论 Case B 复合理论的普适有效性
On the universal validity of Case B recombination theory
AI总结：
本文通过发现极端发射线星系中异常低的 Ha/Hb 比值，质疑 Case B 复合理论的普适性，提出巴尔末自吸收机制，并利用 SDSS 数据证明该效应在高激发星系中不可忽视。
摘要：
在持续搜寻低质量极端发射线星系的过程中，我们发现了一个 Ha/Hb 巴尔末线比值为 2.620 ± 0.078 的星系。低于无尘埃 Case B 值的 Ha/Hb 巴尔末比值在极端发射线星系中相对频繁地出现。这些低比值表明 Case B 假设在这些天体中可能并不成立。在排除了低 Ha/Hb 比值是由观测效应引入的系统误差所致的可能性之后，我们利用总 Hb 光度、[OIII]/[OII] 线比值以及巴尔末线等值宽度的约束，提出该气体对 Ha 和 Lya 光子都是光学厚的，并且散射气体的几何形状和取向导致 Ha 光子相对于高阶巴尔末系光子被优先从视线方向移除。最后，我们利用 SDSS 巡天数据表明，在高激发发射线星系中，巴尔末自吸收可能比先前假设的更为重要，因为在这些星系中 Lya 对氢激发态的泵浦可以有效发挥作用。如果不加以识别，巴尔末自吸收可能导致对星系物理性质的不准确估计。例如，对于球对称散射介质，尘埃消光的影响可能被高估；而对于非球对称分布，则可能被低估。
英文摘要：
In an ongoing search for low-mass extreme emission line galaxies, we identified a galaxy with a Ha/Hb Balmer line ratio of 2.620 +- 0.078. Ha/Hb Balmer ratios lower than the dust-free Case~B value appear relatively frequently in extreme emission line galaxies. These low values suggest that the Case~B assumption may not be valid in these objects. After ruling out the possibility that the low Ha/Hb ratio is due to systematic errors introduced by observational effects, we use constraints from the total Hb luminosity, the [OIII]/[OII] line ratio and the Balmer line equivalent widths, to suggest that the gas is optically thick to both Ha and Lya photons, and the geometry and orientation of the scattering gas causes Ha photons to be preferentially removed from the line of sight with respect to higher order Balmer series photons. Finally, we use data from the SDSS survey to show that Balmer self-absorption may be more important than previously assumed in high excitation emission line galaxies, where Lya pumping of the hydrogen excited state can be effective. If not recognized, Balmer self-absorption could lead to inaccurate estimates of galaxy physical properties. As an example, the effect of dust extinction could be over-estimated, for spherically symmetric scattering medium, or under-estimated, for a not spherically-symmetric distribution.
机构、总结、中文摘要均由AI分析而来，可能存在错误，请以论文正文为准。
