Shelley-Anne Harrisberg

Field of Interests

My main field of interest concerns supermassive black holes (BHs) and their possible connection to galaxy evolution. Although massive, BHs are compact objects with small spatial extents (typically on the scale of astronomical units) that are tiny in comparison to that of their host galaxies (typically on the scale of kiloparsecs). Moreover, their gravitational influence, while significant, extends only over the innermost region of the galaxy (typically on scales of parsecs). Nonetheless, several strong correlations have been found to exist between BHs and various properties of their host galaxies that suggest some form of co-evolution. This co-evolution is believed to be in the form of self-regulating mechanisms such as feedback from Active Galactic Nuclei (AGN). However, while theoretical models of galaxy evolution support this, the exact nature of this co-evolution is not fully understood, nor is the formation process of the BHs themselves. It is hoped that the afore-mentioned correlations might provide a key to a more robust understanding of exactly these issues.

To better examine these correlations, a large and robust dataset of BH measurements over the entire range of galaxy morphologies and masses is required. One of the most common (and, to date, accurate) method of measuring BH masses is via the so-called dynamical methods. This involves selecting a tracer (such as stars, gas or maser emission) in the region of the BH’s gravitational sphere of influence and measuring its circular velocity through dynamical modelling. A relatively recent addition to the available tracers has been cold molecular gas (previous measurements with gas were with hot, ionized gas), which can be observed in the millimetre/submillimetre wavelengths. This has been made possible with the advent of the Atacama Large Millimetre/sub-millimetre Array (ALMA) and its predecessor, the Combined ARRAY for Research in Millimetre-wave Astronomy (CARMA), both of which have provided resolutions necessary to resolve the BH’s gravitational sphere of influence or regions close enough to accurate measure or constrain the mass.

For my Master Thesis project, I am conducting a mass measurement of the putative BH in the massive elliptical galaxy NGC 2513 with the aim of adding to the database of BH mass measurements in high mass galaxies using cold molecular gas dynamics. All dynamical methods involve deriving an accurate measurement of the luminous mass of the galaxy (mostly stars, but also gas and dust if significant enough). Subtracting this from the total mass – which is derived from the dynamical model of the tracer – will give the mass of the BH. To derive an accurate model of the galaxy’s luminous mass, I use a high-resolution infrared image of the galaxy from the Hubble Space Telescope (HST) and model its two- dimensional surface brightness contours (isophotes) using the Multi-Gaussian Expansion method, whilst taking care to mask the galaxy’s prominent dust lane. Failure to take dust obscuration of a galaxy’s luminosity can lead to under-estimating the luminous mass of the galaxy. The luminous mass is obtained in the process of the dynamical modelling by multiplying the deprojected intrinsic luminosity with the mass-to-light ratio of the galaxy.

To get the total mass, I first combine and then image high- and low-resolution 12CO(2-1) data using the Common Astronomy Software Applications package (CASA) to produce a 3-D velocity cube (R.A., Dec, Velocity) which can be modelled. The high-resolution data is required to resolve the sphere of influence, while the low-resolution data is necessary for sufficient sensitivity. The modelling is performed with the KINematic Molecular Simulation (KinMS) routines developed by Davis et al. (2012), which coupled with an MCMC routine provides a best-fit BH mass.