Study of the Lynx-Cancer void galaxies-V. The extremely isolated galaxy UGC4722

We present a detailed study of the extremely isolated Sdm galaxy UGC4722 (M_B = -17.4) located in the nearby Lynx-Cancer void. UGC4722 is a member of the catalogue of isolated galaxies, and has also been identified as one of the most isolated galaxies in the Local Supercluster. Optical images of the galaxy however show that it has a peculiar morphology with an elongated ~ 14 kpc long plume. New observations with the Russian 6-m telescope (BTA) and the Giant Metrewave Radio Telescope (GMRT) of the ionised and neutral gas in UGC4722 reveal the second component responsible for the disturbed morphology of the system. This is a small, almost completely destroyed, very gas-rich dwarf (M_B = -15.2, M_HI/L_B ~4.3). We estimate the oxygen abundance for both galaxies to be 12+log(O/H) ~ 7.5-7.6, which is 2-3 times lower than what is expected from the luminosity-metallicity relation for similar galaxies in denser environments. The ugr colours of the plume derived from Sloan Digital Sky Survey (SDSS) images are consistent with a simple stellar population with a post starburst age of 0.45-0.5 Gyr. This system hence appears to be the first known case of a minor merger with a prominent tidal feature consisting of a young stellar population.