Master's thesis defence by Fiona Hjorth Gammelgaard

Investigating the Host Galaxy Contribution to AGN spectra

Abstract

In this project, I investigate how accurately one can measure different components in spectra of active galactic nuclei (AGN), most importantly, for which the host galaxy contribution to the measured total continuum can be trusted. AGNs are highly luminous central regions of galaxies, where large amounts of energy are emitted when gas is accreted onto the supermassive black hole in the galaxy center.

To study the emission from the active nucleus alone, it is necessary to subtract the light from the stars in the galaxy, which is included in the observed spectra. To reach my scientific goal, I simulate realistic AGN spectra with varied contributions and signal to noise ratios.

I develop a spectral decomposition code to fit and measure the AGN components and the host galaxy contribution in the simulated spectra. I carry out statistical analysis of the measurement results to understand how the host galaxy measurement is affected by physical parameters, measurement noise and redshift. I find that the most reliable host galaxy fractions are 70% and 80%. For spectra with host contributions of 25% and especially 10%, the fit program often fails to measure the host fraction. I find that to obtain the most accurate and precise results, a signal to noise ratio of at least ∼ 15 is necessary. I find that the host galaxy measurement is significantly affected by the redshift and the slope of the AGN continuum.

The results also indicate that measurements of real observational data may be misleading, since it is often not possible to distinguish between host galaxy fractions that differ by approximately +-10%, in some cases even more. This introduces additional measurement uncertainty. As a consequence of these findings, measurement uncertainties must always be considered, as an inaccurate host galaxy measurement can affect estimates of other AGN properties such as the mass of the central black hole.

Supervisor

Marianne Vestergaard

External examiner

Hans Kjeldsen (AU)