The ionizing background determines the ionization balance and the thermodynamics of the cosmic gas. It is therefore a fundamental ingredient to theoretical and empirical studies of both the IGM and galaxy formation. We present here a new calculation of its spectrum that satisfies the empirical constraints we recently obtained by combining state-of-the-art luminosity functions and intergalactic opacity measurements. In our preferred model, star-forming galaxies and quasars each contribute substantially to the HI ionizing field at z<3, with galaxies rapidly overtaking quasars at higher redshifts as quasars become rarer. In addition to our fiducial model, we explore the physical dependences of the calculated background and clarify how recombination emission contributes to the ionization rates. We find that recombinations do not simply boost the ionization rates by the number of reemitted ionizing photons as many of these rapidly redshift below the ionization edges and have a distribution of energies. A simple analytic model that captures the main effects seen in our numerical radiative transfer calculations is given. Finally, we discuss the effects of HeII reionization by quasars on both the spectrum of the ionizing background and on the thermal history of the IGM. In regions that have yet to be reionized, the spectrum is expected to be almost completely suppressed immediately above 54.4 eV while a background of higher-energy (>~0.5 keV) photons permeates the entire universe owing to the frequency-dependence of the photoionization cross section. We provide an analytic model of the heat input during HeII reionization and its effects on the temperature-density relation.
The reionization of the second electron of helium shapes the physical state of intergalactic gas at redshifts between 2 < z < 5. Because performing full in situ radiative transfer in hydrodynamic simulations is computationally expensive for large volumes, the physics of HeII reionization is often approximated by a uniform UV background model that does not capture the patchiness of reionization. We have devised a model that implements the effects of HeII reionization using semi-analytic calculations of the thermal state of intergalactic gas-- a way to bypass a full radiative transfer simulation…
The End of Helium Reionization at z~2.7 Inferred from Cosmic Variance in HST/COS HeII Lyman Alpha Absorption Spectra
本文报道了在两个z~3类星体的HST/COS光谱中探测到的强烈变化的星系际HeII吸收。从我们对这些和三个档案视线中的HeII吸收的均匀分析中，我们发现平均HeII有效光学深度从z~2.3处的τ ~1显著增加到z~3.2处的τ>5，但在2.7< z <3处的2<τ <5的大散射在~10 Mpc的尺度上。这种分散主要是由于HeII部分和HeII电离背景的波动，而不是密度变化，探测的同时代HI森林。HeII吸收的半解析模型需要HeII电离背景的强烈降低来解释在z>2.7时吸收的强烈增加，这可能表明HeII再电离在z>2.7时是不完全的。同样，最近的三维数值模拟的HeII再电离定性地同意观察到的趋势，只有当HeII再电离完成在z=2.7或甚至更低，如我们的五个视线中的两个在z<2.8的大tau>3所示。通过在2.7< z <3时将样本大小加倍，我们新发现的HeII视线首次探测了氦完全电离时第二次再电离时期的多样性。
We present statistics of HeII Lya transmission spikes and large-scale absorption troughs using archival high-resolution ($R=λ/Δλ\simeq 12,500$-$18,000$) far-UV spectra of eight HeII-transparent quasars obtained with the Cosmic Origins Spectrograph on the Hubble Space Telescope. The sample covers the redshift range 2.5<z<3.8, thereby probing the rapidly evolving HeII absorption at the end of the HeII reionization epoch. The measured lengths of the troughs decrease dramatically from L>100cMpc at z>3 to L~30cMpc at z~2.7, signaling a significant progression of HeII reionization at these redshifts…
We present new high-resolution (R~14,000) spectra of the two brightest HeII-transparent quasars in the far-UV (FUV) at z>3.5, HE2QSJ2311-1417 (z=3.70) and HE2QSJ1630+0435 (z=3.81), obtained with the Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope (HST). In the predominantly saturated HeII absorption spectra, both sightlines show several isolated resolved (full width at half maximum FWHM>50 km/s) transmission spikes in HeII Ly$α$ and HeII Ly$β$. The incidence of such spikes decreases with increasing redshift, but both sightlines show significant spikes at z>3.5, signaling the pr…
Measuring photo-ionization rate and mean free path of HeII ionizing photons at $2.5 \leq z \leq 3.6$: Evidence for late and rapid HeII reionization Part-II
The excursion set reionization framework is widely used, due to its speed and accuracy in reproducing the 3D topology of reionization. However, it is known that it does not conserve photon number. Here, we introduce an efficient, on-the-fly recipe to approximately account for photon conservation. Using a flexible galaxy model shown to reproduce current high-$z$ observables, we quantify the bias in the inferred reionization history and galaxy properties resulting from the non-conservation of ionizing photons. Using a mock 21-cm observation, we perform inference with and without correcting for i…
Efficient and accurate simulations of the reionization epoch are crucial to exploring the vast uncharted parameter space that will soon be constrained by measurements of the 21 cm power spectrum. One of these parameters, $R_{\rm max}$, is meant to characterize the absorption of photons by residual neutral gas inside of ionized regions, but has historically been implemented in a very simplistic fashion acting only as a maximum filtering scale. We leverage the correspondence between excursion set methods and the integrated flux from ionizing sources to define two physically-motivated prescriptio…
