astr112 the galaxy lecture 11 prof. john hearnshaw 13. the interstellar medium: dust 13.5...

ASTR112 The GalaxyLecture 11

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13. The interstellar medium: dust 13.5 Interstellar polarization14. Galactic cosmic rays 15. The galactic magnetic field

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Interstellar polarization

• Polarized light has electric field confined to one plane transverse to propagation• Stars emit light which is unpolarized• Partial polarization is possible after starlight has passed through a dust cloud of aligned elongated dust grains• Degree of polarization can be expressed in magnitudes using a polarizing filter on a polarimeter )/(log5.2

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The observations:

• Polarization is limited to stars near galactic plane, | b| 5º• Mostly the observed polarizations are small Δmp0.03, but occasionally as high as ~0.15 mag.• All highly polarized stars are also highly reddened by IS dust• But, some reddened stars are not polarized at all• Ratio of polarization to extinction is:

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The explanation

• Polarization requires alignment of rotating dust grains in the weak galactic magnetic field (actually it is the rotation axes which are aligned)• Polarization requires the grains to be elongated, not spherical• Polarization is strong when we see distant stars through a transverse magnetic field (l = 140º and 320º), but weak when we look along the field lines (l = 30º and 260º)• Direction of the field is approximately along spiral arms


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Observations of interstellar polarization as functionof galactic coordinates. The plane and amount ofpolarization is shown by the short lines for each star


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Galactic cosmic rays

• Cosmic rays are high energy particles, mainly protons (90 % by number) or α-particles (4He nuclei) (9 %). Remainder are nuclei of heavier elements, especially 12C, 16O, 14N, 20Ne, 24Mg, 28Si and 56Fe.• Cosmic ray energies are in the range 109 to 1020 eV; <109 eV, CR merge with solar wind and deflected by Earth’s mag. field; at ≥1020 eV very few or no CR exist. Far fewer high energy CR than low; flux E-2.7


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The energy spectrum ofgalactic cosmic rays.Note the smooth and featureless spectrum.Note also the very lowflux of high energy particles.


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• CR travel in the Galaxy at speeds >0.99 c

• CR fill the whole galactic disk and arrive on Earth travelling in all directions

• CR are confined to the Galaxy by a weak galactic magnetic field Bgal ~ 3 × 10-6 G• CR particles bent into curved path of radius r = E/ceB by a mag. field. • At E = 1010 eV, r ~0.7 AU – CR tightly confined; E = 1020 eV, r ~ 36 kpc - size of Galaxy, no confinement


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Composition of cosmic rays

• Composition of CR shows some similarities with that of Sun

• But CR have much higher abundance of light elements lithium, beryllium and boron (Li, Be and B) than in Sun (e.g. Li/H ~ 10–11 in Sun; ~ 4 × 10–6 in CR)

• Compared with stars, CR have higher abundance of elements heavier than O, and they are deficient in elements H, He.


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Abundances of elements in CR show Li, Be and B much enhanced


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Interaction of cosmic rays with the ISM

• Heavier cosmic ray particles (e.g. C, N, O nuclei) crash into IS gas clouds, mainly HI, and the high energy collisions cause fragments of these nuclei to be broken off. Some of these fragments are nuclei of the elements Li, Be and B.• Such nuclear reactions are known as spallation reactions• Spallation causes the abundance of Li, Be, B to slowly build up in CR over their lifetime.• Composition of CR thus slowly but continuously changing with time over millions of years


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awEGRET (1991) satellite all-sky gamma-ray survey showing the Galaxy in gamma-rays. The gamma rays are emitted when cosmic rays interact with the interstellar medium.


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Age of cosmic rays

• The typical path length of CR particles through ISM can be determined from the observed amount of Li, Be and B in CR, based on there being ~ 106 H atoms m-3 in ISM• Path length through ISM found is ~ 2 × 106 light years• Velocity of CR is V ~ c• Hence mean age of CR particles is ~ 2 × 106 years• Size of Galaxy is ~ 105 light years, so CR must travel in curved paths (this is indirect evidence for a mag. field)• Oldest CR are age ~ 4 × 106 years (twice mean age)


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Motion of a charged particle in a magnetic field.The path is a helix oriented along the field lines.


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Original composition of cosmic rays

• The original (t = 0) composition of CR can be predicted by extrapolating their slowly changing composition backwards through 4 × 106 years• This t = 0 composition is dominated by 12C, 16O with a little 14N, 20Ne, 24Mg, 28Si, 56Fe. This is the composition of CR at their source


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CR abundances at their source (supernovae?) are predicted to be rich in alpha particles and also C and O nuclei. The arriving cosmic rays contain small amounts of Li, Be and B.


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Source of cosmic rays

• Presumed source of CR is supernova explosions• There are probably 2 or 3 supernovae/century in a typical spiral galaxy, including the Milky Way• CR lose their energy by colliding with ISM in a few million years. Hence supply of new CR must be continuous• Energy density of CR in Galaxy ~ 106 eV/m3

• Total energy of all CR in whole galactic disk ~1048 J• Energy replacement rate ~1034 J/s


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The Crab nebula

The Crab Nebulais the remnant ofa star that explodedin 1054 AD. It wasobserved by Chinese astronomers


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The Vela super-nova remnant

The Vela supernovaremnant, 10,000years after the explosion


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Thye origin of cosmic rays may be from the accelerationof atoms in the ISM by shock waves from nearby supernova explosions


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Energy released in supernova explosions

• Each supernova releases energy of about 1044 J• This energy is initially in form of kinetic energy of ejected material, photons and neutrinos• Mean energy released by 3 supernovae/century (3 × 1044/3 × 109) J/s ~1035 J/s (as 1 century ~ 3 × 109 s)• The energy released by supernovae is about 10 × greater than that required to account for the energy of CR• CR may be accelerated to high energy in shock fronts in ISM near the supernova site


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The galactic magnetic field

Evidence for a galactic magnetic field• Faraday rotation of plane of polarization of radio waves• IS dust grain alignment causing polarization of some stars reddened by IS dust• Zeeman splitting of 21-cm line of HI• Cosmic ray confinement in Galaxy


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• All methods give consistent estimates of the field at

B ~ 3 × 10–6 gauss

(cf. B ~ 0.3 G)

• Magnetic field appears to be oriented along the Galaxy’s spiral arms


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End of lecture 11

astr112 the galaxy lecture 11 prof. john hearnshaw 13. the interstellar medium: dust 13.5...

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