Molecular gas during the post-starburst phase: Low gas fractions in green-valley Seyfert post-starburst galaxies

Post-starbursts (PSBs) are candidate for rapidly transitioning from starbursting to quiescent galaxies. We study the molecular gas evolution of PSBs at z ~ 0.03-0.2. We undertook new CO (2-1) observations of 22 Seyfert PSB candidates using the Arizona Radio Observatory Submillimeter Telescope. This sample complements previous samples of PSBs by including green-valley PSBs with Seyfert-like emission, allowing us to analyse for the first time the molecular gas properties of 116 PSBs with a variety of AGN properties. The distribution of molecular gas to stellar mass fractions in PSBs is significantly different from normal starforming galaxies in the CO Legacy Database (COLD) GASS survey. The combined samples of PSBs with Seyfert-like emission line ratios have a gas fraction distribution that is even more significantly different and is broader (~0.03-0.3). Most of them have lower gas fractions than normal star-forming galaxies. We find a highly significant correlation between the WISE 12 and 4.6 μm flux ratios and molecular gas fractions in both PSBs and normal galaxies. We detect molecular gas in 27 per cent of our Seyfert PSBs. Taking into account the upper limits, the mean and the dispersion of the distribution of the gas fraction in our Seyfert PSB sample are much smaller (μ = 0.025, σ = 0.018) than previous samples of Seyfert PSBs or PSBs in general (μ ~ 0.1-0.2, σ ~ 0.1-0.2).