The Cosmic Near Infrared Background: Remnant Light from Early Stars

The redshifted ultraviolet light from early stars at z~10 contributes to the cosmic near infrared background. We present detailed calculations of its spectrum with various assumptions about metallicity and mass spectrum of early stars. We show that if the near infrared background has a stellar origin, metal-free stars are not the only explanation of the excess near infrared background; stars with significant metals (Z=1/50) can produce the same amount of background intensity as the metal-free stars. We quantitatively show that the predicted average intensity at 1-2 microns is essentially determined by the efficiency of nuclear burning in stars, which is not very sensitive to metallicity. We predict $\nu I_\nu/\dot{\rho}_*$ ~ 3.5-8 nW m^-2 sr^-1, where $\dot{\rho}_*$ is the mean star formation rate at z=7-15 (in units of Msun yr^-1 Mpc^-3) for stars more massive than 5 Msun. On the other hand, since we have very little knowledge about the form of mass spectrum of early stars, 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 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.