New Constraints on IGM Thermal Evolution from the Ly α Forest Power Spectrum

Journal Article · 06 February 2019 · The Astrophysical Journal (Online)

DOI: https://doi.org/10.3847/1538-4357/aafad1 · OSTI ID:1544084

Walther, Michael  ^[1]; Oñorbe, Jose  ^[2]; Hennawi, Joseph F.  ^[3]; Lukić, Zarija ^[4]

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1. Univ. of California, Santa Barbara, CA (United States); Max Planck Inst. for Astronomie, Heidelberg (Germany); Heidelberg Univ. (Germany)
2. Max Planck Inst. for Astronomie, Heidelberg (Germany); Univ. of Edinburgh, Scotland (United Kingdom). The Royal Observatory
3. Univ. of California, Santa Barbara, CA (United States); Max Planck Inst. for Astronomie, Heidelberg (Germany)
4. Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)

We observe the thermal evolution of the intergalactic medium (IGM) over 3 Gyr of cosmic time $$1.8\lt z\lt 5.4$$ by comparing measurements of the Lyα forest power spectrum to a suite of ~70 hydrodynamical simulations. We conduct Bayesian inference of IGM thermal parameters using an end-to-end forward modeling framework whereby mock spectra generated from our simulation grid are used to build a custom emulator that interpolates the power spectrum between thermal grid points. The temperature at mean density T ₀ rises steadily from $${T}_{0}\sim 6000\,{\rm{K}}$$ at z = 5.4, peaks at 14,000 K for z ~ 3.4, and decreases at lower redshift, reaching T ₀ ~ 7000 K by z ~ 1.8. This evolution provides conclusive evidence for photoionization heating resulting from the reionization of $$\mathrm{He}\,{\rm{II}}$$, as well as the subsequent cooling of the IGM due to the expansion of the universe after all reionization events are complete. Our findings are broadly consistent with previous measurements of thermal evolution based on a variety of approaches, but the sensitivity of the power spectrum, the combination of high-precision measurements of large-scale modes ($$k\lesssim 0.02\,{\rm{s}}\ {\mathrm{km}}^{-1}$$) from the Baryon Oscillation Spectroscopic Survey with our recent determination of the small-scale power, our large grid of models, and our careful statistical analysis enable us to break the well-known degeneracy between the temperature at mean density T ₀ and the slope of the temperature–density relation γ that has plagued previous analyses. At the highest redshifts, z ≥ 5, we infer lower temperatures than expected from the standard picture of IGM thermal evolution leaving little room for additional smoothing of the Lyα forest by free streaming of warm dark matter.

• Research Organization: Lawrence Berkeley National Laboratory (LBNL), Berkeley, CA (United States); Lawrence Berkeley National Laboratory (LBNL), Berkeley, CA (United States). National Energy Research Scientific Computing Center (NERSC); Univ. of California, Oakland, CA (United States)
• Sponsoring Organization: USDOE Office of Science (SC), Advanced Scientific Computing Research (ASCR) (SC-21), Scientific Discovery through Advanced Computing (SciDAC) (SC21.1 ); USDOE Office of Science (SC), Advanced Scientific Computing Research (ASCR) (SC-21). Scientific Discovery through Advanced Computing (SciDAC)
• Grant/Contract Number: AC02-05CH11231
• OSTI ID: 1544084
• Journal Information: The Astrophysical Journal (Online), Journal Name: The Astrophysical Journal (Online) Journal Issue: 1 Vol. 872; ISSN 1538-4357
• Publisher: Institute of Physics (IOP)Copyright Statement
• Country of Publication: United States
• Language: English