宇宙再電離時期的kSZ效應(yīng)和X射線背景與21 cm信號的互相關(guān)研究

馬清波

(中國科學(xué)院紫金山天文臺南京210033)

博士學(xué)位論文摘要選登

宇宙再電離是宇宙從黑暗時期到完全電離過渡的重要階段, 也是宇宙學(xué)研究的一個非常重要的課題, 但是目前為止人們對宇宙再電離仍然缺乏足夠精確的觀測, 其中最大的問題是微弱的有效信號往往淹沒于巨大的前景噪聲中因而很難提取出來.本工作中研究了宇宙再電離時代的動力學(xué)蘇尼阿耶夫-澤爾多維奇效應(yīng)(Kinetic Sunyaev-Zel’dovich, kSZ)、X射線背景以及與中性氫的21 cm信息的互相關(guān).由于不同觀測途徑之間的互相關(guān)能夠有效去除不相干噪聲的干擾, 這些研究成果有助于從未來的觀測結(jié)果中提取出有用的宇宙再電離信號.

由于kSZ效應(yīng)和21 cm信息(kSZ-21 cm)的互相關(guān)在小尺度上可以完全忽略,因此重點研究了kSZ平方(kSZ2)和21 cm信息(kSZ2-21 cm)的互相關(guān).首先利用半解析數(shù)值模擬(21CMFAST)構(gòu)建了宇宙再電離時代的kSZ效應(yīng)的2維分布以及隨紅移演化的21 cm信息, 在此基礎(chǔ)上計算了kSZ2-21 cm互相關(guān)及其隨紅移的演化.研究表明, 當(dāng)宇宙的平均電離率xe0.7時, kSZ2的擾動主要由電離泡的分布主導(dǎo),因此kSZ2-21 cm互相關(guān)為負(fù); 而當(dāng)0.8 xe<1時, kSZ2-21 cm互相關(guān)由中性氫的分布主導(dǎo), 所以互相關(guān)為正.不同的是在非常高的紅移處, 當(dāng)xe< 0.15, 21 cm信息的擾動由與物質(zhì)密度擾動正相關(guān)的自旋溫度主導(dǎo), kSZ2-21 cm互相關(guān)為正.kSZ2-21 cm互相關(guān)信號的觀測需要首先用維納濾波濾除原初宇宙微波背景輻射(CMB)在大尺度上的各向異性污染.研究發(fā)現(xiàn), 平方公里陣列望遠(yuǎn)鏡(SKA)和當(dāng)前的地基CMB實驗?zāi)軌蛞院芨叩男旁氡扔^測kSZ2-21 cm互相關(guān), 如SKA～10 h的積分和1.7′分辨率以及3.4μK噪聲水平的CMB實驗在xe=0.2、0.5、0.9時的信噪比分別能達(dá)到51、60、37.

宇宙再電離時代的X射線背景主要由X射線雙星、中心吸積黑洞以及受激波加熱的星際介質(zhì)貢獻(xiàn).利用宇宙學(xué)流體動力學(xué)和輻射轉(zhuǎn)移數(shù)值模擬研究了這些高能輻射源對宇宙X射線背景的貢獻(xiàn)以及與21 cm信息的互相關(guān).研究發(fā)現(xiàn)這些輻射源對宇宙X射線背景的貢獻(xiàn)只占當(dāng)前軟頻段0.5–2 keV的觀測結(jié)果的～5%以及硬頻段2–8 keV的～4%, 這些源對宇宙X射線背景分布的自相關(guān)功率譜的貢獻(xiàn)小于當(dāng)前觀測結(jié)果的～2%, 因此幾乎無法直接用宇宙X射線背景的觀測研究宇宙再電離.借助于21 cm信息的紅移信息, 宇宙X射線背景和21 cm信息的互相關(guān)(Xray-21 cm)能夠研究高紅移X射線源的許多性質(zhì).研究表明, 在宇宙再電離的早期, Xray-21 cm互相關(guān)為正, 而當(dāng)星系際介質(zhì)被高度電離時, Xray-21 cm互相關(guān)則為負(fù).Xray-21 cm互相關(guān)從正到負(fù)的過渡與高紅移X射線源的模型以及研究的空間尺度的大小有關(guān).一般來說, X射線源的輻射越強(qiáng)過渡發(fā)生的越早, 同時在越小的空間尺度上過渡也越早.以SKA望遠(yuǎn)鏡作為21厘米實驗的參考儀器, 并假設(shè)高精度X射線巡天實驗?zāi)軌蚍直娌⒁瞥椛淞髁? 10?15erg·cm?2·s?1的X射線源, 2者的聯(lián)合觀測的信噪比< 1, 而如果X射線巡天實驗?zāi)軌蜃R別>10?17erg·cm?2·s?1的X射線源, 那么在xe=0.5時累積的信噪比最高可以達(dá)到～5.

