Resolved Profiles of Stellar Mass, Star Formation Rate, and Predicted CO-to-H ₂ Conversion Factor Across Thousands of Local Galaxies

Abstract We present radial profiles of surface brightness in UV and IR bands, estimate stellar mass surface density (Σ ⋆ ) and star formation rate surface density (Σ SFR ), and predict the CO-to-H 2 conversion factor ( α CO ) for over 5000 local galaxies with stellar mass M ⋆ ≥ 10 9.3 M ⊙ . We build these profiles and measure galaxy half-light radii using GALEX and WISE images from the z 0MGS program, with special care given to highly inclined galaxies. From the UV and IR surface brightness profiles, we estimate Σ ⋆ and Σ SFR and use them to predict α CO with state-of-the-art empirical prescriptions. We validate our (kpc-scale) α CO predictions against observational estimates, finding the best agreement when accounting for CO-dark gas as well as CO emissivity and excitation effects. The CO-dark correction plays a primary role in lower-mass galaxies, whereas CO emissivity and excitation effects become more important in higher-mass and more actively star-forming galaxies, respectively. We compare our estimated α CO to observed galaxy-integrated SFR to CO luminosity ratio as a function of M ⋆ . A large compilation of literature data suggests that star-forming galaxies with M ⋆ = 10 9.5–11 M ⊙ show strong anticorrelations of SFR/ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>L</mml:mi> <mml:msub> <mml:mrow> <mml:mo accent="true">′</mml:mo> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">CO</mml:mi> <mml:mo stretchy="false">(</mml:mo> <mml:mn>1</mml:mn> <mml:mspace width="0.1em"/> <mml:mtext>–</mml:mtext> <mml:mspace width="0.1em"/> <mml:mn>0</mml:mn> <mml:mo stretchy="false">)</mml:mo> </mml:mrow> </mml:msub> <mml:mo>∝</mml:mo> <mml:msubsup> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>⋆</mml:mo> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.29</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> and SFR/ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>L</mml:mi> <mml:msub> <mml:mrow> <mml:mo accent="true">′</mml:mo> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">CO</mml:mi> <mml:mo stretchy="false">(</mml:mo> <mml:mn>2</mml:mn> <mml:mspace width="0.1em"/> <mml:mtext>–</mml:mtext> <mml:mspace width="0.1em"/> <mml:mn>1</mml:mn> <mml:mo stretchy="false">)</mml:mo> </mml:mrow> </mml:msub> <mml:mo>∝</mml:mo> <mml:msubsup> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>⋆</mml:mo> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.40</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> . The estimated α CO trends, when combined with a constant molecular gas depletion time t dep , can only explain ≈1/3 of these SFR/ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>L</mml:mi> <mml:msub> <mml:mrow> <mml:mo accent="true">′</mml:mo> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">CO</mml:mi> </mml:mrow> </mml:msub> </mml:math> trends. This suggests that t dep being systematically shorter in lower-mass star-forming galaxies is the main cause of the observed SFR/ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>L</mml:mi> <mml:msub> <mml:mrow> <mml:mo accent="true">′</mml:mo> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">CO</mml:mi> </mml:mrow> </mml:msub> </mml:math> variations. We publish all data products from this work, including galaxy sizes, UV and IR surface brightness profiles, Σ ⋆ , Σ SFR , and α CO estimates.