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Cosmic Evolution of the H2 Mass Density and the Epoch of Molecular Gas

Garratt, T. K.; Coppin, K. E. K.; Geach, J. E.; Almaini, O.; Hartley, W. G.; Maltby, D. T.; Simpson, C. J.; Wilkinson, A.; Conselice, C. J.; Franco, M.; Ivison, R. J.; Koprowski, M. P.; Lovell, C. C.; Pope, A.; Scott, D.; van der Werf, P.

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T. K. Garratt

K. E. K. Coppin

J. E. Geach

Professor of Astrophysics

W. G. Hartley

C. J. Simpson

A. Wilkinson

C. J. Conselice

M. Franco

R. J. Ivison

M. P. Koprowski

C. C. Lovell

A. Pope

D. Scott

P. van der Werf


We present new empirical constraints on the evolution of ${\rho }_{{{\rm{H}}}_{2}}$, the cosmological mass density of molecular hydrogen, back to z ≈ 2.5. We employ a statistical approach measuring the average observed 850 μm flux density of near-infrared selected galaxies as a function of redshift. The redshift range considered corresponds to a span where the 850 μm band probes the Rayleigh–Jeans tail of thermal dust emission in the rest frame, and can therefore be used as an estimate of the mass of the interstellar medium. Our sample comprises of ≈150,000 galaxies in the UK InfraRed Telescope Infrared Deep Sky Survey Ultra-Deep Survey field with near-infrared magnitudes KAB ≤ 25 mag and photometric redshifts with corresponding probability distribution functions derived from deep 12-band photometry. With a sample approximately 2 orders of magnitude larger than in previous works we significantly reduce statistical uncertainties on ${\rho }_{{{\rm{H}}}_{2}}$ to z ≈ 2.5. Our measurements are in broad agreement with recent direct estimates from blank field molecular gas surveys, finding that the epoch of molecular gas coincides with the peak epoch of star formation with ${\rho }_{{{\rm{H}}}_{2}}\approx 2\times {10}^{7}\,{M}_{\odot }\,{\mathrm{Mpc}}^{-3}$ at z ≈ 2. We demonstrate that ${\rho }_{{{\rm{H}}}_{2}}$ can be broadly modeled by inverting the star formation rate (SFR) density with a fixed or weakly evolving star formation efficiency. This "constant efficiency" model shows a similar evolution to our statistically derived ${\rho }_{{{\rm{H}}}_{2}}$, indicating that the dominant factor driving the peak star formation history at z ≈ 2 is a larger supply of molecular gas in galaxies rather than a significant evolution of the SFR efficiency within individual galaxies.


Garratt, T. K., Coppin, K. E. K., Geach, J. E., Almaini, O., Hartley, W. G., Maltby, D. T., …van der Werf, P. (2021). Cosmic Evolution of the H2 Mass Density and the Epoch of Molecular Gas. Astrophysical Journal, 912(1), Article 62.

Journal Article Type Article
Acceptance Date Mar 6, 2021
Online Publication Date May 5, 2021
Publication Date May 5, 2021
Deposit Date Jun 7, 2021
Publicly Available Date May 6, 2022
Journal Astrophysical Journal
Print ISSN 0004-637X
Electronic ISSN 1538-4357
Publisher American Astronomical Society
Peer Reviewed Peer Reviewed
Volume 912
Issue 1
Article Number 62
Keywords Space and Planetary Science; Astronomy and Astrophysics
Public URL
Publisher URL
Additional Information Article Title: Cosmic Evolution of the H2 Mass Density and the Epoch of Molecular Gas; Journal Title: The Astrophysical Journal; Article Type: paper; Copyright Information: © 2021. The American Astronomical Society. All rights reserved.; Date Received: 2020-01-16; Date Accepted: 2021-03-06; Online publication date: 2021-05-05


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