The virus-P exploration of nearby galaxies (VENGA): The x co gradient in NGC 628

Abstract

We measure the radial profile of the 12CO(1-0) to H2 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 (ΣSFR). We estimate the molecular gas surface density (ΣH2) from ΣSFR 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 XCO 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(XCO) = 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 ΣH2 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. © 2013. The American Astronomical Society. All rights reserved.