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 \la 3″ ( \la 270 pc in linear scale ) is mainly gaseous in origin .
We detect H \alpha emission out to radii as large as 6″ .
At radii \ga 3″ 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 \la 120 pc .
This places Pox 186 among the most compact dwarf galaxies known .
The derived \alpha 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 4.5m Multiple Mirror Telescope ( MMT ) The MMT Observatory is a joint facility of the Smithsonian Institution and the University of Arizona . and 3.6m ESO telescope Based on observations collected at the European Southern Observatory , Chile , ESO program 71.B-0032 ( A ) . spectra are 12 + log ( O/H ) = 7.76 \pm 0.02 and 7.74 \pm 0.01 ( \sim Z _ { \odot } /15 ) 12+log ( O/H ) _ { \odot } = 8.92 ( Anders & Grevesse [ 1989 ] ) . , respectively , in accordance with previous determinations .
The helium mass fractions found in this region are Y = 0.248 \pm 0.009 ( MMT ) and Y = 0.248 \pm 0.004 ( 3.6m ) suggesting a high primordial helium abundance .