ESTIMATE OF THE INTERGALACTIC INFRARED RADIATION-FIELD FROM GAMMA-RAY OBSERVATIONS OF THE GALAXY MRK421

THE magnitude of the intergalactic infrared radiation field (IIRF) is of fundamental importance to investigations of the evolution of galaxies. Bursts of star formation, which seem to be critically important to galaxy evolution, are characterized by large infrared luminosities(1), yet ie have little knowledge of the frequency and intensity of starbursts in the early Universe. Unfortunately, direct observational determinations of the IIRF are plagued by the difficulty of separating the extragalactic emission from Galactic foreground radiation and the zodiacal light in the Solar System, and have so far produced only upper limits that are far above theoretical expectations(2). We have previously proposed(3) a way to use ground-based observations of high-energy gamma-rays to probe the IIRF, and here we apply our method to the spectrum(4,5) of the BL Lac object Mrk421. The intensity we derive for the IIRF is consistent with the conversion of at least 30% of the energy from stellar nucleosynthesis to infrared radiation, both from emission by cool stars and as a result of absorption and re-emission by interstellar dust grains.