The cosmic evolution of Supermassive Black holes: A panchromatic study of the nuclear environment

活动星系核或AGN是位于宇宙中大多数星系中心的不断增长的超大质量黑洞(SMBH)。作为宇宙中能量最高的现象之一，它们驱动物质和能量进出其所在的星系，影响着恒星形成速度、气体温度和气体组成等星系的各种性质。因此，了解活动星系核的演化对于理解星系的形成和演化具有重要意义。此外，有证据表明，早期宇宙中的星系(红移z~2)在恒星形成率(SFR)、气体密度、气体量、金属丰度等方面与局部宇宙中的星系有很大的不同。因此，人们可能还会期待AGN及其在星系内的直接核环境的互补演化。我的项目将使用最新的望远镜和数据分析技术来寻找这种演化。研究活动星系核的主要方法是观察它的发射特性。虽然大多数辐射来自SMBH附近的吸积盘，但它与活动星系核周围的物质相互作用，因此携带了关于这种物质的组成和几何性质的信息。发射特性通常以光谱能量分布(SED)的形式表示，该光谱能量分布是光源(或其一部分)的辐射量作为波长的函数，通常通过一系列光度滤光片来测量。精确的SED模拟是这个项目的主要分析方法。根据统一模型，活动星系核被尘埃云包围，称为从SMBH中心跨越0.1-10个PC的被称为“遮蔽环”(或简称“环”)的尘埃云。它还指出，这个环相对于观察者的视线的不同方向导致了基于SED形状的活动星系核的不同子类。早期的“环面”模型基于活动星系核的光学性质，只有几个自由参数，如环面的方向和光源的光度。然而，多波长研究暗示，这个环的几何形状比之前想象的要复杂得多。因此，这些复杂的模型需要更多的参数来完全描述环面，这使得在计算和理论上一次约束所有这些参数变得困难。这一困难表现为在任何给定波长产生发射的许多相互竞争的现象。为了准确地约束圆环参数，在对SED建模时，可靠地分离这些现象是很重要的。当活动星系核的本征辐射与环面周围的尘埃相互作用而变红时，活动星系核发出的红外辐射就会产生。然而，这种发射受到来自宿主星系恒星形成区的红外辐射的污染。由于以往红外观测的空间分辨率较低，很难分离SED的这些分量，并区分不同的环面模型。然而，由于不同的发射机制来自星系中不同的空间区域，源的高分辨率成像有助于在物理上分离这些发射机制。这将使我们能够更精确和准确地约束环面参数，以寻找这些参数在宇宙时间内进化的迹象。使用詹姆斯·韦伯太空望远镜(JWST)的高分辨率成像来找到这些进化特征并了解它们是我博士项目的最终目标。

Active Galactic Nuclei or AGN are growing supermassive black holes (SMBH) at the centre of most galaxies in the universe. As one of the most energetic phenomena in the universe, they drive material and energy in and out of their host galaxy, affecting various galaxy properties such as star formation rate, gas temperature, and gas composition. Therefore, understanding the evolution of the AGNs is of great importance for understanding galaxy formation and evolution. Moreover, there is evidence to suggest that galaxies in the early universe (redshift z~2) differ significantly from galaxies in the local universe in terms of their star formation rates (SFR), gas densities, amount of gas, metallicity, etc. Therefore, one may also expect a complementary evolution in AGN and their immediate nuclear environment within galaxies. My project will look for this evolution using the newest telescopes and data analysis techniques.The main way of studying an AGN is by looking at its emission properties. While most of the emitted radiation originates from accretion disk in the close proximity of the SMBH, it interacts with the material surrounding the AGN and as such carries the information about the composition and the geometrical properties of this material. The emission properties are typically represented in form of a spectral energy distribution (SED), the amount of radiation from the source (or part of it) as a function of wavelength, generally measured through a series of photometric filters. Accurate SED modelling is the prime analytic approach of this project.According to the unified model, AGNs are surrounded by dust clouds called the ``obscuring torus'' (or simply ``torus'') spanning 0.1-10 pc from the central SMBH. It also states that different orientations of this torus with respect to the observer's line-of-sight gives rise to various sub-classes of AGNs based on SED shapes. The early ``torus'' models were based on the optical properties of the AGN and had only a few free parameters like orientation of the torus and the luminosity of the source. However, the multi-wavelength studies have hinted that the geometry of this torus is more complex than previously thought. Consequently, these complex models require many more parameters to fully describe the torus, making it difficult to constrain all these parameters at once -- both computationally and theoretically.This difficulty manifests itself in the form of a number of competing phenomena producing emissions at any given wavelength. In order to accurately constrain the torus parameters, it is important to reliably disentangle these phenomena while modelling the SED. The infrared (IR) emission from the AGN is produced when the intrinsic radiation is reddened by interacting with the surrounding dust in the torus. This emission is, however, contaminated by the IR emission from the star forming regions of the host galaxy. With the poor spatial resolution of previous IR observations, it has been difficult to separate these components of SED, and distinguish between various torus models. However, since different emission mechanisms originate from different spatial regions in a galaxy, the high resolution imaging of the source can help in physically separating these emission mechanisms. This will allow us to constrain the torus parameters more precisely and accurately enough to look for the signs of evolution in these parameters over cosmic times. Finding these evolution signatures using high resolution imaging from the James Webb Space Telescope (JWST) and understanding them is the end goal of my PhD project.