A Bayesian approach to tilted-ring modelling of galaxies
- Authors: Maina, Eric Kamau
- Date: 2020
- Subjects: Bayesian statistical decision theory , Galaxies , Radio astronomy , TiRiFiC (Tilted Ring Fitting Code) , Neutral hydrogen , Spectroscopic data cubes , Galaxy parametrisation
- Language: English
- Type: text , Thesis , Masters , MSc
- Identifier: http://hdl.handle.net/10962/145783 , vital:38466
- Description: The orbits of neutral hydrogen (H I) gas found in most disk galaxies are circular and also exhibit long-lived warps at large radii where the restoring gravitational forces of the inner disk become weak (Spekkens and Giovanelli 2006). These warps make the tilted-ring model an ideal choice for galaxy parametrisation. Analysis software utilizing the tilted-ring-model can be grouped into two and three-dimensional based software. Józsa et al. (2007b) demonstrated that three dimensional based software is better suited for galaxy parametrisation because it is affected by the effect of beam smearing only by increasing the uncertainty of parameters but not with the notorious systematic effects observed for two-dimensional fitting techniques. TiRiFiC, The Tilted Ring Fitting Code (Józsa et al. 2007b), is a software to construct parameterised models of high-resolution data cubes of rotating galaxies. It uses the tilted-ring model, and with that, a combination of some parameters such as surface brightness, position angle, rotation velocity and inclination, to describe galaxies. TiRiFiC works by directly fitting tilted-ring models to spectroscopic data cubes and hence is not affected by beam smearing or line-of-site-effects, e.g. strong warps. Because of that, the method is unavoidable as an analytic method in future Hi surveys. In the current implementation, though, there are several drawbacks. The implemented optimisers search for local solutions in parameter space only, do not quantify correlations between parameters and cannot find errors of single parameters. In theory, these drawbacks can be overcome by using Bayesian statistics, implemented in Multinest (Feroz et al. 2008), as it allows for sampling a posterior distribution irrespective of its multimodal nature resulting in parameter samples that correspond to the maximum in the posterior distribution. These parameter samples can be used as well to quantify correlations and find errors of single parameters. Since this method employs Bayesian statistics, it also allows the user to leverage any prior information they may have on parameter values.
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Calibration and interpretation of A 2.3 GHz continuum survey
- Authors: Greybe, Andrew
- Date: 1984
- Subjects: Radio astronomy , Astronomical observatories , Galaxies
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5502 , http://hdl.handle.net/10962/d1007210 , Radio astronomy , Astronomical observatories , Galaxies
- Description: This thesis continues the Rhodes 2.3 GHz Survey of the Southern Sky. It consists of two parts : a data processing part and an astronomical analysis part. In the data processing part the data for the regions 4HR to 15HR, -80° to -61° and 12HR to 23HR . -27° to -7° are presented in contour map format. A beam pattern of the Hartebeesthoek telescope at 13 cm is constructed from drift scans of the radio source TAU A. This is used to investigate the data filtering techniques applied to the Rhodes Survey. It is proposed that a set of widely spaced scans which have been referred to the South Celestial Pole can provide a single calibrated baselevel for the Rhodes Survey. The observing technique and the necessary reduction programs to create a coarse grid of antenna temperatures of the Southern Sky using these observation are developed. Preliminary results for this technique are presented as a map of the region 18HR to 6HR, 90° to 30° with a 5°x5° resolution. On the astronomical side two studies are undertaken. The region 13HR to 23HR, -61° to -7° is searched for large extended areas of emission. 7 features occurring at intermediate galactic latitudes are found. They are interpreted as follows: one of them is the classical HII region surrounding the star Zeta Ophiuchi (l",b")=(6.7°,22.4°), and the rest are combinations of thermal and nonthermal emission from galactic features. The galactic equator profile for 24°> L > -58° is studied. It is dominated by a plateau of emission for L < -26°. This is interpreted as a combination of thermal and nonthermal radiation emitted by a ring of gas symmetric about the galactic centre with a radius of 4 - 6 kpc.
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