Simulations of Normal Spiral Galaxies

Results are presented of numerical simulations of normal isolated late type spiral galaxies. Specifically the galaxy NGC 628 is used as a template. The method employs a TREESPH code including stellar particles, gas particles, cooling and heating of the gas, star formation according to a Jeans criterion, and Supernova feedback. A regular spiral disc can be generated as an equilibrium situation of two opposing actions. On the one hand cooling and dissipation of the gas, on the other hand gas heating by the FUV field of young stars and SN mechanical forcing. The disc exhibits small and medium scale spiral structure of which the multiplicity increases as a function of radius. The theory of swing amplification can explain, both qualitatively and quantitatively, the emerging spiral structure. In addition, swing amplification predicts that the existence of a grand design m=2 spiral is only possible if the disc is massive. The simulations show that the galaxy is then unstable to bar formation. A general criterion is derived for the transition between bar stable and unstable, depending on disc mass contribution and on disc thickness. It seems that bar stability hardly depends on the presence of gas. A detailed quantitative analysis is made of the emerging spiral structure and a comparison is made with observations. That demonstrates that the structure of the numerical isolated galaxies is not as strong and has a larger multiplicity compared to the structure of some exemplary real galaxies. It is argued that a grand design can only be generated by a central bar or by tidal forces resulting from an encounter with another galaxy.Comment: Accepted for publication in: Monthly Notices of the Royal Astron. Soc. The astro-ph copy has a few figures with degraded resolution. A copy with high quality graphics (4281 kb) can be downloaded from the kapteyn institute weg page at http://www.astro.rug.nl (Goto preprints and preprints 2003

The kinematics of the bulge and the disc of NGC 7331

Presented are spectroscopic emission and absorption line observations along the major axis of the Sb galaxy NGC 7331. The kinematics of the ionized gas and the stars appears to be regular, but contrary to what one might expect, the emission line gas rotates slower than the stars in the inner regions. This may be caused by an inner inclined and warped gas layer. In the bulge region the absorption line profiles have a shallow extension towards the systemic velocity, but no counterrotation is observed which is contrary to previous claims. These claims might have been based on a wrong interpretation of the employed analysis method. A kinematical model has been made in order to explain the observed sizes and shapes of the absorption line profiles. It appeared necessary to combine a rapidly rotating disc having a radially decreasing velocity dispersion with a slowly rotating constant dispersion bulge. Then, simultaneously, the observed stellar radial velocities, the velocity dispersions and the observed asymmetry of the line profile could be explained satisfactorily. An even better fit to the data can be achieved when the disc is relatively thinner and colder inside the bulge region. For the disc a M/L ratio of 1.6 +/- 0.7 is derived in the I band. On the other hand, a rotation curve fit gives a M/L ratio of 6.8 +/- 1 for the bulge. An analysis of a sample of galactic discs and bulges shows that on average the mass-to-light ratio of the bulge is three times as large as the mass-to-light ratio of the disc in the I band. For the B-band this ratio goes up to 7.2.Comment: Latex A+A macr

Stealing the riches: using the human genome project for livestock research

The document attached has been archived with permission from the Australian Association of Cattle Veterinarians.The human genome project has brought a new era not only to medical genetics, but also to livestock molecular genetics. By 'borrowing' everything from techniques and research strategies to actual data from the human genome project, geneticists are making significant progress in applying DNA biotechnologies to livestock production. These applications include parentage testing, identity testing, and diagnostic testing of genetic disorders. Another important application is selection of superior animals for breeding programs by identifying those carrying specific genes. Examples of these applications will be discussed from our work on cattle and sheep molecular genetics

Circularity of objects in images

The most commonly used measure of circularity of objects in images is shown to give incorrect results. An alternative measure of circularity based on the distance between a set and a discrete disk is described. The alternative measure gives circularity zero (distance zero) for discrete disks and values in the range (0,1) for discrete sets which are not disks

MOND rotation curves for spiral galaxies with Cepheid-based distances

Rotation curves for four spiral galaxies with recently determined Cepheid-based distances are reconsidered in terms of modified Newtonian dynamics (MOND). For two of the objects, NGC 2403 and NGC 7331, the rotation curves predicted by MOND are compatible with the observed curves when these galaxies are taken to be at the Cepheid distance. For NGC 3198, the largest distance for which reasonable agreement is obtained is 10% smaller than the Cepheid-based distance; i.e., MOND clearly prefers a smaller distance. This conclusion is unaltered when new near-infrared photometry of NGC 3198 is taken as the tracer of the stellar mass distribution. For the large Sc spiral, NGC 2841, MOND requires a distance which is at least 20% larger than the Cepheid-based distance. However, the discrepancy of the Tully-Fisher and SNIa distances with the Cepheid determination casts some doubt upon the Cepheid method in this case.Comment: Accepted for publication in astronomy and astrophysics 9 pages, 9 figure

An investigation of the structure and kinematics of the spiral galaxy NGC 6503

The spiral galaxy NGC 6503 exhibits a regular kinematical structure except for a remarkable drop of the stellar velocity dispersion values in the central region. To investigate the dynamics of the disc a theoretical framework has been described. This includes a mass decomposition of the galaxy into a family of disc/halo realizations compatible with the observed photometry and rotation curve. For this family stellar velocity dispersion values and stability parameters were calculated, showing that the more massive discs, although having larger dispersions, are less stable. However, a reliable theoretical description of the inner regions where the drop occurs cannot be given. That is why we have resorted to numerical calculations. Pure stellar 3d simulations have been performed for the family of decompositions. A clear result is that disc/dark halo mass ratios approaching those of the maximum disc limit generate a large bar structure. This is incompatible with the observed morphology of NGC 6503. For the larger radii the stellar kinematics resulting from the simulations essentially agrees with that predicted by the theory, but the central velocity dispersion drop could not be reproduced. A close inspection reveals that the central nuclear region is very small and bright. Therefore, tentatively, this nucleus was considered as an isothermal sphere and a core fitting procedure was applied. For an adopted equal mass-to-light ratio of disc and nucleus, a velocity dispersion of 21.5 km/s is predicted, in excellent agreement with the observed central value. The observed dispersion drop can thus be explained by a separate kinematically distinct galactic component.Comment: 14 pages, Latex, use mn.sty style fil

