Tidal Tail in the Luminous Infrared Merger Arp 299

We present VLA H i observations and UH88 ′′ deep optical Band R-band observations of the IR luminous merger Arp 299 (= NGC 3690 + IC 694). These data reveal a gas-rich (M HI = 3.3 × 10 9 M ⊙) optically faint (µ B ∼ >27 mag arcsec −2 , µ R ∼ >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 star burst (∼ <20 Myr). The observations reveal a most remarkable structure within the tidal tail: it appears to be composed of two parallel filaments separated by ∼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 which will merge into a single object in ∼ 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. Arp 299 is a nearby (V hel =3080 km s −1) 2 peculiar system comprised of two highly distorted disk galaxies, with NGC 3690 (= Mrk 171A = UGC 6471 = VV118b) to the west and IC 694 (= Mrk 171B = UGC 6472 = VV118a) 3 to the east. There is also a compact spheroidal galaxy lying approximately 1 ′ to the NW which is a the same velocity as Arp 299 (MCG+10-17-2a, V hel =3100 km s −1 , Fairall 1971, Sargent 1972). Figure 1 presents a B-band image of this system, taken from the optical data described later in this paper. Each of the above named systems are labeled, as is the unrelated background bridge-tail-bar system Arp 296. Arp 299 appears to be in an advanced stage of merging, with the two disks in contact, but the nuclei still separated by about 20 ′′ (4.7 kpc) 4. which locate two major concentrations of molecular gas at peaks in …