NGC 4254: An act of harassment uncovered by the Arecibo Legacy Fast Alfa survey

We present an H I map constructed from the Arecibo Legacy Fast ALFA survey of the surroundings of the strongly asymmetric Virgo Cluster Sc galaxy NGC 4254. Noted previously for its lopsided appearance, rich interstellar medium, and extradisk H I emission, NGC 4254 is believed to be entering the Virgo environment for the first time and at high speed. The ALFALFA map clearly shows a long H I tail extending similar to 250 kpc northward from the galaxy. Embedded as one condensation within this H I structure is the object previously identified as a dark galaxy by Davies et al.: VIRGOHI 21. A body of evidence including its location within and velocity with respect to the cluster and the appearance and kinematics of its strong spiral pattern, extradisk H I, and lengthy H I tail is consistent with a picture of galaxy harassment as proposed by Moore et al. The smoothly varying radial velocity field along the tail as it emerges from NGC 4254 can be used as a timing tool, if interpreted as resulting from the coupling of the rotation of the disk and the collective gravitational forces associated with the harassment mechanism.