A 180 Kiloparsec Tidal Tail in the Luminous Infrared Merger Arp 299

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Abstract
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 (MH I = 3.3 × 109 M⊙) optically faint (μB 27 mag arcsec-2, μR 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 (750 Myr since first periapse), which is currently experiencing a very young starburst (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.
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