Limits on non-canonical heating and turbulence in the intergalactic medium from the low redshift Lyman-alpha forest

Abstract

We examine the column density distribution function (CDDF) and Doppler parameter distribution from hydrodynamical simulations and Cosmic Origins Spectrograph (COS) observations of the Lyα forest at redshift 0 ≤ z ≤ 0.2. Allowing for a factor of two uncertainty in the metagalactic H I photoionisation rate, our hydrodynamical simulations are in good agreement (1-1.5σ ) with the shape and amplitude of the observed CDDF at H I column densities 10 13.3 cm −2 ≤ N HI ≤ 10 14.5 cm −2 . However, the Doppler widths of the simulated lines remain too narrow with respect to the COS data. We argue that invoking AGN feedback does not resolve this discrepancy. We also disfavour enhanced photoheating rates as a potential solution, as this requires an unphysically hard UV background spectrum. If instead appealing to a non-canonical source of heating, an additional specific heat injection of u 6.9 eV m −1 p is required at z 2.5 for gas that has N HI 10 13.5 cm −2 by z = 0.1. Alternatively, there may be an unresolved line of sight turbulent velocity component of v turb 8.5 km s −1 (N HI /10 13.5 cm −2 ) 0.21 for the coldest gas in the diffuse IGM.

Measuring the thermal and ionization state of the low-z IGM using likelihood free inference

“…We explain these effects qualitatively in section § 3.3 and show that they are consistent with previous work. Moreover, previous work (Viel et al 2017;Gaikwad et al 2017;Nasir et al 2017) reported a discrepancy in the 1D marginalized 𝑏 distribution for low redshift IGM between the observation and current simulations, implying the existence of additional heating or turbulence that is stronger than expected (Bolton et al 2021). While these works mainly focus on 1D marginalized distributions of 𝑏 and column density distribution function (CDDF), our new inference algorithm, which successfully emulate both 1D marginalized and 2D joint 𝑏-𝑁 H distribution, would allow us to investigate such problem using the joint distribution together with d𝑁/d𝑧 statistics.…”

“…have larger 𝑏 parameter) than numerical model predictions (Gaikwad et al 2017;Viel et al 2017;Nasir et al 2017). While it has been speculated that such a discrepancy might be resolved by an additional source of turbulence (Bolton et al 2021), an alternative explanation would be that there are additional sources of heating, and the IGM is actually hotter than expected, with 𝑇 0 conceivably approaching 10000K.…”