The cosmic near infrared background: Remnant light from early stars

The redshifted ultraviolet light from early stars at z similar to 10 contributes to the cosmic near infrared background (NIRB). We present detailed calculations of its spectrum with various assumptions about metallicity and mass spectrum of the early stars. We show that if the NIRB has a stellar origin, metal-free stars are not the only explanation of the excess NIRB; stars with significant metals (e.g., Z=1/50 Z(circle dot)) can produce the same amount of background intensity as metal-free stars. This is because the average intensity at 1-2 microns is determined by the efficiency of nuclear burning in stars, which is not very sensitive to metallicity. We predict v*Iv/(rho) over dot*similar to 4 - 8nW m(-2) sr(-1), where (rho) over dot* is the mean star formation rate at z=7-15 (solar masses per year per cubic megaparsec) for stars more massive than 5 solar masses. On the other hand, since we have very little knowledge about the form of the mass spectrum of early stars, the uncertainty in the average intensity due to the mass spectrum could be large. An accurate determination of the near-infrared background allows us to probe the formation history of early stars, which is difficult to constrain by other means. While the star formation rate at z=7-15 inferred from the current data is significantly higher than the local rate at z<5, it does not rule out the stellar origin of the cosmic near-infrared background. In addition, we show that a reasonable initial mass function, coupled with this star formation rate, does not overproduce metals in the universe in most cases and may produce as little as less than 1% of the metals observed in the universe today.