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Jacobus C. Kapteyn is regarded as one of the coryfees of the University of Groningen. Part of his legacy is two paintings of him by Dutch painter Jan Pieter Veth. One, showing him at his desk, decorates the Kapteyn Room in the Kapteyn Astronomical Institute, and the other one, displaying him in academic attire, is in the University's gallery of professors in the central Academy Building. The first was offered to the Kapteyns on the occasion of his 40-th anniversary as professor in 1918 and the second to the University after his retirement in 1921. It has been suggested that there must have been a third portrait that now is lost. Former director Adriaan Blaauw has proposed that the one in the Academy Building actually was first offered in 1918, but at Mrs. Kapteyn's request replaced by the one now in the Kapteyn Room. The first version was then later adapted to the requirements of the gallery of professors by Veth himself by overpainting it with academic attire. A preliminary trial version by Veth, in the possession of Kapteyn's greatgrandson, shows what it would have looked like before the adaption. The following reports on new evidence: the biography of Jan Veth that historian Johan Huizinga, friend of Veth, wrote, and letters Veth wrote to his wife while he was working on these paintings. This provides strong support of Blaauw's sequence of events with a few modifications. No third painting has ever been produced., Comment: This paper has been submitted for publication to the Journal of Astronomical History and Heritage. It has 14 figures, some of them illustrating paintings by Veth. A version with high-resolution figures is available at www.astro.rug.nl/~vdkruit/Veth.pdf

Antonie (Anton) Pannekoek (1873-1960) is remembered as one of the initiators of the field of stellar atmospheres. A second part of his research concerned Galactic astronomy. He was convinced that the sidereal system was built up of clouds of stars in a smooth, low-density stratum. In addition there were dark clouds together with streaks with little or no extinction in between. Pannekoek looked at bright star clouds and estimated their distance from their contribution to star counts. He found values of tens of kpc, which would mean their distribution was similar in extent to that of Shapleys globular cluster system. Later he had to reduce his distance by a factor over two, and later still retract the method. He developed a rigorous method of estimating distances of dark clouds from modeling star counts off and on the cloud, preceding Wolf's quick and dirty method. He should have received more credit for this. He started isophotal maps of the northern and southern Milky Way, first from visual observations, later from photographic surface photometry using out-of-focus exposures. I compare Pannekoeks maps with detailed photographic surface photometry of the south by the group in Bochum and to the almost all-sky mapping by the Pioneer 10 spacecraft, free of zodiacal light, from beyond the asteroid belt. This shows Panneloeks maps to be surprisingly accurate. The legacy of Pannekoek in the area of Galactic research consists of his mapping of the structure of the nearby part of the Galaxy, the distances of dark clouds, and isophotal maps of the Milky Way. His other contributions turned out inconclusive or wrong as a result of his conviction, resulting from his many years of observing and mapping the Milky Way, that the nearby distribution is characterized primarily by more or less isolated clouds of stars and by dust restricted to isolated dark clouds and streaks., Comment: 92 pages, 32 figures, 4 tables. This paper has been accepted for publication in the March issue by the Journal of Astronomical History and Heritage. For a version with high-resolution figures please go to the my homepage, where a link will be provided

In this contribution I discuss the Kapteyn Astronomical Laboratory during the period of Pieter Johannes van Rhijn's directorate, which lasted from 1921 to 1957. It had developed under the founder Jacobus Cornelius Kapteyn into one of the leading astronomical research institutes in the world. When van Rhijn took over at the retirement of Kapteyn, it was in the process of coordinating Kapteyn's Plan of Selected Areas. Van Rhijn's research was solid and professional work, but in his papers he invariably stopped before discussing how his findings did fit into the larger scheme of things. He maybe was unimaginative but it did lack the link to the larger view towards the emerging picture of the structure of the Galaxy. Van Rhijn was unfortunate to be hampered throughout almost his complete directorate by factors, that severely limited his attempts to obtain more funding in spite of local support by his university. These were of course in the first place the Great Depression of the 1930s and the Second World War and its aftermath, while during most of the 1940s he suffered from tuberculosis. But also the remote location of Groningen compared to Leiden, where a major infrastructure led by three important protegees of Kapteyn was in place, and the governmental bias towards support for Leiden over Groningen was an important factor. Finally I examine the developments in the 1950s and the circumstances that made Adriaan Blaauw accept his appointment as van Rhijn's successor in 1957 and initiate the beginnings of the revival under his leadership., Comment: Accepted by the Journal of Astronomical History and Heritage. For a version with high-res figures please see www.astro.rug.nl/~vdkruit/PJvRhijn.pdf

Honorary degrees and particularly doctorates are important instruments to enhance the standing of universities and professors, in addition to receiving these as a measure of a scientist's recognition. Jacobus C. Kapteyn from the University of Groningen in the Netherlands, one of the most prominent astronomers of his times, received three of these and has persuaded his university to award at least three, possibly five. I examine the background of the selection of the latter in view of developments in Kapteyn's times in his career, international astronomy and political and cultural circumstances., Comment: Accepted for publication in the Journal for Astronomical History and Heritage. For a version with hi-res figures see https://www.astro.rug.nl/%7Evdkruit/honocaus.pdf

We use data from the IAC Stripe82 Legacy Project to study the surface photometry of 22 nearby, face-on to moderately inclined spiral galaxies. The reprocessed and combined Stripe 82 $g'$, $r'$ and $i'$ images allow us to probe the galaxy down to 29-30 $r'$-magnitudes/arcsec$^2$ and thus reach into the very faint outskirts of the galaxies. Truncations are found in three galaxies. An additional 15 galaxies are found to have an apparent extended stellar halo. Simulations show that the scattering of light from the inner galaxy by the Point Spread Function (PSF) can produce faint structures resembling haloes, but this effect is insufficient to fully explain the observed haloes. The presence of these haloes and of truncations is mutually exclusive, and we argue that the presence of a stellar halo and/or light scattered by the PSF can hide truncations. Furthermore, we find that the onset of the stellar halo and the truncations scales tightly with galaxy size. Interestingly, the fraction of light does not correlate with dynamic mass. Nineteen galaxies are found to have breaks in their profiles, the radius of which also correlates with galaxy size., Comment: Accepted for publication by Monthly Notices of the Royal Astronomical Society. High-res versions of the paper and the online appendix are available at http://www.astro.rug.nl/~vdkruit/jea3/homepage/Petersetal-VII.pdf and ...PaperVII-online.pdf