A Face-On Tully-Fisher Relation

We construct the first "face-on" Tully-Fisher (TF) relation for 24 galaxies with inclinations between 16 degrees and 41 degrees. The enabling measurements are integral-field, echelle spectroscopy from the WIYN 3.5m telescope, which yield accurate kinematic estimates of disk inclination to 15 degrees. Kinematic inclinations are of sufficient accuracy that our measured TF scatter of 0.42 mag is comparable to other surveys even without internal-absorption corrections. Three of four galaxies with significant kinematic and photometric asymmetries also have the largest deviations from our TF relation, suggesting that asymmetries make an important contribution to TF scatter. By measuring inclinations below 40 degrees, we establish a direct path to linking this scatter to the unprojected structure of disks and making non-degenerate dynamical mass-decompositions of spiral galaxies.Comment: 13 pages, 3 figures (2 color). Accepted for publication in ApJ Letter

Dark and luminous matter in the NGC 3992 group of galaxies, I. The large barred spiral NGC 3992

Detailed neutral hydrogen observations have been obtained of the large barred spiral galaxy NGC 3992 and its three small companion galaxies, UGC 6923, UGC 6940, and UGC 6969. For the main galaxy, the HI distribution is regular with a low level radial extension outside the stellar disc. However, at exactly the region of the bar, there is a pronounced central HI hole in the gas distribution. Likely gas has been transported inwards by the bar and because of the emptyness of the hole no large accretion events can have happened in recent galactic times. The gas kinematics is very regular and it is demonstrated that the influence of the bar potential on the velocity field is negligible. A precise and extended rotation curve has been derived showing some distinct features which can be explained by the non-exponential radial light distribution of NGC 3992. The decomposition of the rotation curve gives a slight preference for a sub maximal disc, though a range of disc contributions, up to a maximum disc situation fits nearly equally well. For such a maximum disc contribution, which might be expected in order to generate and maintain the bar, the required mass-to-light ratio is large but not exceptional.Comment: accepted for publication in Astronomy and Astrophysics. A copy with high resolution graphics will shortly become available at http://www.astro.rug.nl/Preprints/preprints.htm

Exploring Disk Galaxy Dynamics Using IFU Data

In order to test the basic equations believed to dictate the dynamics of disk galaxies, we present and analyze deep two-dimensional spectral data obtained using the PPAK integral field unit for the early-type spiral systems NGC 2273, NGC 2985, NGC 3898 and NGC 5533. We describe the care needed to obtain and process such data to a point where reliable kinematic measurements can be obtained from these observations, and a new more optimal method for deriving the rotational motion and velocity dispersions in such disk systems. The data from NGC 2273 and NGC 2985 show systematic variations in velocity dispersion with azimuth, as one would expect if the shapes of their velocity ellipsoids are significantly anisotropic, while the hotter disks in NGC 3898 and NGC 5533 appear to have fairly isotropic velocity dispersions. Correcting the rotational motion for asymmetric drift using the derived velocity dispersions reproduces the rotation curves inferred from emission lines reasonably well, implying that this correction is quite robust, and that the use of the asymmetric drift equation is valid. NGC 2985 is sufficiently close to face on for the data, combined with the asymmetric drift equation, to determine all three components of the velocity ellipsoid. The principal axes of this velocity ellipsoid are found to be in the ratio sigma_z:sigma_phi:sigma_R ~ 0.7:0.7:1, which shows unequivocally that this disk distribution function respects a third integral of motion. The ratio is also consistent with the predictions of epicyclic theory, giving some confidence in the application of this approximation to even fairly early-type disk galaxies.Comment: 15 pages, 7 figures, accepted for publication in MNRA

High Latitude HI in NGC 2613: Buoyant Disk-Halo Outflow

We combine new VLA D array HI data of NGC 2613 with previous high resolution data to show new disk-halo features in this galaxy. The global HI distribution is modeled in detail using a technique which can disentangle the effects of inclination from scale height and can also solve for the average volume density distribution in and perpendicular to the disk. The model shows that the galaxy's inclination is on the low end of the range given by Chaves & Irwin (2001) and that the HI disk is thin (z_e = 188 pc), showing no evidence for halo. Numerous discrete disk-halo features are observed, however, achieving z heights up to 28 kpc from mid-plane. One prominent feature in particular, of mass, 8X10^7 Msun and height, 22 kpc, is seen on the advancing side of the galaxy at a projected galactocentric radius of 15.5 kpc. If this feature achieves such high latitudes because of events in the disk alone, then input energies of order ~ 10^{56} ergs are required. We have instead investigated the feasibility of such a large feature being produced via buoyancy (with drag) within a hot, pre-existing X-ray corona. Reasonable plume densities, temperatures, stall height (~ 11 kpc), outflow velocities and ages can indeed be achieved in this way. The advantage of this scenario is that the input energy need only be sufficient to produce blow-out, a condition which requires a reduction of three orders of magnitude in energy. If this is correct, there should be an observable X-ray halo around NGC 2613.Comment: 32 pages 7 gif figures, accepted by Ap