The Influence of Interactions and Minor Mergers on the Structure of Galactic Disks: I.Observations and Disk Models

This paper is the first part in our series on the influence of tidal interactions and minor mergers on the radial and vertical disk structure of spiral galaxies. We report on the sample selection, our observations, and data reduction. Surface photometry of the optical and near infrared data of a sample of 110 highly-inclined/edge-on disk galaxies are presented. This sample consists of two subsamples of 61 non-interacting galaxies (control sample) and of 49 interacting galaxies/minor merging candidates. Additionally, 41 of these galaxies were observed in the near infrared. We show that the distribution of morphological types of both subsamples is almost indistinguishable, covering the range between 0 <= T <= 9. An improved, 3-dimensional disk modelling- and fitting procedure is described in order to analyze and to compare the disk structure of our sample galaxies by using characteristic parameters. We find that the vertical brightness profiles of galactic disks respond very sensitive even to small deviations from the perfect edge-on orientation. Hence, projection effects of slightly inclined disks may cause substantial changes in the value of the disk scale height and must therefore be considered in the subsequent study.Comment: LaTeX, 36 pages, 5 figures, complete series of papers incl. all figures of higher quality is available at http://aurora.as.arizona.edu/~schwarz

Properties of tidally-triggered vertical disk perturbations

We present a detailed analysis of the properties of warps and tidally-triggered perturbations perpendicular to the plane of 47 interacting/merging edge-on spiral galaxies. The derived parameters are compared with those obtained for a sample of 61 non-interacting edge-on spirals. The entire optical (R-band) sample used for this study was presented in two previous papers. We find that the scale height of disks in the interacting/merging sample is characterized by perturbations on both large (~disk cut-off radius) and short (~z0) scales, with amplitudes of the order of 280pc and 130pc on average, respectively. The size of these large (short) -scale instabilities corresponds to 14% (6%) of the mean disk scale height. This is a factor of 2 (1.5) larger than the value found for non-interacting galaxies. A hallmark of nearly all tidally distorted disks is a scale height that increases systematically with radial distance. The frequent occurrence and the significantly larger size of these gradients indicate that disk asymmetries on large scales are a common and persistent phenomenon, while local disturbances and bending instabilities decline on shorter timescales. Nearly all (93%) of the interacting/merging and 45% of the non-interacting galaxies studied are noticeably warped. Warps of interacting/merging galaxies are ~2.5 times larger on average than those observed in the non-interacting sample, with sizes of the order of 340pc and 140pc, respectively. This indicates that tidal distortions do considerably contribute to the formation and size of warps. However, they cannot entirely explain the frequent occurrence of warped disks.Comment: LaTeX, 35 pages, 6 figures, all figures and appendix of higher quality available at http://aurora.as.arizona.edu/~schwarz

Three-dimensional modelling of edge-on disk galaxies

We present detailed three-dimensional modelling of the stellar luminosity distribution for the disks of 31 relatively nearby (<= 110 Mpc) edge-on spiral galaxies. In contrast to most of the standard methods available in the literature we take into account the full three-dimensional information of the disk. We minimize the difference between the observed 2D-image and an image of our 3D-disk model integrated along the line of sight. Thereby we specify the inclination, the fitting function for the z-distribution of the disk, and the best values for the structural parameters such as scalelength, scaleheight, central surface brightness, and a disk cut-off radius. From a comparison of two independently developed methods we conclude, that the discrepancies e.g. for the scaleheights and scalelengths are of the order of ~10%. These differences are not due to the individual method itself, but rather to the selected fitting region, which masks the bulge component, the dust lane, or present foreground stars. Other serious limitations are small but appreciable intrinsic deviations of real disks compared to the simple input model. In this paper we describe the methods and present contour plots as well as radial profiles for all galaxies without previously published surface photometry. Resulting parameters are given for the complete sample.Comment: LaTeX, 25 pages, 28 figures higher quality figures available at http://www.astro.ruhr-uni-bochum.de/astro/publications/pub2000.htm

Ventilation pathways in the tropical Atlantic and Pacific Oceans with a focus on the Oxygen Minimum Zones : development and application of a nested high-resolution global model system

The main goal of this study is to identify the pathways, source regions and age of the waters in the Oxygen Minimum Zones in the tropical Atlantic and Pacific Oceans. To decipher these aspects, Lagrangian trajectory analyses in conjunction with passive tracer simulations reflecting a surface dye and an artificial age tracer are utilized within a global high-resolution nested model system. The tracers and trajectories implemented here give insights on both, the beginning and the end, of the path of a water parcel from the surface into the OMZs, respectively

