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ALMACAL VI: Molecular gas mass density across cosmic time via a blind search for intervening molecular absorbers

Klitsch, Anne; Péroux, Céline; Zwaan, Martin A; Smail, Ian; Nelson, Dylan; Popping, Gergö; Chen, Chian-Chou; Diemer, Benedikt; Ivison, RJ; Allison, James R; Muller, Sébastien; Swinbank, A Mark; Hamanowicz, Aleksandra; Biggs, Andrew D; Dutta, Rajeshwari

ALMACAL VI: Molecular gas mass density across cosmic time via a blind search for intervening molecular absorbers Thumbnail


Anne Klitsch

Céline Péroux

Martin A Zwaan

Dylan Nelson

Gergö Popping

Chian-Chou Chen

Benedikt Diemer

RJ Ivison

James R Allison

Sébastien Muller

Aleksandra Hamanowicz

Andrew D Biggs

Rajeshwari Dutta


We are just starting to understand the physical processes driving the dramatic change in cosmic star-formation rate between z ∼ 2 and the present day. A quantity directly linked to star formation is the molecular gas density, which should be measured through independent methods to explore variations due to cosmic variance and systematic uncertainties. We use intervening CO absorption lines in the spectra of mm-bright background sources to provide a census of the molecular gas mass density of the Universe. The data used in this work are taken from ALMACAL, a wide and deep survey utilizing the ALMA calibrator archive. While we report multiple Galactic absorption lines and one intrinsic absorber, no extragalactic intervening molecular absorbers are detected. However, thanks to the large redshift path surveyed (Δz = 182), we provide constraints on the molecular column density distribution function beyond z ∼ 0. In addition, we probe column densities of N(H2) > 1016 atoms cm−2, five orders of magnitude lower than in previous studies. We use the cosmological hydrodynamical simulation IllustrisTNG to show that our upper limits of ρ(H2) ≲ 108.3M⊙Mpc−3 at 0 < z ≤ 1.7 already provide new constraints on current theoretical predictions of the cold molecular phase of the gas. These results are in agreement with recent CO emission-line surveys and are complementary to those studies. The combined constraints indicate that the present decrease of the cosmic star-formation rate history is consistent with an increasing depletion of molecular gas in galaxies compared to z ∼ 2.

Journal Article Type Article
Acceptance Date Sep 17, 2019
Online Publication Date Sep 21, 2019
Publication Date Nov 1, 2019
Deposit Date Oct 4, 2019
Publicly Available Date Oct 4, 2019
Journal Monthly Notices of the Royal Astronomical Society
Print ISSN 0035-8711
Electronic ISSN 1365-2966
Publisher Royal Astronomical Society
Peer Reviewed Peer Reviewed
Volume 490
Issue 1
Pages 1220-1230
Public URL


Accepted Journal Article (1.3 Mb)

Copyright Statement
This article has been accepted for publication in the Monthly notices of the Royal Astronomical Society ©: 2019 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.

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