Evidences for an expanding shell in the Blue Compact Dwarf Galaxy Haro 2

الأصل · EN

Long-slit observations of the blue compact galaxy Haro 2 have been performed to detect the Halpha emission originating in the partially ionized wind outflowing at 200 km/s, that had been previously detected with the Hubble Space Telescope. A shallow broadening of the Halpha 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. At the positions of the dips the Halpha 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. A comparison of the Halpha and Lyₐlpha profiles shows that Lyₐlpha is significantly broader than Halpha, with an additional emission in the red wing. We interpret this redshifted source of Lyₐlpha emission as line photons backscattered by the receding part of the expanding shell. These observations outline the extremely high sensitivity of the Lyₐlpha line to the structure and kinematics of the interstellar medium.