Astroparticle Physics: Galaxy properties and supernova model

Physics of Fundamental Interactions and Experimental Techniques

Acceleration of Cosmic Rays

1 / 48The physical properties of the Galaxy Today it is an established fact that the Sun is part of a system of stars, the Galaxy (or Milky Way), which is very similar to the spiral galaxies that we observe in the Universe. The recent images of the Galaxy using observations at different wavelengths show that it is basically a disk with a central bulge surrounded by a halo of globular clusters (a globular cluster is a spheroidal conglomeration of stars. Globular clusters are bound together by gravity, with a higher concentration of stars towards their centers). It is convenient to distinguish two components: a spheroidal and a disk structure. Both contain stellar populations and other material with very different characteristics. These two components have different chemical compositions, kinematic and dynamic properties and a diverse evolutionary history. Distances and sizes are usually expressed by astronomers in parsec (symbol: pc). One parsec corresponds to about 3.26 light-years or to 1 pc = 3.086 x 1018 cm

2 / 48The physical properties of the Galaxy PROFILE VIEW Globular star cluster GALACTIC DISK GALAXY HALO RIPE FOR LIFE? Loose stars and some 150 dense stellar clusters orbit within the Milky Way's halo. Life-sustaining planets are unlikely here, because heavy elements are too sparse to build Earthlike worlds. bulge of cast, gas, and oid stars surtpunch the core, Experts we divided over whether this Wes could support the A CHAOTIC CORE CORE ttori themurof the sunmakes K.T.C 3 U 10.000 Bột-1 N C M 20.000 M A 3 u E S R E 10.000 S Our galactic path steers us clear of hazards Kamion End MANDEL CANALES AND MATTHEW WE CHPMASTYEL NGMA STAFF SLAN MCNAUGHTONL ART ANTOINE COLLANION SOUCIS GUILLERMO GONZALEZ, BALL STAFÉ UNEVERSITY MICHAEL COMMANLOCE, NORTHERN ARLTONA UNIVERSITY KARUS ASTROMOLOGY NALAPL INTERNATIONAL JOURNAL OF ASTRONOLOGY 40.000 light-years ARM Blice sichintente burits of Sublin Kost le toafe T YOU ARE HERE SOLAR SYSTEM Sol& system orbut The solar system is comfortably nestled in a safe harbor between major spiral arms, and its nearly circular orbit helps it avoid the galaxy's perilous inner regions

3 / 48The physical properties of the Galaxy The spheroidal component has a very massive nucleus (smaller than 3 pc of radius) with a black hole at its center, with mass 2 × 10° solar masses, a bulge with radius of ~3 kpc and an extended halo of about 30 kpc. The disk is very thin (~ 200-300 pc thick) and a radius of about 15 kpc. The Sun is about 8.5 kpc from the center. The galactic volume, assuming a flat disk having a radius of ~ 15 kpc and a thickness of ~ 300 pc, corresponds to: VG = [](15 x 103)2 x 300] x (3 x 1018)3 = 5 x 1066 3 cm The volume of the galactic halo is more than an order of magnitude larger. Stars and globular clusters are the characteristic components of the spherical region, where gas and dust are relatively scarce. Spectroscopy indicates that the stars in the spheroid component are metal-poor, and very old. The disk is instead characterized by the presence of large amounts of dust and gas, which give rise to absorption of the interstellar radiation, and by young and metal rich stars. This situation is analogous to that observed in other spiral galaxies.

Galactic Magnetic Field

4 / 48The Galactic Magnetic Field Different estimates exist on the average intensity of the regular galactic magnetic field, which depends on the distance from the galactic center (see picture). We assume approximately: B~4µG The galactic field is oriented mainly parallel to the plane, with a small vertical component along the z-axis. The models of the large-scale structure of the galactic magnetic field provide a regular distribution of the B lines that follows the distribution of matter, i.e. a spiral shape (see picture). The spatial extension of regions in which the magnetic field is coherent is of the order of 1-10 pc. As the Galaxy is filled with such a magnetic field, we obtain the following Larmor radii for protons at different energies: TL(E = 1012eV) ~ 1015 cm=3x 10-4 pc TL(E =1015eV) ~1018cm=0.3 pc TL(E = 1018 eV) ~1021 cm =300 pc

5 / 48The Galactic Magnetic Field These values should be compared with the Galaxy dimensions (radius of ~ 15 kpc). Particles below 1018 eV are strongly constrained inside the galactic volume by the galactic magnetic field. Bị (HG) MILKY WAY 12 10 8 6 4 2 Ro 0 0 5 10 15 R (kpc) Total magnetic field strength in the Galaxy as a function of the distance from the galactic center. The position of the Sun is indicated by the arrow The direction and strength of the regular magnetic field in the Galactic plane is represented by the length and direction of the arrows. The intensity of the field inside the circle of radius 4 kpc representing the bulge is assumed to be 6.4µG kpc 25 1= 100° 20 15 10 5- 0 G.C. -10- -15 -20 -25 -25 -20 -15 -10 o 5 10 15 20 25 kpc

