Surveying the Whirlpool at Arcseconds with NOEMA (SWAN): III. $^{13}$CO/C$^{18}$O ratio variations across the M51 galaxy

TL;DR

This study uses the SWAN NOEMA survey to map $^{13}$CO(1-0) and C$^{18}$O(1-0) across M51 at $ oughly$125 pc, analyzing the $^{13}$CO(1-0)/C$^{18}$O(1-0) ratio $R^{13}_{18}$ as a function of galactocentric radius and $ ext{SFR}$ surface density. Employing the ratio-of-medians and spectral stacking, the authors find a galaxy-wide $ ilde{R}^{13}_{18} oughly 4.33$, with environment-dependent values (e.g., higher in the southern arm) and a modest positive radial trend but a negative trend with $ ext{SFR}$ surface density, especially in the molecular ring and northern arm. Through a discussion of line excitation, chemical abundances (fractionation, selective photodissociation, selective nucleosynthesis), and opacity, the work argues that selective nucleosynthesis and opacity variations are the primary drivers of the observed variations at cloud scales, rather than excitation effects. The results underscore the role of galactic environment in shaping CO isotopologue chemistry in a grand-design spiral and highlight the value of high-resolution, multi-line isotopologue mapping for understanding the physical and chemical state of the ISM and its link to star formation.

Abstract

CO isotopologues are common tracers of the bulk molecular gas in extragalactic studies, providing insights into the physical and chemical conditions of the cold molecular gas, a reservoir for star formation. Since star formation occurs within molecular clouds, mapping CO isotopologues at cloud-scale is important to understanding the processes driving star formation. However, achieving this mapping at such scales is challenging and time-intensive. The Surveying the Whirlpool Galaxy at Arcseconds with NOEMA (SWAN) survey addresses this by using the Institut de radioastronomie millimétrique (IRAM) NOrthern Extended Millimeter Array (NOEMA) to map the $^{13}$CO(1-0) and C$^{18}$O(1-0) isotopologues, alongside several dense gas tracers, in the nearby star-forming galaxy M51 at high sensitivity and spatial resolution ($\approx$ 125 pc).We examine the $^{13}$CO(1-0) to C$^{18}$O(1-0) line emission ratio as a function of galactocentric radius and star formation rate surface density to infer how different chemical and physical processes affect this ratio at cloud scales across different galactic environments: nuclear bar, molecular ring, northern and southern spiral arms. In line with previous studies conducted at kiloparsec scales for nearby star-forming galaxies, we find a moderate positive correlation with galactocentric radius and a moderate negative correlation with star formation rate surface density across the field-of-view (FoV), with slight variations depending on the galactic environment. We propose that selective nucleosynthesis and changes in the opacity of the gas are the primary drivers of the observed variations in the ratio.

Figures (10)

• Figure 1: The figure presents integrated intensity (moment-0) maps of the $^{13}$CO(1–0) (left) and C$^{18}$O(1–0) (right) emission lines. Gray points denote non-detections, i.e. sightlines with S/N $\leq$ 3, while coloured points indicate detections with S/N > 3. Overlaid contours correspond to the $^{12}$CO(1–0) emission 2013pety at the 30 K km s$^{-1}$ level, shown for reference.
• Figure 2: The complete dataset is presented in this figure, while the version masked for AGN activity and used in the analysis is provided in Appendix \ref{['AGN Acivity']}. The left panel shows the $\mathrm{R}^{13}_{18}$ line ratio map, while the central panel presents the $\Sigma_{\text{SFR}}$ map, limited to regions where the line ratio is significantly measured. In these maps, light gray points indicate non-detections (S/N $\leq$ 3 in both lines), intermediate gray denotes lower limits (S/N $\leq$ 3 in C$^{18}$O) and dark gray indicates upper limits (S/N $\leq$ 3 in $^{13}$CO). Coloured points mark detections with S/N > 3 in both lines. The overlaid contours represent the 30 K km s$^{-1}$ level of $^{12}$CO(1–0) emission for reference. The right panel shows the PAWS environmental mask 2014colombo over the SWAN FoV, where different colours denote distinct environments: nuclear bar (blue), molecular ring (pink), northern spiral arm (orange), southern spiral arm (red), and interarm (green). Points with black outlines correspond to the significant detections shown in the other panels.
• Figure 3: The figure displays $\mathrm{{R}^{13}_{18}}$ plotted against galactocentric radius. In the top panel, coloured points represent sightlines where both emission lines have S/N > 3. Downward triangles denote lower limits, where the ratio has S/N $\leq$ 3 and C$^{18}$O(1-0) also has S/N $\leq$ 3. Upward triangles indicate upper limits, where the ratio has S/N $\leq$ 3 and $^{13}$CO(1-0) also has S/N $\leq$ 3. Diamonds represent non-detections, where both lines have either S/N $\leq$ 3 or S/N > 3, but result in the ratio having S/N $\leq$ 3. White hexagons correspond to points obtained via spectral stacking with the error bars corresponding to the propagated statistical uncertainties. The bottom panels highlight the points of each environment (see right map in Fig. \ref{['fig:maps']}), with grey points matching the coloured ones in the top panel and the coloured points highlighting the respective environment. The black dashed line marks the $\widetilde{{\mathrm{R}}}^{13}_{18}$ for the FoV, while the coloured dashed line indicates the $\widetilde{{\mathrm{R}}}^{13}_{18}$ for the specific environment. The colour saturation for points in both the top and bottom panels reflects the kernel density estimate (KDE).
• Figure 4: The figure displays $\mathrm{{R}^{13}_{18}}$ plotted against SFR surface density. The description is analogous to Fig. \ref{['fig:env_plots_radius']}, with an added black dashed line indicating the FoV's median $\Sigma_{\text{SFR}}$. The black line extending in the northern spiral arm panel arises from a low-value non-detection point.
• Figure 5: The figure display deviations of significant (S/N $>$ 3) points from the average trends. The left panel illustrate the deviation of significant data points from the ratio of medians for the entire FoV while the right panels show how these significant points deviate from the stacked average within the corresponding $\Sigma_{\mathrm{SFR}}$ bin for the full FoV.