Galactic Superwinds Circa 2001

4. SUPERWIND DEMOGRAPHICS

Lehnert & Heckman (1996) discussed the analysis of the optical emission-line properties of a sample of $\sim$ 50 disk galaxies selected to be bright and warm in the far-infrared (active star-formers) and to be viewed within $\sim$ 30° of edge-on. They defined several indicators of minor-axis outflows: 1) an excess of ionized gas along the minor axis (from H $\alpha$ 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 L_FIR, larger L_FIR/L_OPT, 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 NaI $\lambda$ 5893 (NaD) absorption feature was produced primarily by interstellar gas (rather than stars). In 12 of these (63%) the NaD centroid was blueshifted by $\sim$ 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 $\Sigma_{*}^{}$ $\geq$ 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).