Multiple-line study of molecular gas in spiral galaxy NGC 2903

Abstract

Multiple molecular lines with radiative transfer modeling are a powerful tool to probe the physics of starforming gas in galaxies. We investigate the gas properties in the center of spiral galaxy NGC 2903 using low-J CO lines,i.e. 12 CO(1-0), 12 CO(2-1), 12 CO(3-2), 13 CO(1-0), and HCN(1-0). We apply a nonlocal thermodynamic equilibriumradiative transfer code to derive beam-averaged molecular gas properties. We use two methods (i.e. χ2 minimizationand likelihood) to define the best model representing the observed line ratios best. The line ratio diagnostics suggest thatCO gas in the center of NGC 2903 is thinner and the dense gas fraction is similar compared to that of spirals, starbursts,and early-type galaxies (ETGs), while the gas in the center of the galaxy is warmer than that of ETGs and colder thanthat of starbursts. Based on the best-fitting model results, we find that the beam-averaged gas kinetic temperature isTK = 20 K, H2 volume number density is log(n(H2)) = 4.2 cm −3, and CO column number density is log(N (CO)) =19.0 cm −2in the center of NGC 2903. Both methods, i.e. the line ratio diagnostics and modeling, indicate an ISM inthe center of NGC 2903 having intermediate temperature and star formation activity (also supported by star formationrates), thinner CO gas with similar dense gas fraction, and higher H2 volume number density compared to that of spirals,ETGs, and starbursts.