Pox 186: An ultracompact galaxy with dominant ionized gas emission

We present a ground-based optical spectroscopic and HST U, V, I photometric study of the blue compact dwarf (BCD) galaxy Pox 186. It is found that the emission of the low-surface brightness (LSB) component in Pox 186 at radii less than or similar to3" (5270 pc in linear scale) is mainly gaseous in origin. We detect Ha emission out to radii as large as 6". At radii greater than or similar to3" the light of the LSB component is contaminated by the emission of background galaxies complicating the study of the outermost regions. The surface brightness distribution in the LSB component can be approximated by an exponential law with a scale length alpha less than or similar to 120 pc. This places Pox 186 among the most compact dwarf galaxies known. The derived a is likely to be an upper limit to the scale length of the LSB component because of the strong contribution of the gaseous emission. The oxygen abundance in the bright H II region derived from the 4.5 m Multiple Mirror Telescope (MMT) and 3.6 m ESO telescope spectra are 12 + log (O/H) = 7.76 +/- 0.02 and 7.74 +/- 0.01 (-Z(circle dot)/15) , respectively, in accordance with previous determinations. The helium mass fractions found in this region are Y = 0.248 +/- 0.009 (MMT) and Y = 0.248 +/- 0.004 (3.6 m) suggesting a high primordial helium abundance.