Exploring the Ends of the Rainbow: Cosmic Rays in Star-Forming Galaxies

Transcription

1 Exploring the Ends of the Rainbow: Cosmic Rays in Star-Forming Galaxies Brian Lacki With Todd Thompson, Eliot Quataert, Eli Waxman, Abraham Loeb 21 September 2010

2 The Cosmic SED

3

4

5 Nonthermal Thermal Nonthermal

6 ???????? Nonthermal Thermal Nonthermal?

7 Cosmic Rays in (Our) Star-Forming Galaxy Galactic ExtraGalactic

8 Cosmic Rays in (Our) Star-Forming Galaxy Cosmic ray p+ emit pionic gamma-rays in dense gas Cosmic ray e- (and e+) emit bremsstrahlung in dense gas and Inverse Compton in radiation fields. Gamma rays Credit: NASA/DOE/International LAT Team

9 Cosmic Rays in (Our) Star-Forming Galaxy Cosmic ray e- (and e+) emit synchrotron in magnetic fields Radio Credit: Haslam et al. / MPIfR / SkyView

10 Cosmic Rays in Star-Forming Galaxies Credit: Tavani et al. (2010) -- AGILE Star-formation processes source of most of CRs at Earth Includes primary protons (and nuclei), primary electrons Also secondary particles when interacting with gas Presumed to be accelerated in supernova remnants May be stellar winds, superbubbles, pulsars, etc.

11 Cosmic Rays in Star-Forming Galaxies Important part of ISM Feedback? Ionization in molecular clouds Star-formation tracers Synchrotron radio (FIR-radio correlation) Contribution to X-rays? Background to indirect DM searches, pulsars Gamma-rays from CR protons, electrons Secondary e+/e-

12 The FIR-Radio Correlation Very tight correlation between FIR & radio of non-agn galaxies -- Milky Way to ULIRGs (1012 Lsun) Factor ~2 scatter over 4 dex in luminosity Linear except at lowest luminosities Radio as a star-formation tracer? Synchrotron connected to both B and CR electrons From Bell (2003)

13 Far Infrared Herschel Radio VLA

14 Dust absorbs UV light, re-emits it as far IR FIR UV SNe CR e- Radio Supernovae accelerate cosmic ray electrons These emit synchrotron radio in magnetic fields

15 Star-forming Galaxies on the FRC Normal Galaxies Starbursts ULIRGs Form ~1 Msun / yr over several kpc Form ~1 Msun / yr within ~200 pc Form ~100 Msun / yr within ~200 pc B ~ 10 µg B ~ 200 µg B ~ 2000 µg Urad ~ 1 ev cm-3 Urad ~ 250 ev cm-3 Urad ~ 40,000 ev cm-3 n ~ 1 cm-3 n ~ 500 cm-3 n ~ 10,000 cm-3 Σg <~ 0.01 g cm-2 Σg ~ g cm-2 Σg ~ 1-10 g cm-2

16 Calorimetry? FIR UV SNe CR e- Radio All light emitted in UV absorbed by dust All energy in CR e- radiated as synchrotron

17 Or Conspiracy? FIR UV SNe CR e- Radio Maybe most UV light and CR e- escape, but radio/fir constant for other reasons

18 The Diffusion-Loss Equation Diffusion in space Advection Continuous Source by winds energy losses term Like advection in energy space Particle lifetime Includes escape from modeled region Describes how cosmic rays evolve as they emit radiation and move through galaxy Derived from Boltzmann equation related to hydrodynamic equations

19 The Diffusion-Loss Equation Continuous Source energy losses term Like advection in energy space Particle lifetime Includes escape from modeled region Describes how cosmic rays evolve as they emit radiation and move through galaxy Derived from Boltzmann equation related to hydrodynamic equations

20 The Simplest Version Q(E) x t(e) ~ N(E) Rate at which particles injected at energy E Time particles survive with energy E Number of particles with energy E Shorter t(e) at high E steeper (softer) spectrum Shorter t(e) at low E flatter (harder) spectrum

21 The Life of a CR Electron Flatten Spectra Steepen Spectra Flatten Spectra Atom e- Diffusive Escape Synchrotron Convective Escape Inverse Compton Ionization Bremsstrahlung Atom

22 Secondary e+/- γ e+/ν Cosmic ray protons hit ISM protons and make pions Secondary e+/e- Gamma-rays & neutrinos

23 Model Cosmic Ray Spectra Low Density Galaxy High Density Starburst

24 Starbursts Are Calorimeters FIR CR electrons escape Normal Galaxies Starbursts Neither UV photons nor CR electrons escape UV photons escape Radio A conspiracy sets radio luminosities of low density galaxies

25 Proton calorimetry Pion losses convert CR protons to gammarays, neutrinos, e+/- Secondary e+/e- are produced in starbursts Where is their radio emission?

26 Our Conspiracy Theory Radio FIR

27 Our Conspiracy Theory Radio Brem Ion. FIR IC Considering only non-synchrotron losses or only secondaries breaks the FRC.

28 Our Conspiracy Theory Secondary e+/e- Radio Brem Ion. IC FIR The radio luminosity of starbursts is determined by several factors

29 The High-Σg Conspiracy FIR Factor of ~10-20 Factor of ~10-20 Radio

30 The High-Σg Conspiracy FIR UV SNe CR e- CR p+ All light emitted in UV absorbed by dust CRs do not escape from host galaxies Radio emission set by many factors Radio γ-rays +ν

31 The high-z FRC CMB more intense at high redshift => Greater IC losses But how do bremsstrahlung and ionization change things? This ratio sets the synchrotron radio luminosity. When the CMB dominates tlife, the radio emission is suppressed. tsynch But tlife is dominated by bremsstrahlung, ionization, and IC from starlight. tlife The CMB must compete with these processes. High-Σg conspiracy preserves FRC! FRC buffered by other loss processes

32 The Old View CMB Radio FIR The CMB only has to compete with synchrotron. UB vs UCMB criterion

33 Buffering Secondary e+/e- Radio Brem Ion. IC CMB FIR CMB isn't just competing with synchrotron it's competing with every other loss process.

