Carbon monoxide (CO) in the Magellanic irregular galaxy NGC 55 and the N(H2)/W(CO) ratio in irregular galaxies

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Using SEST 15 m telescope on La Silla in Chile, NGC 55 was searched and successfully mapped in the J = 1 -> CO line. The distribution of CO corresponds well with the intense H I cloud near the bar of NGC 55. The extent of the CO cloud is about 975 pc along the major axis and 390 pc perpendicular to the major axis. Assuming virial equilibrium, the total molecular mass of the cloud is estimated to be 8 x 10^7^ M_sun_, implying that 80% of the cloud is molecular. The factor that transforms the velocity-integrated CO line, W(CO), into a column density of molecular hydrogen, N(H_2_), is found to be N(H_2_)/W(CO) = 60 x 10^20^ (molecules times higher than values for our Galaxy. This is confirmed by an independent estimate of the molecular gas mass as derived from dust masses which can be inferred from IRAS FIR fluxes. From a comparison to the metallicity seems to be inversely proportional to the nitrogen abundance. The present-day star formation rate is low, M^dot^ ~ 0.3 M_sun_ yr^-1^, but must have been higher in the past.