van der Thijs Hulst

The H I Halo of NGC 891

Neutral hydrogen observations of the nearby, edge-on spiral galaxy NGC 891 reveal the presence of an H I halo extending up to at least 5 kpc from the plane. This halo gas appears to rotate 25 to 100 km s(-1) more slowly than the gas in the plane. If this velocity difference is due to the gradient in the gravitational potential, then it may serve to discriminate between disk and spheroidal mass models. The classic picture of a large outer flare in the H I layer of NGC 891 may no longer be valid. A correlation is seen between the distributions of H I, H alpha and radio continuum emission; this, in accordance with galactic fountain models, supports the picture of a substantial disk-halo circulation related to star formation activity in the disk of NGC 891. There is now also clear evidence for the presence of a rapidly rotating (upsilon(rot) similar or equal to 230 km s(-1)) disk or ring of H I in the central part of NGC 891.

Spin Alignment of Dark Matter Halos in Filaments and Walls

The MMF technique is used to segment the cosmic web as seen in a cosmological N-body simulation into wall-like and filament-like structures. We find that the spins and shapes of dark matter halos are significantly correlated with each other and with the orientation of their host structures. The shape orientation is such that the halo minor axes tend to lie perpendicular to the host structure, be it a wall or filament. The orientation of the halo spin vector is mass-dependent. Low-mass halos in walls and filaments have a tendency to have their spins oriented within the parent structure, while higher mass halos in filaments have spins that tend to lie perpendicular to the parent structure.

The Multiscale Morphology Filter: Identifying and Extracting Spatial Patterns in the Galaxy Distribution

Aims. We present here a new method, MMF, for automatically segmenting cosmic structure into its basic components: clusters, fi laments, and walls. Importantly, the segmentation is scale independent, so all structures are identified without prejudice as to the ir size or shape. The method is ideally suited for extracting catalogues of clusters, wa lls, and filaments from samples of galaxies in redshift surve ys or from particles in cosmological N-body simulations: it makes no prior assumptions about the scale or shape of the structures. Methods. Our Multiscale Morphology Filter (MMF) method has been developed on the basis of visualization and feature extraction t echniques in computer vision and medical research. The density or intensity field of the sample is smoothed over a range of scales. Th e smoothed signals are processed through a morphology response filter whose for m is dictated by the particular morphological feature it see ks to extract, and depends on the local shape and spatial coherence of the intensity field. The morphology signal at each location is then defi ned to be the one with the maximum response across the full range of smoothing scales. The success of our method in identifying anisotropic features such as filaments and walls depends critically on the use of an optimally defined intensity field. This is accomplished by applying the DTFE re construction methodology to the sample particle or galaxy distribution. Results. We have tested our MMF Filter against a set of heuristic models of weblike patterns such as are seen in the Megaparsec cosmic matter distribution. To test its effectiveness in the context of more realistic configurations w e also present preliminary results from the MMF analysis of an N-body model. Comparison with alternative prescriptions for feature extraction shows that MMF is a remarkably strong structure finder

The Westerbork HI survey of spiral and irregular galaxies. III. HI observations of early-type disk galaxies

We present H observations of 68 early-type disk galaxies from the WHISP survey. They have morphological types between S0 and Sab and absolute B-band magnitudes between –14 and –22. These galaxies form the massive, high surface-brightness extreme of the disk galaxy population, few of which have been imaged in H before. The H properties of the galaxies in our sample span a large range; the average values of MHI/LB and DHI/D25 are comparable to the ones found in later-type spirals, but the dispersions around the mean are larger. No significant differences are found between the S0/ S0a and the Sa/Sab galaxies. Our early-type disk galaxies follow the same H mass-diameter relation as later-type spiral galaxies, but their effective H surface densities are slightly lower than those found in later-type systems. In some galaxies, distinct rings of H emission coincide with regions of enhanced star formation, even though the average gas densities are far below the threshold of star formation derived by Kennicutt (1989, ApJ, 344, 685). Apparently, additional mechanisms, as yet unknown, regulate star formation at low surface densities. Many of the galaxies in our sample have lopsided gas morphologies; in most cases this can be linked to recent or ongoing interactions or merger events. Asymmetries are rare in quiescent galaxies. Kinematic lopsidedness is rare, both in interacting and isolated systems. In the appendix, we present an atlas of the H observations: for all galaxies we show H surface density maps, global profiles, velocity fields and radial surface density profiles.

