He II邻近分布的特性
Characteristics of He II Proximity Profiles
摘要：
z~3类星体光谱中的邻近剖面是研究星系际介质的重要光谱特征之一，其通量向He Ⅱ Lya波长304（1+z）A的蓝方向扩展。基于24个He II类星体的HST光谱，我们发现它们中的大多数显示出邻近轮廓，对应于源的静止坐标系中的电离半径高达20 Mpc。与z~6类星体的H_i谱相比，He Ⅱ邻近效应更为显著，且在相当长的再电离周期内被观测到。He II邻近区的大小在较高的红移处减小，特别是在z > 3.3处。该趋势类似于H I的趋势，表明在z~4处开始He II再电离。对于类星体SDSS 1253 +6817（z=3.48），He II吸收谷显示出逐渐下降，并为模拟He II再电离提供了一个很好的案例。为了模拟这样一个广泛的轮廓，需要一个类星体辐射场，其分布在4和1里德伯之间，比通常假设的要困难得多。这个类星体的紫外连续谱确实异常陡峭，类星体附近的He II电离水平高于星系际介质的平均水平。这些结果证明了这个类星体的极硬EUV连续体在其周围产生了一个大的电离区。明显的例外是两个z~2.8的最明亮的He II类星体，它们没有明显的邻近轮廓，这可能意味着它们是年轻的。
英文摘要：
The proximity profile in the spectra of z~3 quasars, where fluxes extend blueward of the He II Lya wavelength 304 (1+z) A, is one of the most important spectral features in the study of the intergalactic medium. Based on the HST spectra of 24 He II quasars, we find that the majority of them display a proximity profile, corresponding to an ionization radius as large as 20 Mpc in the source's rest frame. In comparison with those in the H i spectra of the quasars at z~6, the He II proximity effect is more prominent and is observed over a considerably longer period of reionization. The He II proximity zone sizes decrease at higher redshifts, particularly at z > 3.3. This trend is similar to that for H I, signaling an onset of He II reionization at z~4. For quasar SDSS1253+6817 (z=3.48), the He II absorption trough displays a gradual decline and serves a good case for modeling the He II reionization. To model such a broad profile requires a quasar radiation field whose distribution between 4 and 1 Rydberg is considerably harder than normally assumed. The UV continuum of this quasar is indeed exceptionally steep, and the He II ionization level in the quasar vicinity is higher than the average level in the intergalactic medium. These results are evidence that a very hard EUV continuum from this quasar produces a large ionized zone around it. Distinct exceptions are the two brightest He II quasars at z~2.8, for which no significant proximity profile is present, possibly implying that they are young.
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