A 180 kiloparsec tidal tail in the luminous infrared merger Arp 299

We present VLA H I observations and University of Hawaii 88 inch (2.2 m) de ep optical B- and R-band observations of the IR-luminous merger Arp 299 (=N GC 3690 + IC 694). These data reveal a gas-rich (M-H I = 3.3 x 10(9) M-.) o ptically faint (mu(B) greater than or similar to 27 mag arcsec(-2), mu(R) g reater than or similar to 26 mag arcsec(-2)) tidal tail with a length of ov er 180 kpc. The size of this tidal feature necessitates an old interaction age for the merger (greater than or similar to 750 Myr since first periapse ), which is currently experiencing a very young starburst (less than or sim ilar to 20 Myr). The observations reveal a most remarkable structure within the tidal tail: it appears to be composed of two parallel filaments separa ted by approximately 20 kpc. One of the filaments is gas-rich with little i f any starlight, while the other is gas-poor. We believe that this bifurcat ion results from a warped disk in one of the progenitors. The quantities an d kinematics of the tidal H I suggest that Arp 299 results from the collisi on of a retrograde Sab-Sb galaxy (IC 694) and a prograde Sbc-Sc galaxy (NGC 3690) that occurred 750 Myr ago and will merge into a single object in rou ghly 60 Myr. We suggest that the present IR-luminous phase in this system i s due in part to the retrograde spin of IC 694. Finally, we discuss the app arent lack of tidal dwarf galaxies within the tail.