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Published November 2020 | Published + Submitted + Supplemental Material
Journal Article Open

ALMA characterizes the dust temperature of z ∼ 5.5 star-forming galaxies


The infrared (IR) spectral energy distributions (SEDs) of main-sequence galaxies in the early Universe (z > 4) is currently unconstrained as IR continuum observations are time-consuming and not feasible for large samples. We present Atacama Large Millimetre Array Band 8 observations of four main-sequence galaxies at z ∼ 5.5 to study their IR SED shape in detail. Our continuum data (rest-frame 110 μm⁠, close to the peak of IR emission) allows us to constrain luminosity-weighted dust temperatures and total IR luminosities. With data at longer wavelengths, we measure for the first time the emissivity index at these redshifts to provide more robust estimates of molecular gas masses based on dust continuum. The Band 8 observations of three out of four galaxies can only be reconciled with optically thin emission redward of rest-frame 100μm⁠. The derived dust peak temperatures at z ∼ 5.5 (⁠30−43K⁠) are elevated compared to average local galaxies, however, ∼10K below what would be predicted from an extrapolation of the trend at z < 4. This behaviour can be explained by decreasing dust abundance (or density) towards high redshifts, which would cause the IR SED at the peak to be more optically thin, making hot dust more visible to the external observer. From the 850-μm dust continuum, we derive molecular gas masses between 10¹⁰ and 10¹¹M⊙ and gas fractions (gas over total mass) of 30−80 per cent (gas depletion times of 100−220 Myr⁠). All in all, our results provide a first measured benchmark SED to interpret future millimetre observations of normal, main-sequence galaxies in the early Universe.

Additional Information

© 2020 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/open_access/funder_policies/chorus/standard_publication_model). Received: 12 May 2020; Revision received: 18 July 2020; Accepted: 09 August 2020; Published: 22 August 2020. This paper makes use of the following ALMA data: ADS/JAO.ALMA#2018.1.00348.S, ADS/JAO.ALMA#2017.1.00428.L, ADS/JAO.ALMA#2015.1.00388.S, ADS/JAO.ALMA#2015.1.00928.S, ADS/JAO.ALMA#2012.1.00523.S. ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada), MOST and ASIAA (Taiwan), and KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO and NAOJ. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. This work was supported by the Swiss National Science Foundation through the SNSF Professorship grant 157567 'Galaxy Build-up at Cosmic Dawn'. DAR acknowledges support from the National Science Foundation under grant numbers AST-1614213 and AST-1910107. DAR also acknowledges support from the Alexander von Humboldt Foundation through a Humboldt Research Fellowship for Experienced Researchers. Finally, we thank the referee for the very useful comments that improved this manuscript. Data Availability: The data underlying this paper were accessed from the ALMA science archive (http://almascience.nrao.edu/aq/). The program identifiers are ADS/JAO.ALMA#2018.1.00348.S, ADS/JAO.ALMA#2017.1.00428.L, ADS/JAO.ALMA#2015.1.00388.S, ADS/JAO.ALMA#2015.1.00928.S, and ADS/JAO.ALMA#2012.1.00523.S. The individual and stacked best-fitting IR SEDs (Fig. 3) are available in the online version or on request.

Attached Files

Published - staa2545.pdf

Submitted - 2005.07716.pdf

Supplemental Material - staa2545_supplemental_file.zip


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August 20, 2023
October 20, 2023