13 CO ( 1 – 0 ) and 12 CO ( 2 – 1 ) in the center of the barred galaxy NGC 1530

We present 13CO(1–0) and 12CO(2–1) aperture synthesis maps of the barred spiral galaxy NGC 1530. The angular resolutions are respectively3′′.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 (≥ 5×102 cm−3) than in the arcs (' 2×102 cm−3), confirming earlier HCN results. The kinetic temperatures are in the range 20 − 90 K, 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 2.3 M (K km s−1 pc2)−1, significantly lower than the standard Galactic value.