The breaks and truncations in the luminosity profile of face-on spiral galaxies offer valuable insights in their formation history. The traditional method of deriving the surface photometry profile for face-on galaxies is to use elliptical averaging. In this paper, we explore the question whether elliptical averaging is the best way to do this. We apply two additional surface photometry methods, one new: principle axis summation, and one old that has become seldom used: equivalent profiles. These are compared to elliptically averaged profiles using a set of 29 face-on galaxies. We find that the equivalent profiles match extremely well with elliptically averaged profiles, confirming the validity of using elliptical averaging. The principle axis summation offers a better comparison to edge-on galaxies., Comment: Accepted for publication by Monthly Notices of the R.A.S. A hi-res version is available at http://www.astro.rug.nl/~vdkruit/Petersetal-VI.pdf

We present optical and near-infrared archival observations of eight edge-on galaxies. These observations are used to model the stellar content of each galaxy using the FitSKIRT software package. Using FitSKIRT, we can self-consistently model a galaxy in each band simultaneously while treating for dust. This allows us to accurately measure both the scale length and scale height of the stellar disc, plus the shape parameters of the bulge. By combining this data with the previously reported integrated magnitudes of each galaxy, we can infer their true luminosities. We have successfully modelled seven out of the eight galaxies in our sample. We find that stellar discs can be modelled correctly, but have not been able to model the stellar bulge reliably. Our sample consists for the most part of slow rotating galaxies, and we find that the average dust layer is much thicker than what is reported for faster rotating galaxies., Comment: Accepted for publication by Monthly Notices RAS. Hi-res. version available at www.astro.rug.nl/~vdkruit/Petersetal-IV.pdf

We present a new strategy for fitting the structure and kinematics of the HI in edge-on galaxies using a fit to the terminal-velocity channel maps of a HI data cube. The strategy can deal with self-absorbing HI gas and the presence of warps. The method is first tested on a series of models. We demonstrate that fitting optically thin models to real galaxies will lead to an overestimation of the thickness and velocity dispersion, and to a serious underestimation of the HI face-on column densities. We subsequently fit both self-absorption and optically thin models to the HI data of six edge-on galaxies. In three of these we have also measured the velocity dispersion. On average 27 \pm 6 % of the total HI mass of edge-on galaxies is hidden by self-absorption. This implies that the HI mass, thickness and velocity dispersion of galaxies is typically underestimated in the literature., Comment: Accepted for publication by Monthly Notices RAS. Hi-res. version available at www.astro.rug.nl/~vdkruit/Petersetal-III.pdf

We present a new HI modelling tool called \textsc{Galactus}. The program has been designed to perform automated fits of disc-galaxy models to observations. It includes a treatment for the self-absorption of the gas. The software has been released into the public domain. We describe the design philosophy and inner workings of the program. After this, we model the face-on galaxy NGC2403, using both self-absorption and optically thin models, showing that self-absorption occurs even in face-on galaxies. It is shown that the maximum surface brightness plateaus seen in Paper I of this series are indeed signs of self-absorption. The apparent HI mass of an edge-on galaxy can be drastically lower compared to that same galaxy seen face-on. The Tully-Fisher relation is found to be relatively free from self-absorption issues., Comment: Accepted for publication by Monthly Notices RAS. Hi-res. version available at www.astro.rug.nl/~vdkruit/Petersetal-II.pdf

In the previous papers in this series, we have measured the stellar and \hi content in a sample of edge-on galaxies. In the present paper, we perform a simultaneous rotation curve and vertical force field gradient decomposition for five of these edge-on galaxies. The rotation curve decomposition provides a measure of the radial dark matter potential, while the vertical force field gradient provide a measure of the vertical dark matter potential. We fit dark matter halo models to these potentials. Using our \hi self-absorption results, we find that a typical dark matter halo has a less dense core ($0.094\pm0.230$\,M$_\odot$/pc$^3$) compared to an optically thin \hi model ($0.150\pm0.124$\,M$_\odot$/pc$^3$). The HI self-absorption dark matter halo has a longer scale length $R_c$ of $1.42\pm 3.48$\,kpc, versus $1.10\pm 1.81$\,kpc for the optically thin HI model. The median halo shape is spherical, at $q=1.0\pm0.6$ (self-absorbing \hi), while it is prolate at $q=1.5\pm0.6$ for the optically thin. Our best results were obtained for ESO\,274-G001 and UGC\,7321, for which we were able to measure the velocity dispersion in Paper III. These two galaxies have drastically different halo shapes, with one oblate and one strongly prolate. Overall, we find that the many assumptions required make this type of analysis susceptible to errors., Comment: Accepted for publication by MNRAS. Earlier papers in the series are in the refereeing process. High-res. version of this paper is available at www.astro.rug.nl/~vdkruit/Petersetal-V.pdf

Jacobus Cornelius Kapteyn (1851-1922) presented a model for the distribution of stars in space together with a dynamical interpretation in terms of an equilibrium between the gravitational field of the stars and their random motion and rotation. In the vertical direction Kapteyn's results are substantially correct. Usually the Kapteyn Universe is described as being flawed due to neglect of interstellar absorption. Kapteyn was led to adopt this on the basis of widely accepted evidence by Shapley on an absence of reddening of stars in globular clusters. But another, equally important misconception was Kapteyn's interpretation of the two Star Streams as manifestations of two groups of stars rotating around a center in opposite directions. This was supported by the observation of very different mixes in stellar types in the two streams. Had Kapteyn adopted the absorption as he himself had determined it he would not have been able to arrive at a consistent picture., Comment: To appear in LESSONS FROM THE LOCAL GROUP: A Conference in Honour of David Block and Bruce Elmegreen, eds. Freeman, K.C., Elmegreen, B.G., Block, D.L.and Woolway, M., Springer: New York

We present neutral hydrogen observations for a sample of eight nearby, late-type, edge-on galaxies. All of the galaxies have been well resolved in the radial direction, while six have also been well resolved in the vertical direction. We find that each of the galaxies has approximately the same maximum surface brightness temperature throughout its disc. We argue that self-absorption is the main cause of this phenomenon and that subsequent decompositions will require a treatment of this., Comment: Accepted for publication by MNRAS. High-res. version of this paper is available at www.astro.rug.nl/~vdkruit/Petersetal-I.pdf

