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

R. J. Ivison,Maud Galametz,Padelis P. Papadopoulos,Zhi-Yu Zhang,Emmanuel M. Xilouris,M. W. L. Smith

doi:10.1098/rsos.160025

R. J. Ivison, Maud Galametz + Show 4 more

Open Access

https://doi.org/10.1098/rsos.160025

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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.

Highlights

Imaging the thermal continuum from cosmic dust and the carbon monoxide (CO) line emission from its concomitant molecular (H2) gas at millimetre and submillimetre wavelengths has become a powerful tool for deducing structural characteristics of galaxies [1,2,3,4,5,6,7]
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
We must note that once the noise associated with any synthesis imaging observations and the cosmological (1 + z)−3 brightness dimming factor are taken into account, the fundamental constraints set by the rising CMB on the imaging of cold dust and molecular gas distributions of distant galaxies will take effect for redshifts lower than those indicated in this work

Summary

Introduction

Imaging the thermal continuum from cosmic dust and the carbon monoxide (CO) line emission from its concomitant molecular (H2) gas at millimetre and submillimetre wavelengths has become a powerful tool for deducing structural characteristics of galaxies [1,2,3,4,5,6,7]. Rather than assuming a Tdust range—as was the past practice for such studies [9,11]— 5 we will demonstrate the CMB effect on the observable brightness distribution of cold dust continuum at high redshifts by using real Tdust maps of galaxies in the local Universe. Such maps can be obtained from the wealth of IR/submillimetre imaging data available from the Spitzer Space Telescope and the Herschel Space Observatory. We exploit NGC628, M33 and M31, for which there are highquality Tdust maps over most of their extent [13,27,28] and whose levels of SF activity range from the vigorous, in NGC628 and M33 (ΣSFR approx. 10−3 to 10−2 M yr−1 kpc−2 [29,30]), to the quiescent M31 (ΣSFR approx. 10−4 to 10−3 M yr−1 kpc−2 [31])

Archival Herschel data

Temperature maps of cold dust distribution

Archival carbon monoxide data

Temperature maps of carbon monoxide

Radiative transfer modelling with large velocity gradient

Findings

Summary and conclusion