Tracing the Evolution of Local Universe Galaxies by Kinematical Studies of HI

Abstract

This work studies the neutral gas in external galaxies using the neutral atomic hydrogen (HI) line to investigate the kinematics of the gas component. This thesis consists of three parts, one part focusses on early-type galaxies (ETGs), one part on spiral galaxies, and one part on the systematic effects which are inherent to the applied methods.<br /> The first part of this work studies the Tully-Fisher Relation (TFR, Tully & Fisher 1977) for ETGs. This work applies the TFR to study physical properties of ETGs. To this end, the asymptotic rotation velocities for a subset of the ATLAS^3D ensemble of ETGs have been derived using velocity fields. These velocities are complemented by K-band magnitudes from the 2MASS survey as well as stellar mass-to-light ratios from the ATLAS^3D project. It is found that the K-band TFR reveals a high scatter. If instead the baryonic TFR is constructed, the scatter is substantially reduced. Furthermore, the HI TFR appears to be largely consistent with the carbon monoxide (CO) TFR for the ATLAS^3D sample. This implies that the CO line, which is detectable out to larger redshifts, can also be used to study the TFR and thus can in principle be used to study the redshift evolution of galaxies. Finally, the ETG TFR has been compared to that of spiral galaxies. The K-band TFR for ETGs is found to be systematically below the one for spiral galaxies. It is also found that if the average M_*/L_K of spirals is approx. 0.3 M_⊙/L_⊙, the baryonic TFR for ETGs and spirals is identical.<br /> The second chapter studies a sample of spiral galaxies from the “Bluedisk” ensemble. The goal is to gain insight into the dominating mechanism by which galaxies are supplied with gas to form stars in the local universe. Concerning this, there are two hypotheses: firstly, the accretion of cold gas from the external environment. Secondly, condensation of gas via cooling of a hot coronae of ionised gas as predicted in a dark matter scenario. The “Bluedisk” sample consists of a sample of 48 galaxies, half of which are selected using optical properties to be particularly gas rich, and the other half to have normal gas content. The data is investigated for signatures of cold gas accretion, i.e., warps, lopsidedness etc. Moreover, the velocity fields of both of the subsamples are used to search for asymmetries, which could point at enhanced accretion of gas. The result is that there are no signs of enhanced episodic gas accretion from the environment. This is in line with the earlier results from the “Bluedisk” survey, focussing on the morphological aspects of the galaxies.<br /> The final chapter studies a selection of systematic effects in the analysis of HI data. Firstly, the effect of Milky Way (MW) contamination on the kinematics of the short-spacing correction (SSC) data set is investigated. In this case, the MW emission causes errors in the flux measurements. Based on NGC 2403 data, it has been shown that also the kinematics cannot be analysed if the velocity field is used. Instead, the data cube has to be analysed to obtain a good description of the data. Secondly, the influence of distance on velocity fields is studied, as this is relevant for both of the preceding chapters. It is found that only for the most nearby spiral galaxies with distances of roughly 20 Mpc, radial profiles of the most important kinematical quantities; the rotation curve, the disk geometry, and even non-circular motions, can be measured. If one studies galaxies at larger distances, only global kinematical properties can be extracted. Finally, it is shown that the vertical density and velocity structure of a galaxy can have an impact on the deduced parameters from a velocity field.