Modelling the crystalline silicate reservoir in starburst galaxies



Thesis Supervisor at ESO (as of 01.11.2018): Ciska Kemper

Abstract

The interstellar dust reservoir in galaxies is predominantly composed of silicates, in our own Milky Way found to be mostly in amorphous form (glass; Kemper et al. 2004). However, infrared spectroscopy reveals that stellar dust production -- one of the main sources of interstellar dust -- can contain significant amounts of crystalline silicates (Sylvester et al. 1999, Kemper et al. 2001). Upon injection in the interstellar medium these crystalline silicates are gradually amorphitized, due to cosmic ray hits, which cause damage to the lattice structure, and an amorphization time scale can be determined.

One of the most interesting Spitzer results in this area revealed the presence of crystalline silicates in absorption in a sample of 12 out of 77 starburst galaxies (Spoon et al. 2006), with crystallinities in the range from 6-13 %. We put together a simple dust production model for galaxies, which reveals that the high levels of crystallinity can be understood as being due to freshly produced dust that has not yet
been amorphized by cosmic ray hits, which may be possible under certain conditions (Kemper et al. 2011).

In this project, we will expand this dust production model for a starbursting galaxy. The current model solves a set of coupled differential equations describing the interstellar silicate reservoir in terms of crystalline and amorphous components, affected by dust production, amorphization, crystallization, and dust destruction rates. Assuming that the interstellar crystallization rate and the dust formation in the ISM are negligible, and the destruction rate does not depend on lattice structure, this model shows that, the crystalline fraction of silicates in the ISM peaks around 30 Myr after the onset of the starburst, with crystallinities consistent with the measurements by Spoon et al. (2006). After that time, the crystalline fraction declines again as a consequence of the amorphization time scale, governed by the cosmic ray fluence.

In this project, we will drastically expand on this model in the following ways:

* Inclusion of a more realistic description for the dust production by evolved stars

* Inclusion of the formation of dust in the interstellar medium, which is currently ignored

* Parametrization of the cosmic ray fluence based on the expected supernova rates in galaxies

* The model currently assumes instantaneous mixing. We want to improve this by including spatial information on the distribution of star
formation, supernovae and interstellar dust.

* Small grain correction: very small grains may be crystalline but appear spectroscopically as amorphous, due to the relative importance of edge effects. This will be incorporated in the model

* Implementation of metallicity and chemical abundance constraints

With these improvements, we will do further parameter studies to explain the crystalline fractions seen in already detected galaxies,
both starburst and lower star formation rate galaxies (Spoon et al. 2006; Willett et al. 2011; Aller et al. 2012; Stierwalt et al. 2014). Using a suitable radiative transfer code we will also predict the resulting spectral appearance of interesting results from the parameters study. The results from this analysis will be compared with the observational results.