Characteristics and robustness of Agulhas leakage estimates: an inter-comparison study of Lagrangian methods

The inflow of relatively warm and salty water from the Indian Ocean into the South Atlantic via Agulhas leakage is important for the global overturning circulation and the global climate. In this study, we analyse the robustness of Agulhas leakage estimates as well as the thermohaline property modifications of Agulhas leakage south of Africa. Lagrangian experiments with both the newly developed tool Parcels and the well established tool Ariane were performed to simulate Agulhas leakage in the eddy-rich ocean–sea-ice model INALT20 (1/20∘ horizontal resolution) forced by the JRA55-do atmospheric boundary conditions. The average transport, its variability, trend and the transit time from the Agulhas Current to the Cape Basin of Agulhas leakage is simulated comparably with both Lagrangian tools, emphasizing the robustness of our method. Different designs of the Lagrangian experiment alter in particular the total transport of Agulhas leakage by up to 2 Sv, but the variability and trend of the transport are similar across these estimates. During the transit from the Agulhas Current at 32∘ S to the Cape Basin, a cooling and freshening of Agulhas leakage waters occurs especially at the location of the Agulhas Retroflection, resulting in a density increase as the thermal effect dominates. Beyond the strong air–sea exchange around South Africa, Agulhas leakage warms and salinifies the water masses below the thermocline in the South Atlantic

Can Lagrangian Tracking Simulate Tracer Spreading in a High-Resolution Ocean General Circulation Model?

To model tracer spreading in the ocean, Lagrangian simulations in an offline framework are a practical and efficient alternative to solving the advective–diffusive tracer equations online. Differences in both approaches raise the question of whether both methods are comparable. Lagrangian simulations usually use model output averaged in time, and trajectories are not subject to parameterized subgrid diffusion, which is included in the advection–diffusion equations of ocean models. Previous studies focused on diffusivity estimates in idealized models but could show that both methods yield similar results as long as the deformations-scale dynamics are resolved and a sufficient amount of Lagrangian particles is used. This study compares spreading of an Eulerian tracer simulated online and a cloud of Lagrangian particles simulated offline with velocities from the same ocean model. We use a global, eddy-resolving ocean model featuring 1/20° horizontal resolution in the Agulhas region around South Africa. Tracer and particles were released at one time step in the Cape Basin and below the mixed layer and integrated for 3 years. Large-scale diagnostics, like mean pathways of floats and tracer, are almost identical and 1D horizontal distributions show no significant differences. Differences in vertical distributions, seen in a reduced vertical spreading and downward displacement of particles, are due to the combined effect of unresolved subdaily variability of the vertical velocities and the spatial variation of vertical diffusivity. This, in turn, has a small impact on the horizontal spreading behavior. The estimates of eddy diffusivity from particles and tracer yield comparable results of about 4000 m2 s−1 in the Cape Basin

Can Lagrangian Tracking Simulate Tracer Spreading in a High-Resolution Ocean General Circulation Model?

To model tracer spreading in the ocean, Lagrangian simulations in an offline framework are a practical and efficient alternative to solving the advective–diffusive tracer equations online. Differences in both approaches raise the question of whether both methods are comparable. Lagrangian simulations usually use model output averaged in time, and trajectories are not subject to parameterized subgrid diffusion, which is included in the advection–diffusion equations of ocean models. Previous studies focused on diffusivity estimates in idealized models but could show that both methods yield similar results as long as the deformations-scale dynamics are resolved and a sufficient amount of Lagrangian particles is used. This study compares spreading of an Eulerian tracer simulated online and a cloud of Lagrangian particles simulated offline with velocities from the same ocean model. We use a global, eddy-resolving ocean model featuring 1/20° horizontal resolution in the Agulhas region around South Africa. Tracer and particles were released at one time step in the Cape Basin and below the mixed layer and integrated for 3 years. Large-scale diagnostics, like mean pathways of floats and tracer, are almost identical and 1D horizontal distributions show no significant differences. Differences in vertical distributions, seen in a reduced vertical spreading and downward displacement of particles, are due to the combined effect of unresolved subdaily variability of the vertical velocities and the spatial variation of vertical diffusivity. This, in turn, has a small impact on the horizontal spreading behavior. The estimates of eddy diffusivity from particles and tracer yield comparable results of about 4000 m2 s−1 in the Cape Basin