Separation of Line Emission from Star Formation, Shocks, and AGN Ionisation in Galaxies

Abstract

The processes of star formation and the accretion from an active galactic nucleus (AGN) are known to be linked, and together they drive the evolution of their host galaxy. Yet the exact nature of the link between the two processes remains uncertain. The advent of integral field spectroscopy has advanced the study of star formation-AGN mixing, through spatially-resolved spectra showing the mixing across a galaxy. This thesis builds upon the previous work of star formation-AGN mixing, and also considers the contribution from a third ionising source, shocks, subsequently.

The first part of this thesis concerns photoionisation modelling. Specifically, we study the systematic effects and uncertainties present in photoionisation models on the BPT diagram, as a result of using various examples and values of model parameters. On average, we show that differences in the input physics of differing model codes account for a ~0.1 dex in [O III]/H-beta and [N II]/H-alpha ratios.

The second part of this thesis applies photoionisation models to the study of star formation-AGN mixing, in order to calculate the relative contributions of star formation and AGN ionisation to line emission in IFU data. The application of photoionisation models to this study allows the relative contribution of each ionisation source to be calculated using theory, rather than by empirical means. Data used during this work is from the TYPHOON/PrISM survey. We also rebin the data to lower resolutions to study the effects of spatial resolution on star formation-AGN mixing. Results show that lowering the resolution will overestimate the contribution from the AGN. Low-surface-brightness features, such as shocks or diffuse ionised gas (DIG), also appear at lower resolutions as the data is rebinned.

In the third part of the thesis, we consider the inclusion of shocks in star-formation-AGN mixing, leading to the study of star formation-shock-AGN mixing. Conventional methods used to calculate the relative contribution of star formation and AGN to line emission, such as the BPT diagram, prove ineffective in separating line emission from shocks and AGN. We display a new three-dimensional diagnostic diagram, which aids in the simultaneous separation of line emission from all three ionising sources. This new diagram retains emission-line ratio information used in the BPT diagram, while adding distance and velocity dispersion values for each spaxel in the IFU datacube. To demonstrate this diagram, we use the high spatial and spectral resolution data of the Siding Spring Southen Seyfert Spectroscopic Snapshot Survey (S7). Specifically, we use the prototypical Seyfert galaxy NGC 1068 as a test case. Our results show that data on the 3D diagram displays two distinct mixing sequences of spaxels, which we show to be a star formation-AGN sequence, and star formation-shock sequence. Significant scatter is present towards the AGN and shock regions of their relevant sequences, indicating mixing between the emission from shocks and AGN also.

In the fourth part of the thesis, we further the results of the previous section by calculating the relative contribution of line emission from star formation, shocks, and AGN in each spaxel of the galaxy NGC 1068 from the S7. When compared to simply star formation-AGN mixing, the results suggest that the shocked emission mixes primarily with the AGN emission. Hence, not accounting for shocks in the decomposition will greatly overestimate the true contribution of photoionisation from the AGN. The results also show that if H-alpha is to be used as a star formation rate indicator, then separation of the line emission between as many sources as possible should be performed to provide accurate results. The fifth and final part of the thesis provides a brief overview on possible future work in the field.