The Global Schmidt Law in Star Forming Galaxies Robert C. Kennicutt, Jr. The Global Schmidt Law in Star Forming Galaxies Robert C. Kennicutt, Jr. Abstract: Measurements of Hα, HI, and CO distributions in 61 normal spiral galaxies are combined with published far-infrared and CO observations of 36 infrared-selected starburst galaxies to study the form of the global star formation law over the full range of gas densities and star formation rates (SFRs) observed in galaxies. The disk-averaged SFRs and gas densities for the combined sample are well represented by a Schmidt law with index N = 1.4 ± 0.15. The Schmidt law provides a surprisingly tight parametrization of the global star formation law, extending over several orders of magnitude in SFR and gas density. An alternative formulation of the star formation law, in which the SFR is presumed to scale with the ratio of the gas density to the average orbital timescale, also fits the data very well. Both descriptions provide potentially useful “recipes” for modelling the SFR in numerical simulations of galaxy formation and evolution. Subject headings: galaxies: evolution — galaxies: ISM — galaxies: spiral — galaxies: starburst — stars: formation 1. Introduction: A key ingredient in the understanding and modelling of galaxy evolution is the relationship between the large-scale star formation rate (SFR) and the physical conditions in the interstellar medium (ISM). Most current galaxy formation and evolution models treat star formation using simple ad hoc parametrizations, and our limited understanding of the actual form and nature of the SFR-ISM interaction remains as one of the major limitations in these models. Measurements of the star formation law in nearby galaxies can address this problem in two important respects, by providing empirical “recipes” that can be incorporated into analytical models and numerical simulations, and by providing clues to the physical mechanisms that underlie the observed correlations. The most widely applied star formation law remains the simple gas density power law introduced by Schmidt (1959), which for external galaxies is usually expressed in terms of the observable surface densities of gas and star formation: $$ \Sigma_{SFR} = A \Sigma_{gas}^{N} $$ The validity of the Schmidt law has been tested in dozens of empirical studies, with most measured values of N falling in the range 1 – 2, depending on the tracers used and the linear scales considered. On large scales the star formation law shows a more complex character, with a Schmidt law at high gas densities, and a sharp decline in the SFR below a critical threshold density. These thresholds appear to be associated with large-scale gravitational stability thresholds for massive cloud formation. At high gas densities, well above the stability threshold, the form of the Schmidt law appears to be remarkably consistent from galaxy to galaxy, both in terms of its slope and the absolute SFR efficiency.The Global Schmidt Law in Star Forming Galaxies Robert C. Kennicutt, Jr. The Global Schmidt Law in Star Forming Galaxies Robert C. Kennicutt, Jr. Abstract: Measurements of Hα, HI, and CO distributions in 61 normal spiral galaxies are combined with published far-infrared and CO observations of 36 infrared-selected starburst galaxies to study the form of the global star formation law over the full range of gas densities and star formation rates (SFRs) observed in galaxies. The disk-averaged SFRs and gas densities for the combined sample are well represented by a Schmidt law with index N = 1.4 ± 0.15. The Schmidt law provides a surprisingly tight parametrization of the global star formation law, extending over several orders of magnitude in SFR and gas density. An alternative formulation of the star formation law, in which the SFR is presumed to scale with the ratio of the gas density to the average orbital timescale, also fits the data very well. Both descriptions provide potentially useful “recipes” for modelling the SFR in numerical simulations of galaxy formation and evolution. Subject headings: galaxies: evolution — galaxies: ISM — galaxies: spiral — galaxies: starburst — stars: formation 1. Introduction: A key ingredient in the understanding and modelling of galaxy evolution is the relationship between the large-scale star formation rate (SFR) and the physical conditions in the interstellar medium (ISM). Most current galaxy formation and evolution models treat star formation using simple ad hoc parametrizations, and our limited understanding of the actual form and nature of the SFR-ISM interaction remains as one of the major limitations in these models. Measurements of the star formation law in nearby galaxies can address this problem in two important respects, by providing empirical “recipes” that can be incorporated into analytical models and numerical simulations, and by providing clues to the physical mechanisms that underlie the observed correlations. The most widely applied star formation law remains the simple gas density power law introduced by Schmidt (1959), which for external galaxies is usually expressed in terms of the observable surface densities of gas and star formation: $$ \Sigma_{SFR} = A \Sigma_{gas}^{N} $$ The validity of the Schmidt law has been tested in dozens of empirical studies, with most measured values of N falling in the range 1 – 2, depending on the tracers used and the linear scales considered. On large scales the star formation law shows a more complex character, with a Schmidt law at high gas densities, and a sharp decline in the SFR below a critical threshold density. These thresholds appear to be associated with large-scale gravitational stability thresholds for massive cloud formation. At high gas densities, well above the stability threshold, the form of the Schmidt law appears to be remarkably consistent from galaxy to galaxy, both in terms of its slope and the absolute SFR efficiency.