Cross-correlation between kSZ Effect, X-ray Background, and 21 cm Signal from the Epoch of Reionization

MA Qing-bo

(Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210033)

Epoch of cosmic reionization (EoR) is a very important era for the transition of our Universe dark ages to full ionized universe.It is also a very important issue in the study of cosmology.However, until now there are lack of precise measurements of EoR.One of the problems is that the huge foreground noise usually overwhelms the EoR signal, thus it is difficult to resolve the real signal from the observational data.In this thesis, I study the kinetic Sunyaev-Zel’dovich effect (kSZ), the X-ray background during EoR, and their correlations with the 21 cm signal of neutral hydrogen.Since the correlations between different measurements could efficiently remove their uncorrelated noises, our studies would help to measure the EoR signal from the observations in the near future.

Since the kSZ effect is proportional to the line-of-sight velocity, the kSZ-21 cm correlation suffers from cancellation at small angular scales.We thus focus on the correlation between the kSZ-squared field (kSZ2) and 21 cm signals (kSZ2-21 cm).We first build the 2-D distributions of kSZ effect and the 21 cm signals at different redshifts using the seminumerical simulations (21CMFAST), then study the kSZ2-21 cm correlations together with their evolution with redshift.It is found that, when the global ionization fraction is low(xe0.7), the kSZ2fluctuation is dominated by the rare ionized bubbles, that leads to an anti-correlation with the 21 cm signal.With 0.8 xe<1, the correlation is dominated by small pockets of neutral regions, leading to a positive correlation.However, at very high redshifts with xe<0.15,the spin temperature fluctuations change the sign of the correlation from negative to positive, since weakly ionized regions can have strong 21 cm signals in this case.To extract this correlation, Wiener filtering is effective in removing large signals from the primary CMB anisotropy.The expected signal-to-noise ratios for a ～10-hour integration of upcoming Square Kilometer Array (SKA) data cross-correlated with maps from the current generation of CMB observatories with 3.4 μK noise and 1.7′beam over 100 deg2are 51, 60, and 37 for xe=0.2, 0.5, and 0.9, respectively.

The X-ray sources during EoR,that might contribute to the X-ray background, are the X-ray binaries, the accreting nuclear black holes, and the shock heated interstellar medium.We use high resolution hydrodynamical and radiative transfer simulations to study the contribution of these high-z energetic sources to the X-ray background and its correlations with the 21 cm signal.We find that these X-ray sources contribute only ～5% of the unresolved X-ray background in the soft 0.5–2 keV band and ～4% in the hard 2–8 keV band.The same sources contribute to less than ～2% of the measured angular power spectrum of the X-ray background fluctuations.Thus it is impossible to study the EoR using only the X-ray background measurements.Using the redshift information of 21 cm signal, the correlation between X-ray background and 21 cm signal(Xray-21 cm) could be used to study the properties of the X-ray sources during the EoR.I find that Xray-21 cm correlations are positive during the early stages of reionization when most of hydrogen is neutral, while they become negative when the intergalactic medium gets highly ionized,with the transition from positive to negative depending on both the X-ray model and the scale under consideration.With SKA as the reference instrument for the 21 cm experiment, the predicted signal-to-noise ratio for such correlations is < 1 if the corresponding X-ray survey is only able to resolve and remove X-ray sources with observed flux >10?15erg·cm?2·s?1, while the cumulative signal-to-noise ratio at xe=0.5 is ～ 5 if sources with observed flux >10?17erg ·cm?2·s?1are detected.