cosmic ray interactions in extreme molecular media · yoast-hull+ 2013, 2015 12 of 11,000 models...

Cosmic Ray Interactions In Extreme Molecular Media

Tova Yoast-HullTova Yoast-HullUniversity of Wisconsin-Madison

Department of Physics2 June 2015

Collaborators: Erin Boettcher, J. S. Gallagher III, Ellen Zweibel (University of Wisconsin), Susanne Aalto (Chalmers University, Sweden), John Everett (Northwestern University)

Funded by: Center for Magnetic Self-Organization & Wisconsin IceCube Particle Astrophysics Center

Cosmic Ray Populations in Galaxies● Significant fraction of energy

from supernovae goes into CRs and is redistributed via:– Interactions with the ISM

(heating)

– Launching of galactic winds (cooling)

● CRs detected at Earth were scrambled by the galactic B-field; other messengers:– Radio (CR electrons &

magnetic field)

– Gamma-rays, neutrinos (CR protons & ISM density)

Supernova Remnant W28Aharonian+ 2008, A&A, 481

~3

5 pc

Cosmic Rays at Earth● In the Milky Way, we find

equipartition of:– Cosmic rays

– Magnetic fields

– Radiation fields

– Interstellar gas

● What is the distribution of energy in other galaxies?

● What role do cosmic rays play in formation & evolution of galaxies?

Beatty & Westerhoff 2009, ARNPS, 59

From Cosmic Ray Sources to Earth


Cosmic Rays


Cosmic Rays

Gamma-Rays


Cosmic Rays

Gamma-Rays

Neutrinos

YEGZ Starburst Model

SNR

Cosmic rays are accelerated by supernova remnants.

Yoast-Hull+ 2013, ApJ, 768


SNR


∫Emin

Emax

Q(E)EdE=ηνSN E51

V



SNR

MolecularGas

They diffuse through the medium to interact with molecular clouds.

e-p


∫Emin

Emax

Q(E)EdE=ηνSN E51

V


YEGZ Starburst Model – Hadrons

SNR

MolecularGas

p

CR protons collide with Hydrogen and Helium to create charged and neutral pions.

p

p

Magnetic mirroring effects?



SNR

MolecularGas

p

As the pions decay, secondary electrons, positrons, gamma-rays, & neutrinos are created.

e-e+

ν


p

p




SNR

MolecularGas

p

As the pions decay, secondary electrons, positrons, gamma-rays, & neutrinos are created.

e-e+

ν


p

p


N (E)≈Q(E)τ (E)


Starburst Model – Bremsstrahlung

SNR

MolecularGas

CR electrons are accelerated by Hydrogen and Helium nuclei and radiate gamma-rays.

e-

e- e-

e-

p npn

e-


Starburst Model – Inverse Compton

SNR

CR electrons interact with the interstellar radiation field and produce gamma-rays.e-

e- e-

e-

e-

Radiation Field

Starburst Model – Synchrotron

SNR

CR electrons interact with the magnetic fields and produce non-thermal synchrotron.e-

e- e-

e-

e-

B

Magnetic Field

Prototype Galaxy: M82 Starburst Core● Our original model

was developed for and tested on M82.

● Nearest and best studied starburst galaxy

● Observed in radio & gamma-rays

● Contains large-scale galactic wind

Weiss+ 2001, A&A, 365

~450 pc

Model Overview

Model OverviewSupernova Rate

+ Gas Mass

1


+ Gas Mass

1CR Source Functions


+ Gas Mass


2


+ Gas Mass


2 CR Protons


+ Gas Mass


2Spectral Index

+ Wind Speed

CR Protons


+ Gas Mass


2Spectral Index

+ Wind Speed

3

CR Protons


+ Gas Mass


2Spectral Index

+ Wind Speed

3

CR Protons

CR Electrons


+ Gas Mass


2Spectral Index

+ Wind Speed

3Magnetic Field Strength

+ Thermal Radio Contribution

CR Protons

CR Electrons

Observational Information● Radio

– Supernova rate

● Millimeter– Molecular gas content

● Infrared– Radiation field

– Dust temperatures

● Optical– ISM pressures, wind

● X-Rays– Hot gas content, wind

Muxlow+ 1995, L-Band MERLIN Image

M82 Core – Radio Continuum

M82 Radio & Gamma-Ray Spectral Fits

Yoast-Hull+ 2013, 2015

Gamma-ray emission is mainly from neutral pion decay, but also includes bremsstrahlung and inverse Compton.

