Evolving beyond z=0: insights about the future of stars and the intergalactic medium

Abstract

ABSTRACT We present results from seven cosmological simulations that have been extended beyond the present era as far as redshift z ≈ −0.99 or $t\approx 96.0\, {\rm Gyr}$, using the Enzo simulation code. We adopt the calibrated star formation and feedback prescriptions from our previous work on reproducing the Milky Way with Enzo, with modifications to the simulation code, chemistry, and cooling library. We then consider the future behaviour of the halo mass function (HMF), the equation of state (EOS) of the IGM, and the cosmic star formation history (SFH). Consistent with previous work, we find a freeze out in the HMF at z ≈ −0.6 or $t\approx 28.1\, {\rm Gyr}$. The evolution of the EOS of the IGM presents an interesting case study of the cosmological coincidence problem, where there is a sharp decline in the IGM temperature immediately after z = 0. For the SFH, the simulations produce a peak and a subsequent decline into the future. However, we do find a turnaround in the SFH after z ≈ −0.98 or $t\approx 82.4\, {\rm Gyr}$ in some simulations, which we attribute to limitations of the criteria used for star formation. By integrating the SFH in time up to z ≈ −0.92 or $t\approx 55.1\, {\rm Gyr}$, the simulation with the best spatial resolution predicts an asymptotic total stellar mass that is very close to that obtained from extrapolating the fit of the observed SFR. Lastly, we investigate the future evolution of the partition of baryons within a Milky Way-sized galaxy, using both a zoom and a box simulation. Despite vastly different resolutions, these simulations predict individual haloes containing an equal fraction of baryons in stars and gas at the time of freeze out.