Kapteyn Institute Preprints (2003)

Kapteyn Institute Preprint Service.

Preprints of the Kapteyn Institute.
Paper copies can be ordered at:
Kapteyn Institute
P.O. Box 800
9700 AV Groningen
The Netherlands
All the preprints are compressed using gzip.


2003 Preprints

* Preprint 341 New radiative transfer models for obscuring tori in active galaxies. I.M. van Bemmel, C.P. Dullemond
* Preprint 340 Simulations of normal spiral galaxies. Roelof Bottema

New radiative transfer models for obscuring tori in active galaxies


I.M. van Bemmel, C.P. Dullemond

Preprint no. 341

Accepted for publication in Astronomy and Astrophysics.


Two-dimensional radiative transfer is employed to obtain  the broad-band infrared spectrum of active galaxies. In the models
 we vary the geometry and size of the obscuring medium, the surface density, the opacity and the grain size distribution. Resulting
 spectral energy distributions are constructed for different orientations of the toroid. Colour-colour comparisons with observational data are consistent with previous observations that the emission longward of 60 micron is produced by star-formation and unrelated to the presence of an obscuring torus. We also find that the toroid cannot be flat, but is rather conical or flaring. The density is most likely constant with radius, and the size is relatively large with an inner radius around 10 pc. A direct comparison with radio galaxy Cygnus A yields a best fit for a conical disk with constant surface density, and a size from 10 to 30 pc assuming the far-infrared emission is due to star-formation in the host galaxy.

Simulations of normal spiral galaxies

Roelof Bottema

Preprint no. 340

Accepted for publication in Monthly Notices of the Royal Astronomical Society.


Results are presented of numerical simulations of normal isolated late type spiral galaxies. Specifically the galaxy NGC 628 is used as a template. The method employs a TREESPH code including stellar particles, gas particles, cooling and heating of the gas, star formation according to a Jeans criterion, and Supernova feedback. A regular spiral disc can be generated as an equilibrium situation of two opposing actions. On the one hand cooling and dissipation of the gas, on the other hand gas heating by the FUV field of young stars and SN mechanical forcing. The disc exhibits small and medium scale spiral structure of which the multiplicity increases as a function of radius. The theory of swing amplification can explain, both qualitatively and quantitatively, the emerging spiral structure. In addition, swing amplification predicts that the existence of a grand design m=2 spiral is only possible if the disc is massive. The simulations show that the galaxy is then unstable to bar formation, confirming the result of Ostriker & Peebles (1973). The occurrence of this bar instability is further investigated. A general criterion is derived for the transition between bar stable and unstable, depending on disc mass contribution and on disc thickness. It seems that bar stability hardly depends on the presence of gas. A detailed quantitative analysis is made of the emerging spiral structure and a comparison is made with observations. That demonstrates that the structure of the numerical isolated galaxies is not as strong and has a larger multiplicity compared to the structure of some exemplary real galaxies. It is argued that the suggestion of Kormendy & Norman (1979) holds, that a grand design can only be generated by a central bar or by tidal forces resulting from an encounter with another galaxy.

Maintained by:  Gineke Alberts