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) deep optical B- and R-band observations of the IR-luminous merger Arp 299 (=NGC 3690 + IC 694). These data reveal a gas-rich (M-H I = 3.3 x 10(9) M-.) optically faint (mu(B) greater than or similar to 27 mag arcsec(-2), mu(R) greater than or similar to 26 mag arcsec(-2)) tidal tail with a length of over 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 similar to 20 Myr). The observations reveal a most remarkable structure within the tidal tail: it appears to be composed of two parallel filaments separated by approximately 20 kpc. One of the filaments is gas-rich with little if any starlight, while the other is gas-poor. We believe that this bifurcation results from a warped disk in one of the progenitors. The quantities and kinematics of the tidal H I suggest that Arp 299 results from the collision 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 roughly 60 Myr. We suggest that the present IR-luminous phase in this system is due in part to the retrograde spin of IC 694. Finally, we discuss the apparent lack of tidal dwarf galaxies within the tail.