Kapteyn Institute Preprints (2003)
Kapteyn Institute Preprint Service.
Preprints of the Kapteyn Institute.
Paper copies can be ordered at:
P.O. Box 800
9700 AV Groningen
All the preprints are compressed using gzip.
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.
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.
Preprint no. 340
Accepted for publication in Monthly Notices of the Royal Astronomical
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.