GALACTIC WIND AND LYMAN-ALPHA EMISSION IN THE BLUE COMPACT GALAXY HARO-2=MKN-33

We have observed with the Goddard High-Resolution Spectrometer on boar d the Hubble Space Telescope the blue compact galaxy Haro 2 in two wav elength regions around Lyman alpha and O I 1302 Angstrom. We have dete cted 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 1260 km s(-1), lower by about 200 km s(-1) 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 tha t of the bulk of the galaxy as measured in the 2 I-cm line. The absorp tion thus takes place in a gas flowing away from the H II region at a velocity of 200 km s(-1). The column density of neutral hydrogen in th is gas is 7 10(19) atom cm(-2). We show that this is only a small frac tion of the total column density, the outflowing gas being mainly ioni zed by the central star cluster: its total column density is of the or der of 2 10(21) atom cm(-2), and the total expanding mass if distribut ed in a spherical shell immediately surrounding the H II region is of the order of 10(7) M.. The heavy-element abundances in this shell are uncertain, their range being between 1/30 to 1/3 solar or even more, w ith a strong preference for the higher values. This gas may well escap e from the galaxy; the existence of such a galactic wind might have im portant consequences for its chemical evolution. We have also detected a strong Lyman alpha emission, redshifted with respect to both the H II region and the absorbing shell. Lyman alpha photons are obviously e mitted 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 ne utral shell, and scattered and absorbed by the neutral intervening gas . It is the first time that a velocity structure has been seen in a Ly man-alpha emitting galaxy. Our results illustrate the difficulty of in terpreting the Lyman alpha emission of galaxies in terms of rate of st ar formation: the escape of Lyman alpha photons is not only very sensi tive to the amount of intervening neutral gas and to its dust abundanc e, but also to its geometrical and velocity structures.