利用 21 公分功率譜約束宇宙再游離過程

Abstract

再游離時期是一個宇宙相變的過程,當第一批恆星形成時,會釋放 出紫外光去游離中性氫原子。到目前為止,這個過程的觀測證據仍然 非常有限。由於宇宙再游離牽涉許多複雜的物理機制,現今的理論模 型依然有相當大的不確定性。在這個論文中,我們使用一個公開的模 擬套件叫 21cmFAST (Mesinger el al. 2011)。利用這個套件,我們在短 時間內模擬出一個大的宇宙體積,並且探索在宇宙再游離這個過程中, 影響 21 公分功率譜的關鍵參數。為了量化這些關鍵天文物理參數的 衝擊,我們建構出一個費雪訊息矩陣 (Fisher Information Matrix),並且 假設實驗設計去預測觀測結果,和檢視這個實驗假設,在宇宙再游離 過程會有多強的約束。這些實驗設計為 Hydrogen Epoch of Reionization Array (HERA) 和 Square Kilometer Array (SKA1-low),是正在計劃中的 下一代觀測儀器。我們量化這些關鍵參數隨著紅移的變化,最後發現 恆星形成效率,在高紅移 21 ≲ z ≲ 26 時對 21 公分功率譜有顯著的影 響,而在宇宙再游離快結束時則變小。在單位重子釋放的紫外光數目 對功率譜的影響方面,宇宙再游離發生前影響最大,並在紅移 z ∼ 24 有峰值;當來曼 α 耦合機制達到飽和時,紫外光對功率譜的影響則快 速下降。由於X射線有較長的平均自由徑,可以加熱較遠的低密度星 際物質,其加熱機制的影響,衝擊值在紅移 z ∼ 21 時達到高峰。我們 還找到了紫外光的逸散分數,為光子從它們的宿主星系逃逸到星際物 質的比值,此衝擊 21 公分功率譜則在宇宙再游離最後階段達到峰值。

Up to date there has been little observational constraints on the cosmic reionization process, the phase transition of the universe caused by the for- mation of the first luminous objects. Current theoretical modelling of reion- ization is subject to large uncertainties due to the complex astrophysical pro- cesses. In this thesis, we use a publicly available semi-numerical package, 21cmFAST (Mesinger et al. 2011), to simulate a large comoving cosmic vol- ume and explore key astrophysical parameters that impact the neutral hydro- gen 21-cm power spectra during reionization. We construct the Fisher infor- mation Matrix to quantify the impact of these astrophysical parameters, and make forecasts on how well we can constrain the reionization process given an experimental design, such as the Hydrogen Epoch of Reionization Array (HERA) and the Square Kilometer Array (SKA1-low). We quantify the im- pact of model parameters as a function of redshifts. We find the star formation efficiency affects more on power spectra at high redshift 21 ≲ z ≲ 26 and less at late stage of reionization. The number of UV photon per baryon influences more before the reionization starts with a peak at redshift z ∼ 24, and drops rapidly after Lyα coupling saturates. The X-ray heating mechanism peaks at redshift z ∼ 21, and provides the most heating on the low density Intergalac- tic Medium (IGM) due to the long mean free path. We also find the fraction of the UV photons escape from their host galaxies into the IGM impact the 21-cm power spectra at the final stage of the reionization process.