Post-starburst galaxies and the transformation of blue into red galaxies

We present deep single-dish radio observations of a sample of nearby post-starburst galaxies (0.05 < z < 0.1). About 50% of these post-starburst galaxies are detected at 21 cm, with H I masses of ~10⁹M_⊙, up to ~10¹⁰M_⊙. These post-starburst galaxies are as gas-rich as spiral galaxies with comparable luminosities. There appears to exist no direct correlation between the amount of H I present in a post-starburst galaxy and its star formation rate as traced by radio continuum emission. Moreover, the end of the starburst clearly does not necessarily require the complete exhaustion of the neutral gas reservoir. High-resolution radio observations of one post-starburst binary system suggest that most of the neutral gas resides outside the stellar bodies of the galaxies. Most likely, the gas was expelled by supernova and/or AGN feedback. This effectively stops star formation, even though copious amounts of diffuse neutral gas remain in the immediate vicinity. This remaining H I reservoir may eventually lead to further episodes of star formation. This may indicate that some post-starbursts are observed in the inactive phase of the star formation duty cycle.