AGN Diagnostics
At the center of most every galaxy lies a supermassive black hole, millions to billions of times larger than our own sun. Some of these lie dormant and dark, like Sagittarius A* at the center of the Milky Way. Others are actively accreting material, and in this process release an incredible amount of energy and present specific emission signatures in their spectra. Astronomers compare these different emission lines, often by taking the ratio of one line with another, and use them as a surrogate for different properties of the emitting regions. For example: how “hard,” or high-energy, are the emitted photons? What is the ratio of ionized to neutral atoms in the surrounding gas? What is the abundance of free electrons in the gas, able to create forbidden emission lines via collisions? These ratios are plotted in a diagram called a Baldwin-Phillips-Terlevich (BPT) diagram. An objects location on this diagram can give us some idea whether or not it has an accreting Active Galactic Nucleus (AGN), shines purely from star formation, or is some combination of both.
Further Investigation Needed: Ambiguous Objects
AGN and star-forming galaxies usually separate themselves out on a traditional BPT diagram due to the different mechanisms powering their emission. However, there are some objects that are ambiguous in nature, located between the Kauffmann (Kauffmann et. al., 2001) and Kewley (Kewley et. al. 2003) lines on the BPT diagram. These lines represent an empirical separation of AGN and star-forming galaxies, and the maximum possible starburst emission a galaxy can have, respectively. Galaxies located between these are widely considered to be composite objects; objects that contain both AGN and star-formation activity and may represent an evolutionary stage in the assumed SMBH-host galaxy connection. My work focuses on these objects and, using different emission features in their spectra, attempts to classify the sources of their emission.
We predict that not every object in the ‘composite’ zone is truly an AGN with some active nuclear star-formation, and that there may be some extreme examples of pure AGN or pure SF galaxies present there that are important to both study individually, and include in wider surveys.
BPT Diagram from Trouille+ (2011). Red objects are AGN, and blue objects are star-forming galaxies. The dashed line is the Kauffmann (2001) line, and the dotted is the Kewley (2003) line. Grey objects fall into the composite zone, which is what I explore in my work.
Areas of Exploration
As shown above, there are a great deal of objects that fall in the composite zone. My current work focuses on:
- Using high-energy emission lines to identify the true nature of ambiguous objects.
- Establishing additional methods of AGN diagnostics using their emission lines.
- Understanding what processes and conditions are responsible for the strength of certain emission lines in AGN, SF, and composite spectra.
- Comparing observational data to simulated spectra from photoionization modelling codes CLOUDY and MAPPINGS V.
- Using a holistic approach to ‘diagnose’ the nuclei of objects in the composite zone (such as broad lines from AGN, or the ‘bump’ created by Wolf-Rayet stars).
