IONIZED-GAS IN THE HALOS OF EDGE-ON STARBURST GALAXIES - EVIDENCE FORSUPERNOVA-DRIVEN SUPERWINDS

Supernova-driven galactic winds (''superwinds'') have been invoked to explain many aspects of galaxy formation and evolution. Such winds sho uld arise when the supernova rate is high enough to create a cavity of very hot shock-heated gas within a galaxy. This gas can then expand o utward as a high-speed wind that can accelerate and heat ambient inter stellar or circum-galactic gas causing it to emit optical line radiati on and/or thermal X-rays. Theory suggests that such winds should be co mmon in starburst galaxies and that the nature of the winds should dep end on the star formation rate and distribution. In order to systemati ze our observational understanding of superwinds (determine their inci dence rate and the dependence of their properties on the star formatio n that drives them) and to make quantitative comparisons with the theo ry of superwinds, we have analyzed data from an optical spectroscopic and narrow-band imaging survey of an infrared flux-limited (S-60 mu m greater than or equal to 5.4 Jy) sample of about 50 IR-warm (S-60 mu m /S-100 mu m > 0.4), starburst galaxies whose stellar disks are viewed nearly edge-on (b/a greater than or equal to 2). This sample contains galaxies with infrared luminosities from x 10(10)-10(12) L. and allows us to determine the properties of superwinds over a wide range of sta r formation rates. We have found that extraplanar emission-line gas is a very common feature of these edge-on, IR-bright galaxies and the pr operties of the extended emission-line gas are qualitatively and quant itatively consistent with the superwind theory. We can summarize these properties as morphological, ionization, dynamical, and physical. 1. Morphological properties.-Extraplanar filamentary and shell-like emiss ion-line morphologies on scales of hundreds of parsecs to 10 kpc are c ommon, there is a general ''excess'' of line emission along the minor axis, the minor axis emission-line ''excess'' correlates with ''IR act ivity,'' and the minor axis emission-line ''excess'' also correlates w ith the relative compactness of the Ha emission. 2. Ionization propert ies.-line ratios become more ''shocklike'' (high ratios of [IN II] lam bda 6583/H alpha, CS Ill lambda lambda 6716, 6731/H alpha, and [OI] la mbda 6300/H alpha) at more extreme IR properties, the most ''shocklike '' line ratios occur far out along the minor axis, ''shocklike'' line ratios corresponds to broad emission lines, and the most extreme line ratios correspond to the most extreme IR properties, especially for th e emission-line gas farthest out along the minor axis. 3. Dynamical pr operties.-Lines are broader along the minor axis than along the major axis, line widths correlate with the ''IR activity,'' line widths corr elate with line ratios, line widths do not correlate with rotation spe ed, minor axis shear (a measure of the systematic velocity change alon g the minor axis) correlates with ''IR activity,'' minor axis shear co rrelates with axial ratio and implies that a face-on galaxy would have an outflow/inflow speed of 170(-80)(+150) km s(-1), and the starburst s show statistically blueward line profile asymmetries. 4. Physical pr operties.-Pressures in the nuclei of these galaxies are 3 orders of ma gnitude higher than the ambient pressure in the interstellar medium of our galaxy, and the pressure falls systematically with radius. While none of these results are in themselves proof of the superwind model, we believe that when the results are taken as a whole, the superwind h ypothesis is very successful in explaining what we have observed. In a ddition, these results have implications for galaxy evolution and the nature of the intergalactic medium. Those galaxies with the best evide nce for driving superwinds are those with large IR luminosities (L(IR) X 10(44) ergs s(-1)), large IR excesses (L(IR)/L(OPT) greater than or similar to 2), and warm far-IR colors (S-60 mu m/S-100 mu m greater t han or similar to 0.5). Integrating over the local far-IR luminosity f unction for galaxies meeting the above criteria, multiplying by the ag e of the universe, and then dividing by the local space density of gal axies implies that superwinds have carried out approximate to 5 x 10(8 ) M. in metals and 10(59) ergs in kinetic plus thermal energy per aver age (Schecter L) galaxy over the history of the universe. We note tha t these two quantities are approximately equal to the mass of metals c ontained inside an average galaxy and the gravitational binding energy of an average galaxy, respectively. Even with the conservative assump tions of this calculation (we have neglected that star formation rates were presumably higher in the early universe), it is obvious that sup erwinds may have a major impact on the evolution of indiviual galaxies and the intergalactic medium by injecting mass, metals, and kinetic e nergy into the galactic halo and potentially the intergalactic medium.