P. C. van der Kruit

Truncations of stellar disks and warps of HI-layers in edge-on spiral galaxies

Context. Edge-on spiral galaxies often have stellar disks with relatively sharp truncations and there is extensive observational material of warped HI-layers in the outer parts. Warps appear to start preferentially near the optical boundaries of the disks, but the possible relation between truncations and warps has not been studied in detail. Aims. The aim is to make a comparative study of warps and truncations in edge-on galaxies. Samples with detailed surface photometry or HI-mapping have little overlap. The Garcia-Ruiz et al. (2002, AA 1981b, AA 1982a, AA 1982b, AA (ii) when an HI-warp is present it starts at 1.1 truncation radii ( statistically allowing all possible geometries); (iii) this supplements the rules for warps formulated by Briggs ( 1990, ApJ, 352, 15), 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 sometimes with features in the rotation curve and often with steep declines in 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; ( vi) in an appendix the definition and derivation of the Holmberg radius is discussed. Contrary to what is often assumed in the literature Holmberg radii are not corrected for inclination. Conclusions. 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 ( either in a monolithic process in a short period or hierarchically on a somewhat more protracted timescale) and the warped outer disk forms as a result of much later infall of gas with a higher angular momentum in a different orientation. The results are also consistent in this picture with an origin of the disk truncations that is related to the maximum specific angular momentum available during its formation.

Structure and kinematics of edge-on galaxy discs - V. The dynamics of stellar discs

The stellar disc kinematics in a sample of 15 intermediate- to late-type edge-on spiral galaxies are studied using a dynamical modelling 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 modelling constrains the disc surface density and stellar radial velocity dispersion at a fiducial radius through the free parameter root M/L (sigma(z)/sigma(R))(-1), where sigma(z)/sigma(R) is the ratio of vertical and radial velocity dispersion and M/L is the disc mass-to-light ratio. For 13 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.

Herschel/SPIRE observations of the dusty disk of NGC 4244

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 (SED), 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(d) = 2-5 h(*)), higher dust masses than previous models of NGC 4244 (M-d = 0.47-1.39 x 10(7) M-circle dot) and a face-on optical depth of tau(f.o.)(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 sub-mm 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.

Structure and kinematics of edge-on galaxy discs - I. Observations of the stellar kinematics

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 improved cross-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 modelling 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 h3 and h4 curves. In two cases this kinematic asymmetry probably represents the ‘figure-of-eight’ pattern synonymous with a barred potential. We emphasize, however, that the signatures seen in the h3 and h4 curves may also be due to the disc seen in projection.

The opacity of spiral galaxy disks. VIII. Structure of the cold ISM

The quantity of dust in a spiral disk can be estimated using the dusts 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 to sections of 14 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 the typical ISM geometry. The apparent optical depth measurement comes from the number of distant galaxies seen in Hubble Space Telescope (HST) images through the foreground disk, calibrated with the synthetic field method (SFM). We discuss what can be learned from the SFM measurement alone regarding ISM geometry. We measure the IR flux in images from the 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, and the SED model finds mostly cold dust ( T <25 K). The ratios between the measured average and apparent optical depths of the disk sections imply optically thin (tau(c =) 0.4) clouds in these disks. Optically thick disks are likely to have more than a single cloud along the line of sight.

The opacity of spiral galaxy disks. IV. Radial extinction profiles from counts of distant galaxies seen through foreground disks

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. and Holwerda et al. developed the Synthetic Field Method (SFM), which analyzes 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 the area of a galaxy disk without making assumptions about either the distribution of absorbers or of the disk starlight. However, it is limited by poor statistics on 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 Hubble Space Telescope (HST)/WFPC2 archival fields of 29 spiral galaxies. 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 and for composites from our sample. The effects of inclination, spiral arms, and Hubble type on the radial extinction profile are discussed. The dust opacity of the disk apparently arises from two distinct components: an optically thicker ( A(I) = 0.5-4 mag) but radially dependent component associated with the spiral arms and a relatively constant optically thinner disk ( A(I) = 0.5 mag). These results are in complete agreement with earlier work on occulted galaxies. The early-type spiral disks in our sample show less extinction than the later types. Low surface brightness galaxies, and possibly Sds, appear effectively transparent. The average color of the field galaxies seen through foreground disks does not appear to change with radius or opacity. This gray behavior is most likely due to the patchy nature of opaque clouds. The average extinction of a radial annulus and its average surface brightness seem to correlate for the brighter regions. This leads to the conclusion that the brighter parts of the spiral disk, such as spiral arms, are also the ones with the most extinction associated with them.

