Inferring the IGM thermal history during reionization with the Lyman α forest power spectrum at redshiftz≃ 5

Abstract

We use cosmological hydrodynamical simulations to assess the feasibility of constraining the thermal history of the intergalactic medium during reionisation with the Lyα forest at z ≃ 5 . The integrated thermal history has a measureable impact on the transmitted flux power spectrum that can be isolated from Doppler broadening at this redshift. We parameterise this using the cumulative energy per proton, u 0 , deposited into a gas parcel at the mean background density, a quantity that is tightly linked with the gas density power spectrum in the simulations. We construct mock observations of the line of sight Lyα forest power spectrum and use a Markov Chain Monte Carlo approach to recover u 0 at redshifts 5 < ∼ z < ∼ 12. A statistical uncertainty of ∼ 20 per cent is expected (at 68 per cent confidence) at z ≃ 5 using high resolution spectra with a total redshift path length of ∆z = 4 and a typical signal-to-noise ratio of S/N = 15 per pixel. Estimates for the expected systematic uncertainties are comparable, such that existing data should enable a measurement of u 0 to within ∼ 30 per cent. This translates to distinguishing between reionisation scenarios with similar instantaneous temperatures at z ≃ 5, but with an energy deposited per proton that differs by 2-3 eV over the redshift interval 5 < ∼ z < ∼ 12. For an initial temperature of T ∼ 10 4 K following reionisation, this corresponds to the difference between early (z re = 12) and late (z re = 7) reionisation in our models.

Probing the thermal state of the intergalactic medium at z &gt; 5 with the transmission spikes in high-resolution Ly α forest spectra

“…In Appendix B we use such rescaling to show how the three flux statistics we consider here depend on thermal parameters T0 and γ and the photo-ionization rate, and demonstrate that it is the width distribution of the transmission spikes which is most sensitive to the thermal state of the gas. We further show that, for the reionization and thermal history models we consider in this work, the spike statistics are only very weakly affected by pressure (Jeans) smoothing at the typical gas densities probed by the Lyα transmission spikes (Gnedin & Hui 1998;Theuns et al 2000;Peeples et al 2010;Kulkarni et al 2015;Lukić et al 2015;Nasir et al 2016;Maitra et al 2019;Wu et al 2019). We use the spike width distribution to obtain an estimate of the best fit values and uncertainty for the thermal parameters 8 .…”

The Sherwood simulation suite: overview and data comparisons with the Lyman α forest at redshifts 2 ≤z≤ 5

“…9), a later reionization at z r = 9 results in less pressure smoothing and hence more power on scales log(k/s km −1 ) > −1.7. This indicates that it should be possible to constrain the integrated thermal history during reionization using the line-of-sight Ly α forest power spectrum at high redshift (Nasir, Bolton & Becker 2016). However, the magnitude of the effect at z 4 lies within the 1σ bootstrapped uncertainty for the McDonald et al (2000) absorption path length.…”

“…It produces more pixels with F > 0.6 in the PDF, and increases the power spectrum at scales log(k/s km −1 ) > −1.5. This indicates that it should be possible to constrain the integrated thermal history during reionization using the line-of-sight Ly α forest power spectrum at high redshift (Nasir et al 2016).…”

Observing the tail of reionization: neutral islands in the z = 5.5 Lyman-α forest

“…Walther et al (2019) parameterized the smoothing with a length scale (termed the pressure smoothing scale) that characterizes these effects in the 1D flux power spectrum. Alternatively, Boera et al (2019) casted these effects into the cumulative heat per proton injected into gas at the mean density, u 0 (Nasir et al 2016). Our mock Lyα forest spectra cannot reliably model the small-scale flux power spectrum because our hydro simulation does not have sufficient resolution.…”