The Rotation Velocity - Density Relation
  We have assembled 21-cm linewidths for 376 of the 732 galaxies in a magnitude-limited redshift survey of the the Perseus-Pisces region.   We analyze a distance limited subset of 271 galaxies (131 widths) to examine the relationship between linewidth and local density. The sample indicates that galaxies with linewidths $\gtrsim 480$ km/s are absent from regions where the galaxy density is $\lesssim 0.03$ galaxies Mpc$^{-3}$ ($M_{B(0)}< -18.3$). This effect is in the direction predicted by standard CDM models. Galaxies with linewidths $\lesssim$ 480 km/s appear throughout the sample.   The observational constraints could be substantially improved with a larger sample, IR photometry, and more uniform 21-cm data. 
Lorraine A. Allen and Margaret J. Geller (Harvard-Smithsonian Center   for Astrophysics)