%0 Thesis
%A Asin, Oliver Andre Laurent
%T Gone with the breeze: A subsonic model for describing the Fermi bubbles features
%I University of Potsdam
%V Dissertation
%M PUBDB-2025-00322
%P 138
%D 2024
%Z Dissertation, University of Potsdam, 2024
%X More than a decade ago, two Galactic bubbles lobes were detected by the Fermi-LAT instruments in a range of 10 GeV to 500 GeV, i.e., in the γ-ray energy band. Their emissions reach a latitude of   50◦ and a longitude of   40◦ , corresponding to a height of   10 kpc and a width of 7 kpc for an observer at 8 kpc from the Galactic Center. The brightness intensity of the emissions appears to be constant within the lobes. Moreover, sharp edges are observed at the boundaries.The Fermi bubbles could be a counterpart of the Microwave haze as they both exhibit a hard spectra and a matching width at their bases. More recently, the eROSITA bubbles have been observed in the X-ray band. They extend to a height of 14 kpc, enshrouding the Fermi bubbles and are likely connected to them.To date, there is no clear consensus about the mechanism responsible for the formation of this bubble structure. Such a mechanism must be able to provide an explanation for the emission mechanism and the origin of the event giving rise to the bubbles. The emission of the Fermi bubbles could be explained by some hadronic or leptonic cosmic rays energy losses.The majority of the models proposed for describing the Fermi bubbles assume a supersonic velocity profile with a velocity ranging from 500 to 1000 km s−1 . The observations of the UV absorption lines provided by cold clouds are able to establish a velocity profile for the Fermi bubbles. The velocities range from 100 to 300 km s−1 . A maximal velocity of   300 km s−1 at a height of   1 kpc from the Galactic Center has been observed. Moreover, beyond this height, the velocity decelerates continuously. Those observations seem to contradict the supersonic velocity models proposed so far.In this thesis, a subsonic model is proposed for explaining the velocity profile and the emission of the Fermi bubbles. This model, called a Galactic breeze, exhibits a deceleration that could be in agreement with the observations. A hydrodynamic analytic model has been first explored. The Galactic breeze is considered to be thermally-driven in an isothermal hot Galactic halo. The first step is to define a Galactic gravitational potential. For the model, three components have been set up, a bulge, a disc and a dark matter halo. At a distance called the critical radius, the subsonic profile reaches its maximum velocity. The value of this critical radius depends on the gravitational potential and the thermal velocity. In order to have a subsonic velocity as close as possible to 300 km s−1 at the critical radius, the largest possible thermal velocity for the hot Galactic halo must be considered. Based on the observations of the Gaia mission, a fitting-range for the Galactic gravitational potential has been provided. For the Galactic breeze model, the gravitational potential has been normalised to reach the upper limit of this fitting range. Considering a critical radius at a height of 1 kpc from the Galactic Center the corresponding thermal velocity for the hot Galactic halo is 250 km s−1 corresponding to   400 eV. 2Following these results, a numerical code has been used for solving the hydrodynamic equations and to provide a spatial distribution for the subsonic velocity profile. A Galactic breeze profile exhibiting a deceleration similar to the observations has been obtained. However, the maximal velocity reachable with this model is lower than the observations.The next step has been to use the velocity profile provided by the numerical simulation to numerically solve a transport code for the propagation of cosmic rays. An isotropic and homogeneous diffusion has been considered and the gamma-rays emission finds its origin in the pp interactions. For an injected luminosity of Linj = 1.4 × 1040 erg s−1 , the Galactic breeze model is able to form a bubble structure that matches approximately with the observations. However, the bubbles formed are too wide when compared with the Fermi bubbles as the sharpness of the edges is not obtained.Following these results, the Galactic breeze model has been extended to include the spatial distribution of a magnetic field. An analytic study of the influence of a magnetic field distribution on a subsonic profile has been done. This revealed that an azimuthal magnetic field distribution can have an effect on the position of the critical radius. Depending on the profile of the magnetic field, the critical radius can be shifted farther from the Galactic Center implying that the thermal velocity can be increased. However, it is extremely difficult to find a magnetic field distribution able to provide a maximal velocity of 300 km s−1 for the Galactic breeze and keep the critical radius at 1 kpc. As a first step, the model has been explored by considering values of just a few µG in order to conserve the Galactic breeze profile obtained with hydrodynamic simulations. A MHD numerical code has been used and the magnetic field has been injected with the velocity, allowing to study their mutual influence. Several configurations have been considered that have demonstrated the effect of a compression induced by the velocity profile on the azimuthal magnetic field. This compression leads to a magnetic tension effect that slightly disrupts the velocity profile.In the same way as for the hydrodynamic model, both the velocity and the magnetic field distribution have been used to numerically solve the transport of cosmic rays. However, the diffusion is this time inhomogeneous. In order to compare the results with the hydrodynamic model and ensure the consistency of the results, an isotropic diffusion coefficient has been considered. For each configuration, a different coherent length has been fixed to have a diffusion length of   0.1 pc inside the bubble structure, similar to the hydrodynamic model and the observations. The next step has been to consider an anisotropic diffusion coefficient. For this, a perpendicular diffusion coefficient ten times less than the parallel diffusion coefficient has been considered. With such anisotropy, the Galactic breeze model is able to provide γ-ray emissions, reproducing the sharpness observed for the Fermi bubbles.
%F PUB:(DE-HGF)11
%9 Dissertation / PhD Thesis
%R 10.3204/PUBDB-2025-00322
%U https://bib-pubdb1.desy.de/record/622284