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

Abstract

We present 13CO(1-0) and 12CO(2-1) aperture synthesis maps of the barred spiral galaxy NGC1530. 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 (>= 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 20-90K, 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 2.3 Msolar (K km/s pc^2)^{-1}, significantly lower than the standard Galactic value.