Correlations of ALMA CO(2-1) with JWST mid-infrared fluxes down to scale of $\lesssim$100 parsec in nearby star-forming galaxies from PHANGS

TL;DR

We examine CO(2-1) emissions in 19 PHANGS galaxies and their correlations with JWST MIR tracers of PAH and dust down to ~100 pc using the regression framework \\raddest\\ to properly handle uncertainties and outliers. The results reveal broadly log-linear CO–MIR/dust relations that depend on ionization conditions and exhibit a bimodal intercept across galaxies, with the most significant galaxy-to-galaxy variation driven by ${b_{ m KS}}$. The analysis further shows that the coupling strength and non-linear deviations are scale dependent, becoming stronger and more non-linear at smaller scales, and that non-log-linear behavior is most pronounced in the brightest regions, particularly for the dust band. Across MIR bands, slopes are similar while intercepts differ, indicating band-specific emissivity and PAH/dust ratios modulate the mean relation but not the overall coupling strength. The findings highlight the crucial role of local ionization environments and spatial scale in shaping molecular gas–dust–PAH correlations and motivate comparisons with detailed ISM simulations.

Abstract

We investigate the correlations of CO (2-1) emission (${I_{\rm CO}}$) with PAH (${I_{\rm F770W, PAH}}$ and ${I_{\rm F1130W}}$) and dust (${I_{\rm F2100W}}$) emission down to scales of $\lesssim$ 100 pc, by applying \raddest, a novel regression technique recently developed by T. Jing & C. Li (2025) that effectively handles uncertainties and outliers in datasets, to 19 nearby star-forming galaxies in the PHANGS sample. We find that for the majority of the data points in all galaxies, the scaling of ${I_{\rm CO}}$ with ${I_{\rm F770W, PAH}}$, ${I_{\rm F1130W}}$, and ${I_{\rm F2100W}}$ can be well described by log-linear relations, though with substantial dependence on ionization conditions (i.e., HII-like, composite-like, and AGN-like). Under given ionization conditions, significant galaxy-to-galaxy variations are identified, and are primarily attributed to variations of intercept $b$, which exhibits clear bimodality. This bimodality is related to the overall host galaxy star formation strength. The differences in slope $k$ and intrinsic scatter $σ$ across different MIR bands (${I_{\rm F770W, PAH}}$, ${I_{\rm F1130W}}$, and ${I_{\rm F2100W}}$) are minor compared to their galaxy-to-galaxy variations. All parameters ($k$, $b$, and $σ$) depend on the spatial scale of measurement, suggesting that the coupling among CO, PAH, and dust is regulated by different mechanisms at varying scales. We identify non-log-linear behaviors in the brightest regions, where deviations are primarily characterized by flattening of slope. No significant (3$σ$) correlations are found between global properties and the best-fit parameters. We discuss the comparison to previous studies and plausible physics behind the statistical results obtained in this work.

Figures (13)

• Figure 1: Examples of the three different cases of non-log-linear behavior (case (a), (b), and (c), from left to right). In each panel, the lower sub-panel displays the logarithm of the median observed CO(2-1) flux in each observed ${\rm F2100W}$ or ${\rm F770W}_{\rm PAH}$ flux bin as black circles (real data), blue circles (mock data generated by single log-linear fitting), and green circles (mock data generated by piecewise log-linear fitting, available only in the left panel). Error bars represent the 1$\sigma$ uncertainty of the median. The best-fit relations for the single and piecewise log-linear fits are shown as blue and green lines, respectively. The two vertical dotted lines mark the 5% and 95% percentiles of the observed ${\rm F2100W}$ or ${\rm F770W}_{\rm PAH}$ flux distribution. These lines are also shown in the top sub-panel, where the histogram illustrates the marginalized distribution of the observed MIR bands. Note that data points with negative MIR bands measurements are excluded when plotting the marginalized distribution in logarithmic space but are included when calculating the 5% and 95% percentiles and applying regression analysis.
• Figure 2: Scaling relation between ${I_{\rm CO}}$ and ${I_{\rm F770W, PAH}}$, ${I_{\rm F1130W}}$, and ${I_{\rm F2100W}}$ (top to bottom) in different ionization conditions (all regions, H ii-like regions, composite-like regions, and AGN-like regions from left to right). In each panel, the contour showcase the distribution of spaxels from all galaxies, and the bold line is corresponding best-fit result. The thin lines are best-fit results for each galaxy.
• Figure 3: Best-fit slope $k_{\rm KS}$, intercept $b_{\rm KS}$, and intrinsic scatter $\sigma_{\rm KS}$ based on KS-test based method for each galaxy, of different MIR bands (blue for ${I_{\rm F770W, PAH}}$, green for ${I_{\rm F1130W}}$, and red for ${I_{\rm F2100W}}$), and under different ionization conditions (all regions, H ii-like regions, composite-like regions, and AGN-like regions from left to right). In each panel, the galaxies are sorted on the x-axis by total stellar mass (and also by galaxy ID, as shown in \ref{['tab:galaxies']}). The horizontal dashed lines with different colors show the median value of the best-fit parameters across different MIR bands. Error bars represent 1$\sigma$ uncertainties. In each row, panels in the first three columns share the same y-axis range, which is indicated as a shaded region in the last column panel.
• Figure 4: The correlation between the slope $k_{\rm KS}$ and the intercept $b_{\rm KS}$ is shown for each MIR band (top to bottom) and each ionization condition (left to right). The vertical dashed line in the top-left panel indicates $b_{\rm KS} = 0$, the criterion used to classify galaxies into high-$b$ and low-$b$ subgroups. The high-$b$ galaxies are represented by red dots, while the low-$b$ galaxies are shown as blue crosses. In each row, the first three panels share identical x-axis and y-axis ranges; these ranges are highlighted as gray shaded regions in the last panel of each row.
• Figure 5: Left: Distribution of low-$b$ (blue crosses) and high-$b$ (red circles) galaxies on $\log {\rm SFR}$ vs. $\log M_{\star}$ diagram. The gray contour represents the distribution of a volume-limited sample of nearby galaxies, constructed from the MaNGA MaNGA_Bundy2015MaNGA_Blanton2017MaNGA_Wake2017 sample with galaxy weight corrections. Right: Same as the left panel, but the x-axis is $\log L_{\rm CO}$.