arXiv subjects

P. C. van der Kruit

Publications and source records attributed to P. C. van der Kruit.

47 records · Page 3Linked to original sources

arXiv2004

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.(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)

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arXiv2004

Radial truncations in stellar discs in galaxies

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.

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arXiv2004

Structure and kinematics of edge-on galaxy discs -- III. The rotation curves in the gas

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

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arXiv2004

Structure and kinematics of edge-on galaxy discs -- II. Observations of the neutral hydrogen

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

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arXiv2004

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 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.

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arXiv2003

The Opacity of Spiral Galaxies from Counts of Distant Background Galaxies

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.

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arXiv2001

Structure and dynamics of disks in galaxies

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.

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arXiv2001

Kinematics and dynamics of the "superthin" edge-on disk galaxy IC 5249

We present spectroscopic observations of the stellar motions in the disk of the superthin edge-on spiral galaxy IC 5249 and re-analyse synthesis observations of the HI. We find that the HI rotation curve rises initially to about 90-100 km/s, but contrary to the conclusion of Abe et al. (1999) flattens well before the edge of the optical disk. Over most part of the optical disk we have been able to establish that the (tangential) stellar velocity dispersion is 25-30 km/s. From earlier surface photometry we adopt a value for the radial scalelength of the disk of 7 +/- 1 kpc, a vertical scaleheight of 0.65 +/- 0.05 kpc and a disk truncation radius of 17 +/- 1 kpc. The very thin appearance of IC 5249 on the sky is the result of a combination of a low (face-on) surface brightness, a long scalelength and a a sharp truncation at only about 2.5 scalelengths. From various arguments we derive the stellar velocity dispersions at a position one radial scalelength as sigma_R about 35 km/s, sigma_{theta} about 30 km/s and sigma_z about 20 km/s. This is comparable to the values for the disk of our Galaxy in the solar neighborhood.

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arXiv2000

Truncations in stellar disks

The presence of radial truncations in stellar disks is reviewed. There is ample evidence that many disk galaxies have relatively shaprt truncations in their disks. These often are symmetric and independent of the wavelength band of the observations. The ratio of the truncation radius R_{max} to the disk scalelength h appears often less then 4.5, as expected on a simple model for the disk collapse. Current samples of galaxies observed may however not be representative and heavily biased towards disks with the largest scalelengths. Many spiral galaxies also have HI warps and these generally start at the truncation radius of the stellar disk. The HI surface density suddenly becomes much flatter with radius. In some galaxies the start of the warp and the position of the disk truncation radius is accompanied by a drop in the rotation velocity. In the regiosn beyond the dis truncation in the HI layer some star formation does occur, but the heavy element abundance and the dust content are very low. All evidence is consistent with the notion that the outer gas parts of the disks constitute recently accreted material, at least accreted after formation of what is now the stellar thin disk. Although various models exist for the origin of the truncations in the stellar disks, this at present remains unclear.

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arXiv1999

On the axis ratio of the stellar velocity ellipsoid in disks of spiral galaxies

The spatial distribution of stars in a disk of a galaxy can be described by a radial scale length and a vertical scale height. The ratio of these two scale parameters contains information on the axis ratio of the velocity ellipsoid, i.e. the ratio of the vertical to radial stellar velocity dispersions of the stars, at least at some fiducial distance from the center. The radial velocity dispersion correlates well with the amplitude of the rotation curve and with the disk integrated magnitude, as was found by Bottema (1993). These relations can be understood as the result of the stellar disk being (marginally) stable against local instabilities at all length scales. This is expressed by Toomre's well-known criterion, which relates the sheer in the rotation to a minimum value that the radial stellar velocity dispersion should have for stability for a given surface density. Via the Tully-Fisher (1977) relation, the velocity dispersion then becomes related to the integrated magnitude and hence to the scale length. The vertical velocity dispersion relates directly to the scale height through hydrostatic equilibrium. It can be shown that the ratio of the two length scales relates to the axis ratio of the velocity ellipsoid only through the Toomre parameter Q and in particular does not require a choice of the mass-to-light ratio or a distance scale. We have applied this to the statistically complete sample of edge-on galaxies, for which de Grijs (1997) has performed surface photometry and has determined the length scales in the stellar light distribution.

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arXiv1997

The z-structure of disk galaxies towards the galaxy planes

We present a detailed study of a statistically complete sample of highly inclined disk galaxies in the near-infrared K' band. Since the K'-band light is relatively insensitive to contamination by galactic dust, we have been able to follow the vertical light distributions all the way down to the galaxy planes. The mean levels for the sharpness of the K'-band luminosity peaks indicate that the vertical luminosity distributions are more peaked than expected for the intermediate sech(z) distribution, but rounder than exponential. Since projection of not completely edge-on galaxies onto the plane of the sky causes vertical luminosity profiles to become rounder, we have performed simulations that show that it is possible that all our galaxies can have intrinsically exponential vertical surface brightness distributions. We find that the profile shape is independent of galaxy type, and varies little with position along the major axis. The fact that we observe this in all our sample galaxies indicates that the formation process of the galaxy disks perpendicular to the galaxy planes is a process intrinsic to the disks themselves.

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