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Images of dust continuum and carbon monoxide (CO) line emission are powerfultools for deducing structural characteristics of galaxies, such as disk sizes,H$_2$ gas velocity fields and enclosed H$_2$ and dynamical masses. We report ona fundamental constraint set by the cosmic microwave background (CMB) on theobserved structural and dynamical characteristics of galaxies, as deduced fromdust continuum and CO-line imaging at high redshifts. As the CMB temperaturerises in the distant Universe, the ensuing thermal equilibrium between the CMBand the cold dust and H$_2$ gas progressively erases all spatial and spectralcontrasts between their brightness distributions and the CMB. For high-redshiftgalaxies, this strongly biases the recoverable H$_2$ gas and dust massdistributions, scale lengths, gas velocity fields and dynamical mass estimates.This limitation is unique to mm/submm wavelengths and unlike its known effecton the global dust continuum and molecular line emission of galaxies, it cannotbe addressed simply. We nevertheless identify a unique signature ofCMB-affected continuum brightness distributions, namely an increasing ratherthan diminishing contrast between such brightness distributions and the CMBwhen the cold dust in distant galaxies is imaged at frequencies beyond theRaleigh-Jeans limit. For the molecular gas tracers, the same effect makes theatomic carbon (CI) lines maintain a larger contrast than the CO lines againstthe CMB.

Gone with the heat: A fundamental constraint on the imaging of dust and molecular gas in the early Universe