34 The High-z FRC FIR Radio

35 Submillimeter galaxies (SMGs) SMGs Compact starbursts They re huge and they re puffy. Some high-z starbursts (SMGs) are very wide (kiloparsecs), but with same surface density => low volume density. Weak bremsstrahlung and ionization Synchrotron depends on B

36 SMGs (B ~ Σg0.7 ~ 400 µg) Secondary e+/e- Radio Brem Ion. IC FIR High-Σg conspiracy in compact starbursts

37 SMGs (B ~ Σg0.7 ~ 400 µg) IC Secondary e+/e- Radio FIR The low volume density of SMGs would mean little bremsstrahlung and ionization they would be radio-bright

38 SMGs (B ~ Σg0.7 ~ 400 µg) FIR SMGs are radio-bright by a factor 2-5 Radio

39 SMGs (B ~ ρ ~ 100 µg) Secondary e+/e- Radio Brem Ion. IC FIR High-Σg conspiracy in compact starbursts

40 SMGs (B ~ ρ ~ 100 µg) IC Radio FIR The low volume density of SMGs means low magnetic field strengths and low synchrotron they would be normal or radio-dim

41 SMGs (B ~ ρ ~ 100 µg) FIR SMGs are radio-dim by a factor 1-2 Radio

42 Spectral Slopes of SMGs SMGs have steep radio spectra Compact starbursts have flat radio spectra Compact starbursts have flat radio spectra SMGs have steep radio spectra SMGs should have steeper radio spectra than compact starbursts from IC or synchrotron cooling α ~ instead of

43 Starbursts Seen in Gamma Rays Fermi has detected several starburst galaxies in GeV gammarays HESS and VERITAS detect M82 and NGC 253 at ~TeV

44 The Calorimetry Fraction Fcal ~ Lπ / LCR ~ 3 Lγ / LCR Fcal (M82) ~ Fcal (NGC 253) ~ Assumes that gamma-rays are pionic Much higher than Milky Way (~1 10%) Probably not fully calorimetric, but many uncertainties May be a sign of winds

45 The FIR-Gamma Ray Correlation?

46 The FIR-Gamma Ray Correlation? FIR-Gamma Ray Correlation? Gamma-ray data from Abdo et al. (2010,2010), Acero et al. (2009), Acciari et al. (2009), Blom et al. (1999), Porter et al. (2009), Strong et al. (2000, 2010); FSF from Sanders et al. (2003), Blom et al. (1999), Freudenreich (1999), Williams (2003), Harris & Zaritsky (2004,2009)

47 Comparing Radio to Gamma Rays Argument from Eli Waxman: ν Lν (GeV γ) ~ 2 ν Lν (GeV e ) +/- 1/3 pionic energy into gamma rays; 1/6 into secondary e+/- ν Lν (GeV e ) ~ 2 fsynch ν Lν (GHz) +/- One dex in e+/- energy corresponds to 2 dex in synchrotron frequency (ν ~ E2) ν Lν (GeV γ) ~ 4 fsynch ν Lν (GHz)

48 Comparing Radio to Gamma Rays ν Lν (GeV γ) ~ 4 fsynch ν Lν (GHz) fsynch (M82) ~ 1/8 fsynch (NGC 253) ~ 1/8 1/17 Assumes that gamma-rays are pionic Strong support for a conspiracy setting radio luminosity fsynch even smaller if radio is partly from primaries

49 How Far Does the Synchrotron Go? Thermal Dust Synchrotron Thermal (Free-Free) The radio and FIR spectrum of M82 in Klein et al. (1988)

50 Diffuse hard Xray emission M82 in radio M82 in X-rays MERLIN/VLA Muxlow et al. (1994) Chandra Credit: NASA/SAO/G.Fabbiano et al.

51 Pair Production FIR photon e TeV gamma-ray Starbursts are filled with far-infrared photons TeV gamma-rays converted into e+/beyond Klein-Nishina cutoff little IC cooling e+/- cool by emitting synchrotron X-rays e-

52 Electrons at TeV M82 Pair e+/ekleinnishina bump

53 How Far Does the Synchrotron Go? M82 Synchrotron Opaque to 30 TeV gammarays It goes at least to the X-rays Radio from ~GeV e+/-, X-rays from ~TeV e+/high magnetic field strength high energy emission

54 How Far Does the Synchrotron Go? Synchrotron Opaque to 30 TeV gammarays Optimistic It goes at least to the X-rays Radio from ~GeV e+/-, X-rays from ~TeV e+/high magnetic field strength high energy emission

55 Neutrinos and TeV Gamma-Rays M82 - ν Arp γ-ray Neutrinos and TeV gamma-rays can constrain the synchrotron X-ray fraction of starbursts.

56 Conclusions Emission from cosmic rays span the entire EM spectrum From ~10 MHz to 10 TeV Dominate radio and gamma-rays Some X-ray contribution The physics of CRs in star-forming galaxies is complicated. Conspiracies set radio luminosity of galaxies Implications for high z studies of galaxies Starbursts are gamma-ray sources Gamma rays indicate a conspiracy in radio emission