Cosmic rays in early Star-Forming Galaxies and their effects on the - - PowerPoint PPT Presentation

Cosmic rays in early Star-Forming Galaxies and their effects on the Interstellar Medium

Ellis Owen

ellis.owen.12@ucl.ac.uk

Mullard Space Science Laboratory, University College London, United Kingdom National Tsing Hua University, Taiwan (ROC)

Collaborators: Kinwah Wu (UCL-MSSL, UK) Idunn Jacobsen (UCL-MSSL, UK) Pooja Surajbali (MPIK, Heidelberg, Germany)

International Cosmic Ray Conference, Busan, Korea, July 2017

EQ J100054+023435 – Multiwavelength image with HST, Spitzer, Chandra, Keck, Galex, CFHT, Subaru, UKIRT, JCMT, VLA & IRAM. Credit NASA (2008)

Outline

• Early Star-Forming Galaxies
• Propagation and Interaction of Cosmic Rays

– Direct – Indirect

• Energy Deposition and Cosmic Ray Heating
• Remarks

2

Starburst Galaxies at High Redshift

• Starburst galaxies characterized by high star formation rates (SFR)

3

> 10 M yr1 à many Supernovae à abundant cosmic rays

Starburst Galaxies at High Redshift

• Starburst galaxies characterized by high star formation rates (SFR)
• Why are high redshifts of interest?

– Galaxies with very high SFRs seem to be abundant at high redshifts – Possible implications on cosmic reionization (Sazonov & Sunyaev 2015)

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> 10 M yr1 à many Supernovae à abundant cosmic rays

Starburst Galaxies at High Redshift

• Starburst galaxies characterized by high star formation rates (SFR)
• Why are high redshifts of interest?

– Galaxies with very high SFRs seem to be abundant at high redshifts – Possible implications on cosmic reionization (Sazonov & Sunyaev 2015)

• Parametric model protogalaxy, very active to demonstrate concept
• SFR = , environment defined by

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> 10 M yr1

Density field Radiation field Magnetic field

à many Supernovae à abundant cosmic rays

1000 M yr1

Energy Transport by Cosmic Rays

• Cosmic rays may be influenced by magnetic fields

– Low & Intermediate energies – Larmor radius

• Can hamper their propagation into intergalactic space

– Containment vs. Diffusion

4

∂n ∂t = r · [D(E, r, t)rn] + Q(r, E)

• As a first estimate, assume Bohm diffusion ~1 scattering per gyro-radius

D = 1 3c rL ' c rL

Cosmic Ray Diffusion & Containment

• Strong containment
• Steady-state solution

with cosmic ray densities

• Around ~1012 times

high than free- streaming case

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10−2 10−1 100 101 102

r/kpc

10−38 10−35 10−32 10−29 10−26 10−23 10−20 10−17

dN dEdV /erg · cm−3eV−1

Free-streaming profile Saturated magnetic field, steady-state profile

Cosmic Ray Interactions (Direct)

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+ pion multiplicities at higher energies

p + γ → p + e+ + e− p + γ → ∆+ → ( p + π0 → p + 2γ n + π+ → n + µ+ + νµ n + e+ + νe + ¯ νµ + νµ

Photopion Interaction

Interactions with Radiation Fields (p𝛅)

Photopair Interaction

Interaction by particles scattering off ambient photons (starlight, CMB…)

p + p → 8 > > > > > > < > > > > > > : p + ∆+ → 8 > < > : p + p + π0 p + p + π+ p + n + π+ n + ∆++ → ( n + p + π+ n + n + 2(π+)

Cosmic Ray Interactions (Direct)

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+ pion multiplicities at higher energies Neutron and photon interactions produce pions Pions decay to photons, muons, neutrinos, electrons, positrons, antineutrinos n + γ → π’s π → γ, µ, e, ν . . .

Interactions with Matter (pp)

p + p → 8 > > > > > > < > > > > > > : p + ∆+ → 8 > < > : p + p + π0 p + p + π+ p + n + π+ n + ∆++ → ( n + p + π+ n + n + 2(π+)

Cosmic Ray Interactions (Direct)

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+ pion multiplicities at higher energies Neutron and photon interactions produce pions Pions decay to photons, muons, neutrinos, electrons, positrons, antineutrinos n + γ → π’s π → γ, µ, e, ν . . .

