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We present deep optical imaging of three face-on disk galaxies together with a detailed description of the reduction and calibration methods used, in order to measure the intrinsic shape of their outer stellar edges. Whereas it is now well accepted that disks of spiral galaxies are not infinite exponential beyond galactocentric distances of about 3-5 radial scalelengths, the genuine structure of the truncation region is not yet well known. Our data quantitatively establish a smooth truncation behaviour of the radial surface brightness profiles and is best described by a two-slope model, characterised by an inner and outer exponential scalelength separated at a relatively well defined break radius. This result disagrees with the frequently assumed sharply truncated nature of the radial surface brightness profiles and implies the presence of stars and even star-formation beyond the break radius. In addition, we do not find a strong influence of a nearby companion on the ratio of the break radius to the radial scalelength. Our results denote new observational constraints for the search of the physical explanation for these smooth disk truncations., Comment: LaTeX, 10 pages, 17 figures, accepted to be published in A&A, minor changes to the quality of figures

We present the results from our deep optical imaging survey \mu_lim=26-27 V-mag/arcsec^2 of a morphologically selected sample of 72 edge-on disk galaxies. The question of the global structure of galactic stellar disks, especially the radial surface brightness profile at large galactocentric distances, is addressed. We find that typical radial profiles are better described by a two-slope exponential profile --characterised by an inner and outer scalelength separated at a break radius-- rather than a sharply-truncated exponential model. Results are given for three face-on equivalents, serving as the crucial test to assure the findings for the edge-on sample without possible geometrical line-of-sight effects., Comment: 4 pages, with 6 figures, to appear in "Disks of Galaxies: Kinematics, Dynamics and Perturbations" (ASP Conference Series), eds E.Athanassoula and A. Bosma

We present the first high-resolution study of the disk-halo interface in an edge-on galaxy (NGC 5775) in which every component of the interstellar medium is represented and resolved. New single-dish CO J=2-1 and CO J=1-0 data, ROSAT X-ray data, and HIRES IRAS data are presented along with HI data which emphasizes the high latitude features. In conjunction with previously published radio continuum (6 and 20 cm) and H alpha data, we find spatial correlations between various ISM components in that all components of the ISM are present in the disk-halo features (except for CO for which there is insufficient spatial coverage). The HI features extend to about 7 kpc above the plane. In one case, a loop is detected in position-velocity space, and are also observed over a large velocity range. This implies that the disk-halo features represent expanding supershells. However, the shells may be incomplete and partially open-topped, suggesting that we are observing the breakup of the supershells as they traverse the disk-halo interface. There is some evidence for acceleration with z height and both redshifted and blueshifted velocities are present, although the gas which is lagging with respect to galactic rotation dominates. The radio continuum spectral index is flatter around the shell rims, suggesting that shocks may be important in these regions (thermal contribution can not account for this flattening). The H alpha emission is located interior to the HI. For feature F3, the H alpha emission forms the interior "skin" of the HI shell, yet there appears to be a minimum of in-disk star formation immediately below the feature. We present a picture of a "typical" HI supershell which accelerates and breaks up through the disk-halo interface. Such a feature is likely internally generated via an energetic event in the disk., Comment: 14 pages plus 12 jpg figures, accepted for publication by the Astron. & Astrophy. Journal

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/

We briefly discuss a possible interconnection of vertical HI structures observed in the Milky Way Galaxy with large scale blow-outs caused by the explosions of multiple clustered SNe. We argue that the observed OB associations can produce only about 60 such events, or approximately one chimney per 3 kpc$^2$ within the solar circle. We also discuss the overall properties of HI shells in nearby face-on galaxies and the distribution of H$\alpha$ and dust in edge-on galaxies. We argue that the presence of dust in galactic haloes may indicate that radiation pressure is the most probable mechanism capable of transporting dust to large heights above the galactic plane. In order to make this possible, the galactic magnetic field must have a strong vertical component. We mention that SNe explosions can initiate the Parker instability which in turn creates large scale magnetic loops with a strong vertical component. Recent observations of nearby edge-on galaxies favour this suggestion., Comment: 11 pages, 4 Figs, Talk at the JENAM, May 29 -- June 3, 2000, Moscow

