Separating star formation, AGN, and Shocks in Active Galaxies

Abstract

The cosmic evolution of galaxies and supermassive black holes (SMBHs) is believed to be correlated through interlinked physical processes. Numerical simulations suggest that active galactic nuclei (AGNs) feedback plays a vital role in governing the growth of the host galaxy. However, observational studies found that AGN does not significantly enhance or deficit the star formation rate (SFRs) in the host galaxy at z<2, making the failure of AGN feedback a mystery. More recently, a study using integral field spectroscopy (IFU) data on galaxy NGC 5728 reveals the presence of positive and negative AGN feedbacks in a single galaxy, suggesting the similarity in total SFRs between AGN-host galaxies and star-forming galaxies may result from the balancing effects of both positive and negative AGN feedback within the galaxy.

To better understand how AGNs influence host galaxy star formation and, ultimately, govern (or fail to govern) the growth and evolution of their host galaxies, this thesis tackles the critical bottleneck in studies of the interplay between AGN and star formation and AGN feedback mechanisms: the separation of star formation, AGN activity, and shocks in active galaxies.

To address the inconsistencies in existing AGN models, we first construct a reliable AGN photoionization model capable of predicting emission line properties across a broad wavelength range for Seyfert galaxies. Using the new AGN model, we then develop a set of consistent AGN metallicity diagnostics across UV, optical, and infrared wavelengths, which increase the number of available AGN metallicity diagnostics in the literature from around five to more than twenty. Utilizing the new AGN model and consistent HII model and the time-dependent shocks and precursor models, we then build a theoretical three-dimensional (3D) diagram that can simultaneously separate star formation, AGN, and shocks in galaxies with integral field spectroscopy (IFU) data. The inclusion of theoretical models in the new 3D diagram independently constrains the parameter space for each mechanism and provides information on the gas metallicity, ionization states, and shock velocity along with the separation.

By applying these new theoretical models on the 3D diagram, we separate the star formation, AGN, and shock in the central ~5 kpc region in NGC 5728. In addition to confirming the star-forming ring and AGN bicone outflow in the galaxy center, we also detect a fast shock-dominated rectangular structure with a length of ~0.5 kpc at the base of the AGN outflow. The presence of fast shocks at the base of AGN outflow is likely associated with a nuclear accretion disk. Applying similar studies to a large sample of galaxies will offer valuable insights into how black hole accretion influences host galaxy growth. Show more