Interactions with Matter (pp)

Dominates

Cosmic Ray Interactions (Indirect)

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Electron Injection

Qe(γe) ' Υ 6 400 me mp Qp(γp)

(Schober+ 2015, Lacki & Beck 2013)

Injection profile can be estimated from the CR source term

Cosmic Ray Interactions (Indirect)

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Electron Injection

Qe(γe) ' Υ 6 400 me mp Qp(γp)

(Schober+ 2015, Lacki & Beck 2013)

Sunyaev-Zel’dovich (SZ) Effect X-Ray Emission

Injection profile can be estimated from the CR source term Inverse-Compton scattering off CMB

CMB Photon X-Ray Photon Energetic electrons

LSZ ≈ 1048erg s−1

(upper limit)

Energy Deposition

• Absorption coefficient: energy absorbed at a point
• Cross section depends on interaction (radiation/particles)
• In general, can account for attenuation from emission up to absorption

point by RT

• Then heating is ~ energy absorbed at a point after attenuation
• Cross section: Klein-Nishina (X-rays)… Thomson limit with UV

9

H(r) = F0 α(r) exp ✓ − Z r

re

α(r0) dr0 ◆ Iν(r) = Iν,0 exp ✓ − Z r

r0

n(r0)σνdr0 ◆

α(r) = n(r)σ

Radiation

Energy Deposition

• Absorption coefficient: energy absorbed at a point
• Cross section depends on interaction (radiation/particles)

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α(r) = n(r)σ

Cosmic Rays SZ X-rays

– Cross section is dominating pp interaction – Scale to account for the containment – Emission profile from CR electron secondary injection – Heating then as per conventional treatment (previous slide)

No B field With B field

Energy Deposition

11

Stellar heating around 10-22 erg cm-3 s-1 X-ray heating Cosmic Rays (saturated B field) Note – Cosmic ray MC calculation using 1000 points

CR Heating: ISM

Summary & Remarks

• Cosmic rays are abundant in star forming galaxies

– Of particular interest at high redshift

• Containment by magnetic field appears to be important global effect

– Focuses CR heating into ISM above conventional stellar heating

• Accompanied by an X-ray heating effect due to SZ effect

– Higher than direct CR heating outside the galaxy

• Impacts

– Subsequent star formation (e.g. by heating star forming regions) – Thermal properties of surroundings – Pre-heating IGM during reionization

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Backup: Cosmic Ray Sources/Hillas Criterion

• Cosmic rays: charged

energetic particles (assume protons)

• Sources: supernova remnants

(SNRs) can accelerate CRs up to 1017-18 eV

• Diffusive shock acceleration

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Emax ≤ qBR

Adapted from Jacobsen+2015

Backup: Star-Forming Galaxies at High-z

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Magnetic Field

10−4 10−3 10−2 10−1 100 101

Age of Galaxy/Myr

10−31 10−27 10−23 10−19 10−15 10−11 10−7 10−3

Magnetic Field Strength/G

• Two scale components:

– Local scale, ~10-3 pc – Galactic ordered field ~1kpc

• SN driven
• Initial B field ~10-20 G

permeates protogalaxy

(Sigl+1997; Howard & Kulsrud 1997)

• Turbulent dynamo drives B

field up to µG levels seen in current epoch (Schober+2013)

Model follows J. Schober + 2013

SNe à Turbulence à B field

Backup: Cosmic Ray Interactions

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1010 1012 1014 1016 1018 1020

Energy/eV

10−4 10−3 10−2 10−1 100 101 102 103 104 105

Effective Path Length/Mpc

1 2 3 4 5 6 7 8

Particle Path Lengths

CMB & cosmological losses Interactions with stellar radiation fields Interactions with density fields

Backup: Cosmic Ray Diffusion

• Fundamental diffusion solution (Gaussian)
• Principle of superposition

16

Z tmax n(t)dt

n1(t) n2(t) = n1(t + dt) n3(t) = n2(t + dt) = n1(t + 2dt) ni(t) = · · · = n1(t + (i − 1)dt)

Σi{

Time (to deal with continuous injection)

n(r, t) = Q(rs) (4πDt)3/2 exp ⇢ −(r − rs)2 4Dt

Backup: Cosmic Ray Diffusion

• Fundamental diffusion solution (Gaussian)
• Principle of superposition

17

x x x x x x x x x x x x x x x x x x x x x x x

Space (to deal with source distribution – weighted by galaxy density profile)

n(r, t) = Q(rs) (4πDt)3/2 exp ⇢ −(r − rs)2 4Dt

Backup: Energy Deposition

18

γ − Ray Emission/erg cm−3 s−1

Heating: Cross-Check with GALPROP

Stellar heating X-ray heating Cosmic Rays (saturated B field)

Cosmic Ray Heating Galprop Comparison

Cosmic Rays (initial)