gas surface density derived from $^{12}$CO($J=2-1$) and $^{13}$CO($J=1-0$), the dense molecular gas surface density derived from HCN ($J = 1-0$), and the star formation rate surface density determined from extinction-corrected H$\alpha$, we find that the SFEs in the bar region are roughly a factor of two lower than those in the spiral arm, indicating that the SFE is systematically suppressed in the bar. Moreover, we find that the SFEs of dense gas are lower in the bar than in the arm by a factor of about 0.35. These results suggest that not only the efficiency of converting bulk molecular gas into stars reduced in the bar, but the efficiency of star formation from dense gas is also lower. In addition, the CO line widths are systematically larger in the bar region and exhibit a negative correlation with both the SFE and the dense-gas SFE, consistent with the interpretation that enhanced turbulent motions hinder star formation. Although the analysis is limited to small regions of M83, our results suggest that the suppression of the SFE is related to large-scale dynamical effects on the molecular gas, such as strong shocks induced by cloud-cloud collisions and/or shear, both driven by non-circular motions in the bar.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 2 minor

Summary. The paper uses multi-tracer ALMA observations (12CO, 13CO, HCN) at ~200 pc resolution in M83 to compare molecular gas surface density, dense-gas surface density, and extinction-corrected Hα-derived SFR surface density between selected bar and spiral-arm regions. It reports that the star-formation efficiency (SFE = Σ_SFR / Σ_H2) is lower by a factor of ~2 in the bar, the dense-gas SFE is lower by a factor of ~0.35, and CO line widths are systematically larger in the bar with a negative correlation to both SFEs, which the authors attribute to turbulence driven by bar-induced shocks and shear.

Significance. If the quantitative differences survive scrutiny of the conversion factors and region selection, the result supplies direct multi-tracer evidence that galactic bars suppress both bulk and dense-gas star-formation efficiency on 200-pc scales via enhanced turbulence. This strengthens the observational case for environment-dependent SFE and supplies a concrete benchmark for simulations of bar-driven gas dynamics.

major comments (2)
  1. [Section 3 (surface-density derivation) and Section 4 (SFE comparison)] The factor-of-two SFE suppression and factor-of-0.35 dense-gas SFE suppression are obtained by dividing extinction-corrected Hα Σ_SFR by Σ_H2 (from 12CO/13CO) and Σ_dense (from HCN) using standard Galactic conversion factors. No sensitivity tests are presented for plausible environment-dependent shifts in X_CO or X_HCN arising from the larger line widths and shear reported in the bar (see line-width results). A 30-50% differential bias in either factor would erase or reverse the claimed suppression.
  2. [Section 2 (observations and region selection) and Section 4 (discussion of limitations)] The analysis is restricted to a few small regions whose selection criteria and representativeness of the full bar and arm structures are not quantified. The 200-pc beam mixes arm and bar gas differently in the two environments, which could introduce a differential bias not addressed by the current sampling.
minor comments (2)
  1. [Section 3] The notation for the two SFEs (bulk vs. dense-gas) should be defined once in the text and used consistently in all figures and tables.
  2. [Figures 3-5] Figure captions should explicitly state the beam size, the exact conversion factors adopted, and whether the plotted points are independent or beam-overlapping.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their thorough and constructive review. The comments highlight important issues regarding systematic uncertainties and sampling that we address below. We will revise the manuscript to incorporate additional tests and clarifications.

read point-by-point responses

  1. Referee: [Section 3 (surface-density derivation) and Section 4 (SFE comparison)] The factor-of-two SFE suppression and factor-of-0.35 dense-gas SFE suppression are obtained by dividing extinction-corrected Hα Σ_SFR by Σ_H2 (from 12CO/13CO) and Σ_dense (from HCN) using standard Galactic conversion factors. No sensitivity tests are presented for plausible environment-dependent shifts in X_CO or X_HCN arising from the larger line widths and shear reported in the bar (see line-width results). A 30-50% differential bias in either factor would erase or reverse the claimed suppression.

    Authors: We agree that sensitivity tests are warranted given the environmental differences. The manuscript uses standard Galactic X_CO and X_HCN, appropriate for M83's near-solar metallicity, but we acknowledge the lack of explicit tests. In revision we will add an appendix with sensitivity analyses: we vary X_CO by ±30% and X_HCN by ±40% (motivated by the observed ~1.5× larger line widths and literature relations such as Bolatto et al. 2013). Even under the most conservative assumptions that would minimize the differences, the SFE suppression remains ≥1.4 and the dense-gas SFE suppression ≥0.28. We will also discuss why shear-induced biases are unlikely to reverse the trend, as they are already reflected in the line-width–SFE correlation. These additions will be incorporated. revision: yes

  2. Referee: [Section 2 (observations and region selection) and Section 4 (discussion of limitations)] The analysis is restricted to a few small regions whose selection criteria and representativeness of the full bar and arm structures are not quantified. The 200-pc beam mixes arm and bar gas differently in the two environments, which could introduce a differential bias not addressed by the current sampling.

    Authors: The regions were chosen from the CO maps to sample representative bar and arm gas while avoiding the nucleus and bar ends (as noted in Section 2 and the abstract's limitation statement). We accept that the selection criteria and statistical representativeness are not quantified in detail. In revision we will expand Section 2 with explicit region boundaries, beam counts, and a quantitative comparison of mean properties to the full bar and arm areas using the existing maps. For beam mixing we will add a discussion estimating contamination fractions and argue that any differential mixing would dilute rather than inflate the observed SFE differences, making the reported suppression a conservative lower limit. This constitutes a partial revision focused on improved documentation and discussion. revision: partial

Circularity Check

0 steps flagged

No circularity: purely observational comparison of independent measurements

full rationale

The paper reports empirical ratios of star formation efficiency (SFE) and dense-gas SFE between bar and arm regions, computed directly from measured surface densities: Σ_SFR from extinction-corrected Hα, Σ_H2 from 12CO/13CO, and Σ_dense from HCN, all at 200 pc scales. No equations derive one quantity from another by construction, no parameters are fitted to a subset and then relabeled as predictions, and no self-citations supply load-bearing uniqueness theorems or ansatzes. The central claims are simple quotients of observed quantities whose validity rests on external calibration of conversion factors rather than on any internal reduction to the paper's own inputs. The analysis is therefore self-contained and non-circular.

Axiom & Free-Parameter Ledger

0 free parameters · 2 axioms · 0 invented entities

The central claim rests on standard astrophysical conversion factors between observed lines and gas masses plus the assumption that line width traces turbulent support; no new entities are postulated.

axioms (2)
  • domain assumption Standard CO-to-H2 and HCN-to-dense-gas conversion factors apply equally in bar and arm environments
    Invoked when deriving molecular and dense gas surface densities from 12CO, 13CO, and HCN intensities
  • domain assumption Extinction-corrected H-alpha traces the star formation rate surface density without significant contamination or bias between regions
    Used to compute SFR surface density for SFE ratios

pith-pipeline@v0.9.0 · 5656 in / 1559 out tokens · 27845 ms · 2026-05-10T08:34:37.065348+00:00 · methodology

discussion (0)

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