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We present a ground-based optical spectroscopic and HST U, V, I photometricstudy of the blue compact dwarf (BCD) galaxy Pox 186. It is found that theemission of the low-surface brightness (LSB) component in Pox 186 at radii <3'' (< 270 pc in linear scale) is mainly gaseous in origin. We detect Halphaemission out to radii as large as 6''. At radii > 3'' the light of the LSBcomponent is contaminated by the emission of background galaxies complicatingthe study of the outermost regions. The surface brightness distribution in theLSB component can be approximated by an exponential law with a scale lengthalpha < 120 pc. This places Pox 186 among the most compact dwarf galaxiesknown. The derived alpha is likely to be an upper limit to the scale length ofthe LSB component because of the strong contribution of the gaseous emission.The oxygen abundance in the bright HII region derived from the 4.5m MultipleMirror Telescope (MMT) and 3.6m ESO telescope spectra are 12+log(O/H)=7.76+/-0.02 and 7.74+/-0.01 (~ Zsun/15), respectively, in accordance withprevious determinations. The helium mass fractions found in this region areY=0.248+/-0.009 (MMT) and Y=0.248+/-0.004 (3.6m) suggesting a high primordialhelium abundance.Comment: 11 pages, 9 Postscript figure

Pox 186: An ultracompact galaxy with dominant ionized gas emission