Molecular gas content of HI monsters and implications to cold gas content evolution in galaxies

We present ¹²CO (J=1→0) observations of a sample of local galaxies (0.04<z<0.08) with a large neutral hydrogen reservoir, or 'H I monsters'. The data were obtained using the redshift search receiver on the five college radio astronomy observatory (FCRAO) 14 m telescope. The sample consists of 20 HI-massive galaxies with M_HI > 3 × 10¹⁰M_⊙ from the Arecibo Legacy Fast ALFA (ALFALFA) survey and 8 low surface brightness galaxies (LSBs) witha comparable MHI(>1.5 × 1010M⊙). Our sample selection is purely based on the amount of neutral hydrogen, thereby providing a chance to study how atomic and molecular gas relate to each other in these HI-massive systems. We have detected CO in 15 out of 20 ALFALFA selected galaxies and 4 out of 8 LSBs with molecular gas mass M_H2 of (1-11)× 10⁹M_⊙. Their total cold gas masses of (2-7) × 10¹⁰M_⊙ make them some of the most gas-massive galaxies identified to date in the Local Universe. Observed trends associated with H_I, H₂, and stellar properties of the HI massive galaxies and the field comparison sample are analysed in the context of theoretical models of galaxy cold gas content and evolution, and the importance of total gas content and improved recipes for handling spatially differentiated behaviours of disc and halo gas are identified as potential areas of improvement for the modelling.