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

ASTRONOMY AND ASTROPHYSICS 13 CO(1–0) and 12 CO(2–1) in the center of the barred galaxy NGC 1530 Abstract. We present 13 CO(1–0) and 12 CO(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 12 CO(1–0) map, with a nuclear feature (a ring or unresolved spiral arms) surrounded by two curved arcs. The average line ratios are 12 CO(1–0)/ 13 CO(1–0)= 9.3 and 12 CO(2– 1)/ 12 CO(1–0)= 0.7. The 12 CO/ 13 CO 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 (≥ 5×10 2 cm −3) than in the arcs (≃ 2×10 2 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 12 CO(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 luminos-ity to molecular gas mass conversion factor between 0.3 and 2.3 M ⊙ (K km s −1 pc 2) −1 , significantly lower than the standard Galactic value.