The Evolution of the Baryons Associated with Galaxies Averaged over Cosmic Time and Space

Fabian Walter,Rachel S Somerville,Marcel Neeleman,Pascal Oesch,Elisabete Da Cunha,Hanae Inami,L Y Aaron Yung,Gergö Popping,Roberto Decarli,Leindert Boogaard,Jeff Wagg,Ian Smail,Rychard Bouwens,Danail Obreschkow,Emanuele Daddi,Dominik A Riechers ,F Bertoldi ,R J Ivison ,P Cox ,Joseph F Hennawi ,Hans─Walter Rix ,M Sargent ,T Díaz-Santos ,P Van Der Werf ,Paulo C Cortés ,M Aravena ,J A Hodge ,F E Bauer ,Roberto J Assef ,A Weiß ,T Contini ,C L Carilli ,B Magnelli ,Jorge González-López

doi:10.3847/1538-4357/abb82e

Abstract

We combine the recent determination of the evolution of the cosmic density of molecular gas (H_2) using deep, volumetric surveys, with previous estimates of the cosmic density of stellar mass, star formation rate and atomic gas (HI), to constrain the evolution of baryons associated with galaxies averaged over cosmic time and space. The cosmic HI and H_2 densities are roughly equal at z~1.5. The H_2 density then decreases by a factor 6^{+3}_{-2} to today's value, whereas the HI density stays approximately constant. The stellar mass density is increasing continuously with time and surpasses that of the total gas density (HI and H_2) at redshift z~1.5. The growth in stellar mass cannot be accounted for by the decrease in cosmic H_2 density, necessitating significant accretion of additional gas onto galaxies. With the new H_2 constraints, we postulate and put observational constraints on a two step gas accretion process: (i) a net infall of ionized gas from the intergalactic/circumgalactic medium to refuel the extended HI reservoirs, and (ii) a net inflow of HI and subsequent conversion to H_2 in the galaxy centers. Both the infall and inflow rate densities have decreased by almost an order of magnitude since z~2. Assuming that the current trends continue, the cosmic molecular gas density will further decrease by about a factor of two over the next 5 Gyr, the stellar mass will increase by approximately 10%, and cosmic star formation activity will decline steadily toward zero, as the gas infall and accretion shut down.

Highlights

The principal goal in galaxy evolution studies is to understand how the cosmic structure and galaxies that we see today emerged from the initial conditions imprinted on the Cosmic Microwave Background (CMB)
We have used measurements of the cosmic molecular gas density to put new constraints on the baryon cycle and the gas accretion process for gas that is gravitationally bound to galaxies
We find that the cosmic H2 density is less than or equal to the cosmic H I density over all times, briefly approaching equality at z ∼ 1.5

Summary

Introduction

The principal goal in galaxy evolution studies is to understand how the cosmic structure and galaxies that we see today emerged from the initial conditions imprinted on the Cosmic Microwave Background (CMB). The winds, UV photons and supernovae from the ensuing star formation, along with possible episodic accretion onto the supermassive black hole at the center (active galactic nuclei), provide effective ‘feedback’ to the surrounding gas. This may – at least temporarily – suppress the formation of further stars, or may even expel the cold gas from the centers of the potential wells (e.g., Dekel & Silk 1986; Silk & Rees 1998; Croton et al 2006; Somerville et al 2008). Of particular interest in this baryon cycle is the question: how much gas was present both within and around galaxies to explain the formation of stars in galaxies through cosmic times?

Methods

Findings

Discussion

Conclusion