S. P. C. Peters

The shape of dark matter haloes – III. Kinematics and structure of the H i disc

We present a new strategy for fitting the structure and kinematics of the H I in edge-on galaxies using a fit to the terminal-velocity channel maps of a H I data cube. The strategy can deal with self-absorbing H I gas and the presence of warps. The method is first tested on a series of models. We demonstrate that fitting optically thin models to real galaxies will lead to an overestimation of the thickness and velocity dispersion, and to a serious underestimation of the H I face-on column densities. We subsequently fit both self-absorption and optically thin models to the H I data of six edge-on galaxies. In three of these, we have also measured the velocity dispersion. On average, 27 ± 6 per cent of the total H I mass of edge-on galaxies is hidden by self-absorption. This implies that the H I mass, thickness and velocity dispersion of galaxies is typically underestimated in the literature.

The Way We Measure: Comparison of Methods to Derive Radial Surface Brightness Profiles

The breaks and truncations in the luminosity profile of face-on spiral galaxies offer valuable insights in their formation history. The traditional method of deriving the surface photometry profile for face-on galaxies is to use elliptical averaging. In this paper, we explore the question whether elliptical averaging is the best way to do this. We apply two additional surface photometry methods, one new: principal axis summation, and one old that has become seldom used: equivalent profiles. These are compared to elliptically averaged profiles using a set of 29 face-on galaxies. We find that the equivalent profiles match extremely well with elliptically averaged profiles, confirming the validity of using elliptical averaging. The principal axis summation offers a better comparison to edge-on galaxies.

The shape of dark matter haloes – IV. The structure of stellar discs in edge-on galaxies

We present optical and near-infrared archival observations of eight edge-on galaxies. These observations are used to model the stellar content of each galaxy using the FITSKIRT software package. Using FITSKIRT, we can self-consistently model a galaxy in each band simultaneously while treating for dust. This allows us to measure accurately both the scalelength and scaleheight of the stellar disc, plus the shape parameters of the bulge. By combining these data with the previously reported integrated magnitudes of each galaxy, we can infer their true luminosities. We have successfully modelled seven out of the eight galaxies in our sample. We find that stellar discs can be modelled correctly, but we have not been able to model the stellar bulge reliably. Our sample consists for the most part of slowly rotating galaxies and we find that the average dust layer is much thicker than is reported for faster rotating galaxies.

The shape of dark matter haloes - V. Analysis of observations of edge-on galaxies

In previous papers in this series, we measured the stellar and H I content in a sample of edge-on galaxies. In the present paper, we perform a simultaneous rotation curve and vertical force field gradient decomposition for five of these edge-on galaxies. The rotation curve decomposition provides a measure of the radial dark matter potential, while the vertical force field gradient provides a measure of the vertical dark matter potential. We fit dark matter halo models to these potentials. Using our H I self-absorption results, we find that a typical dark matter halo has a less dense core (0.094 ± 0.230 M⊙ pc-3) than that for an optically thin H I model (0.150 ± 0.124 M⊙ pc-3). The H I self-absorption dark matter halo has a longer scale-length Rc of 1.42 ± 3.48 kpc, versus 1.10 ± 1.81 kpc for the optically thin H I model. The median halo shape is spherical at q = 1.0 ± 0.6 for self-absorbing H I, while it is prolate at q = 1.5 ± 0.6 for the optically thin case. Our best results were obtained for ESO 274-G001 and UGC 7321, for which we were able to measure the velocity dispersion in Paper III. These two galaxies have very different halo shapes, with one oblate and one strongly prolate. Overall, we find that the many assumptions required make this type of analysis susceptible to errors.

The shape of dark matter haloes - II. The GALACTUS H I modelling & fitting tool

We present a new H I modelling tool called GALACTUS. The program has been designed to perform automated fits of disc-galaxy models to observations. It includes a treatment for the self-absorption of gas. The software has been released into the public domain. We describe the design philosophy and inner workings of the program. After this, we model the face-on galaxy NGC 2403 using both self-absorption and optically thin models, showing that self-absorption occurs even in face-on galaxies. These results are then used to model an edge-on galaxy. It is shown that the maximum surface brightness plateaus seen in Paper I of this series are indeed signs of self-absorption. The apparent H I mass of an edge-on galaxy can be drastically lower compared with that same galaxy seen face-on. The Tully-Fisher relation is found to be relatively free from self-absorption issues.