Mid-infrared luminosity as an indicator of the total infrared luminosity of galaxies

The infrared (IR) emission plays a crucial role for understanding the star formation in galaxies hidden by dust. We first examined four estimators of the IR luminosity of galaxies, LFIR (Helou et al. 1988), LTIR (Dale et al. 2001a), revised version of LTIR (Dale & Helou 2002) (we denote LTIR2), and LIR (Sanders & Mirabel 1996) by using the observed SEDs of well-known galaxies. We found that LIR provides excellent estimates of the total IR luminosity for a variety of galaxy SEDs. The performance of LTIR2 was also found to be very good. Using LIR, we then statistically analyzed the IRAS PSCz galaxy sample (Saunders et al. 2000) and found useful formulae relating the MIR monochromatic luminosities [L(12 μm) and L(25μm)], and LIR. For this purpose we constructed a subsample of 1420 galaxies with all IRAS four band (12, 25, 60, and 100μm) flux densities. We found linear relations between LIR and MIR luminosities, L(12μm) and L(25μm). The prediction error with 95-% confidence level is a factor of 4–5. Hence, these formulae are useful for the estimation of the total IR luminosity only from 12μm or 25μm observations. We further tried to make an ‘interpolation’ formula for galaxies at 0 < z < 1. For this purpose we construct the formula of the relation between 15-μm luminosity and the total IR luminosity. We conclude that the 15-μm formula can be used as an estimator of the total IR luminosity from 24μm observation of galaxies at z ≃ 0.6.