In this letter we present first results from spectroscopic observations of Diffuse Ionized Gas (DIG) in the halo of NGC5775 obtained with FORS1 attached to UT1/Antu of the Very Large Telescope (VLT). At our slit position perpendicular to the disk (41" SE of the nucleus) the emission of [NII]6583, [OIII]5007, and [OII]3727 is detected out to 9 kpc into the halo, allowing possible ionization mechanisms of the DIG to be examined. Photoionization models which assume a dilute radiation field are able to fit the data for the disk, but they cannot account for the line ratios measured in the halo (e.g., [OI]/Halpha or [OII]/Halpha). In particular they fail to predict the observed increase of [OIII]5007/Halpha with increasing z. The most striking result concerns the kinematics of the halo gas. Velocities at high galactic latitudes drop from the midplane value to reach the systemic velocity at z ~ 9 kpc. An analysis of VLA archive data of the polarized radio-continuum emission at 4.86 GHz and 1.49 GHz reveals that magnetic fields in the halo have a strong component perpendicular to the disk and are aligned with the Halpha and radio-continuum spurs in the halo. This can result either from a strong wind action or, more likely, from the generation of dipolar magnetic fields. We briefly discuss the interrelation of the magnetic field structure and gas dynamics, in particular the role of magnetic fields in gas outflows, as well as the possible heating of DIG by magnetic reconnection., Comment: 6 pages, 5 figures. Accepted for publication in A&A letters. For high quality figures, see http://www.astro.ruhr-uni-bochum.de/astro/publications/pub2000.html

Two flocculent galaxies NGC 3521 and NGC 5055 has been observed at 10.55GHz with the Effelsberg 100m telescope. In both cases polarized emission reveals substantial radial component of regular magnetic field - similar to that in grand-designed spirals. Comparison with H_alpha distribution obtained at Lowell Observatory is presented. Polarization models discussed, support modern non-standard dynamo concepts for magnetic field generation in galaxies., Comment: 11 pages, 10 figures, A&A in print

We have observed 60 edge-on galaxies in the NIR in order to study the stellar distribution in galaxies with box/peanut-shaped bulges. The much smaller amount of dust extinction at these wavelengths allows us to identify in almost all target galaxies with box/peanut-shaped bulges an additional thin, central component in cuts parallel to the major axis. This structure can be identified with a bar. The length of this structure scaled by the length of the bulge correlates with the morphologically classified shape of the bulge. This newly established correlation is therefore mainly interpreted as the projection of the bar at different aspect angles. Galaxies with peanut bulges have a bar seen nearly edge-on and the ratio of bar length to thickness, 14 +/- 4, can be directly measured for the first time. In addition, the correlation of the boxiness of bulges with the bar strength indicates that the bar characteristic could partly explain differences in the bulge shape. Furthermore, a new size relation between the box/peanut structure and the central bulge is found. Our observations are discussed in comparison to a N-body simulation for barred galaxies (Pfenniger & Friedli 1991). We conclude that the inner region of barred disk galaxies are build up by three distinct components: the spheroidal bulge, a thin bar, and a b/p structure most likely representing the thick part of the bar., Comment: LaTeX, 14 pages, 17 figures, accepted to be published in A&A

In order to examine the excitation and ionization mechanism of extraplanar diffuse ionized gas (DIG) we have obtained optical longslit spectra of seven edge-on spiral galaxies. In four objects the brightest emission lines can be traced out to distances of typically 1.5 kpc above the disk. For NGC1963 and NGC3044 line ratios such as [NII]6583/Ha or [SII]6717/Ha as well as [OIII]5007/Hb could be measured for the halo DIG. This allows us to discuss the DIG in the halo of these objects in the framework of diagnostic diagrams. For these two objects, the line ratios of [OIII]5007/Hb decrease with increasing z, different from the recently reported trend in NGC891 (Rand 1998). We find that emission lines from the DIG in the disks are in good agreement with photoionization models using a dilute radiation field. However, with increasing z these models fail to predict the measured [OI]6300/Ha and HeI5876/Ha line ratios for NGC1963. Diagnostic diagrams reveal for NGC1963 the need for a second ionization mechanism of the halo DIG (besides photoionization). This additional source could be shock ionization. The same diagrams demonstrate an intermediate classification for NGC3044. Plots of [SII]/[NII] vs. emission measure reveal significant changes towards the halo and seem to trace local small scale density fluctuations of the extraplanar DIG., Comment: Accepted for publication in A&A, 15 pages, 17 figures. For high resolution ps-files, see http://www.astro.ruhr-uni-bochum.de/

