The AMIGA sample of isolated galaxies IX. Molecular gas properties

Aims: We characterize the molecular gas content using CO emission of a redshift-limited subsample of isolated galaxies from the AMIGA (Analysis of the interstellar Medium of Isolated GAlaxies) project. Methods: We present the 12CO(1-0) data for 273 AMIGA galaxies. We constructed a redshift-limited sample containing galaxies with 1500\kms <v<5000\kms and excluded objects with morphological evidence of possible interaction. This sample ($n=173$) is the basis for our statistical analysis. It is dominated, both in absolute number and in detection rate, by galaxies of type T=3-5 (Sb-Sc). Most galaxies were observed with a single pointing towards their centers. Therefore, we performed an extrapolation to the total molecular gas mass expected in the entire disk based on the assumption of an exponential distribution. We then studied the relationships between \mhtwo\ and other galactic properties (\lb,\dopttwo,\lk, \lfir, and \mhi). Results: We find correlations between \mhtwo\ and \lb, \dopttwo, \lk, and \lfir. The tightest correlation of \mhtwo\ holds with \lfir\ and, for $T=3-5$, with \lk, and the poorest with \dopttwo. The correlations with \lfir and \lk are very close to linearity. The correlation with \lb is nonlinear so that \mhtwo/\lb\ increases with \lb. The molecular and the atomic gas masses of our sample show no strong correlation. We find a low mean value, log(\mhtwo/\mhi)=-0.7 (for $T=3-5$), and a strong decrease in this ratio with morphological type. The molecular gas column density and the surface density of the star formation rate (the Kennicutt-Schmidt law) show a tight correlation with a rough unity slope. We compare the relations of \mhtwo\ with \lb and \lk found for AMIGA galaxies to samples of interacting galaxies from the literature and find an indication for an enhancement of the molecular gas in interacting galaxies of up to 0.2-0.3 dex.