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

TL;DR

This paper reveals that the rising cosmic microwave background at high redshifts fundamentally biases the imaging of dust and molecular gas in distant galaxies, affecting structural and dynamical measurements in mm/submm observations.

Contribution

It identifies the CMB's impact on dust and CO line imaging at high redshift and proposes signatures to detect CMB effects in observational data.

Findings

CMB temperature rise erases brightness contrasts in dust and gas images.

Contrast between dust emission and CMB increases beyond the Rayleigh-Jeans limit.

Atomic carbon lines retain higher contrast than CO lines against the CMB.

Abstract

Images of dust continuum and carbon monoxide (CO) line emission are powerful tools for deducing structural characteristics of galaxies, such as disk sizes, H$_2$ gas velocity fields and enclosed H$_2$ 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 H$_2$ gas progressively erases all spatial and spectral contrasts between their brightness distributions and the CMB. For high-redshift galaxies, this strongly biases the recoverable H$_2$ gas and dust mass distributions, scale lengths, gas velocity fields and dynamical mass estimates. This limitation is unique to mm/submm 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 (CI) lines maintain a larger contrast than the CO lines against the CMB.