标题： 探测星系形成与He II冷却线 显示英文标题

标题： Probing Galaxy Formation with He II Cooling Lines

摘要： 利用高分辨率宇宙学模拟，我们研究了氢和氦引力冷却辐射。 我们重点关注HeII冷却线，这些线源于与发射HI线的气体具有不同温度历史（T_max ~ 10^5K）的气体。 我们检查了三个主要原子冷却线，HI 1216A、HeII 1640A 和 HeII 304A 是否可观察到，发现HI 1216A 和 HeII 1640A 在z=2-3时的冷却发射可能通过地面深窄带（R>100）成像和/或光谱观测检测到。 虽然HI 1216A冷却发射的预期强度强烈依赖于模拟中IGM自屏蔽阶段的处理方式，但我们的HeII 1640A线预测更为稳健，因为HeII 1640A 发射率在T~10^4.5 K以下可以忽略不计，且对紫外背景的敏感度较低。 尽管HeII 1640A 冷却发射比HI 1216A 至少弱一个数量级，并且不像HI 1216A 那样可能无法用当前观测设施解析空间结构，但它更适合研究星系形成过程中的气体吸积，因为它透明且较少受到恒星形成星系再结合线的污染。 HeII 1640A 线可用于区分驱动所谓的“莱曼阿尔法团块”的机制——包括引力冷却辐射、恒星群体的光电离和由恒星爆发驱动的超风——因为（1）HeII 1640A 发射限于非常低的金属丰度（log(Z/Z_sun) < -5.3）和第三星族恒星，（2）通过HeII 1640A 线宽明确探测到团块的动力学特性，对于冷却辐射而言，其线宽较窄（σ < 400 km/s），小于典型风速。 显示英文摘要

摘要： Using high resolution cosmological simulations, we study hydrogen and helium gravitational cooling radiation. We focus on the HeII cooling lines, which arise from gas with a different temperature history (T_max ~ 10^5K) than HI line emitting gas. We examine whether three major atomic cooling lines, HI 1216A, HeII 1640A and HeII 304A are observable, finding that HI 1216A and HeII 1640A cooling emission at z=2-3 are potentially detectable with deep narrow band (R>100) imaging and/or spectroscopy from the ground. While the expected strength of HI 1216A cooling emission depends strongly on the treatment of the self-shielded phase of the IGM in the simulations, our predictions for the HeII 1640A line are more robust because the HeII 1640A emissivity is negligible below T~10^4.5 K and less sensitive to the UV background. Although HeII 1640A cooling emission is fainter than HI 1216A by at least a factor of 10 and, unlike HI 1216A, might not be resolved spatially with current observational facilities, it is more suitable to study gas accretion in the galaxy formation process because it is optically thin and less contaminated by the recombination lines from star-forming galaxies. The HeII 1640A line can be used to distinguish among mechanisms for powering the so-called "Lyman alpha blobs" -- including gravitational cooling radiation, photoionization by stellar populations, and starburst-driven superwinds -- because (1) HeII 1640A emission is limited to very low metallicity (log(Z/Z_sun) < -5.3) and Population III stars, and (2) the blob's kinematics are probed unambiguously through the HeII 1640A line width, which, for cooling radiation, is narrower (sigma < 400 km/s) than typical wind speeds.