利用红外软度图探索电离辐射的硬度。I. 星暴星系与（超）亮红外星系的相似有效温度标度
Exploring the hardness of the ionising radiation with the infrared softness diagram. I. Similar effective temperature scales for starbursts and (ultra)luminous infrared galaxies
AI总结：
本文利用红外软度参数和软度图研究星暴星系及（超）亮红外星系的电离辐射硬度，通过光致电离模型和贝叶斯式代码发现两类天体具有相似的等效有效温度标度，表明其电离均可由大质量恒星解释，无需更硬电离源。
摘要：
我们探索了红外波段的软度参数，其主要目的是表征发射线星云中入射电离辐射的硬度。该参数由恒星形成区中对应于连续电离阶段的中红外波长范围跃迁组合获得。我们汇编了一个恒星形成星系（SFGs）样本的观测数据，其中包括亮红外星系和超亮红外星系（LIRGs 和 ULIRGs），以研究软度参数及其在二维中的等效表达，即软度图。我们将它们与光致电离模型的预测进行比较，以确定每种情况下电离连续谱能量分布的形状。我们还使用测得的发射线比作为 HCmistry-Teff-IR 的输入，这是一个与光致电离模型预测进行贝叶斯式比较的代码，以量化等效有效温度（T*）和电离参数。我们发现（U）LIRGs 中软度参数的平均值（-0.57）与其余 SFGs 的平均值（-0.51）在误差范围内相似，这可以解释为表明入射辐射场相似。这一结果通过 HCm-Teff-IR 的分析得到证实，该分析同时指出（U）LIRGs 的 T* 标度略低，尽管在误差范围内相似，即使在这些天体的模型中考虑了更高的尘埃气体质量比也是如此。这些推导出的 T* 值与来自大质量恒星的电离相容，无需任何更硬的电离源，无论是对（U）LIRGs 还是其余的 SFGs。然而，（U）LIRGs 中推导出的 T* 与金属丰度没有表现出任何相关性。这可以解释为一个迹象，表明它们相似的平均 T* 值是由于大质量恒星的高能入射通量被与气体混合的加热尘埃衰减所致。
英文摘要：
{We explored the {softness parameter} in the infrared, whose main purpose is the characterisation of the hardness of the incident ionising radiation in emission-line nebulae. This parameter is obtained from the combination of mid-infrared wavelength range transitions corresponding to consecutive ionisation stages in star-forming regions. We compiled observational data from a sample of star-forming galaxies (SFGs), including luminous and ultraluminous infrared galaxies (LIRGs and ULIRGs), to study the softness parameter and its equivalent expression in two dimensions, the softness diagram. We compared them with predictions from photoionisation models to determine the shape of the ionising continuum energy distribution in each case. We also used the measured emission-line ratios as input for HCmistry-Teff-IR, a code that performs a Bayesian-like comparison with photoionisation model predictions in order to quantify the equivalent effective temperature (T*) and the ionisation parameter. We found similar average values within the errors of the softness parameter in (U)LIRGs (-0.57) in the rest of the SFGs (-0.51), which could be interpreted as indicative of a similar incident radiation field. This result is confirmed from the analysis using HCm-Teff-IR, which simultaneously points to a slightly lower, although similar within the errors, T* scale for (U)LIRGs, even when a higher dust-to-gas mass ratio is considered in the models for these objects. These derived T* values are compatible with the ionisation from massive stars, without any need of harder ionising sources, both for (U)LIRGs and the rest of the SFGs. However, the derived T* in (U)LIRGs do not show any correlation with metallicity. This could be interpreted as a sign that their similar average T* values are due to the attenuation of the energetic incident flux from massive stars by the heated dust mixed with the gas.
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