Long-slit spectrophotometry of the multiple knots of the polar ring galaxy IIZw71

Abstract

We carried out long-slit spectroscopic observations of the star forming knots along the polar ring of the dwarf galaxy IIZw71 in the spectral range 3500 - 10000 angstroms taken with the William erschel Telescope (WHT). The spectroscopic observations were complemented with available photometry of the galaxy in the narrow Halpha filter. We measured the rotation curve of the ring, from which we infer a ratio M/L_B = 3.9 inside the star forming ring. We measured the auroral [OIII] line in the two brightest knots, allowing us to measure oxygen, sulphur, nitrogen, argon and neon chemical abundances following the direct method. Different empirical calibrators were used to estimate the oxygen abundance in the two faintest knots. The metallicities obtained are very similar for all the knots, but lower than previously reported in the literature from integrated spectra. The N/O abundance, as derived from the N2O2 parameter, is remarkably constant over the ring, indicating that local polution processes are not conspicuous. Using synthetic stellar populations (SSPs) calculated with the code STARLIGHT, we studied the age distribution of the stellar populations in each knot, finding that in all of them there is a combination of a very young population with less than 10 Myr, responsible for the ionisation of the gas, with other populations older than 100 Myr, probably responsible for the chemical evolution of the knots. The small differences in metallicity and the age distributions among the different knots are indicative of a common chemical evolution, probably related to the process of interaction with the companion galaxy IIZw70.