Radio Emission in Galaxies

Jim CondonNRAO, Charlottesville

MPI Heidelberg 2010 Feb 22

“The” historical, empirical, global FIR/radio flux-density correlation for star-forming galaxies at z ~ 0

• qFIR = log (FIR / S1.4) ~ 2.3

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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

The mouse and the elephant

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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

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?

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MPI Heidelberg 2010 Feb 22

AGN contamination, especially in radio flux-limited samples

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Dust temperature and ionization:extended starburst versus compact AGN

MPI Heidelberg 2010 Feb 22

qFIR is a better AGN indicator than q25 or q12

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MPI Heidelberg 2010 Feb 22

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.

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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

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

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Physical constraints from images at sub-arcsec resolution

(Arp 220) (Mrk 231) (IC 694)

FIR Tb ~ Tcolor soτ > 1 at λ < 25μBIC ~ Bmin E ~ milliG

Radio size << thermal FIR sizeso AGN

Radio Tb ~ 104 K soτ ~ 1 implies thermal(not AGN)

MPI Heidelberg 2010 Feb 22 15

Compact starbursts: higher qfir caused 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, 127LSpectrum 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