Evidences for an expanding shell in the blue compact dwarf galaxy Haro 2

Long-slit observations of the blue compact galaxy Haro 2 have been performed around Hα and Hβ. The main aim of these observations was to detect the Hα emission originating in the partially ionized wind outflowing at 200 km/s, that had been previously detected with the Hubble Space Telescope (HST). A shallow broadening of the Hα line wings has been observed, consistent with the existence of an expanding shell. The rotation curve shows two dips at the same systemic velocity as the nucleus. We interpret this feature as an evidence that the expanding shell is decoupled from the disk rotation. At the positions of the dips the Hα line is clearly broadened with respect to the central core. This broadening is produced by the outer layers of the expanding shell. From the position of these dips we estimate the size of the shell to be around 20 " in diameter, with a corresponding kinematical age between 5 and 6 Myr. This shell has most certainly been powered by the massive star formation process that takes place in the central region of this galaxy. A comparison of the Hα and Lyα profiles shows that Lyα is significantly broader than Hα, with an additional emission in the red wing. We interpret this redshifted source of Lyα emission as line photons backscattered by the receding part of the expanding shell. These observations outline the extremely high sensitivity of the Lyα line to the structure and kinematics of the interstellar medium (ISM). Finally the analysis of stellar Balmer lines in the Hβ region indicates that stars less massive than 10 M have probably been formed.