Our aim is to explore the relation between gas, atomic and molecular, and dust in spiral galaxies. Gas surface densities are from atomic hydrogen and CO line emission maps. To estimate the dust content, we use the disk opacity as inferred from the number of distant galaxies identified in twelve HST/WFPC2 fields of ten nearby spiral galaxies. The observed number of distant galaxies is calibrated for source confusion and crowding with artificial galaxy counts and here we verify our results with sub-mm surface brightnesses from archival Herschel-SPIRE data. We find that the opacity of the spiral disk does not correlate well with the surface density of atomic (Hi) or molecular hydrogen (H2) alone implying that dust is not only associated with the molecular clouds but also the diffuse atomic disk in these galaxies. Our result is a typical dust-to-gas ratio of 0.04, with some evidence that this ratio declines with galactocentric radius, consistent with recent Herschel results. We discuss the possible causes of this high dust-to-gas ratio; an over-estimate of the dust surface-density, an under-estimate of the molecular hydrogen density from CO maps or a combination of both. We note that while our value of the mean dust-to-gas ratio is high, it is consistent with the metallicity at the measured radii if one assumes the Pilyugin & Thuan calibration of gas metallicity., Comment: Accepted by AN, 15 pages, 10 figures, 1 table

We present Herschel/SPIRE images at 250, 350, and 500 {\mu}m of NGC 4244, a typical low-mass, disk-only and edge-on spiral galaxy. The dust disk is clumpy and shows signs of truncation at the break radius of the stellar disk. This disk coincides with the densest part of the Hi disk. We compare the Spectral Energy Distribution, including the new SPIRE fluxes, to 3D radiative transfer models; a smooth model disk and a clumpy model with embedded heating. Each model requires a very high value for the dust scale-length (h(dust) = 2 - 5 h(stars)), higher dust masses than previous models of NGC 4244 (Md = 0.47 - 1.39 \times 10e7 Msun) and a face-on optical depth of {\tau}(V) = 0.4 - 1.12, in agreement with previous disk opacity studies. The vertical scales of stars and dust are similar. The clumpy model much better mimics the general morphology in the submm images and the general SED. The inferred gas-to-dust mass ratio is compatible with those of similar low-mass disks. The relatively large radial scale-length of the dust disk points to radial mixing of the dusty ISM within the stellar disk. The large vertical dust scale and the clumpy dust distribution of our SED model are both consistent with a scenario in which the vertical structure of the ISM is dictated by the balance of turbulence and self-gravity., Comment: 4 pages, 3 figures, 1 table, accepted for A&A Letters

The properties of gas in the 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. Until now mostly massive galaxies have been investigated for their halo properties. Here we report on deep HI and H{\alpha} observations of the edge-on dwarf galaxy UGC 1281 in order to determine the existence of extra-planar gas and the kinematics of this galaxy. This is the first time a dwarf galaxy is investigated for its gaseous halo characteristics. We have obtained H{\alpha} integral field spectroscopy using PPAK at Calar Alto and deep HI observations with the WSRT of this edge-on dwarf galaxy. These observations are compared to 3D models in order to determine the distribution of HI in the galaxy. We find that UGC 1281 has H{\alpha} emission up to 25"(655 pc) in projection above the plane and in general a low H{\alpha} flux. Compared to other dwarf galaxies UGC 1281 is a normal dwarf galaxy with a slowly rising rotation curve that flattens off at 60 km/s and a central depression in its HI distribution. Its HI extends 70" (1.8 kpc) in projection from the plane. This gas can be explained by either a warp partially in the line-of-sight warp or a purely edge-on warp with rotational velocities that decline with a vertical gradient of 10.6 \pm 3.7 km/s/kpc. The line-of-sight warp model is the preferred model as it is conceptually simpler. In either model the warp starts well within the optical radius., Comment: Accepted for publication in MNRAS. 16 pages, 14 figures

We derive total (atomic + molecular) hydrogen densities in giant molecular clouds (GMCs) in the nearby spiral galaxy M33 using a method that views the atomic hydrogen near regions of recent star formation as the product of photodissociation. Far-UV photons emanating from a nearby OB association produce a layer of atomic hydrogen on the surfaces of nearby GMCs. Our approach provides an estimate of the total hydrogen density in these GMCs from observations of the excess far-UV emission that reaches the GMC from the OB association, and the excess 21-cm radio HI emission produced after these far-UV photons convert H2 into HI on the GMC surface. The method provides an alternative approach to the use of CO emission as a tracer of H2 in GMCs, and is especially sensitive to a range of density well below the critical density for CO(1-0) emission. We describe our "PDR method" in more detail and apply it using GALEX far-UV and VLA 21-cm radio data to obtain volume densities in a selection of GMCs in the nearby spiral galaxy M33. We have also examined the sensitivity of the method to the linear resolution of the observations used; the results obtained at 20 pc are similar to those for the larger set of data at 80 pc resolution. The cloud densities we derive range from 1 to 500 cm-3, with no clear dependence on galactocentric radius; these results are generally similar to those obtained earlier in M81, M83, and M101 using the same method., Comment: Accepted for publication in MNRAS. 25 figures, 16 tables, including online-only materials

