The Role of Luminous Infrared Galaxies at z ∼ 1 as Revealed by the 15 μm Extragalactic Background Light

Abstract

Deep mid-infrared sky surveys have revealed a strong excess, compared to the local universe, of faint but intrinsically bright galaxies at redshifts close to z ∼ 0.8. A preliminary study of a sample of optical counterparts with spectroscopic redshifts in the Hubble Deep Field North (HDFN) and in one of the Canada-France Redshift Survey (CFRS) fields, indicates that they are massive (M ∼ 10¹¹ M⊙) and bright (several 10¹¹ L⊙) galaxies. The contribution of these galaxies, brighter than 50 μJy (AB ∼ 20), to the 15 μm extragalactic background light (E.B.L.) is 2.2 ± 0.8 nW m ^-2 sr ^-1. This energy density is as high as 60% (30%) of the one produced by the 60 times more numerous galaxies detected in the HDFN in the B ₄₅₀-band (I ₈₁₄-band) down to AB = 29. The ratio of the energy radiated by luminous infrared galaxies at 15 μm over that radiated by all types of galaxies in the optical was therefore two orders of magnitude higher at z ∼ 0.8 than today. Depending on their spectral energy distribution, these galaxies must produce more than 30% of the 140 μm EBL, where the cosmic infrared background peaks. Hence, they produce a significant fraction of the overall energy radiated by galaxies during the hubble time, may it be due to nucleosynthesis or to accretion around a black hole.