AI 中文总结
该研究基于JWST观测的高、低红移小红点，发现其宽氢线比值远高于B型复合值，部分需高气体密度或尘埃消光解释，表明LRD中普遍存在与宽线区空间相关的尘埃，或为AGN尘埃环的低柱密度对应体。
AI 中文摘要
我们利用来自DAWN JWST档案中JWST/NIRSpec中高分辨率光谱的5个高红移（z>2）小红点（LRD）源，以及来自文献中的15个低红移（z=0.1–0.9）源，研究其宽氢线比值，所有源均检测到置信度>5σ的宽Hβ。在仔细测量宽线流量时，我们考虑了吸收特征和相邻发射线，发现宽Hα/Hβ比值非常高，范围为6至30，远高于B型复合值。我们采用平面平行Cloudy光致电离模型，利用平板两侧的总线发射，结合1个高红移和2个低红移且检测到宽帕邢线的LRD子集的巴尔末线与帕邢线比值，研究超出B型模型的宽氢线比值的物理起源。我们发现，在宽线Hα/Hβ–Paβ/Paγ平面上，1个低红移LRD可由高气体数密度（log(n_H/cm⁻³)~10–11）再现，而其余2个LRD则需要额外的尘埃消光E(B-V)≥0.2–1.0，即使考虑了非B型效应后仍是如此。由于窄Hα/Hβ比值未显示出如此大的尘埃消光，这些结果表明，遮蔽尘埃与宽线区在空间上相关联。即便没有帕邢线测量，高红移和低红移样本中分别有2个和8个LRD表现出Hα/Hβ>13，这无法由非B型模型再现，表明尘埃消光可能在LRD中普遍存在，至少约半数的LRD存在该现象。此类尘埃可能代表活动星系核（AGN）尘埃环的低柱密度对应体，可解释弱热尘埃发射。
英文摘要
We investigate broad hydrogen line ratios in little red dots (LRDs) using five high-redshift ($z>2$) sources from JWST/NIRSpec medium/high-resolution spectra in the DAWN JWST Archive and fifteen low-redshift sources ($z=0.1$--$0.9$) from the literature, all with broad H$β$ detected at $>5σ$. After carefully measuring the broad-line fluxes while accounting for absorption features and neighboring emission lines, we find that the broad H$α$/H$β$ ratios are very high, ranging from 6 to 30, well above the Case B recombination value. Using plane-parallel \textsc{Cloudy} photoionization models with the total line emission from both sides of the slab, we investigate the physical origin of the broad hydrogen line ratios beyond Case B, jointly modeling the Balmer and Paschen line ratios for the subset of one high-$z$ and two low-$z$ LRDs with detected broad Paschen lines. We find that one low-$z$ LRD is reproduced by a high gas number density ($\log (n_{\mathrm H}/{\rm cm^{-3}})\sim10$--11) on the broad-line H$α$/H$β$--Pa$β$/Pa$γ$ plane, whereas the remaining two LRDs additionally require substantial dust extinction of $E(B-V)\gtrsim0.2$--$1.0$, even after accounting for non-Case B effects. Since the narrow H$α$/H$β$ ratios do not indicate such large dust extinction, these results demonstrate that the obscuring dust is spatially associated with the broad-line region. Even without Paschen-line measurements, two and eight LRDs in the high- and low-redshift samples, respectively, exhibit H$α$/H$β>13$, which cannot be reproduced by the non-Case B models, suggesting that dust obscuration might be common among LRDs, occurring in at least about half of the population. Such dust may represent a lower-column-density counterpart of the dusty torus in AGNs, reconciling the weak hot-dust emission.
Comments26 pages, 10 figures in the main text, 8 figures in the Appendix, 2 Tables. Submitted to ApJ. Comments are welcome
