Title: Probing small-scale cosmological fluctuations with the 21 cm forest: effects of neutrino mass, running spectral index and warm dark matter Show Chinese title

Title: 用21厘米森林探测小尺度宇宙学涨落：中微子质量、谱指数跑动和温暗物质的影响

Abstract: Although the cosmological paradigm based on cold dark matter and adiabatic, nearly scale-invariant primordial fluctuations is consistent with a wide variety of existing observations, it has yet to be sufficiently tested on scales smaller than those of massive galaxies, and various alternatives have been proposed that differ significantly in the consequent small-scale power spectrum (SSPS) of large-scale structure. Here we show that a powerful probe of the SSPS at $k\gtrsim 10$ Mpc$^{-1}$ can be provided by the 21 cm forest, that is, systems of narrow absorption lines due to intervening, cold neutral hydrogen in the spectra of high-redshift background radio sources in the cosmic reionization epoch. Such features are expected to be caused predominantly by collapsed gas in starless minihalos, whose mass function can be very sensitive to the SSPS. As specific examples, we consider the effects of neutrino mass, running spectral index (RSI) and warm dark matter (WDM) on the SSPS, and evaluate the expected distribution in optical depth of 21 cm absorbers out to different redshifts. Within the current constraints on quantities such as the sum of neutrino masses $\sum m_\nu$, running of the primordial spectral index $d n_s/d \ln k$ and WDM particle mass $m_{\rm WDM}$, the statistics of the 21 cm forest manifest observationally significant differences that become larger at higher redshifts. In particular, it may be possible to probe the range of $m_{\rm WDM} \gtrsim 10$ keV that may otherwise be inaccessible. Future observations of the 21 cm forest by the Square Kilometer Array may offer a unique and valuable probe of the SSPS, as long as radio sources such as quasars or Population III gamma-ray bursts with sufficient brightness and number exist at redshifts of $z \gtrsim$ 10 - 20, and the astrophysical effects of reionization and heating can be discriminated. Show Chinese abstract

Abstract: 尽管基于冷暗物质和绝热的、接近尺度不变的原初涨落的宇宙学范式与各种现有的观测结果一致，但它尚未在比大质量星系尺度更小的范围内得到充分检验，同时也有多种显著不同的替代方案被提出，这些替代方案在大尺度结构的小尺度功率谱（SSPS）上存在显著差异。 在这里，我们展示出，在$k\gtrsim 10$Mpc$^{-1}$尺度上SSPS的一个有力探针可以由21厘米森林提供，即由于宇宙再电离时期高红移背景射电源光谱中的冷中性氢引起的窄吸收线系统。 预计这些特征主要由无星最小晕塌缩气体引起，其质量函数对SSPS非常敏感。 作为具体例子，我们考虑了中微子质量、跑动谱指数（RSI）和温暗物质（WDM）对SSPS的影响，并评估了预期的21厘米吸收体光学深度分布到不同红移的情况。 在当前对中微子总质量$\sum m_\nu$、原初谱指数的跑动$d n_s/d \ln k$和WDM粒子质量$m_{\rm WDM}$的限制范围内，21厘米森林的统计量表现出可观测到的显著差异，且这些差异在更高红移时更大。 特别是，它可能能够探测到$m_{\rm WDM} \gtrsim 10$keV 这一范围，否则这一范围可能是无法触及的。 只要在红移$z \gtrsim$10-20之间存在足够亮度和数量的射电源（如类星体或第三族伽马射线暴），并且再电离和加热的天体物理效应可以区分，平方公里阵列未来的21厘米森林观测可能会提供一个独特而有价值的SSPS探针。