A dominant population of optically invisible massive galaxies in the early Universe

Abstract

Our current knowledge of cosmic star-formation history during the first two billion years (corresponding to redshift z > 3) is mainly based on galaxies identified in rest-frame ultraviolet light 1 . However, this population of galaxies is known to under-represent the most massive galaxies, which have rich dust content and/or old stellar populations. This raises the questions of the true abundance of massive galaxies and the star-formation-rate density in the early universe. Although several massive galaxies that are invisible in the ultraviolet have recently been confirmed at early epochs 2,3,4 , most of them are extreme starbursts with star-formation rates exceeding 1000 solar masses per year, suggesting that they are unlikely to represent the bulk population of massive galaxies. Here we report submillimeter (wavelength 870 µm) detections of 39 massive star-forming galaxies at z > 3, which are unseen in the spectral region from the deepest ultraviolet to the near-infrared. With a space density of about 2 × 10 −5 per cubic megaparsec (two orders of magnitudes higher than extreme starbursts 5 ) and star-formation rates of ∼200 solar masses per year, these galaxies represent the bulk population of massive galaxies that have been missed from previous surveys. They contribute a total star-formation-rate density ten times larger than that of equivalently massive ultraviolet-bright galaxies at z > 3. Residing in the most massive dark matter halos at their redshifts, they are probably the progenitors of the largest present-day galaxies in massive groups and clusters. Such a high abundance of massive and dusty galaxies in the early universe challenges our understanding of massive-galaxy formation.Observations of galaxies across cosmic time have revealed that more massive galaxies have assembled their stellar masses at earlier epochs, with a significant population of massive ellipticals already in place at redshifts z ∼ 3 − 4 6,7,8 . The early assembly of these massive galaxies has posed serious challenges to current galaxy formation theories. Understanding their formation processes requires studies of their progenitors formed at even higher redshifts. However, most currently known high-redshift galaxies, including mainly Lyman-break galaxies (LBGs) and few extreme starbursts, are found inadequate to account for the large population of these early formed ellipticals, due to either low stellar masses and star formation rates, SFRs (for LBGs 9 ) or low space densities (for the extreme starbursts). This suggests that the main progenitors of massive galaxies at z > 3 remain to be found. Identification of these currently missing massive galaxies is key to our understanding of both massive-galaxy formation and the cosmic SFR density in the early universe.The main targets of this study are a population of galaxies that are Spitzer/Infrared Array Camera (IRAC)-bright yet undetected in even the deepest near-infrared (NIR: H-band) imaging with Hubble Space Telescope (HST), that is, H-dropouts. (Throughout this Letter we use the sho...

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