Starbursts and Dust at High Redshift

Dust reprocessing of UV light in local starbursts

One of the biggest uncertainties in directly estimating the total star formation rate of the universe at high-z is correcting for obscuration by dust. This is because Lyman-dropout systems are typically observed in the rest frame ultraviolet (UV) where the absorption and scattering properties of dust are at their most extreme. We solve this problem by using local starbursts as templates to calibrate the obscuration effects of dust and then applying the results to the Lyman limit systems, since they have properties very similar to starbursts. This figure illustrates our method. The y axis is the ratio of far infrared (FIR) to UV fluxes, which quantifies the dust reprocessing of UV radiation, the x axis is the UV spectral slope (or color with blue to the left). The data points are local UV selected starbursts. The red line shows a simple empirical fit to this relationship. When applied to a sample of U-dropouts in the Hubble Deep Field, we find a star formation rate density of 0.19 Msun Mpc-3 yr-1 (at z = 2.75), about a factor of nine times higher than originally estimated by Madau et al. (1996). The dotted cyan line shows the effects of dust reprocessing using the SMC extinction curve. This model, used by others in the literature, can not recover the UV radiation reprocessed into the FIR in local starbursts, and implies only a factor of three absorption at 1600 Å. Our case for substantial dust obscuration at high-z is given in Meurer (1997), and Meurer, Heckman, & Calzetti (1999).

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