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

International audienceWe present the results of high S/N long-slit spectroscopy with the Multiple Mirror and the SAO 6-m telescopes, optical imaging with the Wise 1-m telescope and H I observations with the Nançay Radio Telescope of the very metal-deficient (12+ log (O/H)=7.64) luminous (MB= -18.1m) blue compact galaxy (BCG) HS 0837+4717. The blue bump near lambda4650, characteristic of WR stars is detected in the central supergiant H II 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 ˜1000. Evidence for fast motions in this region is also seen as broad, low-contrast components in the Halpha, Hbeta and strong [O III] lines lambdalambda4959,5007. 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 ˜6. 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 Vrot ˜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 ˜70 km s-1 (relative to the extrapolated rotation curve), and the apparent extent of ˜4 arcsec (3.3 kpc) imply a dynamical age of ˜14 Myr and the full energetic equivalent of ˜2.3× 104 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 ˜2 kpc. Their continuum flux differs by a factor of four. The brightest one is related to the current starburst with the age of ˜3.7 Myr. The slightly redder fainter component could be an older starburst (˜25 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