BL Lac objects are active nuclei, hosted in massive elliptical galaxies, the emission of which is dominated by a relativistic jet closely aligned with the line of sight. This implies the existence of a parent population of sources with a misaligned jet that have been identified with low-power radiogalaxies. The spectrum of BL Lacs, dominated by non-thermal emission over the whole electromagnetic range, together with bright compact radio cores, high luminosities, rapid and large amplitude flux variability at all frequencies and strong polarization makes these sources an optimal laboratory for high energy astrophysics. A most distinctive characteristic of the class is the weakness or absence of spectral lines, that historically hindered the identification of their nature and ever thereafter proved to be a hurdle in the determination of their distance. In this paper, we review the main observational facts that contribute to the present basic interpretation of this class of active galaxies. We overview the history of the BL Lac objects research field and their population as it emerged from multi-wavelength surveys. The properties of the flux variability and polarization, compared with those at radio, X-ray and gamma-ray frequencies, are summarized together with the present knowledge of the host galaxies, their environments, and central black hole masses. We focus this review on the optical observations, which played a crucial role in the early phase of BL Lacs studies, and in spite of extensive radio, X-ray, and recently gamma-ray observations, could represent the future major contribution to the unveiling of the origin of these sources. In particular, they could provide a firm conclusion on the long debated issue of the cosmic evolution of this class of active galactic nuclei and on the connection between formation of supermassive black holes and relativistic jets.

中文翻译：

---

BL Lacertae 天体的光学视图

BL Lac 天体是活跃的原子核，位于巨大的椭圆星系中，其发射由与视线紧密对齐的相对论射流主导。这意味着存在已被识别为低功率射电星系的未对准射流源的母群。BL Lacs 的频谱以整个电磁范围内的非热辐射为主，加上明亮紧凑的无线电核心、高光度、所有频率下快速和大幅度的通量变化以及强极化，使这些源成为高能天体物理学的最佳实验室. 该类最显着的特征是谱线的弱点或缺失，这在历史上阻碍了对其性质的识别，并且此后被证明是确定它们距离的障碍。在本文中，我们回顾了有助于目前对此类活动星系的基本解释的主要观测事实。我们概述了 BL Lac 天体研究领域的历史及其从多波长调查中出现的群体。与无线电、X 射线和伽马射线频率相比，通量变异性和极化的特性与宿主星系、它们的环境和中心黑洞质量的现有知识一起进行了总结。我们将这篇综述集中在光学观测上，它在 BL Lacs 研究的早期阶段发挥了关键作用，尽管进行了广泛的无线电、X 射线和最近的伽马射线观测，可能代表未来对揭示这些来源的起源的重大贡献。特别是，他们可以就这类活动星系核的宇宙演化以及超大质量黑洞的形成与相对论喷流之间的联系这一长期争论不休的问题提供一个明确的结论。