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/

We present the second part of a detailed statistical study focussed on the effects of tidal interactions and minor mergers on the radial and vertical disk structure of spiral galaxies. In the first part we reported on the sample selection, observations, and applied disk models. In this paper the results are presented, based on disk parameters derived from a sample of 110 highly-inclined/edge-on galaxies. This sample consists of two subsamples of 49 interacting/merging and 61 non-interacting galaxies. Additionally, 41 of these galaxies were observed in the NIR. We find significant changes of the disk structure in vertical direction, resulting in ~1.5 times larger scale heights and thus vertical velocity dispersions. The radial disk structure, characterized by the cut-off radius and the scale length, shows no statistically significant changes. Thus, the ratio of radial to vertical scale parameters, h/z0, is ~1.7 times smaller for the sample of interacting/merging galaxies. The total lack of typical flat disk ratios (h/z0)>7 in the latter sample implies that vertical disk heating is most efficient for (extremely) thin disks. Statistically nearly all galactic disks in the sample (93%) possess non-isothermal vertical luminosity profiles like the sech (60%) and exp (33%) distribution, independent of the sample and passband investigated. This indicates that, in spite of tidal perturbations and disk thickening, the initial vertical distribution of disk stars is not destroyed by interactions or minor mergers., Comment: LaTeX, 16 pages, 7 figures, accepted for publication in A&A, complete series of papers incl. all figures in high quality available at http://aurora.as.arizona.edu/~schwarz/

We present a classification for bulges of a complete sample of ~1350 edge-on disk galaxies derived from the RC3 (Third Reference Catalogue of Bright Galaxies, de Vaucouleurs et al. 1991). A visual classification of the bulges using the Digitized Sky Survey (DSS) in three types of b/p bulges or as an elliptical type is presented and supported by CCD images. NIR observations reveal that dust extinction does almost not influence the shape of bulges. There is no substantial difference between the shape of bulges in the optical and in the NIR. Our analysis reveals that 45% of all bulges are box- and peanut-shaped (b/p). The frequency of b/p bulges for all morphological types from S0 to Sd is > 40%. In particular, this is for the first time that such a large frequency of b/p bulges is reported for galaxies as late as Sd. The fraction of the observed b/p bulges is large enough to explain the b/p bulges by bars., Comment: LaTeX, 35 pages, 10 figures, accepted to be published in A&AS

We present narrowband H-alpha imaging data of a small survey of nearby edge-on spiral galaxies, aiming at the detection of `extraplanar' diffuse ionized gas (DIG). A few of our studied edge-on spirals show signs of disk-halo interaction (DHI), where extended line emission far above the galactic plane of these galaxies is detected. In some cases an extraplanar diffuse ionized gas (eDIG) layer is discovered, e.g., NGC4634, NGC 3044, while other galaxies show only filamentary features reaching into the halo (e.g., IC 2531) and some galaxies show no sign of eDIG at all. The extraplanar distances of the DIG layer in our narrowband H-alpha images reach values of z<= 2 kpc above the galactic plane. The derived star formation rates (SFRs) from the H-alpha flux of the studied galaxies range from 0.05-0.7 M_{sun}/yr, neglecting a correction for internal absorption. The variation of the SFR values among our sample galaxies reflects the diversity of star formation within this sample. A diagnostic diagram is introduced, which allows to predict the existence of gas halos in `quiescent' galaxies based on the ratio S_{60}/S_{100} versus L_{FIR} / D^2_{25} in this diagram. We compare the positions of the non--starburst galaxies with starburst galaxies, since these galaxies populate distinct positions in these diagrams., Comment: LaTeX, 15 pages, 13 figures, accepted for publication in A&A, higher quality figures available at http://www.astro.ruhr-uni-bochum.de/astro/publications/pub2000.html

