ht Milky Way

In 1925, Hubble built a galaxy classiication based on their morphologiesα[67]. his classiication results in mainly threeHubble typesas illustrated in igureα4.1. he majority of known galaxies have smooth featureless mater distribution, they are calledelliptical galaxies, while others have spiral arms. Two third of thespiral galaxiesare shaped with a central bar which is the case of the Milky Way. here is also few percent of irregular galaxies.

he Milky way is composed of a disk, a bulge and presumably a dark mater haloα[68]:

• he diskcontains most of the stars as well as the gas and dust populating the interstellar medium. he solar system is in the disk, located in the local Orion-Cygnus arm. It is at a distance from the Galactic center of about 8.5 kpc as shown in igureα4.3and at 15 pc above the midplane. It is diicult to deine the disk radius as we are located within the disk, however we can estimate that the apparent radius of the stellar disk is roughly 15 kpc while the gas extends to about 25 kpc. he disk width is about 1 kpc.

• he bulgeis located around the center of the Galaxy, it corresponds to a much denser zone.

It has an elongated shape ⁸igureα4.3⁹ extending on about 3 kpc with a width of 2 kpc as can be seen in igureα4.2.

• he dark mater halo has a mass ten times larger than the disk, it gravitationally holds the Galaxy together. It has an oblate spheroid shape, in other words, a latened sphere shape. From its efect on the Magellanic Clouds, its diameter is estimated to be at least 100–120 kpc.

ht Inttrsttllar Mtsium

he interstellar medium is composed of 70.4 ⁵ ⁸resp. 90.8 ⁵⁹ of hydrogen in term of mass ⁸resp.

number of nuclei⁹ and 28.1 ⁵ ⁸9.1 ⁵⁹ of helium, with only 1.5 ⁵ of heavier elements. hese chem-ical elements are present in the form of atomic, ionized and molecular gas as well as dust. hey are concentrated in the spiral arms and within the 150 pc surrounding the Galactic plane. he clouds present in the interstellar medium occupy 1–2 ⁵ of the volume but contain about half of the interstellar medium mass. he diferent components of the interstellar medium are de-scribed below for hydrogen as it is the most abundant element. Seeα[70] and references therein for more details.

• he neutral atomic gas⁸denoted H_�⁹ is present in two thermal phases, the cold phase with temperatures of 50–100 K and the warm one at 6 000–10 000 K. he cold phase is located in dense clouds ⁸called H_� regions⁹ with 20–50 hydrogen atoms per cmm. Located in the inter-cloud medium, the density of the warm phase is much lower ⁸∼0.3 cm m⁹.

• he ionised gasis due to ultraviolet radiations emited by hot and massive stars. he free electrons and ions continuously recombine and ionise again. he equilibrium between recombination and ionisation rates determines the size of the ionised region. he ionised gas is also present in two phases, a warm one at 6 000–10 000 K and a hot one with tem-peratures above 10  K. heir densities are respectively 0.04 cm m and 0.003 cm m. he warm phase is mainly located in inter-cloud regions while the hot phase extends into the Galac-tic halo. Indeed, the hot ionised gas is generated by supernova explosion and stellar winds that can eject the gas on large scales.

• he molecular gasis located in cool ⁸> 100 K⁹ and dense ⁸100–1 000 cm m⁹molecular clouds where there is a higher chance of atom meeting each other and low chance of collisional disruption. he ultraviolet lux must be low to prevent ionisation. he molecular clouds are mostly located along the spiral arms and particularly at a distance of 4–7 kpc from the Galactic center.

• he dustis mainly made of carbon, oxygen and silicon with a typical size of a dust particle of 0.1 to 1 μm. It represents 0.1 ⁵ of the total mass of the stars.

ht Magnttir Fitlss

he magnetic ields in the Galaxy have a strong inluence on cosmic ray propagation. hey consist in two components, a regular and an irregular one. heir strengths being of the same order of magnitude, the Galactic cosmic ray propagation takes place under highly turbulent conditions.

he regular component has a local intensity of ∼1.4 μG. It evolves slowly with a small radial component whose magnitude is not known and a strong azimuthal component. As viewed from the North Galactic Pole, the direction of this azimuthal component is clockwise. It is stronger toward the Galactic centre reaching ∼4.4 μG at a radius of 4 kpc. he regular ield is composed of two layers, one localised in the disk and the other one in the Galactic halo. he transition occurs roughly at a distance of k 0.4 kpc from the midplaneα[71]. he regular magnetic ield of

the Galactic halo is weaker and extends until ∼1.4 kpc. It is not known if it is symmetric ⁸forming a quadrupole⁹ or anti-symmetric ⁸a dipole⁹ above and below the Galactic plane.

he irregular component has a local strength of ∼5 μG, it is associated with turbulent inter-stellar plasma. It is also thought to consist of two layers, in the disk and halo. he strength of the disk component varies in each spiral arm and is inversely proportional to the distance to the Galactic center for radii larger than 5 kpc. he halo component decreases exponentially in the radial direction and is Gaussian in the vertical one. Its vertical extension is comparable to the halo layer of the regular magnetic ield. he irregular ield has a coherence length scale of typically 100 pc.

Despite being one order of magnitude weaker than in the disk, the halo magnetic ield has a great inluence on the cosmic ray propagation because of its extension in height.