The formation and evolution of galactic disks is particularly important for understanding how galaxies form and evolve, and the cause of the variety in which they appear to us. Ongoing large surveys, made possible by new instrumentation at wavelengths from the ultraviolet (GALEX), via optical (HST and large groundbased telescopes) and infrared (Spitzer) to the radio are providing much new information about disk galaxies over a wide range of redshift. Although progress has been made, the dynamics and structure of stellar disks, including their truncations, are still not well understood. We do now have plausible estimates of disk mass-to-light ratios, and estimates of Toomre's $Q$ parameter show that they are just locally stable. Disks are mostly very flat and sometimes very thin, and have a range in surface brightness from canonical disks with a central surface brightness of about 21.5 $B$-mag arcsec$^{-2}$ down to very low surface brightnesses. It appears that galaxy disks are not maximal, except possibly in the largest systems. Their HI layers display warps whenever HI can be detected beyond the stellar disk, with low-level star formation going on out to large radii. Stellar disks display abundance gradients which flatten at larger radii and sometimes even reverse. The existence of a well-defined baryonic Tully-Fisher relation hints at an approximately uniform baryonic to dark matter ratio. Thick disks are common in disk galaxies and their existence appears unrelated to the presence of a bulge component; they are old, but their formation is not yet understood. Disk formation was already advanced at redshifts of $\sim 2$, but at that epoch disks were not yet quiescent and in full rotational equilibrium. Downsizing is now well-established. The formation and history of star formation in S0s is still not fully understood., Comment: This review has been submitted for Annual Reviews of Astronomy & Astrophysics, vol. 49 (2011); the final printed version will have fewer figures and a somewhat shortened text. A pdf-version of this preprint with high-resolution figures is available from http://www.astro.rug.nl/~vdkruit/jea3/homepage/disks-ph.pdf. (table of contents added; 71 pages, 24 figures, 529 references)

The aim of this study is to make a bibliometric comparison of the performance of research astronomers in the Netherlands Research School for Astronomy (NOVA) with astronomers elsewhere by using the NASA Astrophysics Data System (ADS). We use various indices for bibliometric performance for a sample of NOVA astronomers to compare to samples of astronomers worldwide, and from the United States. We give much weight to normalising bibliometric measures by number of authors, and number of years since first publication. In particular we calculate the `Hirsh-index' normalized to number of authors and for first-author papers. Secondly, we consider the results of the 'Nederlands Observatorium van Wetenschap en Technologie' (NOWT; Netherlands Observatory of Science and Technology), which regularly publishes a report 'Science and Technology Indicators'. We reproduce those results using publication lists from institutions in the Netherlands, again using ADS, and examine and discuss the conclusions and indications in these reports. We find that the NOVA researchers perform much better in bibliometric measures than samples drawn from IAU or AAS membership lists. A more suitable comparison is one with the (tenured) staff of the top-15 US institutions and there the NOVA staff performs in these respects as good or almost as good as that of American top institutes. From a citation analysis through the use of ADS we conclude that the impact ratio of Dutch astronomical publications is rising which is opposite to what is reported by NOWT. This difference is most likely caused by a better separation of astronomy and physics in ADS than in World of Knowledge. ADS probably finds more citations in conference proceedings, while the inclusion of citations to articles with their pre-print identifier could also help explain the difference (especially since the citation windows in the reports are short)., Comment: 37 pages, 21 figures, 9 tables

We present a new observational method to evaluate the star formation law as formulated by Schmidt: the power-law expression assumed to relate the rate of star formation in a volume of space to the local total gas volume density. Volume densities in the clouds surrounding an OB association are determined with a simple model which considers atomic hydrogen as a photodissociation product on cloud surfaces. The photodissociating flux incident on the cloud is computed from the far-UV luminosity of the OB association and the geometry. We have applied this "PDR Method" to a sample of star-forming regions in M33 using VLA 21-cm data for the HI and GALEX imagery in the far-UV. It provides an estimate of the total volume density of hydrogen (atomic + molecular) in the gas clouds surrounding the young star cluster. A logarithmic graph of the cluster UV luminosity versus the surrounding gas density is a direct measure of the star formation law. However, this plot is severely affected by observational selection, rendering large areas of the diagram inaccessible to the data. An ordinary least-squares regression fit therefore gives a strongly biased result. Its slope primarily reflects the boundary defined when the 21-cm line becomes optically thick, no longer reliably measuring the HI column density. We use a maximum-likelihood statistical approach which can deal with truncated and skewed data, taking into account the large uncertainties in the derived total gas densities. The exponent we obtain for the Schmidt law in M33 is 1.4 \pm 0.2., Comment: Accepted for publication in ApJ

In this review I concentrate on three areas related to structure of disks in spiral galaxies. First I will review the work on structure, kinematics and dynamics of stellar disks. Next I will review the progress in the area of flaring of HI layers. These subjects are relevant for the presence of dark matter and lead to the conclusion that disk are in general not `maximal', have lower M/L ratios than previously suspected and are locally stable w.r.t. Toomre's Q criterion for local stability. I will end with a few words on `truncations' in stellar disks., Comment: Invited review at "Galaxies and their Masks" for Ken Freeman's 70-th birthday, Sossusvlei, Namibia, April 2010. A version with high-res. figures is available at http://www.astro.rug.nl/~vdkruit/jea3/homepage/Namibiachapter.pdf

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., Comment: Accepted for publication by Astronomy & Astrophysics. For a higher resolution version see http://www.astro.rug.nl/~vdkruit/jea3/homepage/12565.pdf

This is the third paper in a series in which we attempt to put constraints on the flattening of dark halos in disk galaxies. For this purpose we need to analyse the observed XV diagrams in edge-on galaxies to accurately measure the radial HI surface density, the rotation curve and the HI velocity dispersion. We present the results of the modelling of HI observations of 8 HI-rich, late-type, edge-on galaxies. In all of these we find differential rotation. Most systems display HI velocity dispersions of 6.5 to 7.5 km s$^{-1}$ and all except one show radial structure. There is an increase in the mean HI velocity dispersion with maximum rotation velocity, at least up to 120 km s$^{-1}$. Next we analyse the observations to derive the radial variation of the thickness (flaring) of the HI layer. We find that with the exception of the asymmetric IC5052, all of the galaxies in our sample are good candidates for 3D mass modelling to measure the dark halo shape. The flaring profiles are symmetric and have a common shape, increasing linearly inside the stellar disks and exponential outside where the gravitational potential is dominated by the dark halo. In the best example, UGC7321, we find in the inner regions small deviations from the midplane and accompanying increases in thickness of the HI layer that are possibly a result of perturbations by a relatively strong bar., Comment: Accepted for publication by Astronomy & Astrophysics. Higher resolution version available at http://www.astro.rug.nl/~vdkruit/jea3/homepage/12567.pdf

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., Comment: Accepted for publication by Astronomy & Astrophysics. For a higher resolution version see http://www.astro.rug.nl/~vdkruit/jea3/homepage/12566.pdf

