Tabatabaei, Fatemeh: Thermal and Nonthermal Emission from the Nearby Galaxy M33 : A Multi-Scale Study of Infrared and Radio Emission. - Bonn, 2008. - Dissertation, Rheinische Friedrich-Wilhelms-Universität Bonn.
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author = {{Fatemeh Tabatabaei}},
title = {Thermal and Nonthermal Emission from the Nearby Galaxy M33 : A Multi-Scale Study of Infrared and Radio Emission},
school = {Rheinische Friedrich-Wilhelms-Universität Bonn},
year = 2008,
note = {A multi-wavelength study of radio and IR emission from the nearby galaxy M33 is presented. We focus on three main topics: 1) energy sources of IR emission and its correlation with radio continuum emission at different spatial scales, 2) separation of thermal and nonthermal components of the radio continuum emission without assuming a constant nonthermal spectral index, and 3) distribution of the linearly polarized emission and magnetic fields in M33.
Highly resolved and sensitive Spitzer MIPS images of M33 at 24, 70, and 160μm enabled us to study the morphology of different dust components and their role in attenuation of the Hα emission from this galaxy. Radio continuum observations with the VLA and the 100-m Effelsberg telescope led to high sensitive maps of total and linearly polarized radio continuum emission at low (1.42 GHz) and high frequencies (8.35 GHz). A 2D-wavelet transformation was used to find dominant scales of emitting structures, separate the diffuse emission components from compact sources, and compare IR emitting structures to those at radio and Hα wavelengths.
We found that the IR emission is powered predominantly by young O/B stars, specifically at 24μm and 70μm. At least up to scales of 0.8 kpc, the cold dust (emitting at 160µm) is also effectively heated by UV photons from massive ionizing stars, however, the average radiation field also contributes to heating the cold dust at larger scales. At scales smaller than 4 kpc, emission from both the warm and the cold dust show better correlation with the thermal radio than with the nonthermal radio emission, indicating a more important role of UV photons from O/B stars than of cosmic ray electrons in heating the dust at these scales. Interestingly, there is a characteristic scale range where the nonthermal radio-IR correlation is maximum: scales of giant star-forming regions, spiral arms and the central extended region of M33, 0.8-2 kpc, indicating regions with high-density cosmic rays and/or stronger (turbulent) magnetic field.
We developed a new thermal/nonthermal separation method based on a de-reddened Hα map as a template for the thermal radio emission. For the first time, we derived a map of the nonthermal spectral index in a galaxy by means of this method, directly indicating energy loss of cosmic ray electrons when diffusing away from their origins in star-forming regions towards interarm regions and the outer parts of the galaxy. Furthermore, this energy loss is more significant at 8.35 GHz than at 1.42 GHz. Assuming equipartition between the magnetic field and cosmic ray electrons, we obtained the scale length of the cosmic ray electrons and of the magnetic field of about 12 kpc and 24 kpc, respectively.
The large-scale magnetic field exhibits a well ordered spiral structure with almost the same orientation as that of the optical spiral arms. There is a north-south asymmetry in the received polarized emission that is frequency-dependent and most probably caused by Faraday depolarization effects. About 10% of the nonthermal emission from M33 at 8.35 GHz is polarized, mostly due to the strong polarized emission from a magnetic arm in the north-west of the galaxy. The average total and regular magnetic field strengths in M33 are about 6.4μG and 2.5μG, respectively.},

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