Supermassive black holes are found to be ubiquitous in all massive galaxies and their mass is observed to correlate with properties of the host galaxy bulge. Therefore, these black holes are believed to play an important role in galaxy formation and evolution. Most notably so through various feedback mechamisms as the black hole is growing by accreting matter in its phase as an Active Galactic Nucleus (AGN). The amount of possible feedback from the black hole is largely governed by its mass. Accurately measuring masses of black holes in AGN is therefore a crucial part of understanding the physics of AGN and their possible impact on galaxy formation and evolution. Many factors contribute to the uncertainty of the mass estimates in AGN, but studies have shown that the quality of and the way data are measured is important. Specifically, the accuracy to which we can measure broad emission line widths in AGN spectra as a proxy for the velocity of the gas in the Broad Line Region (BLR) has shown to be important. In this work, I aim to provide a comprehensive account of how much the uncertainty on the mass is affected by uncertainties related to measurements of broad emission line widths in AGN spectra. I find that for low data quality and for certain parameterisations of the line width, the impact on the accuracy of the black hole mass can easily be up to 0.5 dex. In Chapters 2 and 3 I determine the accuracy and precision of four different line width parameters used for black hole mass estimates as a function of spectral S/N. I do so for a sample of objects with spectra representative of the general observed AGN population and covering the broad Hβ, Mg II, and C IV emission lines. I find that the MAD and IPV widths show a higher accuracy and precision than the FWHM and σline at any given S/N level.
In addition, the IPV widths allow us to measure the line width at many fractions of the emission line flux, providing more flexibility and the option of measuring the line shape in a robust way using ratios of different IPV widths. In Chapter 4 I show that narrow absorption negatively affects the accuracy of the four line parameters if it is not accounted for in the spectra. The results in Chapters 2 – 4 can be used to assess realistic uncertainties for mass estimates based on existing spectral data and to plan optimally for future observations. I recommend to aim for S/N≥20 pixel−1 in the continuum and further to have a spectral resolution of 100 km s−1 or better in order to be able to account for narrow absorption lines. The second part of this thesis zooms out further from the central black hole. Spectral mid-IR features from Poly-cyclic Aromatic Hydrocarbon (PAH) molecules are used to infer recent and ongoing star formation around AGN where other more traditional methods typically fail. This method rests upon the assumption that the PAH features are excited exclusively by star formation and not the AGN itself. We test this assumption by using mid-IR high angular resolution spectroscopy for a sample of nearby AGN. We trace the radial profile of the 11.3 μm PAH feature in each AGN and find that the radial profile for this emission feature is similar amongst the different AGN. This indicates that they are excited by a compact central source and not diffuse star formation. Through comparisons with CLOUDY simulations of an AGN as the excitation source and observations of nuclear star clusters, we find that the most likely explanation for the radial profiles of the 11.3 μm PAH feature is excitation by the AGN.
This shows that the 11.3 μm PAH feature probably is contaminated by the AGN within approximately 1 kpc and that care should be taken when using this feature to estimate star formation that close to the active nucleus.