Probing the fundamental properties of AGN
Probing the fundamental properties of AGN
One of the most puzzling discoveries in astronomy is finding supermassive black holes - each weighing more than a billion times our Sun - in the very young universe, less than a billion years after the Big Bang. It's like finding a 40-year-old person who's only 4 years old - it challenges our understanding of how these cosmic giants could grow so massive so quickly.
The James Webb Space Telescope has recently deepened this mystery by discovering even more unexpectedly massive black holes in the early universe. These early black holes show interesting patterns: they display certain telltale signs of active feeding (broad emission lines in their light spectrum), but they're surprisingly dim in X-ray light. This unusual behavior suggests they might be consuming matter at extraordinary rates, beyond what we typically expect is possible.
To understand these cosmic extremes, we study how matter falls into black holes through structures called accretion disks. In most black holes, these disks are thin and flat, like a pizza. However, when black holes feed very rapidly, their disks puff up like a bloated pizza, creating what we call a 'slim disk' (see Fig. 1) This different structure changes how we see the black hole from Earth - depending on our viewing angle, it can appear unusually dim in X-rays and show other distinctive features.
Fig1. Slim accretion disk model for highly accreting quasars. Picture adapted from Ni et al. (2018). The height of the inner region of the accretion disk likely scales with accretion rate, giving anisotropic radiation.
This geometry also affects a crucial region near the black hole called the broad-line region - a collection of fast-moving gas clouds that help us measure the black hole's mass. The challenge is that our traditional methods for weighing black holes were developed by studying nearby, slow-growing ones. These methods might not work well for the rapidly feeding black holes we see in the early universe.
A major focus of my PhD dissertation was investigating how structural changes in the accretion disks of highly accreting quasars affect the estimation of fundamental properties like black hole mass and accretion rate, and exploring their connection to X-ray producing inner regions.