Cosmic evolution of the H2 mass density and the epoch of molecular gas (2103.08613v1)

Abstract: We present new empirical constraints on the evolution of $\rho_{\rm H_2}$, the cosmological mass density of molecular hydrogen, back to $z\approx2.5$. We employ a statistical approach measuring the average observed $850\mu{\rm m}$ flux density of near-infrared selected galaxies as a function of redshift. The redshift range considered corresponds to a span where the $850\mu{\rm m}$ 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 (ISM). Our sample comprises of ${\approx}150,000$ galaxies in the UKIDSS-UDS field with near-infrared magnitudes $K_{\rm AB}\leq25$ 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 $z\approx2.5$. 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 $\rho_{\rm H_2}\approx2\times10^7\,{\rm M_\odot}\,{\rm Mpc^{-3}}$ at $z\approx2$. We demonstrate that $\rho_{\rm H_2}$ can be broadly modelled by inverting the star-formation rate 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 $z\approx2$ is a larger supply of molecular gas in galaxies rather than a significant evolution of the star-formation rate efficiency within individual galaxies.

• The paper finds that the peak epoch of molecular gas density at z≈2 aligns with the star formation zenith, challenging models of variable star formation efficiency.
• The study utilizes 850μm flux measurements from a sample of 150,000 galaxies with deep near-infrared photometry to derive precise photometric redshifts.
• Additional analyses using halo mass functions and stellar-halo mass ratios cross-validate CO survey findings, strengthening empirical constraints on galaxy evolution models.

Analysis of Cosmic Evolution of Molecular Hydrogen Mass Density

The paper by Garratt et al. provides an in-depth empirical paper of the cosmic evolution of the molecular hydrogen mass density, $\rho_{\rm H_2}$, over a considerable redshift range, extending back to $z \approx 2.5$. The authors utilize a comprehensive statistical method involving the analysis of the average observed $850\,\mu$m flux density from a substantial sample of approximately $150,000$ galaxies located in the UKIDSS-UDS field. This dataset, which is significantly larger than those used in prior studies, enhances the statistical rigor of their findings.

Central to their methodology is the use of long-wavelength dust continuum emissions in the submillimeter part of the spectrum. At $850\,\mu$m, these emissions probe the Rayleigh-Jeans tail of thermal dust emission. This metric is crucial as it can serve as an indirect, yet effective, estimator of the interstellar medium (ISM) mass. Facilitated by near-infrared selection criteria, the research leverages the deep $12$-band photometry of the UKIDSS-UDS catalog to derive photometric redshifts with precise probability distribution functions.

The paper finds that the peak epoch of molecular gas density aligns closely with the era of peak star formation, reporting $\rho_{\rm H_2}\approx2\times10^7\,{\rm M_\odot}\,{\rm Mpc^{-3}$ at $z\approx2$. This alignment with the peak star formation epoch strongly suggests that higher volumes of molecular gas rather than increased star formation efficiency is the primary driver of this historical peak in star formation. This assertion challenges models which have suggested that star formation efficiency need not remain constant over cosmic timescales. Particularly, the authors demonstrate that a constant star formation efficiency model can reasonably replicate the observed evolution of $\rho_{\rm H_2}$ by merely inverting the known star formation rate density.

Further reinforcing their claims, the paper elucidates several auxiliary approaches, including modeling from the halo mass function and using stellar-halo mass ratios to independently derive $\rho_{\rm H_2}$. This multi-pronged approach provides robust cross-validation of their outcomes and adheres to the empirical data from CO line surveys, offering reasonable alignment albeit variances with some high-excitation CO line derivations.

The implications of this work are multifold:

1. Empirical Constraints: The substantial reduction in statistical uncertainties at redshifts out to $z\approx2.5$ offers vastly improved empirical constraints on models of galactic evolution and star formation histories.
2. Formation Pathways and Mechanisms: By suggesting that the peak in star formation does not necessitate increased efficiencies but rather abundant gas, this work propels further exploration into the formation pathways of H$_2$ under varying cosmological conditions.
3. Theoretical Models: The findings stimulate refinement in theoretical models predicting galaxy formation and evolution, particularly in terms of molecular gas accretion and consumption dynamics over cosmic epochs.

The paper speculates on the future of this line of research, emphasizing the potential of wide-field surveys at $1$\,mm/$3$\,mm wavelengths to probe further back in cosmic history, which would extend these empirical constraints to even higher redshifts.

Overall, Garratt et al.'s paper is a polished and detailed contribution to our understanding of cosmic molecular gas density evolution, bolstering our comprehension of how star formation processes have unfolded across the history of the universe. The methodologies and analyses presented serve as a foundational reference for both contemporary and future astrophysical research in galaxy evolution.