2013ApJ...764..117B

We measure the radial profile of the ¹²CO(1-0) to H₂conversion factor (X_CO) in NGC 628. The Hα emission from the VENGA integral field spectroscopy is used to map the star formation rate (SFR) surface density (Σ_SFR). We estimate the molecular gas surface density (Σ_H2) from Σ_SFR by inverting the molecular star formation law (SFL), and compare it to the CO intensity to measure X_CO. We study the impact of systematic uncertainties by changing the slope of the SFL, using different SFR tracers (Hα versus far-UV plus 24 µm), and CO maps from different telescopes (single-dish and interferometers). The observed X_COprofile is robust against these systematics, drops by a factor of two from R ∼ 7 kpc to the center of the galaxy, and is well fit by a gradient Δlog(X_CO) = 0.06±0.02 dex/kpc. We study how changes in X_CO follow changes in metallicity, gas density, and ionization parameter. Theoretical models show that the gradient in X_CO can be explained by a combination of decreasing metallicity, and decreasing Σ_H2 with radius. Photoelectric heating from the local UV radiation field appears to contribute to the decrease of X_CO in higher density regions. Our results show that galactic environment plays an important role at setting the physical conditions in star-forming regions, in particular the chemistry of carbon in molecular complexes, and the radiative transfer of CO emission. We caution against adopting a single X_CO value when large changes in gas surface density or metallicity are present.