5.2. Mid IR as a SFR indicator

When normalized to the 7.7 µm PAH (polycyclic aromatic hydrocarbon) broad emission line, the spectra of different galaxies exhibit very different 15, 25, 60 or 100 µm over 7.7 µm ratio. This was often interpreted as an indication that measuring the monochromatic luminosity of a galaxy at one mid IR wavelength was useless to determine its bolometric IR luminosity, L_IR = L(8-1000 µm ). However, the variation of the far over mid IR ratio is correlated with L_IR and local galaxies do exhibit a strong correlation of their mid and far IR luminosities (Fig. 9). These correlations can be used to construct a family of template SEDs or correlations from which the L_IR, and therefore SFR, of a galaxy can be derived from its mid IR luminosity (Chary & Elbaz 2001, Elbaz et al. 2002). The L_IR derived from this technique are consistent with those derived by the radio-far IR correlation, when radio-mid-far IR data exist (Elbaz et al. 2002, Garrett 2002, Gruppioni et al. 2003). In the Fig. 10, we have reproduced the plot from Elbaz et al. (2002) complemented with galaxies detected within the ELAIS survey (Rowan-Robinson et al. 2004). Except at low luminosities were the contribution of cirrus to the IR luminosity becomes non negligible, the 1.4 GHz and 15 µm rest-frame luminosities are correlated up to z ~ 1 and therefore predict very consistent total IR luminosities. A similar result was later on obtained using the MIPS instrument onboard the Spitzer Space Observatory at 24 µm (Appleton et al. 2004).

Figure 9. IR luminosity correlations for local galaxies (from Elbaz et al. 2002). a) ISOCAM-LW3 (15 µm ) versus ISOCAM-LW2 (6.75 µm) luminosities ( L) (56 galaxies). b) ISOCAM-LW3 (15 µm) versus IRAS-12 µm luminosities (45 galaxies). c) LIR[8-1000 µm ] versus ISOCAM-LW3 (15 µm) luminosity (120 galaxies). d) LIR[8-1000 µm] versus LW2-6.75 µm luminosities (91 galaxies). Filled dots: galaxies from the ISOCAM guaranteed time (47 galaxies including the open squares). Open dots: 40 galaxies from Rigopoulou et al. (1999). Empty triangles: 4 galaxies from Tran et al. (2001). Galaxies below LIR ~ 1010 L present a flatter slope and have LIR / LB < 1.

Figure 10. 15 µm versus radio continuum (1.4 GHz) rest-frame luminosities. Small filled dots: sample of 109 local galaxies from ISOCAM and NVSS. Filled dots with error bars: 17 HDFN galaxies (z ~ 0.7, radio from VLA or WSRT). Open dots with error bars: 7 CFRS-14 galaxies (z ~ 0.7, Flores et al. 1999, radio from VLA). Open diamonds: 137 ELAIS galaxies (z ~ 0-0.4).

Several studies compared the SFR derived from the IR luminosity with the optical SFR derived from the H emission line (Rigopoulou et al. 2000, Cardiel et al. 2003, Flores et al. 2004, Liang et al. 2004). Rigopoulou et al. (2000) found a large excess of SFR(IR) versus SFR(H) even after correcting the latter for extinction. The Balmer decrement was only measured for limited number of objects in the sample and the extinction correction was derived from broadband photometry, which suffers from strong uncertainties in particular due to the degeneracy between age, metallicity and extinction. However, Cardiel et al. (2003) confirmed the direct measure of the SFR(IR) excess using the combination of NIRSPEC and LRIS, for the distant galaxies, and the Echelle Spectrograph and Imager (ESI), for the closer ones, at the Keck telescope. Using high resolution VLT-FORS2 spectra, Flores et al. (2004) and Liang et al. (2004) were able to measure directly the Balmer decrement (using H / H or H / H) and to subtract the underlying nebular emission lines with a fit of the stellar continuum. Although the SFR(IR) exceeds the estimate from emission lines for the most active objects, the data present a clear correlation between SFR(IR) and SFR(H) which suggests that the star formation regions responsible for the IR luminosity of distant LIRGs are not completely obscured. This correlations also confirms that the mid IR is indeed a good SFR estimator.