Giant disc galaxies: where environment trumps mass in galaxy evolution

We identify some of the most H i-massive and fastest rotating disc galaxies in the local universe with the aim of probing the processes that drive the formation of these extreme disc galaxies. By combining data from the Cosmic Flows project, which has consistently reanalysed archival galaxy H i profiles, and 3.6 μm photometry obtained with the Spitzer Space Telescope, with which we can measure stellar mass, we use the baryonic Tully–Fisher (BTF) relationship to explore whether these massive galaxies are distinct. We discuss several results, but the most striking is the systematic offset of the H i-massive sample above the BTF. These galaxies have both more gas and more stars in their discs than the typical disc galaxy of similar rotational velocity. The ‘condensed’ baryon fraction, fC, the fraction of the baryons in a dark matter halo that settle either as cold gas or stars into the disc, is twice as high in the H i-massive sample than typical, and almost reaches the universal baryon fraction in some cases, suggesting that the most extreme of these galaxies have little in the way of a hot baryonic component or cold baryons distributed well outside the disc. In contrast, the star formation efficiency, measured as the ratio of the mass in stars to that in both stars and gas, shows no difference between the H i-massive sample and the typical disc galaxies. We conclude that the star formation efficiency is driven by an internal, self-regulating process, while fC is affected by external factors. Neither the morphology nor the star formation rate of these galaxies is primarily determined by either their dark or stellar mass. We also found that the most massive H i detected galaxies are located preferentially in filaments. We present the first evidence of an environmental effect on galaxy evolution using a dynamical definition of a filament.