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Register a new userDust emission in the far-IR as a star formation tracer at z=0: systematic trends with luminosity
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We investigate if dust emission in the far-IR continuum provides a robust estimate of star formation rate (SFR) for a nearby, normal late-type galaxy. We focus on the ratio of the 40--1000 micron luminosity (L_dust) to the far-UV (0.165 micron luminosity, which is connected to recent episodes of star formation. Available total photometry at 0.165, 60, 100 and 170 micron limits the statistics to 30 galaxies, which, however, span a large range in observed (and, thus, attenuated by dust) K-band (2.2 micron) luminosity, morphology and inclination (i). This sample shows that the ratio of L_dust to the observed far-UV luminosity depends not only on i, as expected, but also on morphology and, in a tighter way, on observed K-band luminosity. We find that L_dust/L_FUV is proportional to e^(-tau_K) (alpha+0.62) (L_K)^(0.62), where L_FUV and L_K are the unattenuated stellar luminosities in far-UV and K, respectively, and alpha is the ratio of the attenuation optical depths at 0.165 micron (tau_FUV) and 2.2 micron (tau_K). This relation is to zeroth order independent of i and morphology. It may be further expressed as L_dust/L_FUV proportional to (L_K)^delta, where delta = 0.61 - 0.02 alpha, under the observationally-motivated assumption that, for an average inclination, e^(-tau_K) is proportional to (L_K)^(-0.02). We adopt calculations of two different models of attenuation of stellar light by internal dust to derive solid-angle averaged values of alpha. We find that delta is positive and decreases towards 0 from the more luminous to the less luminous galaxies. This means that there is no universal ratio of far-IR luminosity to unattenuated far-UV luminosity for nearby, normal late-type galaxies. (Abridged)
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The combination of both contributions from the observed UV emission and the absorbed radiations reprocessed in the infrared represents the ideal approach to constrain the activity of massive star formation in galaxies. Using recent results from GALEX and Spitzer, we compare the evolutions of the UV and IR energy densities with redshift as well as their contributions to the star formation history at 0<z<1. We find that the comoving IR luminosity is characterized by a much faster evolution than seen in the UV. Our results also indicate that ~70% of the star-forming activity at z~1 is produced by the so-called IR-luminous sources (L_IR > 10^11 L_sol).
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IR emission bands at 3.3, 6.2, 7.7, 8.6 and 11.3 um are generally attributed to IR fluorescence from (mainly) FUV pumped PAHs. As such, they trace the FUV stellar flux and are a measure of star formation. We examined the IR spectral characteristics of Galactic star forming regions, normal and starburst galaxies, AGNs and ULIRGs. The goal is to analyze if PAH bands are a good qualitative and/or quantitative tracer of star formation and hence the application of PAH bands as a diagnostic in order to identify the dominant processes contributing to the IR emission from Seyferts and ULIRGs. We develop a MIR/FIR diagnostic and compare it to known diagnostics, with these also applied to the Galactic sample. This diagnostic is based on the FIR normalized 6.2 um PAH flux and the FIR normalized 6.2 um continuum flux. The Galactic sources form a sequence spanning a range of 3 orders of magnitude, from embedded compact HII regions to exposed PDRs and the (D)ISM. The variation in the 6.2 um PAH/continuum ratio is relative small. Normal and starburst galaxies ressemble exposed PDRs. While Seyfert-2s coincide with the starburst trend, Seyfert-1s are displaced by at least a factor 10 in 6.2 um continuum flux. ULIRGs show a diverse spectral appearance (AGN hot dust continuum, starburst-like or strong dust obscuration in the nucleus). ULIRGs also seems to have more prominent FIR emission than either starburst galaxies or AGNs. We discuss the observed variation in the Galactic sample in view of the evolutionary state and the PAH/dust abundance and the use of PAHs as quantitative tracers of star formation activity. We find that PAHs may be better suited as a tracer of B stars, which dominate the Galactic stellar energy budget, than as a tracer of massive star formation (O stars).
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