Prediction of the Cosmic Evolution of the CO-Luminosity Functions

TL;DR

This paper predicts the evolution of CO emission line luminosity functions across cosmic time using advanced simulations, highlighting challenges in detecting high-redshift galaxies due to CMB effects.

Contribution

It introduces a comprehensive model combining galaxy simulations with CO emission physics to predict CO luminosity functions from z=0 to z=10.

Findings

Detection of regular galaxies at z>7 will be hindered by CMB contrast.

CMB heating does not facilitate high-redshift CO detection as previously thought.

The model provides accessible predictions for future telescope surveys.

Abstract

We predict the emission line luminosity functions (LFs) of the first 10 rotational transitions of CO in galaxies at redshift z=0 to z=10. This prediction relies on a recently presented simulation of the molecular cold gas content in ~3e7 evolving galaxies based on the Millennium Simulation. We combine this simulation with a model for the conversion between molecular mass and CO-line intensities, which incorporates the following mechanisms: (i) molecular gas is heated by the CMB, starbursts (SBs), and active galactic nuclei (AGNs); (ii) molecular clouds in dense or inclined galaxies can overlap; (iii) compact gas can attain a smooth distribution in the densest part of disks; (iv) CO-luminosities scale with metallicity changes between galaxies; (v) CO-luminosities are always detected against the CMB. We analyze the relative importance of these effects and predict the cosmic evolution of the CO-LFs. The most notable conclusion is that the detection of regular galaxies (i.e. no AGN, no massive SB) at high z>7 in CO-emission will be dramatically hindered by the weak contrast against the CMB, in contradiction to earlier claims that CMB-heating will ease the detection of high-redshift CO. The full simulation of extragalactic CO-lines and the predicted CO-LFs at any redshift can be accessed online, prior registration required} and they should be useful for the modeling of CO-line surveys with future telescopes, such as ALMA, the LMT, or the SKA.