A 500 kpc H I extension of the Virgo pair NGC 4532/DDO 137 detected by the Arecibo Legacy Fast ALFA (ALFALFA) Survey

We report the discovery of a similar to 500 kpc H I extension southwest of the Virgo Cluster H I-rich pair NGC 4532/DDO 137, detected as part of the Arecibo Legacy Fast ALFA (ALFALFA) Survey. The feature is the longest and most massive H I tail structure so far found in the Virgo Cluster and, at 1.8 Mpc from M87, the most distant from the main concentration of the intracluster medium. The structure is spatially and spectrally separated into two ridges and is defined by diffuse emission and discrete clumps of mass. All emission 7 (2.5-6.8) x 10(7) M-circle dot. is blueshifted with respect to the NGC 4532/DDO 137 pair emission. Including diffuse emission, the structure has a total mass of up to 7 x 10(8) M-circle dot, has a total mass of up to, equivalent to similar to 10% of the system's H I mass. Optical R-band imaging finds no counterparts to a level of 26.5 mag arcsec(-2). The characteristics of the structure appear most consistent with a tidal origin.