分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： The matter density field at $z\sim 6$ is very challenging to probe. One of the traditional probes of the low density IGM that works successfully at lower redshifts is the Lyman-alpha forest in quasar spectra. However, at the end of reionization, the residual neutral hydrogen usually creates saturated absorption, thus much of the information about the gas density is lost. Luckily, in a quasar proximity zone, the ionizing radiation is exceptionally intense, thus creating a large region with non-zero transmitted flux. In this study we use the synthetic spectra from simulations to investigate how to recover the density fluctuations inside the quasar proximity zones. We show that, under ideal conditions, the density can be recovered accurately with a small scatter. We also discuss how systematics such as the quasar continuum fitting and reionization models affect the results. This study shows that by analyzing the absorption features inside quasar proximity zones we can potentially constrain quasar properties and the environments they reside in.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： In this paper, we study the sizes of quasar proximity zones with synthetic quasar absorption spectra obtained by post-processing a Cosmic Reionization On Computers (CROC) simulation. CROC simulations have both relatively large box sizes and high spacial resolution, allowing us to resolve Lyman limit systems, which are crucial for modeling the quasar absorption spectra. We find that before reionization most quasar proximity zone sizes grow steadily for $\sim 10$ Myr, while after reionization they grow rapidly but only for $\sim 0.1$ Myr. We also find a slow growth of $R_{\rm obs}$ with decreasing turn-on redshift. In addition, we find that $\sim 1-2\%$ of old quasars ($30$ Myr old) display extremely small proximity zone sizes ($<1$ proper Mpc), of which the vast majority are due to the occurrence of a damped Ly$\alpha$ absorber (DLA) or a Lyman limit system (LLS) along the line of sight. These DLAs and LLSs are contaminated with metal, which offers a way to distinguish them from the normal proximity zones of young quasars.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： Using a suite of self-similar cosmological simulations, we measure the probability distribution functions (PDFs) of real-space density, redshift-space density, and their geometric mean. We find that the real-space density PDF is well-described by a function of two parameters: $n_s$, the spectral slope, and $\sigma_L$, the linear rms density fluctuation. For redshift-space density and the geometric mean of real- and redshift-space densities, we introduce a third parameter, $s_L={\sqrt{\langle(dv^L_{\rm pec}/dr)^2\rangle}}/{H}$. We find that density PDFs for the LCDM cosmology is also well-parameterized by these three parameters. As a result, we are able to use a suite of self-similar cosmological simulations to approximate density PDFs for a range of cosmologies. We make the density PDFs publicly available and provide an analytical fitting formula for them.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： High-redshift quasars ionize HeII into HeIII around them, heating the IGM in the process and creating large regions with elevated temperature. In this work, we demonstrate a method based on a convolutional neural network (CNN) to recover the spatial profile for $T_0$, the temperature at the mean cosmic density, in quasar proximity zones. We train the neural network with synthetic spectra drawn from a Cosmic Reionization on Computers simulation. We discover that the simple CNN is able to recover the temperature profile with an accuracy of $\approx 1400$ K in an idealized case of negligible observational uncertainties. We test the robustness of the CNN and discover that it is robust against the uncertainties in quasar host halo mass, quasar continuum and ionizing flux. We also find that the CNN has good generality with regard to the hardness of quasar spectra. Saturated pixels pose a bigger problem for accuracy and may downgrade the accuracy to $1700$ K in the outer parts of the proximity zones. Using our method, one could distinguish whether gas is inside or outside the HeIII region created by the quasar. Because the size of the HeIII region is closely related to the total quasar lifetime, this method has great potential in constraining the quasar lifetime on $\sim $Myr timescales.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Yongda Zhu
George D. Becker
Sarah E. I. Bosman
Laura C. Keating
Valentina D'Odorico
Rebecca L. Davies
Holly M. Christenson
Eduardo Bañados
Fuyan Bian
Manuela Bischetti
Huanqing Chen
Frederick B. Davies
Anna-Christina Eilers
Xiaohui Fan
Prakash Gaikwad
Bradley Greig
Martin G. Haehnelt
Girish Kulkarni
Samuel Lai
Andrea Pallottini
摘要： We present a new investigation of the intergalactic medium (IGM) near reionization using dark gaps in the Lyman-$\beta$ (Ly$\beta$) forest. With its lower optical depth, Ly$\beta$ offers a potentially more sensitive probe to any remaining neutral gas compared to commonly used Ly$\alpha$ line. We identify dark gaps in the Ly$\beta$ forest using spectra of 42 QSOs at $z_{\rm em}>5.5$, including new data from the XQR-30 VLT Large Programme. Approximately $40\%$ of these QSO spectra exhibit dark gaps longer than $10h^{-1}{\rm Mpc}$ at $z\simeq5.8$. By comparing the results to predictions from simulations, we find that the data are broadly consistent both with models where fluctuations in the Ly$\alpha$ forest are caused solely by ionizing ultraviolet background (UVB) fluctuations and with models that include large neutral hydrogen patches at $z<6$ due to a late end to reionization. Of particular interest is a very long ($L=28h^{-1}{\rm Mpc}$) and dark ($\tau_{\rm eff} \gtrsim 6$) gap persisting down to $z\simeq 5.5$ in the Ly$\beta$ forest of the $z_{\rm}=5.85$ QSO PSO J025$-$11. This gap may support late reionization models with a volume-weighted average neutral hydrogen fraction of $ \langle x_{\rm HI}\rangle \gtrsim 5\%$ by $z=5.6$. Finally, we infer constraints on $\langle x_{\rm HI}\rangle$ over $5.5 \lesssim z \lesssim 6.0$ based on the observed Ly$\beta$ dark gap length distribution and a conservative relationship between gap length and neutral fraction derived from simulations. We find $\langle x_{\rm HI}\rangle \leq 0.05$, 0.17, and 0.29 at $z\simeq 5.55$, 5.75, and 5.95, respectively. These constraints are consistent with models where reionization ends significantly later than $z = 6$.
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