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. We have analysed the HI channel maps to accurately measure all four functions that describe the HI kinematics and 3D distribution: 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 UGC7321. We measured the stellar mass distribution ($M=3\times10^8$ \msun with $M/L_R\lesim0.2$), finding that the vertical force of the gas disk dominates the stellar disk at all radii. We find 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 and velocity dispersion of the HI revealed that UGC7321 has a spherical halo density distribution with a flattening of $q = c/a = 1.0 \pm 0.1$., Comment: Accepted by Astronomy & Astrophysics. Higher resolution version available at http://www.astro.rug.nl/~vdkruit/jea3/homepage/12568.pdf Revised versions of papers I, II and III are being refereed and will appear on astro-ph upon acceptance

We present an approach for analysing the morphology and physical properties of Hi features near giant OB asso- ciations in M33, in the context of a model whereby the Hi excess arises from photodissociation of the molecular gas in remnants of the parent Giant Molecular Clouds (GMCs). Examples are presented here in the environs of NGC604 and CPSDPZ204, two prominent Hii regions in M33. These are the first results of a detailed analysis of the environs of a large number of OB associations in that galaxy. We present evidence for "diffusion" of the far-UV radiation from the OB association through a clumpy remnant GMC, and show further that enhanced CO(1-0) emission appears preferentially associated with GMCs of higher volume density., Comment: Accepted to ApJ

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 R25, 0.5 - 50 cm-3 outside R25 . 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., Comment: 9 pages, 11 figures

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 H-$\alpha$ emission. We have taken a quantitative look at this asymmetry at different wavelengths for the first time. We propose that NGC 891 is intrinsically almost symmetric, as seen in Spitzer observations, and the large asymmetry in H-$\alpha$ emission is mostly due to dust attenuation. We quantify the additional optical depth needed to cause the observed H-$\alpha$ asymmetry. A comparison of 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. From the 24 $\mu$m emission we find that the intrinsic asymmetry in star-formation in NGC 891 is small i.e., approximately 30%. The additional asymmetry in H-alpha 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 $\tau$=0.8 and a scale height of 0.5 kpc. This observational technique offers the possibility to quantify the effects of vertical ISM disk stability as an explanation for dust lanes in massive galaxies Dalcanton et al. (2004)., Comment: 4 pages, 3 figures, to appear in the proceedings of ``The Evolving ISM in the Milky Way and Nearby Galaxies'', Pasadena, 2007

I review observations of truncations of stellar disks and models for their origin, compare observations of truncations in moderately inclined galaxies to those in edge-on systems and discuss the relation between truncations and HI-warps and their systematics and origin. Truncations are a common feature in edge-on stellar disks, but the relation of truncations in face-on to those in edge-on galaxies needs further clarification. The origin of truncations is most likely related to a maximum in the specific angular momentum in the material that formed the stellar disks, but this model does probably require some redistribution of angular momentum. HI-warps start just beyond the truncation radius and disks and warps appear distinct components. This suggests that inner disks form initially and settle as rigid, very flat structures, while HI-warps result from later infall of gas with a different orientation of the angular momentum., Comment: Invited Review at the Vatican Symposium: Formation and Evolution of Galaxy Disks, October 2007, proceeding editors Jose G. Funes, SJ and Enrico M. Corsini

HI features near young star clusters in M81 are identified as the photodissociated surfaces of Giant Molecular Clouds (GMCs) from which the young stars have recently formed. The HI column densities of these features show a weak trend, from undetectable values inside R = 3.7 kpc and increasing rapidly to values around 3 x 10^21 cm^-2 near R ~ 7.5 kpc. This trend is similar to that of the radially-averaged HI distribution in this galaxy, and implies a constant area covering factor of ~ 0.21 for GMCs throughout M81. The incident UV fluxes G0 of our sample of candidate PDRs decrease radially. A simple equilibrium model of the photodissociation-reformation process connects the observed values of the incident UV flux, the HI column density, and the relative dust content, permitting an independent estimate to be made of the total gas density in the GMC. Within the GMC this gas will be predominantly molecular hydrogen. Volume densities of 1 < n < 200 cm^-3 are derived, with a geometric mean of 17 cm^-3. These values are similar to the densities of GMCs in the Galaxy, but somewhat lower than those found earlier for M101 with similar methods. Low values of molecular density in the GMCs of M81 will result in low levels of collisional excitation of the CO(1-0) transition, and are consistent with the very low surface brightness of CO(1-0) emission observed in the disk of M81., Comment: 17 pages, 11 figures, 3 tables; Accepted by ApJ

The quantity of dust in a spiral disk can be estimated using the dust's typical emission or the extinction of a known source. In this paper, we compare two techniques, one based on emission and one on absorption, applied on sections of fourteen disk galaxies. The two measurements reflect, respectively the average and apparent optical depth of a disk section. Hence, they depend differently on the average number and optical depth of ISM structures in the disk. The small scale geometry of the cold ISM is critical for accurate models of the overall energy budget of spiral disks. ISM geometry, relative contributions of different stellar populations and dust emissivity are all free parameters in galaxy Spectral Energy Distribution (SED) models; they are also sometimes degenerate, depending on wavelength coverage. Our aim is to constrain typical ISM geometry. The apparent optical depth measurement comes from the number of distant galaxies seen in HST images through the foreground disk. We measure the IR flux in images from the {\it Spitzer} Infrared Nearby Galaxy Survey in the same section of the disk that was covered by HST. A physical model of the dust is fit to the SED to estimate the dust surface density, mean temperature, and brightness in these disk sections. The surface density is subsequently converted into the average optical depth estimate. The two measurements generally agree. The ratios between the measured average and apparent optical depths of the disk sections imply optically thin clouds in these disks. Optically thick disks, are likely to have more than a single cloud along the line-of-sight., Comment: 31 pages, 5 figures, 4 tables, accepted for publication in AJ

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 H$\alpha$ emission. In this letter, we will take a quantitative look at this asymmetry at different wavelengths for the first time. We suggest that NGC 891 is intrinsically almost symmetric and the large asymmetry in H$\alpha$ emission is mostly due to dust attenuation. We will quantify the additional optical depth needed to cause the observed asymmetry in this model. 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. From the 24 $\mu$m emission we find that the intrinsic asymmetry in star formation in NGC 891 is small i.e., $\sim 30%$. The additional asymmetry in H$\alpha$ is modeled as additional symmetric dust attenuation which extends up to $\sim$ 40\arcsec (1.9 kpc) above the plane of the galaxy with a mid-plane value of $\tau$=0.8 and a scale height of 0.5 kpc, Comment: A&A in press. 5 pages, 3 figures

