Galactic wind and Lyman α emission in the blue compact galaxy Haro 2 = MKN 33.

We have observed with the Goddard High-Resolution Spectrometer on board the Hubble Space Telescope the blue compact galaxy Haro 2 in two wavelength regions around Lyman α and O I 1302A. We have detected redshifted absorptions in those lines as well as in lines of N I, Si II and Si III, in front of the hot stars ionizing the central H II region of this galaxy. The heliocentric velocities of the absorption lines are centered at 1260km/s, lower by about 200km/s than the velocity of the H II region measured with the 1.93-m telescope at the Haute Provence Observatory; the latter velocity coincides with that of the bulk of the galaxy as measured in the 21-cm line. The absorption thus takes place in a gas flowing away from the H II region at a velocity of 200km/s. The column density of neutral hydrogen in this gas is 7x10^19^ atom cm^-2^. We show that this is only a small fraction of the total column density, the outflowing gas being mainly ionized by the central star cluster: its total column density is of the order of 2x10^21^atom/cm+2, and the total expanding mass if distributed in a spherical shell immediately surrounding the H II region is of the order of 10^7^M_sun_. The heavy-element abundances in this shell are uncertain, their range being between 1/30 to 1/3 solar or even more, with a strong preference for the higher values. This gas may well escape from the galaxy; the existence of such a galactic wind might have important consequences for its chemical evolution. We have also detected a strong Lyman α emission, redshifted with respect to both the H II region and the absorbing shell. Lyman α photons are obviously emitted by the H II region and the ionized part of the expanding shell. They are possibly scattered and redshifted by the back part of the neutral shell, and scattered and absorbed by the neutral intervening gas. It is the first time that a velocity structure has been seen in a Lyman-α emitting galaxy. Our results illustrate the difficulty of interpreting the Lyman α emission of galaxies in terms of rate of star formation: the escape of Lyman α photons is not only very sensitive to the amount of intervening neutral gas and to its dust abundance, but also to its geometrical and velocity structures.