T. S. van Albada

DARK MATTER IN SPIRAL GALAXIES

Mass models of spiral galaxies based on the observed light distribution, assuming constant M /L for bulge and disc, are able to reproduce the observed rotation curves in the inner regions, but fail to do so increasingly towards and beyond the edge of the visible material. The discrepancy in the outer region can be accounted for by invoking dark matter; some galaxies require at least four times as much dark matter as luminous matter. There is no evidence for a dependence on galaxy luminosity or morphological type. Various arguments support the idea that a distribution of visible matter with constant M /L is responsible for the circular velocity in the inner region, i.e. inside approximately 2.5 disc scalelengths. Luminous matter and dark matter seem to ‘conspire’ to produce the flat observed rotation curves in the outer region. It seems unlikely that this coupling between disc and halo results from the large-scale gravitational interaction between the two components. Attempts to determine the shape of dark halos have not yet produced convincing results.

The Westerbork HI survey of spiral and irregular galaxies - I. HI imaging of late-type dwarf galaxies

Neutral hydrogen observations with the Westerbork Synthesis Radio Telescope are presented for a sample of 73 late-type dwarf galaxies. These observations are part of the WHISP project (Westerbork Hi Survey of Spiral and Irregular Galaxies). Here we present Hi maps, velocity fields, global profiles and radial surface density profiles of Hi, as well as Hi masses, Hi radii and line widths. For the late-type galaxies in our sample, we find that the ratio of Hi extent to optical diameter, defined as 6.4 disk scale lengths, is on average 1.8 +/- 0.8, similar to that seen in spiral galaxies. Most of the dwarf galaxies in this sample are rich in Hi with a typical M-HI/L-B of 1.5. The relative H I content M-HI/L-R increases towards fainter absolute magnitudes and towards fainter surface brightnesses. Dwarf galaxies with lower average H I column densities also have lower average optical surface brightnesses. We find that lopsidedness is as common among dwarf galaxies as it is in spiral galaxies. About half of the dwarf galaxies in our sample have asymmetric global profiles, a third has a lopsided H I distribution, and about half shows signs of kinematic lopsidedness.

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.

Kinematically lopsided spiral galaxies

Asymmetries in the distribution of light and neutral hydrogen are often observed in spiral galaxies, Here, attention is drawn to the presence of large-scale asymmetries in their kinematics. Two examples of kinematically lopsided galaxies are presented and discussed. The shape of the rotation curve - rising more steeply on one side of the galaxy than on the other - is the signature of the kinematic lopsidedness. It is shown that kinematic lopsidedness may be related to lopsidedness in the potential, and that even a mild perturbation in the latter can produce significant kinematic effects. Probably at least half of all spiral galaxies are lopsided.

On the Two Oosterhoff Groups of Globular Clusters

The observational evidence leading to the classification, following Oosterhoff, of globular clusters containing RR Lyrae stars into two distinct groups, is summarized and discussed in the light of results of stellar evolution theory and pulsation theory. The dichotomy is caused, at least in part, by a dichotomy in the ‘transition period’ between the type-ab and type-c stars which reflects a difference in effective temperature at the transition point. When this difference is accounted for, there remains a smaller average difference between the groups, though no longer a clear dichotomy, that is probably a mass and luminosity effect. If this remaining difference is interpreted as a luminosity effect the average difference in luminosity between the two Oosterhoff groups is at most 0.1 mag. It is suggested that Christy’s theoretical relationship between transition period and luminosity cannot be valid, at least not for clusters of different Oosterhoff groups. It is conjectured that the transition-temperature dichotomy may be a reflection of different predominant directions of evolution along the horizontal branch, accompanied by a hysteresis effect in the pulsations.

On the apparent coupling of neutral hydrogen and dark matter in spiral galaxies

We have studied a mass model for spiral galaxies in which the dark matter surface density is a scaled version of the observed H I surface density. Applying this mass model to a sample of 24 spiral galaxies with reliable rotation curves, one obtains good fits for most galaxies. The scaling factors cluster around 7, after correction for the presence of primordial helium. For several cases, however, different, often larger, values are found. For galaxies that cannot be fitted well, the discrepancy occurs at large radii and results from a fairly rapid decline of the Hr surface density in the outermost regions. Because of such imperfections and in view of possible selection effects, it is not possible to conclude here that there is a real coupling between HI and dark matter in spiral galaxies.

Are Dwarf Galaxies Dominated by Dark Matter

Mass models for a sample of 18 late-type dwarf and low surface brightness galaxies show that in almost all cases the contribution of the stellar disks to the rotation curves can be scaled to explain most of the observed rotation curves out to two or three disk scale lengths. The concept of a maximum disk, therefore, appears to work as well for these late-type dwarf galaxies as it does for spiral galaxies. Some of the mass-to-light ratios required in our maximum disk fits, however, are high, up to about 15 in the R band, with the highest values occurring in galaxies with the lowest surface brightnesses. Equally well-fitting mass models can be obtained with much lower mass-to-light ratios. Regardless of the actual contribution of the stellar disk, the fact that the maximum disk can explain the inner parts of the observed rotation curves highlights the similarity in shapes of the rotation curve of the stellar disk and the observed rotation curve. This similarity implies that the distribution of the total mass density is closely coupled to that of the luminous mass density in the inner parts of late-type dwarf galaxies.

Mergers between elliptical galaxies and the thickening of the Fundamental Plane

We have carried out computer simulations to study the effect of merging on the Fundamental Plane (FP) relation. Initially, systems are spherical Jaffe models following a simple scaling relation (M/R2e= constant). They have been put on the FP by imposing different M/L values. Various orbital characteristics have been considered. Our results show that the merger remnants lie very close to the FP of the progenitors. Although non-homology is introduced by the merging process, mergers among homologous galaxies leave a pre-existing FP-relation intact. As a side result we find that variations in the point of view lead to non-negligible scatter about the FP.