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. 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. We have obtained a Fabry-P\'erot data cube in Halpha 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. 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 km/s/kpc. 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., Comment: A&A, in press. 13 pages, 19 figures

Edge-on spiral galaxies often have stellar disks with relatively sharp truncations and warped HI-layers in the outer parts. Warps appear to start preferentially near the optical boundaries of the disks. Here we make a comparative study of warps and truncations in edge-on galaxies. The Garc\'{\i}a-Ruiz et al. (2002) sample with extensive HI-mapping is complemented with luminosity distributions from the Sloan Digital Sky Survey. The method to identify truncations has been tested using the sample of edge-on galaxies of van der Kruit & Searle. Results are: (i.) The majority (17 out of 23) of the galaxies show evidence for truncations. (ii.) When an HI-warp is present it starts at 1.1 truncation radii. (iii.) This supplements the rules for warps formulated by Briggs (1990), if the Holmberg radius is replaced for edge-on systems with the truncation radius. (iv.) The truncation radius and the onset of the warps coincide radially with features in the rotation curve and the HI surface density. The latter is also true for less inclined systems. (v.) Inner disks are very flat and the onset of the warp just beyond the truncation radius is abrupt and discontinuous. These findings suggest that the inner flat disk and the outer warped disk are distinct components with quite different formation histories, probably involving quite different epochs. The inner disk forms initially and the warped outer disk forms as a result of much later infall of gas with a higher angular momentum in a different orientation. In an appendix the Holmberg radius is discussed. Contrary to what is often assumed Holmberg radii are not corrected for inclination., Comment: Accepted for pubication by Astronomy & Astrophysics

Dust emission in the far-infrared (FIR) characterizes the temperature and quantity of interstellar dust in a spiral disk. The three Spitzer/MIPS bands are well suited to measuring the gradient in temperature and the total optical depth in the disk of a spiral galaxy. Another way to estimate the quantity of dust in a spiral disk is the "Synthetic Field Method" (SFM, Gonzalez et al. 1998), which uses the number of distant field galaxies seen through the disk of the nearby spiral. The optical depth estimated from this method can be compared to the values derived from the FIR emission. Since the two techniques depend on different assumptions regarding the dust geometry and emissivity, this comparison between the optical depth profiles can potentially shed light on the structure and quantity of the ISM in spiral disks, especially any colder components. The dust responsible for the opacity from distant galaxy counts appears to be predominantly cold (T < 20 K.). The differences between the radial absorption profiles can be explained by spiral arms in the SFM measurements. Taken over the same aperture, galaxy counts show higher extinction values than the FIR derived ones. The implications for dust geometry can hopefully be explored with a more rigorous estimate of dust mass from the FIR fluxes., Comment: 4 pages, 4 figures to appear in the conference proceedings of "Infrared Diagnostics of Galaxy Evolution", Pasadena, 2005

In this paper we explore the relation between dust extinction and stellar light distribution in disks of spiral galaxies. Extinction influences our dynamical and photometric perception of disks, since it can distort our measurement of the contribution of the stellar component. To characterize the total extinction by a foreground disk, Gonzalez et al. (1998) proposed the ``Synthetic Field Method'' (SFM), which uses the calibrated number of distant galaxies seen through the foreground disk as a direct indication of extinction. The method is described in Gonzalez et al. (1998) and Holwerda et al. (2005a). To obtain good statistics, the method was applied to a set of HST/WFPC2 fields Holwerda et al. (2005b) and radial extinction profiles were derived, based on these counts. In the present paper, we explore the relation of opacity with surface brightness or color from 2MASS images, as well as the relation between the scalelengths for extinction and light in the I band. We find that there is indeed a relation between the opacity (A_I) and the surface brightness, particularly at the higher surface brightnesses. No strong relation between near infrared (H-J, H-K) color and opacity is found. The scalelengths of the extinction are uncertain for individual galaxies but seem to indicate that the dust distribution is much more extended than the stellar light. The results from the distant galaxy counts are also compared to the reddening derived from the Cepheids light-curves Freedman et al. (2001). The extinction values are consistent, provided the selection effect against Cepheids with higher values of A_I is taken into account. The implications from these relations for disk photometry, M/L conversion and galaxy dynamical modeling are briefly discussed., Comment: 9 pages, 2 tables, 10 figures, accepted by A&A

The opacity of foreground spiral disks can be probed from the number of distant galaxies seen through them. To calibrate this number for effects other than the dust extinction, Gonzalez et al (1998) developed the "Synthetic Field Method". A synthetic field is an extincted Hubble Deep Field added to the science field. The relation between the dimming and the number of retrieved synthetic galaxies calibrates the number found in the science field. Here I present results from counts in 32 HST/WFPC2 fields. The relation between opacity and radius, arm and disk, surface brightness and HI are presented. The opacity is found to be caused by a clumpy distribution of clouds in the disk. The brighter parts of the disk -the center and arms- are also the more opaque ones. The dust distribution in spiral disks is found to be more extended than the stellar disk. A comparison between HI column densities and opacity shows little relation between the two., Comment: 7 pages, 5 figures, to appear in the conference proceedings of "Island Universes" conference in honor of Prof. P.C. van der Kruit, edited by R. de Jong