An emulator for the ionizing photon mean free path in ultra-high resolution simulations: the implications of mean free path measurements for the reionization history
University of Washington（华盛顿大学）；Arizona State University（亚利桑那州立大学）；University of California Riverside（加州大学河滨分校）
The Inhomogeneous Ionizing Background Following Reionization
本文研究了再电离后（z ~ 5- 6）时期氢电离本底的空间涨落。光子平均自由程（m. f. p.）随红移而迅速减小，再加上越来越稀有的电离源的聚集，可能导致在这一时期内非常不均匀的电离背景。我们系统地研究了电离通量的概率密度函数（PDF）和功率谱，通过改变一些参数，如m. f. p.，能够容纳恒星的最小晕质量和晕占空比。为了通用，我们利用分析，半数值和数值方法。我们的模型表明，电离背景确实有相当大的波动，在这一时期来源的聚类源，与PDF是一个因素的几个宽的极大似然的一半。的分布也显示出显着的不对称性，与高值的尾巴设置的小尺度上的聚类，和较短的低值的尾巴，这是设置的平均自由path. The电离背景的功率谱是更敏感的源属性比PDF和可以很好地理解分析与类似的框架晕模型（通常用于描述暗物质聚类）。然而，我们发现，Lya森林光谱是非常不敏感的UVB的细节，尽管我们的各种电离背景的PDF和功率谱显着的差异。假设一个统一的电离背景只会低估从Lya森林推断的平均电离率的值几个百分点。相反，Lya森林的分析主要是由密度场的不确定性。因此，我们的研究结果证明了一个统一的电离背景Lya森林分析，即使在这个时代的共同假设。
The reionization process is expected to be prolonged by the small-scale absorbers (SSAs) of ionizing photons, which have been seen as Lyman-limit systems in quasar absorption line observations. We use a set of semi-numerical simulations to investigate the effects of absorption systems on the reionization process, especially their impacts on the neutral islands during the late Epoch of Reionization (EoR). Three models are studied, i.e. the extreme case of no-SSA model with a high level of ionizing background, the moderate-SSA model with a relatively high level of ionizing background, and the de…
Semi-numeric simulations of helium reionization and the fluctuating radiation background
最近He II Lyman-alpha森林观测从2.0 < z < 3.2 show large fluctuations in the optical depth at z >2.7。这些结果指出，波动的氦电离背景，这可能是由于结束氦再电离的这个时代。我们提出了一个快速的，半数值的过程来近似详细的宇宙学模拟。我们计算的暗物质晕的分布，氦的电离态，和密度场在z = 3与最近的模拟广泛的协议。鉴于我们的速度和灵活性，我们调查了一系列电离源和活跃的类星体处方。跨越一个大面积的参数空间，我们发现订单的数量级波动的He II电离率在后再电离制度。在再电离过程中，波动甚至更强，并开发一个双峰分布，与半解析模型和氢当量。这些分布表明，即使在显着的He II分数的低水平电离背景。
At cosmic recombination, there was supersonic relative motion between baryons and dark matter, which originated from the baryonic acoustic oscillations in the early universe. This motion has been considered to have a negligible impact on the late stage of cosmic reionization because the relative velocity quickly decreases. However, recent studies have suggested that the recombination in gas clouds smaller than the local Jeans mass ($\lesssim$ $10^8~M_\odot$) can affect the reionization history by boosting the number of ultraviolet photons required for ionizing the intergalactic medium. Motivat…
We characterise the thermal state of the intergalactic medium (IGM) in ten redshift bins in the redshift range $2 \leq z \leq 4$ with a sample of 103 high resolution, high S/N Ly$α$ forest spectra using four different flux distribution statistics. Our measurements are calibrated with mock spectra from a large suite of hydrodynamical simulations post-processed with our thermal IGM evolution code CITE, finely sampling amplitude and slope of the expected temperature-density relation. The thermal parameters inferred from our measurements of the flux power spectrum, Doppler parameter distribution, …
The reionization epoch concludes when ionizing photons reach every corner of the Universe. Reionization has generally been assumed to be limited primarily by the rate at which galaxies produce ionizing photons, but the recent measurement of a surprisingly short ionizing photon mean free path of $0.75^{+0.65}_{-0.45}$ proper Mpc at $z = 6$ by Becker et al. (2021) suggests that absorption by residual neutral hydrogen in the otherwise ionized intergalactic medium may play a much larger role than previously expected. Here we show that consistency between this short mean free path and the coeval da…
Recent measurements of the ionizing photon mean free path ($λ_{912}^{\rm mfp}$) at $5 < z < 6$ suggest that the IGM was rapidly evolving at those times. We use radiative transfer simulations to explore the implications for reionization, with a focus on the short value of $λ_{912}^{\rm mfp} = 3.57^{+3.09}_{-2.14}$ cMpc/$h$ at $z=6$. We introduce a model for sub-resolution ionizing photon sinks based on radiative hydrodynamics simulations of small-scale IGM clumping. We argue that the rapid evolution in $λ_{912}^{\rm mfp}$ at $z=5-6$, together with constraints on the metagalactic ionizing backgr…
Becker et al. 2021 measured the mean free path of Lyman limit photons in the IGM at $z=6$. The short value suggests that absorptions may have played a prominent role in reionization. Here we study physical properties of ionizing photon sinks in the wake of ionization fronts (I-fronts) using radiative hydrodynamic simulations. We quantify the contributions of gaseous structures to the Lyman limit opacity by tracking the column density distributions in our simulations. Within $Δt = 10$ Myr of I-front passage, we find that self-shielding systems ($N_{\rm HI} > 10^{17.2}$ cm$^{-2}$) are comprised …
