Hidden Star Formation : The Ultraviolet Perspective

Many recent estimates of the star formation rate density at high redshift rely on rest-frame ultraviolet (UV) data. These are highly sensitive to dust absorption. Applying a correlation between the far-infrared (FIR) to UV flux ratio and UV color found in a local starbursts to galaxy samples out to z ∼ 3, one can account for most of the FIR background. However, the correlation is based on a sample that does not include the most extreme starbursts, Ultra Luminous Infrared Galaxies (ULIGs). Our new UV images of ULIGs show that their FIR fluxes are underpredicted by this correlation by factors ranging from 7 to 70. We discuss how ULIGs compare to the various types of high-z galaxies: sub-mm sources, Lyman Break Galaxies, and Extremely Red Objects. 1. Why observe star forming galaxies in the ultraviolet? About 60% of the intrinsic bolometric luminosity of star forming populations is emitted between 912Å and 3000Å with little variation in this fraction with star formation duration (from models of Leitherer et al. 1999). Hence UV light is potentially very useful for measuring star formation. It represents direct emission from hot main-sequence stars, the same stars that will provide the majority of the mechanical energy feedback into the ISM. The UV spectrum is very rich in features that can be used to diagnose the stellar populations and intervening ISM. The overall intrinsic spectral slope in the UV is fairly constant. For young ionizing populations it is set by the Rayleigh-Jeans tail of the Planck 1