Doctor of Philosophy
Planetary Science and Exploration
Gallagher, Sarah C.
In the centres of massive galaxies, active galactic nuclei (AGN) are supermassive black holes, surrounded by an accretion disk of ionized gas, that release tremendous energy in the form of electromagnetic radiation. Because AGN are unresolved through telescopes, we employ reverberation mapping (RM) to study their structure. RM capitalizes on the fact that AGN are variable – the continuum emission from the accretion disk varies, and surrounding gas (in the broad-line region, BLR) responds to those variations with a positive time lag. RM translates the measured time lag into a size of the BLR. Combined with gas velocities (measured from the width of broad emission lines in AGN spectra), RM yields dynamically measured black hole masses, essential for understanding How supermassive black holes grow over cosmic time?.
We conducted a detailed RM study of a highly accreting AGN, Markarian 142, for the first time with the Gemini North and Lijiang Observatories, with simultaneous ultraviolet (UV) continuum and spectroscopic BLR observations. We determined a Hydrogen-beta lag of 8.58 days relative to the UV continuum. Deriving UV lags is important as UV emission more closely tracks the ionizing continuum that dominates the energetics of the BLR than the optical continuum.
Constraining black hole masses in RM is difficult, partly due to uncertainties on emission-line widths. Higher-resolution and high signal-to-noise spectra allow separation of blended lines that originate in different regions of the BLR. In the particular case of Markarian 142, we determined that unresolved narrow lines and flexible narrow-line flux ratios result in excess flux attributed to the narrow-line component of the complex Hydrogen-beta line, broadening the apparent widths measured for these broad lines. We further present recommendations on methodology for line-width measurements in narrow-line AGN to extract more accurate values.
RM studies have improved over time with larger sample sizes that span a broader diversity of the AGN population. To successfully implement large-scale variability campaigns, optimized survey designs are required to maximize science returns. We illustrated the use of such a survey simulation pipeline to optimize AGN RM surveys with a next-generation UV telescope, CASTOR.
Summary for Lay Audience
Residing at the centres of massive galaxies, active galactic nuclei (AGN) are supermassive black holes (millions to billions of solar masses) surrounded by a disk of gas orbiting the central black hole. Gas accreting onto the black hole releases tremendous energy in different forms of light which makes these objects extremely luminous. However, AGN are a thousand times smaller than their host galaxies and hence cannot be mapped with telescopes.
To study AGN, we rely on reverberation mapping that takes advantage of the varying light output in these systems to probe their sizes. The light from the disk illuminates and is absorbed by nearby gas (in the broad-line region, BLR) and re-emitted with a time lag. Reverberation mapping converts time lag into distance (size of the BLR). Using BLR size and gas velocities from broad emission lines in AGN spectra (light from different energies spread on a detector), we can calculate black hole masses in AGN. Studying black holes in a large number of AGN over a wide range of cosmic times is important to understand how supermassive black holes grow over time.
Although AGN have been studied for decades, the structure of AGN that grow very quickly is poorly understood. In our first project, we focused on one such object – Markarian 142. We derived the time lag of the BLR response to the ultraviolet (UV) light from the disk using simultaneous observations from four telescopes on the ground and in space.
To accurately calculate black holes masses in AGN, we need higher-resolution and high signal-to-noise spectra that allow us to separate blended emission lines. This deblending is critical to reliably measure BLR gas velocities. In our second project, we investigated how low spectral resolution affects measurements of narrower broad lines in AGN spectra.
Considering the increasing scale of surveys that aim to measure many AGN black hole masses, it is important that we optimize large-scale surveys with next-generation facilities to maximize science outcomes. In our third project, we illustrated the use of a survey computer simulation pipeline to optimally design variability surveys with a proposed, Canadian-flagship UV telescope, CASTOR.
Khatu, Viraja Chandrashekhar, "Probing the Inner Structure of Active Galactic Nuclei Through Reverberation Mapping" (2022). Electronic Thesis and Dissertation Repository. 8685.