title: Galaxies at redshifts 5 to 6 with systematically low dust content and high [C II] emission.
abstract: The rest-frame ultraviolet properties of galaxies during the first three billion years of cosmic time (redshift z > 4) indicate a rapid evolution in the dust obscuration of such galaxies. This evolution implies a change in the average properties of the interstellar medium, but the measurements are systematically uncertain owing to untested assumptions and the inability to detect heavily obscured regions of the galaxies. Previous attempts to measure the interstellar medium directly in normal galaxies at these redshifts have failed for a number of reasons, with two notable exceptions. Here we report measurements of the forbidden C ii emission (that is, [C II]) from gas, and the far-infrared emission from dust, in nine typical star-forming galaxies about one billion years after the Big Bang (z ≈ 5-6). We find that these galaxies have thermal emission that is less than 1/12 that of similar systems about two billion years later, and enhanced [C II] emission relative to the far-infrared continuum, confirming a strong evolution in the properties of the interstellar medium in the early Universe. The gas is distributed over scales of one to eight kiloparsecs, and shows diverse dynamics within the sample. These results are consistent with early galaxies having significantly less dust than typical galaxies seen at z < 3 and being comparable in dust content to local low-metallicity systems.
authors.affiliation: 1] Infrared Processing and Analysis Center (IPAC), 1200 East California Boulevard, Pasadena, California 91125, USA [2] California Institute of Technology, 1200 East California Boulevard, Pasadena, California 91125, USA.
authors.forename: P L
authors.initials: PL
authors.lastname: Capak
authors.affiliation: 1] National Radio Astronomy Observatory, PO Box 0, Socorro, New Mexico 87801, USA [2] Astrophysics Group, Cavendish Laboratory, J. J. Thomson Avenue, Cambridge CB3 0HE, UK.
authors.forename: C
authors.initials: C
authors.lastname: Carilli
authors.affiliation: New Mexico Institute of Mining and Technology, 801 Leroy Place, Socorro, New Mexico 87801, USA.
authors.forename: G
authors.initials: G
authors.lastname: Jones
authors.affiliation: Department of Astronomy, The University of Texas at Austin, 2515 Speedway, Stop C1400, Austin, Texas 78712, USA.
authors.forename: C M
authors.initials: CM
authors.lastname: Casey
authors.affiliation: Department of Astronomy, Cornell University, 220 Space Sciences Building, Ithaca, New York 14853, USA.
authors.forename: D
authors.initials: D
authors.lastname: Riechers
authors.affiliation: National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, Virginia 22903, USA.
authors.forename: K
authors.initials: K
authors.lastname: Sheth
authors.affiliation: Institute for Astronomy, ETH Zurich, CH-8093 Zurich, Switzerland.
authors.forename: C M
authors.initials: CM
authors.lastname: Carollo
authors.affiliation: Aix Marseille Université, CNRS, LAM (Laboratoire d'Astrophysique de Marseille), UMR 7326, 13388 Marseille, France.
authors.forename: O
authors.initials: O
authors.lastname: Ilbert
authors.affiliation: Argelander-Institut für Astronomie, Auf dem Hügel 71, D-53121 Bonn, Germany.
authors.forename: A
authors.initials: A
authors.lastname: Karim
authors.affiliation: Aix Marseille Université, CNRS, LAM (Laboratoire d'Astrophysique de Marseille), UMR 7326, 13388 Marseille, France.
authors.forename: O
authors.initials: O
authors.lastname: LeFevre
authors.affiliation: National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, Virginia 22903, USA.
authors.forename: S
authors.initials: S
authors.lastname: Lilly
authors.affiliation: California Institute of Technology, 1200 East California Boulevard, Pasadena, California 91125, USA.
authors.forename: N
authors.initials: N
authors.lastname: Scoville
authors.affiliation: Physics Department, University of Zagreb, Bijenička cesta 32, 10002 Zagreb, Croatia.
authors.forename: V
authors.initials: V
authors.lastname: Smolcic
authors.affiliation: 1] Infrared Processing and Analysis Center (IPAC), 1200 East California Boulevard, Pasadena, California 91125, USA [2] California Institute of Technology, 1200 East California Boulevard, Pasadena, California 91125, USA.
authors.forename: L
authors.initials: L
authors.lastname: Yan