HAM presented preliminary estimates of the rates at which starbursts inject mass, metals, and energy into superwinds at the present epoch. These estimates were based on the following assumptions (see HAM for details): First, all galaxies in the local universe with large IR luminosities (LIR > 1044 erg s-1), large IR excesses (LIR / LOPT > 2), and/or warm far-IR colors (flux density at 60 µm greater than 50% of the flux density at 100 µm) drive superwinds. Second, The superwind outflow rates are proportional to the far-IR luminosity, with the constants of proportionality determined from well-studied examples like M 82 and NGC 253.
The recent survey of optical line emission associated with a well-defined
sample of nearly 50 IR-selected edge-on starbursting disk galaxies by L92
and LH have put these two assumptions on a much firmer observational
foundation. They find that such indicators of a superwind as excess optical
line-emission along the minor axis, broad emission-line widths and velocity
shears along the minor axis, and 'shock-type' optical emission-line ratios
(e.g., strong [O I] and [S II] emission relative to
H
) along the minor axis
all correlate with such quantities as the IR luminosity, the IR/optical
luminosity ratio, and the IR color-temperature. They conclude that
superwinds
are probably ubiquitous in galaxies meeting the IR criteria listed in the
preceding paragraph.
Integrating over the local far-IR luminosity function for galaxies meeting
the above criteria
(Bothun, Lonsdale, & Rice
1989)
then yields the superwind
injection rates per unit volume in the local universe. The resulting
injection
rates per unit volume can be normalized by multiplying them by the age
of the universe, and then dividing the result by the local space density of
galaxies. This calculation then implies that superwinds have carried out
approximately 5 × 108
M
in metals and
1059 ergs in kinetic plus thermal
energy per average (Schechter L*) galaxy over the history of the
universe.
For reference, note that these two quantities are approximately equal to
the mass of metals contained inside an average galaxy and the gravitational
binding energy of an average galaxy respectively.
These values pertain to the conservative assumption that the superwind rate has not evolved with look-back time. Since the star-formation rate was presumably higher in the early universe, the true time-integrated values are likely to be about an order-of-magnitude higher (cf. Ostriker & Cowie 1981; HAM). On the other hand, it is unclear what fraction of the energy and mass injected into a superwind by a starburst ultimately escapes the galaxy and is delivered to the intergalactic medium (e.g., less energetic or more strongly confined superwinds may conceivably cool and turn into galactic fountains (cf. Norman & Ikeuchi 1989). In any case, it seems quite likely that superwinds do play an important role in enriching and heating the intergalactic medium (as we will discuss more fully below).