The origin of 26Al in the Galaxy
Résumé
The history of recent galactic nucleosynthesis activity can be studied by
measurements of the 1.809 MeV gamma-ray line arising from the decay of
radioactive 26Al.
The COMPTEL telescope aboard the Compton Gamma-Ray Observatory, launched on
April 5, 1991, permits for the first time an extensive investigation of the
1.8 MeV radiation throughout the entire sky.
The aim of this thesis is to infer the galactic distribution of 26Al from
these measurements and to identify the dominant sources of this
radioactive isotope.
The first part of the thesis is dedicated to the reconstruction of
the 1.8 MeV intensity distribution from the measured data.
It is demonstrated that the use of conventional deconvolution
algorithms, like maximum likelihood or maximum entropy inversion,
leads to lumpy, noise-dominated intensity distributions.
Nevertheless, simulations can help to assess the uncertainties in the
reconstructed images, which permits the scientific exploitation of the
recovered skymaps.
Alternatively, a multiresolution approach is proposed, which largely
reduces the uncertainties in the reconstructed 1.8 MeV intensity
distribution.
In summary, 1.8 MeV emission is mainly concentrated towards the
galactic plane, which clearly demonstrates that the bulk of 26Al is of
galactic rather than local origin.
However, distinct emission features towards Cygnus, Carina, and the
Auriga-Camelopardalis-Perseus region are inconsistent with a smooth
galactic 1.8 MeV emission profile, pointing towards a massive star
origin of 26Al.
The second part of the thesis consists of a multi-wavelength
comparison of COMPTEL 1.8 MeV data which aims in the
identification of the origin of galactic 26Al.
For the comparison, a rigorous Bayesian analysis is applied, which
is the only consistent framework that allows inference based on the
comparison.
It turned out that the 1.8 MeV distribution follows very closely
the distribution of free electrons in the Galaxy which is traced by
thermal bremsstrahlung, observable in the microwave domain.
The similarity of the 1.8 MeV intensity distribution to the
thermal bremsstrahlung distribution implies a direct proportionality
between the 26Al and the massive star column densities, which strongly
supports that massive stars are the origin of galactic 26Al.
In particular, ONeMg-novae and AGB stars can be excluded as dominant
26Al sources since their galactic distribution is not expected to
correlate with the distribution of free electrons.
The correlation between 26Al and free electrons established, the
analysis of 1.8 MeV gamma-ray line emission can complement our knowledge
about star formation and the distribution of ionized gas throughout
the entire Galaxy.
While COMPTEL made the first step in providing the first all-sky map
in the light of the 1.809 MeV line, INTEGRAL, the next generation gamma-ray spectrometer, will allow a detailed study of current star formation
in the Galaxy.
measurements of the 1.809 MeV gamma-ray line arising from the decay of
radioactive 26Al.
The COMPTEL telescope aboard the Compton Gamma-Ray Observatory, launched on
April 5, 1991, permits for the first time an extensive investigation of the
1.8 MeV radiation throughout the entire sky.
The aim of this thesis is to infer the galactic distribution of 26Al from
these measurements and to identify the dominant sources of this
radioactive isotope.
The first part of the thesis is dedicated to the reconstruction of
the 1.8 MeV intensity distribution from the measured data.
It is demonstrated that the use of conventional deconvolution
algorithms, like maximum likelihood or maximum entropy inversion,
leads to lumpy, noise-dominated intensity distributions.
Nevertheless, simulations can help to assess the uncertainties in the
reconstructed images, which permits the scientific exploitation of the
recovered skymaps.
Alternatively, a multiresolution approach is proposed, which largely
reduces the uncertainties in the reconstructed 1.8 MeV intensity
distribution.
In summary, 1.8 MeV emission is mainly concentrated towards the
galactic plane, which clearly demonstrates that the bulk of 26Al is of
galactic rather than local origin.
However, distinct emission features towards Cygnus, Carina, and the
Auriga-Camelopardalis-Perseus region are inconsistent with a smooth
galactic 1.8 MeV emission profile, pointing towards a massive star
origin of 26Al.
The second part of the thesis consists of a multi-wavelength
comparison of COMPTEL 1.8 MeV data which aims in the
identification of the origin of galactic 26Al.
For the comparison, a rigorous Bayesian analysis is applied, which
is the only consistent framework that allows inference based on the
comparison.
It turned out that the 1.8 MeV distribution follows very closely
the distribution of free electrons in the Galaxy which is traced by
thermal bremsstrahlung, observable in the microwave domain.
The similarity of the 1.8 MeV intensity distribution to the
thermal bremsstrahlung distribution implies a direct proportionality
between the 26Al and the massive star column densities, which strongly
supports that massive stars are the origin of galactic 26Al.
In particular, ONeMg-novae and AGB stars can be excluded as dominant
26Al sources since their galactic distribution is not expected to
correlate with the distribution of free electrons.
The correlation between 26Al and free electrons established, the
analysis of 1.8 MeV gamma-ray line emission can complement our knowledge
about star formation and the distribution of ionized gas throughout
the entire Galaxy.
While COMPTEL made the first step in providing the first all-sky map
in the light of the 1.809 MeV line, INTEGRAL, the next generation gamma-ray spectrometer, will allow a detailed study of current star formation
in the Galaxy.