Nonthermal Emission in Starburst Galaxies

Transcription

1 Nonthermal Emission in Starburst Galaxies! Yoel Rephaeli!!! Tel Aviv University & UC San Diego Cosmic Ray Origin! San Vito, March 20, 2014

2 General Background * Stellar-related nonthermal phenomena * Particle acceleration by SN shocks * Starburst-AGN interplay * Contribution to background radiation from starburst (SB) phases * IS (and intracluster) effects of energetic particles

3 Outline * Modeling energetic particles and fields in SBGs * Results for M82 & NGC253 * Implications

4 Images of NGC253

5 Energy loss timescales in a SBN: Energetic protons: Coulomb and p-p collisions τ pp ; (n /100) 1 yr Energetic electrons: Coulomb (electronic excitations), synchrotron, Compton τ ee ; (E /1GeV )(n /100) 1 yr τ S ; (E /100GeV ) 1 (B /100 µg) 2 yr τ C ; (E /100GeV ) 1 (ρ /100ρ CMB ) 1 yr

6 Propagation modes: Diffusion: Assuming that the effective mean free path is comparable to the coherence scale of the magnetic field (across the disk and inner halo), l : pc, the implied diffusion coefficient is D = lc / 3 : cm 2 / s Convection by SN-driven wind, v : km / s When energy loss times are relatively short, and high-energy emission is spatially unresolved, no practical need to distinguish between these propagation modes

7 Basic Approach: The much higher SFR, gas density, and magnetic field in a SBN region justify separate treatments of the evolution of particle spectro-spatial distributions in the primary SB and secondary disk regions Due to the small size of the SB region, and the short inter-sn distance, the source distribution is roughly continuous and uniform When the duration of SB is significantly longer than relevant energy loss times (at E 1 GeV), particle distributions can be assumed to have attained a steady-state

8 Temporal evolution to steady-state: Protons from individual SN (`accelerators ) are followed as they diffuse and lose energy. At a given position, the time-dependent spectro-spatial distribution is the summed up contributions from all SN in the SB region! J p = c 4π N i=1 f i (E,d i,t i ) Numerical solution for f(e,r,t) assuming D=D0 (E/E0) 1/2 and energy losses by p-p interactions Torres, Cillis, Lacki & YR (12)

9 Time evolution of proton flux: R=300 pc; r=50 pc

10 Approximate analytic treatment:! Assume an initial p/e and κ=ρ p /ρe from charge neutrality Deduce e spectral density - in terms of the mean strength of the magnetic field B - directly from measured radio flux Assume particle-field equipartition in the SBN region to determine ρp and B, accounting also for secondary e Calculate γ emission from Compton scattering (on FIR photons), bremsstrahlung, and π 0 decay Persic, YR, & Arieli (08)

11 A more accurate numerical treatment: Assumed an initial p/e ratio and equipartition in the SBN region Particle spectro-spatial distributions were determined using a modified version of the GALPROP code (of Moskalenko & Strong) that solves the diffusion-convection equation Performed an iterative procedure to fit predicted radio spectrum to that measured from SBN to determine ne(e), (related) np(e), and secondary e ± from π ± decays

12 Model parameters: Diffusion coefficient,! D = D 0 (E /1GeV ) 1/2, D 0 : cm 2 / s Convection velocity, v = 500(R / R sb )km / s 2000 km / s Magnetic field scaled to ionized gas density: B = B 0 (n / n 0 ) 2/3, typically: B 0 ~ 200 µg Gas density profile in the disk, n e z/z 0 (1+ (R / R 0 ) 2 FIR radiation field: diluted blackbody at dust temperature

13 Basic observable: radio spectrum M82 Persic, YR, & Arieli (08)

14 M82 e p Spectral density

15 M82

16 TeV Measurements Acciari & (09) Acero & (09)

17 Fermi/LAT Measurements Abdo & (10)

18 Abdo & (10)

19 NGC253 8 GHz 0.33 GHz Carilli (97)

20 Rephaeli, Arieli, & Persic (10)

21 N253 Disk SB Rephaeli, Arieli, & Persic (10)

22 N253 p e se

23 N IC Brems π 0 decay

24 N253

25 Abdo & (10)

26 Predicted X & γ flux of M82:! Total integrated fluxes from the SB region and disk: f (E 100 MeV ); cm 2 / s f (E 100 GeV ); cm 2 / s with the disk contribution of Predicted X-ray flux f C ; (ε /10 kev ) 2.3 cm 2 s 1 F C (ε > 10 kev ) 5 10 ; erg / (cm 2 s) lower than the upper limit deduced from RXTE measurements (Rephaeli & Gruber 02)

27 Predicted X & γ flux of NGC253:!!! Total integrated fluxes from the SB region and disk: f (E 100 MeV ); cm 2 / s f (E 100 GeV ); cm 2 / s with less than half originating outside the SBN Predicted X-ray flux is below the current upper limit

28 Ongoing SBG measurements with NuSTAR Long (500 ks) measurements of NGC253 nuclear region shows that emission is dominated by 3 ULX sources with L(2-10 kev) few erg/s Lehmer & (13) Predicted diffuse emission at higher energies is much weaker than that measured from point-sources

29 Electron and proton energy densities in SBN: Electron energy density, ρe, can be deduced directly from measured radio emission, if the mean strength of the magnetic field, B, is known; otherwise, need to assume particle-field equipartition, necessitating knowledge also of proton energy density, ρp When no γ emission is detected, the assumptions of charge neutrality and equipartition yield κ=ρp /ρe & B, if a theoretical expression is adopted for the proton spectral density Can determine ρp directly when γ emission is measured (when the emission is mostly hadronic)

30 Important considerations: Need to include contributions of low energy electron and protons, essentially down to lowest energy consistent with escape from the (SN) source region Power-law may not be adequate approximation to the particle steady-state spectral density distribution Theoretical expression for synchrotron emissivity has to be calculated (self-consistently) using steady-state spectral density. Rephaeli & Persic (14)

31 Data and deduced values of ρe, ρp & B in SBGs: Within current measurement uncertainties, estimates of proton energy densities are roughly consistent with SN rates and estimated efficiency of particle acceleration Persic & Rephaeli (14)

32 Summary * Recent γ measurements provide important (even if rudimentary) spectral info on energetic protons in star-forming galaxies * Spatial distribution of γ emission is needed for detailed modeling of particle source distribution and propagation mode * Measurement of hard X-ray emission (>30 kev) will help determine the fractional leptonic contribution to the high-energy emission