Diagnostics of active galaxies - I. Modeling the infrared properties of dusty cores starburst galaxies

Aims. Despite extensive observations over the last decades, the central questions regarding the power source of the large IR luminosity of Ultra Luminous Infra Red Galaxies (ULIRGs), and their evolution, are still not fully answered. In this paper we will focus on massive star formation as a central engine and present an evolutionary model for these dust-enshrouded star formation regions.

Methods. An evolutionary model was created using existing star formation and radiative transfer codes (STARBURST99, RADMC and RADICAL) as building blocks. The results of the simulations are compared to data from two IRAS catalogs.

Results. From the simulations it is found that the dust surrounding the starburst region is made up from two components. There is a low optical depth (tau = 0.1, which corresponds to 0.1% of the total dust mass), hot (T similar to 400 K) non-grey component close to the starburst (scale size 10 pc) and a large scale, colder grey component (100 pc, 75 K) with a much larger column (tau = 10). The simulations also show that starburst galaxies can be powered by massive star formation. The parameters for this star forming region are difficult to determine, since the IR continuum luminosity is only sensitive to the total UV input. Therefore, there is a degeneracy between the total starburst mass and the initial mass function (IMF) slope. A less massive star formation with a shallower IMF will produce the same amount of OB stars and therefore the same amount of irradiating UV flux. Assuming the stars are formed according to a Salpeter IMF (Psi(M) proportional to M(-2.35)), the star formation region should produce 10(9) M(circle dot) of stars (either in one instantaneous burst, or in a continuous process) in order to produce enough IR radiation.

Conclusions. Our models confirm that massive star formation is a valid power source for ULIRGs. In order to remove degeneracies and further determine the parameters of the physical environment also IR spectral features and molecular emissions need to be included.