### 4. SUPERWIND DEMOGRAPHICS

Lehnert & Heckman (1996) discussed the analysis of the optical emission-line properties of a sample of 50 disk galaxies selected to be bright and warm in the far-infrared (active star-formers) and to be viewed within 30° of edge-on. They defined several indicators of minor-axis outflows: 1) an excess of ionized gas along the minor axis (from H images) 2) emission-line profiles that were broader along the galaxy minor axis than along the major axis 3) emission-line ratios that were more "shock-like" along the galaxy minor axis than the major axis. All these indicators became stronger in the galaxies with more intense star-formation (larger LFIR, larger LFIR/LOPT, and warmer dust temperatures).

Dahlem, Weaver, & Heckman (1998) used ROSAT and ASCA to search for X-ray evidence for outflows from a complete sample of the seven nearest edge-on starburst galaxies (selected on the basis of far-IR flux, warm far-IR colors, edge-on orientation, and low Galactic HI column). Apart from the dwarf galaxy NGC 55, all the galaxies showed hot gas in their halos. The gas had temperatures of a few times 106 to 107 K, and could be traced out to distances of-order 10 kpc from the disk plane (see also Read, Ponman, & Strickland 1997).

Heckman et al. (2000) obtained spectra of 19 starbursts in which the NaI5893 (NaD) absorption feature was produced primarily by interstellar gas (rather than stars). In 12 of these (63%) the NaD centroid was blueshifted by 102 to 103 km/s relative to the galaxy systemic velocity, and this fraction rose to 79% in galaxies viewed from within 60° of face-on. No comparably redshifted absorption was seen in any galaxies.

At high-redshift, the only readily available tracers of superwinds are the interstellar absorption-lines in the rest-frame ultraviolet. As shown by Franx et al. (1997) and Pettini et al. (2001) the Lyman Break galaxies generically show interstellar absorption-lines that are blueshifted by a few hundred to over a thousand km s-1 relative to the estimated galaxy systemic velocity. These galaxies strongly resemble local starbursts in their high rate of star-formation per unit area (Meurer et al. 1997).

In summary, superwinds are ubiquitous in galaxies with star-formation-rates per unit area 10-1 M yr-1 kpc-2. Starbursts and the Lyman Break galaxies surpass this threshold, while the disks of ordinary present-day spiral galaxies do not (Kennicutt 1998).