Research

Dual Active Galactic Nuclei

Galaxies are thought to grow when smaller galaxies interact and merge into each other, and as they do so, they funnel large amount of molecular gas towards their centers. This often leads to massive star formation in the centers of galaxies and triggering the accretion of one or both galactic nuclei (AGN). The dual AGN phase occurs when both the supermassive black holes are active and are separated by kiloparsec scale distance, and thought to last for about 100 million years. Since most galaxies contain SMBHs at their centers, and galaxies are presumed to grow through mergers, we expect dual AGNs in merger-remnant galaxies. Yet only 35dual AGN systems have been found so far.

In this project, we aim to study close dual AGN systems using high resolution and sensitivity observations at sub-millimeter, infrared, visible and X-ray wavelength to evaluate the impact of gas content, morphology, and mass ratio on AGN dynamics, and in particular, investigate if there are any particular physical conditions that differentiate dual AGNs from inactive galaxies and secular AGN.

Unification of Black Holes Across the Mass Scale

Black holes that accrete surrounding matter are some of the brightest objects in the sky and allow us to study physical processes under conditions of extreme gravity and high orbital velocities. Black holes usually come in two varieties: stellar mass black holes, which are often found in X-ray binaries, and supermassive black holes, at the centres of galactic nuclei. Are these black hole systems analogous to each other, and differ only in scale? The relation between black hole mass, X-ray and radio luminosities, called the fundamental plane of black hole activity, provides some support to this.

The unification between AGNs and XRBs is important for understanding the accretion processes in both kinds of objects. In this project, we investigate the inherent similarities and differences between AGN & XRB, and what can be scaled up or down to understand these two classes of black holes, with the primary objective of comparing various states & properties of XRBs with different classes of AGN.

Breakfast of the Beast: Flaring activity of Sgr A*

At the Centre of the Milky Way lies a supermassive black hole, Sagittarius A* (Sgr A*), 4 million times as massive as the Sun. Sagittarius A* does not have a stable and continuous accretion disk, and is classified as inactive. But it is known to undergo regular flaring events which arise from the innermost region of the accretion flow. These flares have been observed across different wavelengths, from NIR & X-ray to radio and submillimeter frequencies. Multiwavelength observations have shown that the NIR & X-ray flares occur simultaneously, and are followed by the submm & radio flares. The study of the flaring activity provides us indirect constraints on the details of the emission mechanism, and understanding the physics in close vicinity of a supermassive black hole.

We observed the Galactic Centre at 3 mm wavelength using the Australia Telescope Compact Array between 2010 and 2014. We used Structure function analysis and Bayesian blocks representation to detect the flaring events in the light-curves of Sagittarius A*. We detected six flaring events from the observations, lasting 1.5 - 3 hours.  We then used the adiabatically expanding plasmon model to model the flares and derive the properties of the source that gives rise to the flares. This project was part of my PhD thesis, and the results have been published in Monthly Notices of the Royal Astronomical Society, Volume 458, Issue 3