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Peer Reviewed

Deciphering the Thermal and Ionization State of the Intergalactic Medium over the Past 10 Billion Years

Hu, Teng
Advisor(s): Hennawi, Joseph

UC Santa Barbara Electronic Theses and Dissertations (2024)

One of the great successes of modern cosmology is the percent-level concordance between theory and observations of the intergalactic medium (IGM) at z ≳ 1.7. Yet, the Lyα forest at z < 1.7, which can only be studied via HST UV spectra, has pointed out a puzzling discrepancy, i.e., the Doppler b-parameters of these absorption lines are, on average, ∼ 10 km/s wider than those in any existing hydrodynamic simulation. This discrepancy implies that the low-z IGM might be substantially hotter than expected, contradicting one of the fundamental predictions in current cosmology that the IGMshould cool down owing to the Hubble expansion after He ii reionization ( z < 2.5). Moreover, the IGM thermal state degenerates with its ionization state characterized by the UV background (UVB) photoionization rate, ΓHI , which dictates the abundance of the Lyα absorbers, dN /dz. Such a degeneracy requires any reliable measurement to adopt a careful statistical inference procedure. To overcome these difficulties, in this thesis, a novel machine-learning-based inference framework is employed to jointly measure the thermal and ionization state of the IGM, using the 2D distribution of b-parameter and H i column density and dN /dz. This method effectively resolves the degeneracies between the thermal and ionization state of the IGM and achieves high precision, even with limited-sized data. I apply this method to 94 archival HST COS and STIS quasar spectra distributed across the seven redshift bins, yielding a comprehensive evolutionary history of the IGM thermal and ionization state at z < 1.5. The results suggest that the IGM may be significantly hotter than previously expected at low-z and is potentially isothermal, with IGM temperature at mean density, T0 ∼ 30, 000K and power-law index of the temperature-density, γ ∼ 1.0 at z = 0.1. The inferred thermal history suggests that this unexpected IGM temperature possibly emerges around z ∼ 1. Additionally, while the ΓHI measurements align with the theoretical model at z ∼ 1, the values measured at z < 0.5 are substantially lower than predicted, posing challenges to low-z UV background synthesis models.

Cover page: Deciphering the Thermal and Ionization State of the Intergalactic Medium over the Past 10 Billion Years

Article
Peer Reviewed

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

LBL Publications (2024)

ABSTRACT: We perform the first measurement of the thermal and ionization state of the intergalactic medium (IGM) across $0.9 \lt z \lt 1.5$ using 301 Ly $\,\alpha$ 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 ($b{-}N_{{\rm {H\,{\small I}}}{}}$) distributions obtained from Ly$\,\alpha$ forest decomposition. Our results show that the H i photoionization rates, $\Gamma _{{\rm {H\,{\small I}}}{}}$, agree with recent ultraviolet background synthesis models, with $\log (\Gamma _{{\rm{H\,{\small I}}}}/\text{s}^{-1})={-11.79}^{+0.18}_{-0.15}$, ${-11.98}^{+0.09}_{-0.09}$, 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, $T_0$, and the adiabatic index, $\gamma$, as $[\log (T_0/\text{K}), \gamma ]=$ [${4.13}^{+0.12}_{-0.10}$, ${1.34}^{+0.10}_{-0.15}$], $[{3.79}^{+0.11}_{-0.11}$, ${1.70}^{+0.09}_{-0.09}]$, and $[{4.12}^{+0.15}_{-0.25}$, ${1.34}^{+0.21}_{-0.26}]$ at $z=1.4$, 1.2, and 1. Our measurements of $T_0$ at $z=1.4$ and 1.2 are consistent with the trend predicted from previous $z\lt 3$ temperature measurements and theoretical expectations, where the IGM cools down after He ii reionization in the absence of any non-standard heating. However, our $T_0$ measurement at $z=1$ shows unexpectedly high IGM temperature. Given the relatively large uncertainty in these measurements, where $\sigma _{T_0} \sim 5000$ 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 $\Delta z \sim 2$ for each redshift bin, three times the current data set, we can constrain the $T_0$ of IGM within 1500 K, which would be sufficient to constrain the IGM thermal history at $z \lt 1.5$ conclusively.

