寻找亮度最低的BL Lac天体
Looking for the least luminous BL Lac objects
摘要：
在活动星系核中，BL Lac天体显示出极端的性质，这些性质被解释为相对论性光束对靠近视线的等离子体喷流发射的影响。喷流发射的多普勒放大使它们成为研究喷流物理的理想目标。特别是，低功率BL Lac（LPBL）是非常有趣的，因为它们探测在吸积的最低水平的喷流形成和排放过程。然而，它们很难被识别，因为它们的辐射在大多数观测波段都被来自宿主星系的辐射淹没了。在本文中，我们提出了一种新的LPBL选择方法的基础上，除了传统的光学指标的中红外发射。我们考虑了Best & Heckman（2012，MNRAS，421，1569）的无线电选择样本，并将其与WISE全天巡天进行交叉匹配。在一个包括W2-W3颜色和Dn（4000）指数的新诊断平面上，LPBL位于一个几乎没有其他来源的区域。通过过滤小的发射线等效宽度的对象，我们分离出36个LPBL候选红移0.15。它们在1.4GHz的射电光度范围为log L_r = 39.2-41.5 [erg/s]。考虑到我们的样本的完整性，我们分析了BL Lac光度函数（RLF），发现一个戏剧性的缺乏LPBL相对于外推的RLF向低功率。这需要RLF中位于log L_r~40.6 [erg/s]处的中断。BL Lacs数密度的随后峰值可能是发射相对论性喷流所需的最小功率的表现。
英文摘要：
Among active galactic nuclei, BL Lac objects show extreme properties that have been interpreted as the effect of relativistic beaming on the emission from a plasma jet oriented close to the line of sight. The Doppler amplification of the jet emission makes them ideal targets for studying jet physics. In particular, low-power BL Lacs (LPBL) are very interesting because they probe the jet formation and emission processes at the lowest levels of accretion. However, they are difficult to identify since their emission is swamped by the radiation from the host galaxy in most observing bands. In this paper we propose a new LPBL selection method based on the mid-infrared emission, in addition to the traditional optical indices. We considered the radio-selected sample of Best & Heckman (2012, MNRAS, 421, 1569) and cross-matched it with the WISE all-sky survey. In a new diagnostic plane including the W2-W3 color and the Dn(4000) index, LPBL are located in a region scarcely populated by other sources. By filtering objects with small emission line equivalent width, we isolated 36 LPBL candidates up to redshift 0.15. Their radio luminosity at 1.4 GHz spans the range log L_r = 39.2-41.5 [erg/s]. Considering the completeness of our sample, we analyzed the BL Lac luminosity function (RLF), finding a dramatic paucity of LPBL with respect to the extrapolation of the RLF toward low power. This requires a break in the RLF located at log L_r~40.6 [erg/s]. The consequent peak in the BL Lacs number density is possibly the manifestation of a minimum power required to launch a relativistic jet.
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