The opacity of spiral galaxy disks VII. The accuracy of galaxy counts as an extinction probe

Centro de Radiastronom´ia y Astrof´isica, Universidad Nacional Auto´noma de Me´xico, 58190 Morelia, Michoaca´n, Mexico12/04/2005 / 19/07/2005Abstract. The “Synthetic Field Method” (SFM) was introduced by Gonza´ lez et al. (1998) to calibrate numbers of distantgalaxies as a probe of extinction in a foreground spiral disk. Gonza´lez et al. (2003) studied the effect of the foreground diskon these numbers using simulations of current and future instruments for fields in the LMC, M31 and NGC 4536, a galaxy inVirgo. They concluded that: (1) the brighter centers of disks were unsuitable, (2) the granularity of the disk at a fixed su rfacebrightness is the limiting factor in the detection of distant galaxies, and (3) the optimum distance for measurements wouldbe that of the Virgo cluster for the current instruments on board HST. At this distance the foreground disk is smoothed withdistance, 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 iscompared to their distance, granularity, surface brightness and structure. The average surface brightness of the of a fi eld isshown to directly influence the accuracy of the SFM. This rest ricts 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 influe nces thedetection of distant galaxies, limiting the SFM measurements in nearby disks. From the trends in the accuracy and maximumpractical field-of-view considerations, the minimum and maximum distance for SFM application, approximately 5 and 35 Mpcrespectively. Using the same instrument and detection method, the relations with SFM parameters and field characterist ics canbe used to forgo the synthetic fields altogether. For the weal th of ACS fields becoming available in the archive, these rela tionscan be used to select those fields based on expected SFM accura cy.Key words. Methods: data analysis, Methods: observational, Methods: statistical, (ISM:) dust, extinction, Galaxies: ISM,Galaxies: spiral

Comparison of the Galaxy with External Spiral Galaxies

In this chapter some areas of comparative exploration in our and external galaxies are reviewed and discussed. Photometric parameters for the Galaxy are derived. The radial scalelength is 5.0 ± 0.5 kpc, and this makes our Galaxy (and M31) unusually large unless Ho = 65 ± 10 km s−1 Mpc−1. Surface photometry of external edge-on galaxies, in particular of the very similar Sb spiral NGC 891, cannot provide independent evidence for or against the occurance of intermediate components (“thick disks”). The most likely Hubble classification of the Galaxy is Sb II. A crude mass model is given and it is shown that the rotation curve of our Galaxy cannot be fitted with a “maximum disk” model so that no “disk-halo conspiracy” exists. The Galaxy belongs to the class of large spiral galaxies, but about 10 percent of all disk stars occur in such systems. The orientation of the zone of avoidance relative to the Local Supercluster is very favourable; this contrasts sharply with the situation from within M31 or NGC 891.

A comparison of astronomy in fifteen member countries of the organisation for economic co-operation and development

Various data are collected for 15 member countries of the Organisation for Economic Cooperation and Development (OECD) that have to do with the practising of astronomy: (1) using the report of the Astronomy expert meeting of the Megascience Forum of the OECD, the level of astronomy funding, size of the research communities, relative commitment to ground-based versus space-based astronomy, etc.; (2) from other sources the size of the population, Gross National Product and size of the total research community; (3) from the paper ofSchubert et al. (1989) data on publication and citation scores of these countries in astronomy and the total research effort (excluding social and economic sciences). Using these data the 15 countries have been ranked on: (1) the relative level of astronomy funding; (2) the relative level of performance in astronomy; (3) the correspondence between funding and performance in astronomy; (4) the relative level of performance of the total science effort; and (5) the performance in astronomy relative to that in all sciences.The results of this study have been summarized in table 10 below. Other interesting results that can be inferred from the data collected in this paper are: (1) one out of every 75,000 inhabitants of these OECD countries is an astronomical researcher; (2) each citizen of these countries spends on average 2.5

The opacity of spiral galaxy disks: IX. Dust and gas surface densities

per year on astronomical research (either from the ground or in space); (3) the average budget per researcher amounts to roughly 200,000