The VIRUS-P Exploration of Nearby Galaxies (VENGA): The X CO Gradient in NGC 628

We measure the radial profile of the ^(12)CO(1-0) to H_2 conversion 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_(CO) profile 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^(–1). 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.