The ``Synthetic Field Method'' (SFM) was introduced by Gonzalez et al. (1998) to calibrate numbers of distant galaxies as a probe of extinction in a foreground spiral disk. Gonzalez et al. (2003) studied the effect of the foreground disk on these numbers using simulations of current and future instruments for fields in the LMC, M31 and NGC 4536, a galaxy in Virgo. They concluded that: (1) the brighter centers of disks were unsuitable, (2) the granularity of the disk at a fixed surface brightness is the limiting factor in the detection of distant galaxies, and (3) the optimum distance for measurements would be that of the Virgo cluster for the current instruments on board HST. At this distance the foreground disk is smoothed with distance, improving detection of distant background galaxies. Holwerda et al. (2005a) automated the SFM and Holwerda et al. (2005b) applied it to a large set of WFPC2 fields. In this paper, the quality of the extinction measurement in these fields is compared to their distance, granularity, surface brightness and structure. The average surface brightness of the of a field is shown to directly influence the accuracy of the SFM. This restricts meaningful measurements to the disks of spiral galaxies. Large structures such as spiral arms have a similar effect. The granularity or small scale structure in a field influences the detection of distant galaxies, limiting the SFM measurements in nearby disks. From the trends in the accuracy and maximum practical field-of-view considerations, the minimum and maximum distance for SFM application, approximately 5 and 35 Mpc respectively. Using the same instrument and detection method, the relations with SFM parameters and field characteristics can be used to forgo the synthetic fields altogether., Comment: 8 pages, 7 figures, Research Note accepted by A&A

The opacity of spiral galaxy disks, from counts of distant galaxies, is compared to HI column densities. The opacity measurements are calibrated using the ``Synthetic Field Method'' from Gonzalez et al (1998) and Holwerda et al. (2005a). When compared for individual disks, the HI column density and dust opacity do not seem to be correlated as HI and opacity follow different radial profiles. To improve statistics, an average radial opacity profile is compared to an average HI profile. Compared to dust-to-HI estimates from the literature, more extinction is found in this profile. This difference may be accounted for by an underestimate of the dust in earlier measurements due to their dependence on dust temperature. Since the SFM is insensitive to the dust temperature, the ratio between the SFM opacity and HI could very well be indicative of the true ratio. Earlier claims for a radially extended cold dust disk were based on sub-mm observations. A comparison between sub-mm observations and counts of distant galaxies is therefore desirable. We present the best current example of such a comparison, M51, for which the measurements seem to agree. However, this remains an area where improved counts of distant galaxies, sub-mm observations and our understanding of dust emissivity are needed., Comment: 8 pages, 5 figures, 1 table, accepted by A&A

In earlier papers in this series we determined the intrinsic stellar disc kinematics of fifteen intermediate to late type edge-on spiral galaxies using a dynamical modeling technique. From the photometry we find that intrinsically more flattened discs tend to have a lower face-on central surface brightness and a larger dynamica mass-to-light ratio. This observation suggests that at a constant maximum rotational velocity lower surface brightness discs have smaller vertical stellar velocity dispersions.Although the individual uncertainties are large, we find from the dynamical modeling that at least twelve discs are submaximal. The average disc contributes 53$\pm$4 percent to the observed rotation at 2.2 disc scalelengths, with a 1$\sigma$ scatter of 15 percent. This percentage becomes somewhat lower when effects of finite disc flattening and gravity by the dark halo and the gas are taken into account. Since boxy and peanut-shaped bulges are probably associated with bars, the result suggests that at 2.2$h_{\rm R}$ the submaximal nature of discs is independent of barredness. The possibility remains that very high surface brightness discs are maximal.We confirm that the radial stellar disc velocity dispersion is related to the galaxy maximum rotational velocity. The scatter in this $\sigma-v_{\rm max}$ relation appears to correlate with the disc flattening, face-on central surface brightness and dynamical mass-to-light ratio. Low surface brightness discs tend to be more flattened and have smaller stellar velocity dispersions. The findings are consistent with the observed correlation between disc flattening and dynamical mass-to-light ratio., Comment: Accepted for publication by Mon. Not. R.A.S

The stellar disc kinematics in a sample of fifteen intermediate- to late-type edge-on spiral galaxies are studied using a dynamical modeling technique. The sample covers a substantial range in maximum rotation velocity and deprojected face-on surface brightness and contains seven spirals with either a boxy- or peanut-shaped bulge. Dynamical models of the stellar discs are constructed using the disc structure from $I$-band surface photometry and rotation curves observed in the gas. The differences in the line-of-sight stellar kinematics between the models and absorption line spectroscopy are minimized using a least-squares approach. The modeling constrains the disc surface density and stellar radial velocity dispersion at a fiducial radius through the free parameter $\sqrt{M/L}$ $(\sigma_{\rm z}/\sigma_{\rm R})^{-1}$, where $\sigma_{\rm z}/\sigma_{\rm R}$ is the ratio of vertical and radial velocity dispersion and $M/L$ the disc mass-to-light ratio. For thirteen spirals a transparent model provides a good match to the mean line-of-sight stellar velocity dispersion. Models that include a realistic radiative transfer prescription confirm that the effect of dust on the observable stellar kinematics is small at the observed slit positions. We discuss possible sources of systematic error and conclude that most of these are likely to be small. The exception is the neglect of the dark halo gravity, which has probably caused an overestimate of the surface density in the case of low surface brightness discs., Comment: Accepted for publication by the Monthly Notices of the R.A.S

Dust extinction can be determined from the number of distant field galaxies seen through a spiral disk. To calibrate this number for the crowding and confusion introduced by the foreground image, Gonzalez et al.(1998) and Holwerda et al. (2005) developed the ``Synthetic Field Method'' (SFM), which analyses synthetic fields constructed by adding various deep exposures of unobstructed background fields to the candidate foreground galaxy field. The advantage of the SFM is that it gives the average opacity for area of galaxy disk without assumptions about either the distribution of absorbers or of the disk starlight. However it is limited by low statistics of the surviving field galaxies, hence the need to combine a larger sample of fields. This paper presents the first results for a sample of 32 deep HST/WFPC2 archival fields of 29 spirals. The radial profiles of average dust extinction in spiral galaxies based on calibrated counts of distant field galaxies is presented here, both for individual galaxies as well as for composites from our sample. The effects of inclination, spiral arms and Hubble type on the radial extinction profile are discussed. (Abbreviated), Comment: 43 pages, 16 figures, 3 tables, accepted for publication in the Astronomical Journal, (typos, table update, updates abstract)

