Measurements of the thermal and ionization state of the intergalactic medium during the cosmic afternoon

Journal Article · 04 November 2024 · Monthly Notices of the Royal Astronomical Society

DOI: https://doi.org/10.1093/mnras/stae2474 · OSTI ID:2497299

Hu, Teng  ^[1]; Khaire, Vikram  ^[2]; Hennawi, Joseph F.  ^[3]; Tripp, Todd M.  ^[4]; Oñorbe, Jose  ^[5]; Walther, Michael  ^[6]; Lukić, Zarija ^[7]

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1. Univ. of California, Santa Barbara, CA (United States); Aix-Marseille Univ., Marseille (France); Centre National de la Recherche Scientifique (CNRS), Marseille (France); Centre National D’Etudes Spatiales (CNES), Marseille (France)
2. Univ. of California, Santa Barbara, CA (United States); Indian Institute of Space Science & Technology, Thiruvananthapuram (India); Indian Inst. of Technology (IIT), Tirupati (India)
3. Univ. of California, Santa Barbara, CA (United States); Leiden Univ. (Netherlands)
4. Univ. of Massachusetts, Amherst, MA (United States)
5. Universidad de Sevilla (Spain)
6. Ludwig Maximilian Univ. of Munich (Germany); Excellence Cluster ORIGINS, Garching (Germany)
7. Lawrence Berkeley National Laboratory (LBNL), Berkeley, CA (United States)

We perform the first measurement of the thermal and ionization state of the intergalactic medium (IGM) across 0.9 < z < 1.5 using 301 Ly α absorption lines fitted from 12 archival Hubble Space Telescope Space Telescope Imaging Spectrograph quasar spectra. We employ the machine-learning-based inference method that uses joint Doppler parameter–column density (⁠b-N_HI⁠) distributions obtained from Ly α forest decomposition. Our results show that the Γ_HI photoionization rates, ⁠, agree with recent ultraviolet background synthesis models, with log(Γ_HI/s^-1) = $$-11.79^{+0.18}_{-0.15}$$, $$-11.98^{+0.09}_{-0.09}$$⁠, and $$-12.32^{+0.10}_{-0.12}$$⁠, at z = 1.4, 1.2, and 1, respectively. We obtain the IGM temperature at the mean density, T₀⁠, and the adiabatic index, γ⁠, as [log(T₀/K), γ] = $$[4.13^{+0.12}_{-0.10}, 1.34^{+0.10}_{-0.15}]$$, $$[3.79^{+0.11}_{-0.11}, 1.70^{+0.09}_{-0.09}]$$, and $$[4.12^{+0.15}_{-0.25}, 1.34^{+0.21}_{-0.26}]$$ at z = 1.4⁠, 1.2, and 1. Our measurements of T₀ at z = 1.4 and 1.2 are consistent with the trend predicted from previous z < 3 temperature measurements and theoretical expectations, where the IGM cools down after $$He\tiny{II}$$ reionization in the absence of any non-standard heating. However, our T₀ measurement at z = 1 unexpectedly high IGM temperature. Given the relatively large uncertainty in these measurements, where σ_{T$$_0$$} ~ 5000 K, mostly emanating from the limited size of our data set, we cannot conclude whether the IGM cools down as expected. Lastly, we generate mock data sets to test the constraining power of future measurement with larger data sets. The results demonstrate that, with redshift path-length Δz ~ 2 for each redshift bin, three times the current data set, we can constrain the T₀ of IGM within 1500 K, which would be sufficient to constrain the IGM thermal history at z < 1.5 conclusively.

• Research Organization: Lawrence Berkeley National Laboratory (LBNL), Berkeley, CA (United States)
• Sponsoring Organization: USDOE Office of Science (SC), Basic Energy Sciences (BES). Scientific User Facilities (SUF); USDOE Office of Science (SC), High Energy Physics (HEP)
• Grant/Contract Number: AC02-05CH11231
• OSTI ID: 2497299
• Journal Information: Monthly Notices of the Royal Astronomical Society, Journal Name: Monthly Notices of the Royal Astronomical Society Journal Issue: 1 Vol. 536; ISSN 0035-8711
• Publisher: Oxford University PressCopyright Statement
• Country of Publication: United States
• Language: English