Far-infrared counterpart to the optical and H I plume in the Leo triplet of galaxies - NGC 3623, NGC 3627, and NGC 3628

A study of the IR emission in the Leo triplet of interacting galaxies is presented. A new background subtraction routine is applied to IRAS-calibrated raw detector data covering a 4 sq deg field centered on the triplet of galaxies. The discovery of an extragalactic IR counterpart is reported at 60 and 100 microns to the previously known optical and H I tidal plume associated with NGC 3628. The IR plume has a ratio I(100 microns)/N(H) of 7 + or - 4 x 10 to the -15th Jy/sr per H I atoms per sq cm, in striking agreement with Galactic cirrus clouds, and a dust-to-gas ratio of 3.3 x 10 to the -5th. These values are consistent with the tidal model of the plume formation. The IR plume also exhibits a gradient of decreasing temperature along the plume away from NGC 3628 which can be explained by a dilution of the stellar optical and UV radiation field. In addition to the Leo triplet galaxies, almost all background late-type galaxies in the field were detected. 32 refs