The dust extinction in spiral disks can be estimated from the counts of background field galaxies, provided the deleterious effects of confusion introduced by structure in the image of the foreground spiral disk can be calibrated. Gonzalez et al. (1998) developed a method for this calibration, the ``Synthetic Field Method'' (SFM), and applied this concept to a HST/WFPC2 image of NGC4536. The SFM estimates the total extinction through the disk without the necessity of assumptions about the distribution of absorbers or the disk light. The poor statistics, however, result in a large error in individual measurements. We report on improvements to and automation of the Synthetic Field Method which render it suitable for application to large archival datasets. To illustrate the strengths and weaknesses of this new method, the results on NGC 1365, a SBb, and NGC 4536, a SABbc, are presented. The extinction estimate for NGC1365 is A_I = 0.6 (+0.6/-0.7) at 0.45 R_25 and for NGC4536 it is A_I = 1.6 (+1.0/-1.3) at 0.75 R_25. The results for NGC4536 are compared with those of Gonzalez et al. (1998). The automation is found to limit the maximum depth to which field galaxies can be found. Taking this into account, our results agree with those of Gonzalez et al. (1998). We conclude that this method can only give an inaccurate measure of extinction for a field covering a small solid angle. An improved measurement of disk extinction can be done by averaging the results over a series of HST fields, thereby improving the statistics. This can be achieved with the automated method, trading some completeness limit for speed. The results from this set of fields are reported in a companion paper Holwerda et al. (2005b)., Comment: updated references, abstract, 37 pages, 12 figures, 6 tables

We discuss the possible origin of the radial truncations in stellar discs, using measurements that we presented in an earlier paper (Kregel, van der Kruit & de Grijs, 2002). 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 threshhold on star formation., Comment: Accepted for publication by Mon. Not. R.A.S

A technique is introduced for deriving the gaseous rotation curves of edge-on spiral galaxies. The entire major axis position-velocity (XV) diagram is modeled 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 HI XV diagrams of eight edge-on spirals. For seven additional spirals the XV diagrams are of insufficient quality, and the HI rotational velocities derived earlier using the envelope-tracing method are retained. The HI results are augmented with the optical emission line (HII) kinematics to arrive at estimates of the full rotation curves. A detailed comparison of the HI and HII 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 HII is mainly confined to spiral arms and does not extend out to the edge of the HI layer, which may have caused the HII velocity profiles to be significantly narrower than those of the HI. This paper has been accepted by MNRAS and is available in pdf-format at the following URL: http://www.astro.rug.nl/$\sim $vdkruit/jea3/homepage/paperIII.pdf, Comment: Accepted for publication in MNRAS

We present Australia Telescope Compact Array (ATCA) and Westerbork Synthesis Radio Telescope (WSRT) 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 (1 sigma). This paper has been accepted by MNRAS and is available in pdf-format at the following URL: http://www.astro.rug.nl/$\sim $vdkruit/jea3/homepage/paperII.pdf, Comment: Accepted for oublication in MNRAS

We present deep optical long-slit spectra of 17 edge-on spiral galaxies of intermediate to late morphological type, mostly parallel to their major axes and in a few cases parallel to the minor axes.The line-of-sight stellar kinematics are obtained from the stellar absorption lines using the improvedc ross-correlation technique. In general, the stellar kinematics are regular and can be traced well into the disc-dominated region. The mean stellar velocity curves are far from solid-body, indicating that the effect of dust extinction is not large. The line-of-sight stellar disc velocity dispersion correlates with the galaxy maximum rotational velocity, but detailed modeling is necessary to establish whether this represents a physical relation. In four spirals with a boxy- or peanut-shaped bulge we are able to detect asymmetric velocity distributions, having a common signature with projected radius in the mean line-of-sight velocity and the $h_{3}$ and $h_{4}$ curves. In two cases this kinematic asymmetry probably represents the `figure-of-eight' pattern synonymous of a barred potential. We emphasize, however, that the signatures seen in the $h_{3}$ and $h_{4}$ curves may also be due to the disc seen in projection., Comment: Accepted for publication by Mon. Not. R.A.S

We have applied the "Synthetic Field Method" on a sample of ~20 nearby galaxies in order to determine the opacity of their disks. We present preliminary results on the radial dependence of cold dust absorption for 3 examples. The spirals NGC4535 and NGC4725 show significant absorption at a half-light radius. UGC2302, a LSB galaxy, shows much less opacity., Comment: 2 pages; Proceedings, Poster presented at IAU, Sidney, July 22, 2003

We analyse the global structure of the old stellar discs in 34 edge-on spiral galaxies. The radial and vertical exponential scale parameters of the discs are obtained by applying an improved two-dimensional decomposition technique to our I-band photometry. We find a clear increase in the disc scaleheight with maximum rotational velocity, in accordance with observations of the stellar velocity dispersions in galaxy discs. The range and maximum of the intrinsic flattening of the disc light seem to increase with both maximum rotational velocity and total HI mass. We use the disc flattening to estimate the disc contribution to the maximum rotational velocity, resulting in an average of 57+-22 percent. The disc light distributions are further investigated for the presence of radial truncations. We find that the radial light distributions of at least 20 spirals are truncated, corresponding to 60 percent of the sample. For small scalelength spirals, which are the most numerous in the local Universe, the results suggest that the average ratio of disc truncation radius to disc scalelength is at least four., Comment: 25 pages, 7 figures, appendix, Accepted for MNRAS (April 4 2002)

We have automated the ``Synthetic Field Method'' developed by Gonzalez et al.(1998) and used it to measure the opacity of the ISM in the Local Group dwarf galaxy Sextans A by using the changes in counts of background galaxies seen through the foreground system. The Sextans A results are consistent with the observational relation found by Cuillandre et al. (2001) between dust opacity and HI column density in the outer parts of M31., Comment: 4 pages, 6 figures, submitted for the proceedings of The Dynamics, Structure and History of Galaxies: A Workshop in Honour of Prof. Ken Freeman

One the most cited papers in astronomy is Ken Freeman's 1970 paper on exponential disks in galaxies. In this contribution I review what has been done in this area since then and what we can infer about systematic properties of disks in galaxies from surface photometry, HI synthesis observations and measurements of stellar kinematics. Most disks have radial truncations at $3.6 \pm 0.6$ radial scalelengths $h\subr{R}$. Galaxy disk thicknesses $h\subr{R}/h\subr{z}= 7.3 \pm 2.2$ imply that these disk cannot be ``maximum disks''. I briefly discuss a recent study of the ``superthin'' edge-on galaxy IC 5249., Comment: Invited review at "The dynamics, structure and history of galaxies", meeting held in honor of Ken Freeman's 60-th birthday, Dunk Island, Australia, August 2001