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 MPI Heidelberg 2010 Feb 222

How might we update this FIR/radio correlation to 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 223

The mouse and the elephant MPI Heidelberg 2010 Feb 224

FIR/radio correlation: FIR/radio astronomers see the same star-forming galaxy populations MPI Heidelberg 2010 Feb 225

Radio luminosity density functions yield star-formation rate densities and their evolution MPI Heidelberg 2010 Feb 226 Smolcic et al. 2009, ApJ, 690, 610

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 227

AGN contamination, especially in radio flux-limited samples MPI Heidelberg 2010 Feb 228

Dust temperature and ionization: extended starburst versus compact AGN MPI Heidelberg 2010 Feb 229

q FIR is a better AGN indicator than q 25 or q 12 MPI Heidelberg 2010 Feb 2210

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 2211

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 U rad /U B ~ 2 or 3, steady SFR over few X 10 7 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 < solar. Mitigating factors (Lacki et al., arXiv: , ) 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 2212

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 2213

Physical constraints from images at sub- arcsec resolution MPI Heidelberg 2010 Feb 2214 (Arp 220) (Mrk 231)(IC 694) FIR T b ~ T color so τ > 1 at λ < 25μ B IC ~ B min E ~ milliG Radio size << thermal FIR size so AGN Radio T b ~ 10 4 K so τ ~ 1 implies thermal (not AGN)

Compact starbursts: higher q fir caused by finite opacity at < 2 GHz and < 25 μ m MPI Heidelberg 2010 Feb 2215

λ= 18 cm VLBI image of Arp 220 SNe, no AGN MPI Heidelberg 2010 Feb 2216 Lonsdale et al. 2006, ApJ, 647, 185

Back to the future: study star formation via the FIR/thermal radio correlation MPI Heidelberg 2010 Feb 2217 Harwit & Pacini 1975, ApJ, 200, 127L Spectrum of the Galactic HII region W3 q ~ 3.3

Example: NGC 4449 MPI Heidelberg 2010 Feb 2218 Reines et al. 2008, AJ, 135, 2222 VLA image with 1.3 arcsec ~ 25 pc resolution

EVLA and ALMA: New era for radio MPI Heidelberg 2010 Feb 2219