Towards a resolution of the discrepancy between different estimators of star formation rate

Abstract

Different wavelength regimes and methods for estimating the space density of the star-formation rate (SFR) result in discrepant values. While it is recognised that ultra-violet (UV) and H-alpha emission line data must be corrected for the effects of extinction, the magnitude of the required correction is uncertain. Even when these corrections are made there remains a significant discrepancy between SFRs derived from UV and H-alpha measurements compared with those derived from far-infrared (FIR) and radio luminosities. Since the FIR/radio derived SFRs are not affected by extinction, and simple corrections to reconcile the UV and H-alpha measurement with these do not fully account for the discrepancies, a more sophisticated correction may be required. Recent results suggest that at least part of the solution may be a form of extinction which increases with increasing SFR (or luminosity, given the common assumption that SFR is proportional to luminosity). We present an analysis of the effects of a dust reddening dependent on star formation rate applied to estimators of SFR. We show (1) that the discrepancies between H-alpha and FIR/radio SFR estimates may be explained by such an effect, and we present an iterative method for applying the correction; and (2) UV-based estimates of SFR are harder to reconcile with FIR/radio estimates using this method, although the extent of the remaining discrepancy is less than for a non-SFR-dependent correction. Particularly at high redshift, our understanding of extinction at UV wavelengths may require a still more complex explanation.