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.html

We present the analysis of a CCD survey of 31 nearby (<= 110 Mpc) edge-on spiral galaxies. The three-dimensional one-component best fit models provide their disk-scalelengths h and for the first time their disk cut-off radii R_{co}. We confirm for this sample the existence of such sharp truncations, and find a significantly lower mean value of the distance independent ratio R_{co}/h =2.9 +- 0.7 than the standard value of 4.5 often used in the literature. Our data show no correlation of these parameters with Hubble type, whereas we report a correlation between R_{co}/h and the distance based scalelength in linear units. Compared to the Milky Way we find only lower values of R_{co}/h, explained either by possible selection effects or by the completely different techniques used. We discuss our data in respect to present models for the origin of the cut-off radii, either as a relict of the galaxy formation process, or as an evolutionary phenomenon., Comment: LaTeX, 4 pages, 2 figures

We present a model for the interpretation of the gamma-ray background emission from the Galactic halo, which we attribute to interaction of high-energy cosmic rays with dense molecular clumps. In a wide range of clump parameters we calculate the expected gamma-photon flux, taking into account for the first time optical depth effects. This allows us to derive new constraints on masses and sizes of possible molecular clumps and their contribution to the galactic surface density. The observed gamma-ray distribution can be explained best by models with a flattened halo distribution of axis ratio ~ 0.3. If optical depth effects are important, the clumps must have radii of ~ 6 AU and masses of ~ 10^{-3} M_sun. This would result in a total mass of ~ 2 10^{11} M_sun for such clumps and contribute with Sigma ~ 140 M_sun pc^{-2} to the local surface density., Comment: 4 pages, 3 figures, to be published in A&A letters

Spectroscopic measurements of emission line ratios and velocities are presented for ionized gas across the central plane of NGC 55, a late-type galaxy in the Sculptor group. The low metallicity in NGC 55 leads to relatively low line ratios of [S II]/H$\alpha$ and [N II]/H$\alpha$ for H II regions as well as the diffuse ionized gas. These are the first spectroscopical measurements of line ratios in ionized gas exterior to the stellar disc of NGC 55. Analysis of the line ratios and the relative velocities of different features suggests that photoionization is a plausible explanation for the ionization of this diffuse gas. The observed shell structures and the corresponding velocities support the idea of diffuse gas being pushed into the halo by supernova explosions and stellar winds., Comment: 8 pages, 7 figures accepted to be published in A&A main journal

VLA HI observations of the dwarf irregular galaxy NGC 2188 show that the gas and stars have spatial distributions which are substantially different. One end of the optical disk is strongly gas deficient, while neutral gas extends into the halo over distances of more than 2 kpc from the midplane. This and the peculiar velocity field suggest that NGC 2188 is a perturbed system, although it is not obviously an interacting galaxy. In addition, NGC 2188 is remarkable for its interstellar disk-halo connection. An Halpha image of NGC 2188 shows the presence of spectacular features of ionized gas extending from a large star-forming complex up to 500 pc into the halo. Also, peculiar HI filaments and at least one superbubble are present in NGC 2188., Comment: to be published in A&A, 6 pages, latex

We present a study of the radio emission from rotating, charged dust grains immersed in the ionized gas constituting the thick, H$\alpha$-emitting disk of many spiral galaxies. Using up-to-date optical constants, the charge on the grains exposed to the diffuse galactic UV flux has been calculated. An analytical approximation for the grain charge has been derived, which is then used to obtain the grain rotation frequency. Grains are found to have substantial radio emission peaked at a cutoff frequency in the range 10-100~GHz, depending on the grain size distribution and on the efficiency of the radiative damping of the grain rotation. The dust radio emission is compared to the free-free emission from the ionized gas component; some constraints on the magnetic field strength in the observed dusty filaments are also discussed. The model can be used to test the disk-halo interface environment in spiral galaxies, to determine the amount and size distribution of dust in their ionized component, and to investigate the rotation mechanisms for the dust. Numerical estimates are given for experimental purposes., Comment: Plain TeX, ST-801