The distribution of mass in dwarf galaxies and the structure of their ISM
Zusammenfassung der Projektergebnisse
In this project we have used the Very Large Array (VLA) and the Westerbork Synthesis Radio Telescope (WSRT) to carry out Hl measurements of the disks of a carefully selected sample of low-mass, gasrich galaxies down to very low Hl column densities and with high spectral resolution. At the same time, we investigated the stellar disks using deep near-infrared (NlR) photometry, by using data published in the literature as well as by performing such a photometry with the Isaac Newton Telescope (INT) on La Palma. Since both, the data reduction and their analysis was rather time-consuming, we have finalised the full analysis for two of the sample galaxies, leading to a first publication in a refereed Journal, while the data reduction has been finished for all of the objects. The two isolated nearby dwarf irregular galaxies UGCA105 and UGC11891 exhibit a rather fragmented Hi distribution, which is rather extended as compared to their optical disks. For UGCA105 we derive a well-ordered and symmetric velocity field. We find a substantial amount of differential rotation, which is a rare finding in such dwarf galaxies. Also UGC11891 shows quite well-ordered rotation, however the velocity field is slightly asymmetric due to an arm-like extension of the Hi disk. Tilted-ring modelling is carried out using two alternative GIPSY (Groningen Image Processing System) least-squares fitting routines: ROTCUR and TiRiFiC, which operate on the basis of the velocity field and of the entire spectral data cube, respectively. The Hl disks of both galaxies are found to have a warped and diffuse outermost part. Moreover, we have detected kinematically anomalous Hl in UGCA105 (and likely also in UGC11891), which is probably extraplanar, and may hence (at least in part) result from minor mergers or cold accretion from the intergalactic medium. For the mass decomposition, different models of the dark-matter distribution are considered, as well as Hl scaling and MOND. The rotation curves are fitted best by core-dominated dark-matter density profiles. In the case of UGCA105, however, we observe a bump in the rotation curve, which cannot be reproduced by any of the dark-halo models. Only Hl scaling provides a fairly reasonable approximation of the bump, but requires exceptionally large stellar mass-to-light ratios and gas-scaling factors. The underlying cause of this effect is unknown.