The rotation curves shapes of late-type dwarf galaxies

We present rotation curves derived from H i observations for a sample of 62 galaxies that have been observed as part of the Westerbork H i Survey of Spiral and Irregular Galaxies (WHISP) project. These rotation curves have been derived by interactively fitting model data cubes to the observed cubes. This procedure takes the rotation curve shape, the H i distribution, the inclination, and the size of the beam into account, and makes it possible to correct for the effects of beam smearing. A comparison with higher spatial resolution Hα rotation curves available in the literature shows that there is general agreement between the two. The late-type dwarf galaxies in our sample have rotation-curve shapes that are similar to those of late-type spiral galaxies, in the sense that their rotation curves, when expressed in units of disk scale lengths, rise as steeply in the inner parts and start to flatten at two disk scale lengths. None of the galaxies in our sample have solid-body rotation curves that extend beyond three disk scale lengths. The logarithmic slopes between two disk scale lengths and the last measured point on the rotation curve is similar between late-type dwarf and spiral galaxies. Thus, whether the flat part of the rotation curve is reached or not seems to depend more on the extent of the rotation curve than on its amplitude. We also find that the outer rotation curve shape does not strongly depend on luminosity, at least for galaxies fainter than MR ∼− 19. We find that in spiral galaxies and even in the central regions of late-type dwarf galaxies, the shape of the central distribution of light and the inner rise of the rotation curve are related. This implies that galaxies with stronger central concentrations of light also have higher central mass densities, and it suggests that the luminous mass dominates the gravitational potential in the central regions, even in low surface brightness dwarf galaxies.

The mass distribution in early-type disc galaxies : declining rotation curves and correlations with optical properties

We present rotation curves for 19 early-type disc galaxies (S0-Sab). The galaxies span a B-band absolute magnitude range from -17.5 to -22, but the majority have a high luminosity with M-B <-20. Rotation velocities are measured from a combination of H I velocity fields and long-slit optical emission line spectra along the major axis; the resulting rotation curves probe the gravitational potential on scales ranging from 100 pc to 100 kpc. We find that the rotation curves generally rise rapidly in the central regions and often reach rotation velocities of 200-300 km s(-1) within a few hundred parsecs of the centre. The detailed shape of the central rotation curves shows a clear dependence on the concentration of the stellar light distribution and the bulge-to-disc luminosity ratio: galaxies with highly concentrated stellar light distributions reach the maximum in their rotation curves at relatively smaller radii than galaxies with small bulges and a relatively diffuse light distribution. We interpret this as a strong indication that the dynamics in the central regions are dominated by the stellar mass. At intermediate radii, many rotation curves decline, with the asymptotic rotation velocity typically 10-20 per cent lower than the maximum. The strength of the decline is correlated with the total luminosity of the galaxies, more luminous galaxies having on average more strongly declining rotation curves. At large radii, however, all declining rotation curves flatten out, indicating that substantial amounts of dark matter must be present in these galaxies too. A comparison of our rotation curves with the Universal Rotation Curve from Persic et al. reveals large discrepancies between the observed and predicted rotation curves; we argue that rotation curves form a multiparameter family which is too complex to describe with a simple formula depending on total luminosity only. In a number of galaxies from our sample, there is evidence for the presence of rapidly rotating gas in the inner few hundred parsecs from the centres. The inferred central masses and mass densities are too high to be explained by the observed stellar components and suggest the presence of supermassive black holes in these galaxies.

Extended, regular HI structures around early-type galaxies

Abstract: We discuss the morphology and kinematics of the HI of a sample of 30 southern gas-rich early-type galaxies selected from the HI Parkes All-Sky Survey (HIPASS). This is the largest collection of high-resolution HI data of a homogeneously selected sample. Given the sensitivity of HIPASS, these galaxies represent the most HI-rich early-type galaxies. In two-thirds of the galaxies, we find the HI to be in a large, regular disk- or ring-like structure that in some cases is strongly warped. In the remaining cases we find the HI distributed in irregular tails or clouds offset from the galaxy. The giant, regular HI structures can be up to ~200 kpc in diameter and contain up to 10^10 M_sun of HI. The incidence of irregular HI structures appears to be somewhat higher in elliptical galaxies, but the large, regular structures are observed in both elliptical and S0 galaxies and are not strictly connected to the presence of a stellar disk. If these two types of galaxies are the result of different formation paths, this is not strongly reflected in the characteristics of the HI. The size and the regular kinematics of the HI structures imply that the neutral hydrogen must have settled in these galaxies several Gyr ago. Merging as well as gas accretion from the IGM are viable explanations for the origin of the gas in these galaxies. The average column density of the HI is low so that little star formation is expected to occur and these early-type galaxies can remain gas rich for very long periods of time. The large HI structures likely represent key structures for tracing the origin and evolution of these galaxies. Note: Comments: Accepted for publication in AA

