van der Pieter Kruit

Kinematics of diffuse ionized gas in the disk halo interface of NGC 891 from Fabry-Pérot observations

Context. The properties of the gas in halos of galaxies constrain global models of the interstellar medium. Kinematical information is of particular interest since it is a clue to the origin of the gas. Aims. Here we report observations of the kinematics of the thick layer of the diffuse ionized gas in NGC 891 in order to determine the rotation curve of the halo gas. Methods. We have obtained a Fabry-Perot data cube in H alpha to measure the kinematics of the halo gas with angular resolution much higher than obtained from HI 21 cm observations. The data cube was obtained with the TAURUS II spectrograph at the WHT on La Palma. The velocity information of the diffuse ionized gas extracted from the data cube is compared to model distributions to constrain the distribution of the gas and in particular the halo rotation curve. Results. The best fit model has a central attenuation tau H alpha = 6, a dust scale length of 8.1 kpc, an ionized gas scale length of 5.0 kpc. Above the plane the rotation curve lags with a vertical gradient of -18.8 kms(-1) kpc(-1). We find that the scale length of the H alpha must be between 2.5 and 6.5 kpc. Furthermore we find evidence that the rotation curve above the plane rises less steeply than in the plane. This is all in agreement with the velocities measured in the HI.

The dark matter halo shape of edge-on disk galaxies: IV. UGC 7321

This is the fourth paper in a series in which we attempt to put constraints on the flattening of dark halos in disk galaxies. We observed for this purpose the HI in edge-on galaxies, where it is in principle possible to measure the force field in the halo vertically and radially from gas layer flaring and rotation curve decomposition respectively. As reported in earlier papers in this series we have for this purpose analysed the HI channel maps to accurately measure all four functions that describe as a function of galactocentric radius the planar HI kinematics and 3D HI distribution of a galaxy: the radial HI surface density, the HI vertical thickness, the rotation curve and the HI velocity dispersion. In this paper we analyse these data for the edge-on galaxy UGC 7321. We measured the stellar mass distribution (M = 3 x 10(8) M(circle dot) with M/L(R) less than or similar to 0.2), finding that the vertical force of the gas disk dominates the stellar disk at all radii. Measurements of both the rotation curve and the vertical force field showed that the vertical force puts a much stronger constraint on the stellar mass-to-light ratio than rotation curve decomposition. Fitting of the vertical force field derived from the flaring of the HI layer and HI velocity dispersion revealed that UGC 7321 has a spherical halo density distribution with a flattening of q = c/a = 1.0 +/- 0.1. However, the shape of the vertical force field showed that a non-singular isothermal halo was required, assuming a vertically isothermal HI velocity dispersion. A pseudo-isothermal halo and a gaseous disk with a declining HI velocity dispersion at high latitudes may also fit the vertical force field of UGC 7321, but to date there is no observational evidence that the HI velocity dispersion declines away from the galactic plane. We compare the halo flattening of UGC 7321 with other studies in the literature and discuss its implications. Our result is consistent with new n-body simulations which show that inclusion of hydrodynamical modelling produces more spherical halos.

The dark matter halo shape of edge-on disk galaxies - II. Modelling the HI observations: methods

This is the second paper of a series in which we attempt to put constraints on the flattening of dark halos in disk galaxies. For this purpose, we observe the Hi in edge-on galaxies, where it is in principle possible to measure the force field in the halo vertically and radially from gas layer flaring and rotation curve decomposition respectively. To calculate the force fields, we need to analyse the observed XV diagrams to accurately measure all three functions that describe the planar kinematics and distribution of a galaxy: the radial Hi surface density, the rotation curve and the Hi velocity dispersion. In this paper, we discuss the improvements and limitations of the methods previously used to measure these Hi properties. We extend the constant velocity dispersion method to include determination of the Hi velocity dispersion as a function of galactocentric radius and perform extensive tests on the quality of the fits. We will apply this “radial decomposition XV modelling method” to our Hi observations of 8 Hi rich, late-type, edge-on galaxies in the third paper of this series.

Structure and kinematics of edge-on galaxy discs - II. Observations of the neutral hydrogen

We present Australia Telescope Compact Array and Westerbork Synthesis Radio Telescope HI observations of 15 edge-on spiral galaxies of intermediate to late morphological type. The global properties and the distribution and kinematics of the HI gas are analysed and discussed. We determine the rotation curves using the envelope-tracing method. For 10 spiral galaxies with a stellar disc truncation we find an average ratio of the HI radius to the truncation radius of the stellar disc of 1.1 +/- 0.2 (1sigma).

