Annu. Rev. Astron. Astrophys. 1991. 29: 581-625 Copyright © 1991 by Annual Reviews. All rights reserved

7. STAR FORMATION RATES AND EFFICIENCIES IN GALAXIES

The rate at which the ISM in a galaxy evolves is determined in large part by the rate of star formation within molecular clouds. The most widely available measures of the star formation activity in galaxies are global H fluxes (Kennicutt & Kent 1983, Bushouse 1986, Kennicutt et al 1987) and far infrared fluxes from the IRAS survey (Lonsdale et al 1985). Although the H emission traces the young stars, the disadvantages of the H are several. First, the emission traces only high mass stars so that assumptions about the initial mass function (IMF) must be made in order to deduce the overall star formation rate (SFR). Second, the emission suffers extinction, and will therefore provide only a lower limit to the SFR.

An alternative tracer of the rate of high mass star formation in galaxies is provided by the IRAS data. Devereux & Young (1990a) have shown that the global far infrared luminosity observed in spiral galaxies is consistent with that of the high mass OB stars required to ionize the gas. Additionally, only high mass stars are capable of heating the dust in galaxies to the dust temperatures observed throughout spiral disks. In the following, we adopt the view that the SFRs are indicated by the IR luminosities (for a more complete discussion see Devereux & Young 1991). The infrared emission, like the H, is nevertheless strongly biased toward the high mass end of the IMF.

From an analysis of the IRAS data for 1000 nearby galaxies, Devereux & Young (1991) find that the median high mass SFR is similar for spiral galaxies of types Sa-Scd, while the SFR is an order of magnitude lower for the S0 and Sd-Sm galaxies.

In order to examine the morphological type dependence of the star formation rate per unit mass of gas (or the star formation efficiency, SFE), a number of investigators have determined the L_IR / M (H₂) ratio for early and late type spirals. Rengarajan & Verma (1986), Young et al (1989), Thronson et al (1989b), Wilkind & Henkel (1989), Devereux & Young (1991) and Allen & Young (1989) all find that early and late type spirals have similar global SFEs, as shown in Figure 8. Thus, the global star formation rates and efficiencies in disk galaxies do not depend strongly on morphology. Global SFEs do, however, depend on environment as discussed in Section 8.

Figure 8. Histograms of the ratio LIR / M (H2) for each morphological type among spiral galaxies. The vertical tick mark in each panel represents the median value of LIR / M (H2) for that type. The hatched portion of each histogram represents galaxies with H2 upper limits, and LIR / M (H2) lower limits. The constancy of LIR / M (H2) in the mean for spiral galaxies along the Hubble sequence indicates that the global star formation efficiency does not change with type.