Wide-field CO isotopologue emission and the CO-to-H2 factor across the nearby spiral galaxy M101

Carbon monoxide (CO) emission constitutes the most widely used tracer of the bulk molecular gas in the interstellar medium (ISM) in extragalactic studies. The CO-to-H₂ conversion factor, α_12CO(1−0), links the observed CO emission to the total molecular gas mass. However, no single prescription perfectly describes the variation of α_12CO(1−0) across all environments within and across galaxies as a function of metallicity, molecular gas opacity, line excitation, and other factors. Using spectral line observations of CO and its isotopologues mapped across a nearby galaxy, we can constrain the molecular gas conditions and link them to a variation in α_12CO(1−0). Here, we present new, wide-field (10 × 10 arcmin²) IRAM 30-m telescope 1 mm and 3 mm line observations of ¹²CO, ¹³CO, and C¹⁸O across the nearby, grand-design, spiral galaxy M101. From the CO isotopologue line ratio analysis alone, we find that selective nucleosynthesis and changes in the opacity are the main drivers of the variation in the line emission across the galaxy. In a further analysis step, we estimated α_{¹²CO(1−0)} using different approaches, including (i) via the dust mass surface density derived from far-IR emission as an independent tracer of the total gas surface density and (ii) local thermal equilibrium (LTE) based measurements using the optically thin ¹³CO(1-0) intensity. We find an average value of ⟨α_{¹²CO(1 − 0)}⟩ = 4.4 ± 0.9 M_⊙ pc⁻² (K km s⁻¹)⁻¹ across the disk of the galaxy, with a decrease by a factor of 10 toward the 2 kpc central region. In contrast, we find LTE-based α_{¹²CO(1−0)} values are lower by a factor of 2-3 across the disk relative to the dust-based result. Accounting for α_{¹²CO(1−0)} variations, we found significantly reduced molecular gas depletion time by a factor 10 in the galaxy's center. In conclusion, our result suggests implications for commonly derived scaling relations, such as an underestimation of the slope of the Kennicutt Schmidt law, if α_{¹²CO(1−0)} variations are not accounted for.