A dust component 2 kpc above the plane in NGC 891

Context. The halo of NGC 891 has been the subject of studies for more than a decade. One of its most striking features is the large asymmetry in Hα emission. In this letter, we will take a quantitative look at this asymmetry at different wavelengths for the first time. Aims. We suggest that NGC 891 is intrinsically almost symmetric and the large asymmetry in Hα emission is mostly due to dust attenuation. We will quantify the additional optical depth needed to cause the observed asymmetry in this model. Methods. By comparing large strips on the North East side of the galaxy with strips covering the same area in the South West we can quantify and analyze the asymmetry in the different wavelengths. Results. From the 24 µm emission we find that the intrinsic asymmetry in star formation in NGC 891 is small i.e., ∼30%. The additional asymmetry in Hα is modeled as additional symmetric dust attenuation which extends up to ∼40 �� (1.9 kpc) above the plane of the galaxy with a mid-plane value of τ = 0.8 and a scale height of 0.5 kpc

Radial truncations in stellar discs in galaxies

We discuss the possible origin of the radial truncations in stellar discs, using measurements that we presented in an earlier paper. A tentative correlation is found with the de-projected face-on central surface brightness; lower surface brightness discs tend to have a smaller truncation radius in units of scalelength. This and our earlier finding that in smaller spirals the truncation tends to occur at a larger number of scalelengths are best reproduced when the truncation is caused by a constant gas-density threshold on star formation.

Structure and kinematics of edge-on galaxy discs – III. The rotation curves in the gas

A technique is introduced for deriving the gaseous rotation curves of edge-on spiral galaxies. The entire major axis position-velocity (XV) diagram is modelled with a set of rings in a least-squares sense, allowing for the effects of beam-smearing and line-of-sight projection. The feasibility of the technique is demonstrated by applying it to good quality H I XV diagrams of eight edge-on spirals. For seven additional spirals the XV diagrams are of insufficient quality, and the H I rotational velocities derived earlier using the envelope-tracing method are retained. The H I results are augmented with the optical emission line (H II) kinematics to arrive at estimates of the full rotation curves. A detailed comparison of the H I and H II kinematics shows that the discs in our sample are sufficiently transparent at the heights above the plane where we have taken our optical spectra to derive the stellar kinematics. In several of these spirals the H II is mainly confined to the spiral arms and does not extend out to the edge of the H I layer, which may have caused the H II velocity profiles to be significantly narrower than those of H I.

The dark matter halo shape of edge-on disk galaxies . I. HI observations

This is the first paper of a series in which we will attempt to put constraints on the flattening of dark halos in disk galaxies. We observe for this purpose the HI in edge-on galaxies, where it is in principle possible to measure the force field in the halo vertically and radially from gas layer flaring and rotation curve decomposition respectively. In this paper, we define a sample of 8 HI rich late-type galaxies suitable for this purpose and present the HI observations.

Kinematics and dynamics of the "superthin" edge-on disk galaxy IC 5249

We present spectroscopic observations of the stellar motions in the disk of the superthin edge-on spiral galaxy IC 5249 and re-analyse synthesis observations of the HI. We find that the HI rotation curve rises initially to about 90-100 km s(-1), but contrary to the conclusion of Abe et al. (1999) flattens well before the edge of the optical disk. Over most part of the optical disk we have been able to establish that the (tangential) stellar velocity dispersion is 25-30 km s(-1). We argue that the central light concentration in the disk is not a bulge in the classical Population II sense, but most likely represents structure in the disk component. From earlier surface photometry we adopt a value for the radial scalelength of the disk of 7 +/- 1 kpc, a vertical scaleheight of 0.65 +/- 0.05 kpc and a disk truncation radius of 17 +/- 1 kpc. The HI disk has a measurable thickness but from our analysis we conclude that this is due to a small inclination away from perfectly edge-on. The very thin appearance of IC 5249 on the sky is the result of a combination of a low (face-on) surface brightness, a long scalelength and a a sharp truncation at only about 2.5 scalelengths. In terms of the ratio of the radial scalelength and the vertical scaleheight of the disk, IC 5249 is not very at; in fact it is slightly fatter than the disk of our Galaxy. From various arguments we derive the stellar velocity dispersions at a position one radial scalelength out in the disk (R similar to 7 kpc) as respectively sigma (R) similar to 35 km s(-1), sigma (theta) similar to 30 km s(-1) and sigma (z) similar to 20 km s(-1). This is comparable to the values for the disk of our Galaxy in the solar neighborhood. Near the edge of the disk the ratio of radial to vertical velocity dispersion is probably higher. Presumably the angular momentum distribution of the gas that formed the disk in IC 5249 was such that, compared to the Galaxy, a much more extended distribution resulted in spite of the lower overall rotation and mass. The low surface density that arose from that resulted in a thicker HI layer in which star formation proceeded at a much slower rate, but disk heating proceeded at a similar pace.

The volume densities of giant molecular clouds in M 83

Using observed GALEX far-ultraviolet (FUV) fluxes and VLA images of the 21-cm HI column densities, along with estimates of the local dust abundances, we measure the volume densities of a sample of actively star-forming giant molecular clouds (GMCs) in the nearby spiral galaxy M83 on a typical resolution scale of 170 pc. Our approach is based on an equilibrium model for the cycle of molecular hydrogen formation on dust grains and photodissociation under the influence of the FUV radiation on the cloud surfaces of GMCs. We find a range of total volume densities on the surface of GMCs in M83, namely 0.1-400 cm(-3) inside R(25), 0.5-50 cm(-3) outside R(25). Our data include a number of GMCs in the HI ring surrounding this galaxy. Finally, we discuss the effects of observational selection, which may bias our results.