Radio spectrum contains both synchrotron and free-free emission with free-free absorption at low-frequencies.

M82 Chi-Squared Tests


Radio Spectrum Gamma-Ray Spectrum

We use chi-squared tests to optimize magnetic field strength (B), wind speed (vadv), ionized gas density (nion).

Chi-Squared Tests: Joint Solutions


16 of 11,000 Models for p = 2.312 of 11,000 Models for p = 2.2

Differences in the cosmic ray populations show up in our results of modeling the radio and gamma-ray spectra. In particular, the radio data are highly sensitive to supernova rate and average gas density.

Chi-Squared Tests: Joint Solutions


Differences in the cosmic ray populations show up in our results of modeling the radio and gamma-ray spectra. In particular, the radio data are highly sensitive to supernova rate and average gas density.

M82 is a ~50% proton calorimeter! B ~ 300 μG

Merging Galaxy Arp 220● Closest ultraluminous

infrared galaxy– Contains two starburst

nuclei

– Western nucleus is optically thick to the thermal infrared

● Window into the high redshift universe

Sakamoto+ 2008, ApJ, 684

Energy Density Comparison

Milky Way M82 Arp 220 East Arp 220 CND

Supernova Power

2 x 1048

erg yr-17 x 1048

erg yr-17 x 1049

erg yr-11.3 x 1050

erg yr-1

Radiation Field Energy Density 1 eV cm-3 1000 eV cm-3 40,000 eV cm-3 440,000 eV cm-3

Magnetic Field Energy Density

1 eV cm-3

(5 μG)2200 eV cm-3

(300 μG)620,000 eV cm-3

(5 mG)1.2x106 eV cm-3

(7 mG)

Average Gas Density 1 cm-3 260 cm-3 7,700 cm-3 42,000 cm-3

Cosmic Ray Energy Density 1 eV cm-3 450 - 650

eV cm-31100 - 3700

eV cm-32500 - 5140

eV cm-3

Arp 220 Radio Chi-Squared Tests

Yoast-Hull+ 2015, submitted

Eastern Nucleus Western Nucleus

In the calorimeter limit, wind speed does not affect the total cosmic ray electron population. Thus, the gamma-ray spectrum can be reliably predicted with only an observed radio spectrum and star formation rate.

η = 5%, p = 2.3

Arp 220 Radio Chi-Squared Tests


Eastern Nucleus Western Nucleus

In the calorimeter limit, wind speed does not affect the total cosmic ray electron population. Thus, the gamma-ray spectrum can be reliably predicted with only an observed radio spectrum and star formation rate.

η = 10%, p = 2.3

Detectable Gamma-Ray FluxArp 220 is likely detectable at GeV energies by Fermi, but not above ~10 TeV due to gamma-gamma absorption.

Due to the extremely short lifetimes, the Arp 220 nuclei are > 90% proton calorimeters.


Summary● Joint modeling of gamma-ray and radio spectra is

necessary for constraining the properties of CR populations in galaxies.

● Strong magnetic fields & low CR energy densities in starbursts.

● M82: winds are vital to correctly modeling the CR populations.– ~50% CR energy back into ISM, ~50% into the wind

● Arp 220: winds are irrelevant as all regions are so dense, no CRs escape & self-absorption is key– ~100% CR energy back into ISM

cosmic ray interactions in extreme molecular media · yoast-hull+ 2013, 2015 12 of 11,000 models...

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