THE CALIBRATION OF MONOCHROMATIC FAR-INFRARED STAR FORMATION RATE INDICATORS

Abstract

(Abridged) Spitzer data at 24, 70, and 160 micron and ground-based H-alpha\nimages are analyzed for a sample of 189 nearby star-forming and starburst\ngalaxies to investigate whether reliable star formation rate (SFR) indicators\ncan be defined using the monochromatic infrared dust emission centered at 70\nand 160 micron. We compare recently published recipes for SFR measures using\ncombinations of the 24 micron and observed H-alpha luminosities with those\nusing 24 micron luminosity alone. From these comparisons, we derive a reference\nSFR indicator for use in our analysis. Linear correlations between SFR and the\n70 and 160 micron luminosity are found for L(70)>=1.4x10^{42} erg/s and\nL(160)>=2x10^{42} erg/s, corresponding to SFR>=0.1-0.3 M_sun/yr. Below those\ntwo luminosity limits, the relation between SFR and 70 micron (160 micron)\nluminosity is non-linear and SFR calibrations become problematic. The\ndispersion of the data around the mean trend increases for increasing\nwavelength, becoming about 25% (factor ~2) larger at 70 (160) micron than at 24\nmicron. The increasing dispersion is likely an effect of the increasing\ncontribution to the infrared emission of dust heated by stellar populations not\nassociated with the current star formation. The non-linear relation between SFR\nand the 70 and 160 micron emission at faint galaxy luminosities suggests that\nthe increasing transparency of the interstellar medium, decreasing effective\ndust temperature, and decreasing filling factor of star forming regions across\nthe galaxy become important factors for decreasing luminosity. The SFR\ncalibrations are provided for galaxies with oxygen abundance 12+Log(O/H)>8.1.\nAt lower metallicity the infrared luminosity no longer reliably traces the SFR\nbecause galaxies are less dusty and more transparent.\n