Program: GS-2008B-Q-75

Among blue compact dwarf galaxies (BCDs), or HII galaxies, the dwarf emission-line galaxy Mrk 996 (MB = -16.9) occupies a place apart because of the extreme electron density (4.5 x 10^6 cm-3 ) at the center of its star-forming region. Its optical spectrum also shows remarkable properties, suggesting very unusual physical conditions: low-ionization emission lines such as O+ , S+ and N+ have narrow widths (< ~150 km s-1 ), similar to those in other H II regions, but high-ionization emission lines such as the helium lines, the O++ and Ne++ nebular lines and all auroral lines, show very broad line widths, ~400 km s-1 , suggesting different ionization zones with very distinct kinematical properties. These unusual properties can only be understood if the broad lines arise in a very dense inner region while the narrow lines arise in a considerably less dense outer region. The large density gradient can be attributed to a mass outflow caused by stellar winds of Wolf-Rayet stars, the presence of which is clearly indicated by the WN and WC bumps in the optical spectrum. Observing Mrk 996 with GMOS will give us a unique opportunity to study kinematically and chemically the interplay between the stellar winds of a large WR stellar population and the ambient interstellar medium.