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

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.