The Void Galaxy Survey: Optical Properties and H I Morphology and Kinematics

We have carefully selected a sample of 60 galaxies that reside in the deepest underdensities of geometrically identified voids within the Sloan Digital Sky Survey. HI imaging of 55 galaxies with the Westerbork Synthesis Radio Telescope reveals morphological and kinematic signatures of ongoing interactions and gas accretion. We probe a total volume of 485 Mpc(3) within the voids, with an angular resolution of 8 kpc at an average distance of 85 Mpc. We reach column density sensitivities of 5 x 10(19) cm(-2), corresponding to an HI mass limit of 3 x 10(8) M-circle dot. We detect HI in 41 galaxies, with total masses ranging from 1.7 x 10(8) to 5.5 x 10(9) M-circle dot. The upper limits on the 14 non-detections are not inconsistent with their luminosities, given their expected HI mass-to-light ratios. We find that the void galaxies are generally gas-rich, low-luminosity, blue disk galaxies, with optical and HI properties that are not unusual for their luminosity and morphology. The sample spans a range of absolute magnitudes (-16.1 > M-r > -20.4) and colors (0.06 <g - r <0.87), and includes disk and irregular galaxies. We also identify three as early-type galaxies, all of which are not detected in HI. All galaxies have stellar masses less than 3 x 10(10) M-circle dot, and many have kinematic and morphological signs of ongoing gas accretion, suggesting that the void galaxy population is still in the process of assembling. The small-scale clustering in the void, within 600 kpc and 200 km s(-1), is similar to that in higher density regions, and we identify 18 HI-rich neighboring galaxies in the voids. Most are within 100 kpc and 100 km s(-1) of the targeted galaxy, and we find no significant population of HI-rich low-luminosity galaxies filling the voids, contrary to what is predicted by simulations.

A jet‐induced outflow of warm gas in 3C 293

Using long-slit emission-line spectra we detect a fast outflow of ionized gas, with velocities up to 1000 km s −1 ,i nthe nearby powerful radio galaxy 3C 293 (z = 0.045). The fast outflow is located about 1 kpc east of the nucleus, in a region of enhanced radio emission due to the presence of a distorted radio jet. We present results that indicate that this fast outflow is caused by a jet‐ISM interaction. The kinematics of the outflowing ionized gas are very similar to those of a fast outflow of neutral hydrogen gas in this galaxy, suggesting that both outflows are the result of the same driving mechanism. While the mass of the outflowing ionized gas is about 1 × 10 5 M� , the total H I mass involved in the neutral outflow is about 100 times higher (10 7 M� ). This shows that, despite the high energies that must be involved in driving the outflow, most of the gas remains, or again becomes, neutral. Other outflows of ionized gas, although not as pronounced as in the region of the enhanced radio emission, are also seen in various other regions along the axis of the inner radio jets. The regular kinematics of the emission-line gas along the major axis of the host galaxy reveals a rotating ionized gas disc 30 kpc in extent. Ke yw ords: line: profiles ‐ ISM: jets and outflows ‐ ISM: kinematics and dynamics ‐ galaxies: active ‐ galaxies: individual: 3C 293 ‐ galaxies: ISM.

Fast Outflow of Neutral Hydrogen in the Radio Galaxy 3C 293

We report the detection of very broad H I absorption against the central regions of the radio galaxy 3C 293. The absorption profile, obtained with the Westerbork Synthesis Radio Telescope, has a full width at zero intensity of about 1400 km s-1, and most of this broad absorption (~1000 km s-1) is blueshifted relative to the systemic velocity. This absorption represents a fast outflow of neutral gas from the central regions of this active galactic nucleus. Possible causes for such an outflow are discussed. We favor the idea that the interaction between the radio jet and the rich interstellar medium produces this outflow. Some of the implications of this scenario are considerebd.