SPECTROSCOPY OF LUMINOUS INFRARED GALAXIES AT 2 MICRONS. III. ANALYSIS FOR GALAXIES WITH log (L_(IR)/solar luminosity) approx > 11.2

摘要

We have obtained spectra across the K window for the first large sample of luminous galaxies selected from the IRAS survey. This paper contains the principal analysis of the 43 systems in our sample with luminosities of 11.2 approx < log (L_(IR)/solar luminosity) approx < 11.9. The spectra themselves were presented in a companion paper by Goldader et al. (Paper II). The Brγ luminosities are proportional to L_(IR), at levels similar to those of star-forming regions. This strongly suggests that star formation accounts for the bulk of the energy production in these objects, in general agreement with previous studies. Good agreement is found for the continuous star formation models of Leitherer & Heckman with upper mass cutoffs well below 100 solar mass. The models accommodate a range in starburst ages of ～10~7 to 10~9 yr. Instantaneous starburst models fit the data but imply an unrealistically short range of ages for the entire sample. It is difficult to avoid concluding that the initial mass functions are deficient in stars of less than ～1 solar mass.