Interstellar Matter Distribution

6 / 48The Interstellar Matter Distribution In outlying regions of the Galaxy, only a small fraction of the space is occupied by matter in the form of stars. The rest is filled with large masses of gas (molecules, atoms, ions) and tiny solid particles, the interstellar dust. Dusts are made up of ice grains of various species, graphite, silicates and perhaps metals. Most of the ISM is made of neutral (H1) and molecular (H2) hydrogen. As a whole, this gas and dust is called Inter Stellar Matter (ISM). It represents 5-10% of the total mass of the Galaxy. The average density of this medium is: nISM ~ 1 proton / cm3 = 1.6 x 10-24 g / cm3 It is hard to detect the ISM in the visible range of the electromagnetic radiation, and it has been studied mostly using radio-astronomy techniques, which reveal the presence in the ISM of the characteristic lines of many molecules. The presence of ISM is very important for CRs because primary CRs, interacting with it, produce secondary particles, including antimatter. We will see this later on.

Energy Density of Cosmic Rays

7 / 48Energy density of cosmic rays We can integrate the CR differential flux over the threshold energy E0=3 GeV (above which the solar contribution is negligible), over the solid angle and divide for the particle velocity (assumed equal to c). In this way we can obtain the number density of cosmic rays: nCR =1 ×10-10 cm-3 A second important quantity is the energy density, still obtainable by integrating the flux multiplied by the energy. We obtain: PCR=1 eV / cm3 We need to compare these numbers with some other astrophysical quantities, in order to understand if they represent small or large quantities. ISM number density The number density of CRs can be compared with the average number density of the interstellar matter (as we have seen, it corresponds to nISM ~ 1 proton/ cm3). Thus, only about one proton out of ~ 10- not bound in stars in the Galaxy is a relativistic particle, i.e. a cosmic ray.

Starlight Density

8 /48Energy density of cosmic rays The starlight density From photometric measurements of the light coming from galactic stars, astronomers have evaluated the visible photon density: ny vis =2×10-2cm-3 PYvis ~4 ×10-2eV/cm3 assuming 2 eV/photon for the visible light. This is a much smaller value than that of CRs. The density of the Cosmic Microwave Background Radiation The CMB radiation is the thermal radiation filling almost uniformly the observable Universe. The CMB radiation has a thermal black body spectrum at a temperature of 2.725 K. Using the measured number density of the CMB radiation: ny CMB =400 cm-3 PYCMB = 0.3eV / cm3 In this particular case, in spite of the similarity between the numbers of the two radiations, there is no argued connection between the two phenomena of CRs and CMB.

Energy of Cosmic Ray Sources

9 / 48Energy of CR sources Supernova remnants are energetically suitable candidates for the acceleration of CRs with energy below the knee. The main motivation is the equilibrium between the loss of CRs due to their escape out of the galactic volume and the energy provided by supernova shockwaves. The CR sources are uniformly distributed in the Galaxy and the CRs are trapped by the galactic magnetic fields. According to the present observations, the total kinetic energy of CRs corresponds to: PCR X VG = 8 x 1054 erg where VG is the the galactic volume (VG ~ 5 x 1066 cm3). If the particles are completely confined inside the galactic volume, this number should increase with time in the presence of new galactic core-collapse supernova explosions. A competitive effect which decreases PCR is due to the escape of CRs out of the Galaxy with a characteristic escape (or confinement) time Tesc. This quantity corresponds to the average time needed for a CR, trapped by the galactic magnetic field, to reach the boundary. From here, the particle can freely escape, because the magnetic field outside the galaxy is negligible.

10 / 48Energy of CR sources The confinement time (we will see later) is Tesc ~ 107y = 3 x 1014 s. Assuming an almost steady value of the energy density PCR , the energy loss rate due to the escape of CR out of the galactic volume is: PCR= PCR X VG T esc = 8 × 1054 3 × 1014 = 3 × 1040 erg /s Thus, the power required by cosmic accelerators to replenish the galactic volume corresponds to PCR. A supernova explosion of 10 solar masses releases about 1033 erg, 99% in form of neutrinos and 1% in form of kinetic energy of expanding particles (shock wave). The supernova rate fsn in a galaxy like our own is about 3 per century (fsN ~10-9 s-1). If a physical process able to accelerate charged particles exists, it transfers energy from the kinetic energy of the shock wave to CRs with an efficiency n: PSN= 1 x fsN × 1053 x 0.01 = n × 1042 erg/s

11 / 48Energy of CR sources By requiring that PCR = Psy, the quantity n must be of the order of a few percent. In this case, the shock waves from supernova explosions are able to refurbish the Galaxy with new accelerated particles and maintain the stationary energy content of CRs. This condition makes the supernova model energetically compatible with the observations. A transfer mechanism with efficiency of few % is known and it will be described in the following slides. With a rate of about three supernovae per century in a typical Galaxy, the energy required could be provided by a small fraction (~5-10%) of the kinetic energy released by supernova explosions. Let us now remember how a supernove explosion occurs.

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