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

Creative Commons 'BY' version 4.0 license

Article
Peer Reviewed

The impact of the WHIM on the IGM thermal state determined from the low-z Lyman α forest

UC Santa Barbara Previously Published Works (2023)

ABSTRACT: At z ≲ 1, shock heating caused by large-scale velocity flows and possibly violent feedback from galaxy formation, converts a significant fraction of the cool gas (T ∼ 104 K) in the intergalactic medium (IGM) into warm–hot phase (WHIM) with T > 105 K, resulting in a significant deviation from the previously tight power-law IGM temperature–density relationship, $T=T_0 (\rho / {\bar{\rho }})^{\gamma -1}$. This study explores the impact of the WHIM on measurements of the low-z IGM thermal state, [T0, γ], based on the b–$N_{{\rm H\,{\small I}}} $ distribution of the Ly α forest. Exploiting a machine learning-enabled simulation-based inference method trained on Nyx hydrodynamical simulations, we demonstrate that [T0, γ] can still be reliably measured from the b–$N_{{\rm H\,{\small I}}} $ distribution at z = 0.1, notwithstanding the substantial WHIM in the IGM. To investigate the effects of different feedback, we apply this inference methodology to mock spectra derived from the IllustrisTNG and Illustris simulations at z = 0.1. The results suggest that the underlying [T0, γ] of both simulations can be recovered with biases as low as |Δlog (T0/K)| ≲ 0.05 dex, |Δγ| ≲ 0.1, smaller than the precision of a typical measurement. Given the large differences in the volume-weighted WHIM fractions between the three simulations (Illustris 38 per cent, IllustrisTNG 10 per cent, and Nyx 4 per cent), we conclude that the b–$N_{{\rm H\,{\small I}}} $ distribution is not sensitive to the WHIM under realistic conditions. Finally, we investigate the physical properties of the detectable Ly α absorbers, and discover that although their T and Δ distributions remain mostly unaffected by feedback, they are correlated with the photoionization rate used in the simulation.

Cover page: The impact of the WHIM on the IGM thermal state determined from the low-z Lyman α forest

Article
Peer Reviewed

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

UC Santa Barbara Previously Published Works (2022)

ABSTRACT: We present a new approach to measure the power-law temperature density relationship $T=T_0 (\rho/ \bar{\rho })^{\gamma -1}$ and the UV background photoionization rate $\Gamma _{{{{\rm H\, {\small I}}}}{}}$ of the intergalactic medium (IGM) based on the Voigt profile decomposition of the Ly α forest into a set of discrete absorption lines with Doppler parameter b and the neutral hydrogen column density $N_{\rm H\, {\small I}}$. Previous work demonstrated that the shape of the $b-N_{{{{\rm H\, {\small I}}}}{}}$ distribution is sensitive to the IGM thermal parameters T0 and γ, whereas our new inference algorithm also takes into account the normalization of the distribution, i.e. the line-density dN/dz, and we demonstrate that precise constraints can also be obtained on $\Gamma _{{{{\rm H\, {\small I}}}}{}}$. We use density-estimation likelihood-free inference (DELFI) to emulate the dependence of the $b-N_{{{{\rm H\, {\small I}}}}{}}$ distribution on IGM parameters trained on an ensemble of 624 nyx hydrodynamical simulations at z = 0.1, which we combine with a Gaussian process emulator of the normalization. To demonstrate the efficacy of this approach, we generate hundreds of realizations of realistic mock HST/COS data sets, each comprising 34 quasar sightlines, and forward model the noise and resolution to match the real data. We use this large ensemble of mocks to extensively test our inference and empirically demonstrate that our posterior distributions are robust. Our analysis shows that by applying our new approach to existing Ly α forest spectra at z ≃ 0.1, one can measure the thermal and ionization state of the IGM with very high precision ($\sigma _{\log T_0} \sim 0.08$ dex, σγ ∼ 0.06, and $\sigma _{\log \Gamma _{{{{\rm H\, {\small I}}}}{}}} \sim 0.07$ dex).

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

Creative Commons 'BY-NC' version 4.0 license