1999A&A...347...37R

We present ¹³CO(1-0) and ¹²CO(2-1) aperture synthesis maps of the barred spiral galaxy NGC 1530. The angular resolutions are respectively 3.1" and 1.6". Both transitions show features similar to the ¹²CO(1-0) map, with a nuclear feature (a ring or unresolved spiral arms) surrounded by two curved arcs. The average line ratios are ¹²CO(1-0)/¹³CO(1-0)=9.3 and ¹²CO(2-1)/¹²CO(1-0)=0.7. The ¹²CO/¹³CO ratio is lower in the circumnuclear ring (6-8) than in the arcs (11-15). We fit the observed line ratios by escape probability models, and deduce that the gas density is probably higher in the nuclear feature (≥5x10²cm^–3) than in the arcs (≃2x10²cm^–3), confirming earlier HCN results. The kinetic temperatures are in the range 20-90K, but are weakly constrained by the model. The average filling factor of the ¹²CO(1-0) emitting gas is low, ≃0.15. The cm-radio continuum emission also peaks in the nuclear feature, indicating a higher rate of star formation than in the arcs. We derive values for the CO luminosity to molecular gas mass conversion factor between 0.3 and 2.3M_☉/(K.km/s.pc²), significantly lower than the standard Galactic value.