  Multiple molecular lines with radiative transfer modelling are a powerful
tool to probe the physics of star-forming gas in galaxies. We investigate the
gas properties in the centre of the spiral galaxy NGC 2903 using low-J CO
lines, i.e. 12CO(1-0), 12CO(2-1), 12CO(3-2), 13CO(1-0), and HCN(1-0). We apply
a nonlocal thermodynamic equilibrium radiative transfer code to derive
beam-averaged molecular gas properties. We use two methods (i.e. chi-square
minimizations and likelihood) to define the best model representing the
observed line ratios best. The line ratio diagnostics suggest that CO gas in
the centre of NGC 2903 is thinner and the dense gas fraction is similar
compared to that of spirals, starburst, and early-type galaxies (ETGs), while
the gas in the centre of the galaxy is warmer than that of ETGs and colder than
that of starbursts. Based on the best-fitting model results, we find that the
beam-averaged gas kinetic temperature is T_K = 20 K, H_2 volume number density
is log(n(H_2)) = 4.2 cm-3 , and CO column number density is log(N(CO)) = 19.0
cm-2 in the centre of NGC 2903. Both methods, i.e. the line ratio diagnostics
and modelling, indicate an ISM in the centre of NGC 2903 having intermediate
temperature and star formation activity (also supported by star formation
rates), thinner CO gas with similar dense gas fraction, and higher H_2 volume
number density compared to that of spirals, ETGs, and starbursts.
