[论文解读] Stellar Population in LLAGN.II: STIS observations

R. M. González Delgado, R. Cid Fernandes+6 more|LA Referencia (Red Federada de Repositorios Institucionales de Publicaciones Científicas)|Jan 21, 2004

Galaxies: Formation, Evolution, Phenomena被引用 39

一句话总结

本研究利用哈勃空间望远镜STIS和地面望远镜对73个低光度活动星系核（LLAGN）进行高分辨率光谱观测，以探究年轻和中等年龄恒星在电离核区气体中的作用。研究发现，弱-[OI] LLAGN（以低[OI]/Hα比值为特征）宿主中存在显著的中等年龄恒星群体，表明其电离可能由恒星过程驱动；而强-[OI] LLAGN则主要由老年恒星主导，其电离可能由活动星系核（AGN）驱动，支持一种假设：弱-[OI] LLAGN是低光度的复合型赛弗特2型星系的对应体，其核区星暴正在消退。

ABSTRACT

We present a study of the stellar population in Low Luminosity AGN (LLAGN). Our goal is to search for spectroscopic signatures of young and intermediate age stars, and to investigate their relationship with the ionization mechanism in LLAGN. The method used is based on the stellar population synthesis of the optical continuum of the innermost (20-100 pc) regions in these galaxies. For this purpose, we have collected high spatial resolution optical (2900-5700 A) STIS spectra of 28 nearby LLAGN that are available in the Hubble Space Telescope archive. The analysis of these data is compared with a similar analysis also presented here for 51 ground-based spectra of LLAGN. Our main findings are: (1) No features due to Wolf-Rayet stars were convincingly detected in the STIS spectra. (2) Young stars contribute very little to the optical continuum in the ground-based aperture. However, the fraction of light provided by these stars is higher than 10% in most of the weak-[OI] LLAGN STIS spectra. (3) Intermediate age stars contribute significantly to the optical continuum of these nuclei. This population is more frequent in objects with weak than with strong [OI]. Weak-[OI] LLAGN that have young stars stand out for their intermediate age population. (4) Most of the strong-[OI] LLAGN have predominantly old stellar population. A few of these objects also show a feature-less continuum that contributes significantly to the optical continuum. These results suggest that young and intermediate age stars do not play a significant role in the ionization of LLAGN with strong [OI]. However, the ionization in weak-[OI] LLAGN with young and/or intermediate age population could be due to stellar processes.

研究动机与目标

• 确定年轻和中等年龄恒星群体是否对低光度活动星系核（LLAGN）的电离有贡献。
• 比较LLAGN的恒星群体特性与其发射线比值（尤其是[OI]/Hα）的关系，以识别电离机制。
• 探究弱-[OI] LLAGN是否为核区星暴正在消退的复合型赛弗特2型星系的低光度对应体。
• 通过比较哈勃STIS与地面数据，评估空间分辨率对LLAGN中年轻恒星群体检测的影响。

提出的方法

• 从哈勃空间望远镜档案中获取28个近距LLAGN的高空间分辨率STIS光谱（2900–5700 Å），覆盖内区20–100 pc范围。
• 将STIS数据与51个LLAGN的地面光谱进行对比，评估空间分辨率对恒星特征检测的影响。
• 对光学连续谱进行恒星群体合成（EPS）建模，估算不同恒星群体的相对贡献比例。
• 将LLAGN划分为强-[OI]（[OI]/Hα ≥ 0.25）与弱-[OI]（[OI]/Hα ≤ 0.25）两类，以比较其恒星组分差异。
• 利用瞬时爆发模型的演化序列，解释恒星群体年龄分布与电离状态之间的关系。
• 将EPS结果与发射线诊断参数相关联，推断各类LLAGN中主导电离源。

实验结果

研究问题

• RQ1 年轻和中等年龄恒星是否对弱-[OI] LLAGN的电离有显著贡献？
• RQ2 LLAGN中的恒星群体特性与[OI]/Hα发射线比值（电离源的关键诊断参数）之间有何关联？
• RQ3 在恒星群体以老年为主的情况下，强-[OI] LLAGN的电离是否主要由AGN或恒星过程驱动？
• RQ4 空间分辨率在多大程度上影响LLAGN中年轻恒星特征的检测？
• RQ5 弱-[OI] LLAGN是否为核区星暴正在消退的复合型赛弗特2型星系的低光度对应体？

主要发现

• 在LLAGN的STIS光谱中未发现明显的沃尔夫-拉叶星特征。
• 在地面光谱中，年轻恒星对光学连续谱的贡献较小，但在大多数弱-[OI] LLAGN（[OI]/Hα ≤ 0.25）中，其贡献超过10%。
• 中等年龄恒星对LLAGN的光学连续谱有显著贡献，尤其在弱-[OI]源中，其丰度高于强-[OI] LLAGN。
• 大多数强-[OI] LLAGN具有以老年恒星为主的主导群体，少数显示无特征连续谱，主导光学辐射。
• 弱-[OI] LLAGN中存在中等年龄和年轻恒星，提示其电离可能由恒星过程主导，尤其在Hα等效宽度较高的源中。
• 从数据推断的演化序列表明，弱-[OI] LLAGN比强-[OI] LLAGN更年轻，其演化路径为：从以星暴为主导的阶段，随星暴消退而过渡至AGN主导阶段。