HS 0837+4717 - a metal-deficient blue compact galaxy with large nitrogen excess

We present the results of high SIN long-slit spectroscopy with the Multiple Mirror and the SAO 6-m telescopes, optical imaging with the Wise 1-m telescope and HI observations with the Nancay Radio Telescope of the very metal-deficient (12 + log (O/H) = 7.64) luminous (M-B = -18(m).1) blue compact galaxy (BCG) HS 0837+4717. The blue bump near lambda4650, characteristic of WR stars is detected in the central supergiant HII region, as well as the barely seen red bump at lambda5808. The derived number of WR stars in the region of the current starburst is similar to1000. Evidence for fast motions in this region is also seen as broad, low-contrast components in the Halpha, Hbeta and strong [O III] lines lambda4959,5007. While the extinction of the narrow emission lines from the supergiant H II region is low, the very large Balmer decrement of the broad components suggests that the part of current starburst is highly obscured by dust. Abundance ratios X/O for X = Ne, Ar, S, Fe and Cl in the supergiant H II region are in good agreement with the mean values of other very metal-deficient BCGs. Nitrogen, however, is overabundant by a factor of similar to6. This implies an unusually efficient N enrichment in HS 0837+4717, and probably, a non-typical evolution scenario. The Halpha-line position-velocity (P-V) diagrams for directions approximately along the major and minor axes reveal disturbed motions of the ionized gas, mainly in peripheral regions. The SW part of the major axis P-V diagram looks like a rotation curve, with the velocity amplitude V-rot similar to 50-70 km s(-1) at r = 4.3 kpc. Its NE part displays, however, strong deviations, indicating either counter-rotation, or a strong outflow/supershell. If it is considered as indicating a shell-like feature its velocity amplitude of similar to70 km s(-1) (relative to the extrapolated rotation curve), and the apparent extent of similar to4 (3.3 kpc) imply a dynamical age of similar to14 Myr and the full energetic equivalent of similar to2.3 x 10(4) SNe. The latter indicates continuing starbursts during at least this time interval. The long-slit spectra reveal a complex morphology for this galaxy. It consists of two compact regions at a distance of similar to2 kpc. Their continuum flux differs by a factor of four. The brightest one is related to the current starburst with the age of similar to3.7 Myr. The slightly redder fainter component could be an older starburst (similar to25 Myr). The Wise 1-m telescope UBVR integrated photometry reveals a high optical luminosity for this BCG, and the unusual (B-V) and (V-R) colours. The morphology of HS 0837+4717 is highly disturbed, with two small tails emerging to NNW and SSE. Such a disturbed overall morphology, a double-nucleus structure, significantly disturbed velocities of ionized gas, together with the very high power of the starburst suggests a possible explanation of the object as a recent merger. We compare the properties of this BCG and of similar objects known in the literature, and conclude that their high nitrogen excess is most probably related to the short phase of a powerful starburst when many WR stars contribute to the enrichment of ISM.