We present the structural and star formation properties of 59 void galaxies as part of the Void Galaxy Survey (VGS). Our aim is to study in detail the physical properties of these void galaxies and study the effect of the void environment on galaxy properties. We use Spitzer 3.6μ and B-band imaging to study the morphology and color of the VGS galaxies. For their star formation properties, we use Hα and GALEX near-UV imaging. We compare our results to a range of galaxies of different morphologies in higher density environments. We find that the VGS galaxies are in general disk dominated and star forming galaxies. Their star formation rates are, however, often less than 1 M⊙ yr-1. There are two early-type galaxies in our sample as well. In re versus MB parameter space, VGS galaxies occupy the same space as dwarf irregulars and spirals.

We report the discovery of two small intergalactic H II regions in the loose group of galaxies around the field elliptical NGC 1490. The H II regions are located at least 100 kpc from any optical galaxy but are associated with a number of large H I clouds that are lying along an arc 500 kpc in length and that have no optical counterpart on the Digital Sky Survey. The sum of the H I masses of the clouds is almost 1010M⊙ and the largest H I cloud is about 100 kpc in size. Deep optical imaging reveals a very low surface brightness counterpart to this largest H I cloud, making this one of the H I richest optical galaxies known (MHI/LV ~ 200). Spectroscopy of the H II regions indicates that the abundance in these H II regions is only slightly sub-solar, excluding a primordial origin of the H I clouds. The H I clouds are perhaps remnants resulting from the tidal disruption of a reasonably sized galaxy, probably quite some time ago, by the loose group to which NGC 1490 belongs. Alternatively, they are remnants of the merger that created the field elliptical NGC 1490. The isolated H II regions show that star formation on a very small scale can occur in intergalactic space in gas drawn from galaxies by tidal interactions. Many such intergalactic small star formation regions may exist near tidally interacting galaxies.

Studies of the HI in galaxies have clearly shown that subtle details of the HI distribution and kinematics often harbour key information for understanding the structure and evolution of galaxies. Evidence for the accretion of material has grown over the past many years and clear signatures can be found in HI observations of galaxies. We have obtained new detailed and sensitive HI synthesis observations of three nearby galaxies which are suspected of capturing small amounts of HI and show that indeed accretion of small amounts of gas is taking place in these galaxies. This could be the same kind of phenomenon of material infall as observed in the stellar streams in the halo and outer parts of our galaxy and M 31.

We are studying the mass distribution in a sample of 50 early type spiral galaxies, with morphological type betweens SO and Sab and absolute magnitudes Mb between −18 and −22; they form the massive and high-surface brightness extreme of the disk galaxy population. Our study is designed to investigate the relation between dark and luminous matter in these systems, of which very little yet is known.

From a combination of WSRT Hi observations and long-slit optical spectra, we have obtained high-quality rotation curves. the rotation velocities always rise very fast in the center; in the outer regions, they are often declining, with the outermost measured velocity 10-25% lower than the maximum.

We decompose the rotation curves into contributions from the luminous (stellar and gaseous) and dark matter. the stellar disks and bulges always dominate the rotation curves within the inner few disk scale lengths, and are responsible for the decline in the outer parts. As an example, we present here the decompositions for UGC 9133. We are able to put tight upper and lower limits on the stellar mass-to-light ratios.

We are studying the properties of the holes and the high velocity gas in NGC 6946. Here we present some puzzling results.

In this paper we discuss the large-scale structures (> 500 pc) of the neutral atomic hydrogen in the disk of nearby galaxies, with an emphasis on finding evidence for neutral gas features in the disk - halo interface. Most nearby galaxies studied in detail appear to have hole and shell type structures in their HI disks and several show evidence for gas at peculiar velocities, probably examples of gas streaming out of the disk into the halo. Not all of the observed phenomena can be explained by stellar winds and supernova explosions and other explanations will briefly be discussed.