Modeling the Extragalactic Background Light from Stars and Dust

The extragalactic background light (EBL) from the far-infrared through the visible and extending into the ultraviolet is thought to be dominated by starlight, either through direct emission or through absorption and reradiation by dust. This is the most important energy range for absorbing gamma-rays from distant sources such as blazars and gamma-ray bursts and producing electron-positron pairs. In previous work, we presented EBL models in the optical through ultraviolet by consistently taking into account the star formation rate (SFR), initial mass function (IMF), and dust extinction, and treating stars on the main sequence as blackbodies. This technique is extended to include post-main-sequence stars and reprocessing of starlight by dust. In our simple model, the total energy absorbed by dust is assumed to be re-emitted as three blackbodies in the infrared, one at 40 K representing warm, large dust grains, one at 70 K representing hot, small dust grains, and one at 450 K representing polycyclic aromatic hydrocarbons. We find that our best-fit model combining the Hopkins and Beacom SFR using the Cole et al. parameterization with the Baldry and Glazebrook IMF agrees with available luminosity density data at a variety of redshifts. Our resulting EBL energy density is quite close to the lower limits from galaxy counts, though in two cases below the lower limits, and agrees fairly well with other recent EBL models shortward of about 5 {mu}m. Deabsorbing TeV gamma-ray spectra of various blazars with our EBL model gives results consistent with simple shock acceleration theory. We also find that the universe should be optically thin to gamma-rays with energies less than 20 GeV.