The Effect of Filaments and Tendrils on the HI Content of Galaxies

We use the ALFALFA HI survey to examine whether the cold gas reservoirs of galaxies are inhibited or enhanced in large-scale filaments. Our sample includes 9947 late-type galaxies with HI detections, and 4236 late-type galaxies with well-determined HI detection limits that we incorporate using survival analysis statistics. We find that, even at fixed local density and stellar mass, and with group galaxies removed, the HI deficiency of galaxies in the stellar mass range 8.5 <log(M/Mo) < 10.5 decreases with distance from the filament spine, suggesting that galaxies are cut off from their supply of cold gas in this environment. We also find that, at fixed local density and stellar mass, the galaxies that are the most gas-rich are those in small, correlated "tendril" structures within voids: although galaxies in tendrils are in significantly denser environments, on average, than galaxies in voids, they are not redder or more HI deficient. This stands in contrast to the fact that galaxies in tendrils are more massive than those in voids, suggesting a more advanced stage of evolution. Finally, at fixed stellar mass and color, galaxies closer to the filament spine, or in high density environments, are more deficient in HI. This fits a picture where, as galaxies enter denser regions, they first lose HI gas and then redden as star formation is reduced.