title: Gone with the heat: a fundamental constraint on the imaging of dust and molecular gas in the early Universe.
abstract: Images of dust continuum and carbon monoxide (CO) line emission are powerful tools for deducing structural characteristics of galaxies, such as disc sizes, H2 gas velocity fields and enclosed H2 and dynamical masses. We report on a fundamental constraint set by the cosmic microwave background (CMB) on the observed structural and dynamical characteristics of galaxies, as deduced from dust continuum and CO-line imaging at high redshifts. As the CMB temperature rises in the distant Universe, the ensuing thermal equilibrium between the CMB and the cold dust and H2 gas progressively erases all spatial and spectral contrasts between their brightness distributions and the CMB. For high-redshift galaxies, this strongly biases the recoverable H2 gas and dust mass distributions, scale lengths, gas velocity fields and dynamical mass estimates. This limitation is unique to millimetre/submillimetre wavelengths and unlike its known effect on the global dust continuum and molecular line emission of galaxies, it cannot be addressed simply. We nevertheless identify a unique signature of CMB-affected continuum brightness distributions, namely an increasing rather than diminishing contrast between such brightness distributions and the CMB when the cold dust in distant galaxies is imaged at frequencies beyond the Raleigh-Jeans limit. For the molecular gas tracers, the same effect makes the atomic carbon lines maintain a larger contrast than the CO lines against the CMB.
authors.affiliation: Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK; ESO, Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany.
authors.forename: Zhi-Yu
authors.initials: ZY
authors.lastname: Zhang
authors.affiliation: School of Physics and Astronomy, Cardiff University, Queen's Buildings, The Parade, Cardiff CF24 3AA, UK; ESO, Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany; Research Center for Astronomy, Academy of Athens, Soranou Efesiou 4, 115 27 Athens, Greece.
authors.forename: Padelis P
authors.identifier: 0000-0002-0390-4596
authors.initials: PP
authors.lastname: Papadopoulos
authors.affiliation: ESO, Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany; Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK.
authors.forename: R J
authors.identifier: 0000-0001-5118-1313
authors.initials: RJ
authors.lastname: Ivison
authors.affiliation: ESO , Karl-Schwarzschild-Strasse 2 , 85748 Garching, Germany.
authors.forename: Maud
authors.identifier: 0000-0002-0283-8689
authors.initials: M
authors.lastname: Galametz
authors.affiliation: School of Physics and Astronomy , Cardiff University , Queen's Buildings, The Parade, Cardiff CF24 3AA, UK.
authors.forename: M W L
authors.initials: MW
authors.lastname: Smith
authors.affiliation: Institute for Astronomy , National Observatory of Athens , 15236 Penteli, Greece.
authors.forename: Emmanuel M
authors.initials: EM
authors.lastname: Xilouris
keywords: cosmology: cosmic background radiation; galaxies: high-redshift; galaxies: interstellar medium