EVIDENCES FOR AN EXPANDING SHELL IN THE BLUE COMPACT DWARF GALAXY HARO-2

Long-slit observations of the blue compact galaxy Hare 2 have been per formed around H alpha and H beta. The main aim of these observations w as to detect the Her emission originating in the partially ionized win d outflowing at 200 km/s, that had been previously detected with the H ubble Space Telescope (HST). A shallow broadening of the H alpha line wings has been observed, consistent with the existence of an expanding shell. The rotation curve shows two dips at the same systemic velocit y as the nucleus. We interpret this feature as an evidence that the ex panding shell is decoupled from the disk rotation. At the positions of the dips the H alpha line is clearly broadened with respect to the ce ntral core. This broadening is produced by the outer layers of the exp anding shell. From the position of these dips we estimate the size of the shell to be around 20'' in diameter, with a corresponding kinemati cal age between 5 and 6 Myr. This shell has most certainly been powere d by the massive star formation process that takes place in the centra l region of this galaxy. A comparison of the H alpha and Ly alpha prof iles shows that Ly alpha is significantly broader than H alpha, with a n additional emission in the red wing. We interpret this redshifted so urce of Ly alpha emission as line photons backscattered by the recedin g part of the expanding shell. These observations outline the extremel y high sensitivity of the Ly alpha line to the structure and kinematic s of the interstellar medium (ISM). Finally the analysis of stellar Ba lmer lines in the H beta region indicates that stars less massive than 10 M. have probably been formed.