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Radio Emission in Galaxies

Radio Emission in Galaxies. Jim Condon. NRAO, Charlottesville. “The” historical, empirical, global FIR/radio flux-density correlation for star-forming galaxies at z ~ 0. q FIR = log (FIR / S 1.4 ) ~ 2.3 .

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Radio Emission in Galaxies

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  1. Radio Emission in Galaxies • Jim Condon • NRAO, Charlottesville

  2. “The” historical, empirical, global FIR/radio flux-density correlation for star-forming galaxies at z ~ 0 qFIR = log (FIR / S1.4) ~ 2.3 MPI Heidelberg 2010 Feb 22

  3. How might we update this FIR/radio correlationto make it a better tracer of star formation? • Why 1.4 GHz? • Why 60/100 microns? • How can we reduce known limitations? • How can we improve the local FIR/radio correlation within galaxies? • How can we avoid contamination by old stars and AGNs? • How can the correlation best be extended to higher redshifts? • How can we best use new instruments (e.g., EVLA, ALMA)? MPI Heidelberg 2010 Feb 22

  4. The mouse and the elephant MPI Heidelberg 2010 Feb 22

  5. FIR/radio correlation: FIR/radio astronomers see the same star-forming galaxy populations MPI Heidelberg 2010 Feb 22

  6. Radio luminosity density functions yield star-formation rate densities and their evolution Smolcic et al. 2009, ApJ, 690, 610 MPI Heidelberg 2010 Feb 22

  7. Global radio emission in star-forming galaxies ~ 90% synchrotron radiation at 1.4 GHz Problems AGN contamination? ~ 90% diffuse Poorly understood Not optically thin? Why not study free-free emission at higher frequencies instead? MPI Heidelberg 2010 Feb 22

  8. AGN contamination, especially in radio flux-limited samples MPI Heidelberg 2010 Feb 22

  9. Dust temperature and ionization:extended starburst versus compact AGN MPI Heidelberg 2010 Feb 22

  10. qFIR is a better AGN indicator than q25 or q12 MPI Heidelberg 2010 Feb 22

  11. Radio emission from a Seyfert galaxy Predominantly nonthermal radio contamination by an AGN lowers the far-infrared/radio ratio but does not affect the far-infrared/free-free radio ratio. MPI Heidelberg 2010 Feb 22

  12. Basic conspiracy theories Calorimeter theory (Völk, H. J. 1989, A&A, 218, 67) • CR electrons accelerated in SNRs of dust-heating massive stars • Energy losses primarily radiative above ν ~ 5 GHz, fixed IC/synchrotron ratio implies fixed Urad/UB ~ 2 or 3, steady SFR over few X 107 years, steep radio spectra. Leaky Box theory (Chi, X., & Wolfendale, A. W. 1990, MNRAS, 245, 101) • Equipartition of CRs and ISM B fields in a very leaky calorimeter • Flatter radio spectra, q decreases with luminosity when L < 1010 solar. Mitigating factors (Lacki et al., arXiv:0904.4161, 0910.0478) • Other CR losses (e.g., bremsstrahlung keeps radio spectra flatter) and sources (secondary electrons from CR proton collisions, pion decay; gamma rays seen by Fermi in M82 and NGC 253 by Abdo et al. 2010, ApJ, 709, L152) • UV escapes from CR-leaky dwarf galaxies (Bell, E. F. 2003, ApJ, 586, 794) MPI Heidelberg 2010 Feb 22

  13. Infrared Emission, ISM, and Star Formation:Why bother with (nonthermal) radio emission? Aperture synthesis: high angular resolution, accurate absolute positions, high sensitivity, and high dynamic range, but… at short wavelengths, the angular resolution is often too high and the surface-brightness sensitivity too low Astrophysical constraints implied by the FIR/radio correlation Use “failures” to find and study unusual starbursts MPI Heidelberg 2010 Feb 22

  14. Physical constraints from images at sub-arcsec resolution (Arp 220) (Mrk 231) (IC 694) FIR Tb ~ Tcolorso τ > 1 at λ < 25μ BIC ~ Bmin E ~ milliG Radio size << thermal FIR size so AGN Radio Tb ~ 104 K so τ ~ 1 implies thermal (not AGN) MPI Heidelberg 2010 Feb 22

  15. Compact starbursts: higher qfircaused by finite opacity at < 2 GHz and < 25 μm MPI Heidelberg 2010 Feb 22

  16. λ=18 cm VLBI image of Arp 220 SNe, no AGN Lonsdale et al. 2006, ApJ, 647, 185 MPI Heidelberg 2010 Feb 22

  17. Back to the future: study star formation viathe FIR/thermal radio correlation Harwit & Pacini 1975, ApJ, 200, 127L Spectrum of the Galactic HII region W3 q ~ 3.3 MPI Heidelberg 2010 Feb 22

  18. Example: NGC 4449 Reines et al. 2008, AJ, 135, 2222 VLA image with 1.3 arcsec ~ 25 pc resolution MPI Heidelberg 2010 Feb 22

  19. EVLA and ALMA: New era for radio MPI Heidelberg 2010 Feb 22

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