The Radio-IR Correlation: Coupling of Thermal and Non-Thermal Processes Amy Kimball General Exam February 28, 2007

Radio Gamma X-ray Optical Near-IR Mid-IR Infrared H2H2 H

Outline Radio and Infrared emission processes Discovery of the IR-radio correlation The connection between thermal and non-thermal emission in galaxies Dispersion in the IR-radio correlation Conclusion

Typical spectrum (M82) Black-body (dust) Synchrotron (SNe, AGN) Thermal Bremsstrahlung (HII regions) (Radio regime)(Far-infrared regime) Condon 1992

Radio emission Accelerating charged particles emit radiation –In astrophysics: e - –Electric field: Bremsstrahlung –Magnetic field: synchrotron

Radiation from Accelerating, Charged Particles change in electric field due to motion of particle; integrate power radiated in all directions Larmor’s Radiation Formula:

Thermal Bremsstrahlung: accelerating e - in E-field e-e- e-e- h b Ze + Single electron: Electron velocity distribution:

Thermal Bremsstrahlung: Spectrum Log (frequency) Log (intensity) Thermal cutoff I  e (-h /kT) Bremsstrahlung self-absorption I  2

Thermal radio: HII regions O and B stars ionize their surrounding hydrogen

Synchrotron spectrum: accelerating e - in B-field Frequency ( / c ) Relative intensity Spectrum from a single electron

Synchrotron emission: Ensemble of electrons Spectrum depends on energy distribution of the ensemble Cosmic ray electrons (non-thermal) Accelerated in shocks Supernovae AGN jets

Cosmic ray energy spectrum Empirically known to have a power- law energy distribution Where x ~ Fermi acceleration at shock front: –Most particles scattered few times –Few particles scattered many times –Predicts x~2

Synchrotron spectrum: power-law e - ensemble (non-thermal)

Summary: Radio emission in galaxies Thermal Bremsstrahlung –Free electrons interacting with nuclei –HII regions around O and B stars (Non-thermal) Synchrotron –Free electrons interacting with magnetic field –Accelerated by supernovae or AGN jets

Blackbody (Thermal) Radiation Thermal equilibrium:

Stars heat dust in galaxies Dust particularly opaque to UV; Transparent to IR

FIR: Dust Reprocessing

Summary: Infrared emission in galaxies Thermal emission from dust heated by: –Massive O and B stars –Red giants –“cirrus” radiation (all stars) –AGN (dominates!) (can also see infrared synchrotron in some AGN)

Infrared Data Infrared Astronomical Satellite (IRAS) First major infrared space telescope Covered 96% of sky 20,000 galaxies –Late-type (spirals) –ULIRGs –AGNs

IRAS

L FIR (infrared) L FIR : Estimate of  m emission Wavelength (  m) Filter response function 12  m 24  m60  m100  m

Radio emission at a single frequency: usually 1.4 GHz (20cm) –Westerbork or NVSS Synchrotron Thermal Bremss. Condon 1992 S (radio) Subtract thermal radio to obtain pure non- thermal radio

First strong detection: Disk galaxies; no AGN –Virgo cluster spirals –“field” spirals –“starburst nuclei” (HII region-like spectra) Helou, Soifer, & Rowan-Robinson 1985 Far-infrared: Log L FIR (W/m 2 ) Radio: Log S 6.3GHz (mJy)

Spirals, Irregulars, Blue compact dwarfs Wunderlich, Klein, & Wielebinski Radio: Log S 6.3cm [W/Hz] Infrared: Log L FIR [W]

IRAS + NVSS (more radio matches) Log All IRAS galaxies with NVSS match (many more radio matches!) Yun, Reddy, & Condon 2001

Q: ratio L FIR /S Yun, Reddy, & Condon 2001 Log Q Log L 60  m (L  )

AGN do not share relation Sopp & Alexander 1991 Ellipticals SO Radio galaxies Late-type spirals Log (L FIR /L  ) Log (L radio /L  )

What is the connection? Black-body (dust) Synchrotron (SNe, AGN) Thermal Bremsstrahlung (HII regions) Condon 1992 (Radio regime)(Far-infrared regime)

Answer is…. STAR FORMATION-- MASSIVE STARS!! Form in dusty giant molecular clouds; nearly all their luminosity emerges in the far-infrared-- about two-thirds between 40 and 120  m (M > 5 M  ) Their supernova remnants accelerate free electrons which escape into the galaxy, and emit synchrotron (M > 8M  ) Not the complete answer…

L FIR  SFR (star formation rate) From models: (assume starburst history, adopt IMF) (Kennicutt 1998) From observations: M81 Gordon et al UV HH RInfrared Radio

Non-thermal Radio  SFR SNRs produce L synch too low by factor of 10 Cosmic ray electrons escape from SNR and emit (  ~10 7 yrs) long after SNR is gone (  ~10 5 yrs) Simple model: use empirical Galactic L sync  f SN to infer L sync  SFR

Dispersion in Q Yun, Reddy, & Condon 2001 Obric et al Log Q (60  m/20cm)  =0.11 Radio + IR Radio + IR + optical Log L 60  m (L  ) Log Q 60

Galaxies have different star formation histories Kennicutt 1998

Galaxies have different metallicities Tremonti et al ,000 star-forming galaxies

Galaxies have different dust Dumke, Krause, & Wielebinski 2004 Models fit to spectrum of NGC 4631 Relative flux

Galaxies have different magnetic fields Hummel et al. 1988

Enter SPITZER New bigger and better! Examine IR-radio correlation on small scales-- –Does the IR-radio ratio change depending on position inside a galaxy?

Ratio of L 60  m to L 20cm decreases with radius Implies IR disk emission has smaller scale length than radio disk emission Marsh & Helou 1995 Radial distance [kpc]

Even stronger relation to IR surface brightness Ratio tied to star formation regions rather than radius? Infrared: Log f 70  m [Jy] Log Q 70 Line of constant radio surface brightness Murphy et al. 2006

Q maps spiral arms! Gordon et al M81 Log Q arcsec log

Implications of a small global Q dispersion UV photons and relativistic e - produced in same proportions in all star-forming regions Star formation controls magnetic field strength… or vice-versa Cosmic rays influence star formation Supernovae induce star formation

Estimate SFR and L FIR (< 20%) from radio emission for a wide range of galaxies Constraint on galaxy properties? –Magnetic fields, dust, etc. (Can also seek out AGN) Putting the correlation to use

What we learned IR-Radio correlation is one of strongest in astronomy Holds for galaxies whose infrared and radio emission is dominated by star formation Is easily explained qualitatively but not quantitatively (yet) Can be used for good.

Cheers! Committee: Ž eljko, Eric, Scott, Marina Comments: Chris, Daryl, Jillian, Mirela, Lucianne, Nick, Oliver, Peter, Stephanie Outfit: UWAWA

Yun, Reddy, & Condon 2001 Possible trends: –Higher dispersion at higher luminosity? –Steeper at low luminosity?

Condon, Anderson, & Helou 1991 Optically selected spiral and irregular galaxies –Steepens, higher dispersion at lower L FIR

Synchrotron: Single electron

Related correlations? What other correlations should we see –CO-FIR (ref. Devereux&Young1991) Murgia et al Radio-CO relation

And other galaxies? Wrobel & Heeschen 1991 AGN E/SO Wrobel & Heeschen 1988 Radio core or jet/lobe dominated

Radio-loud galaxies Radio emission comes from lobes/jets: decoupled from infrared emission Sanders & Mirabel 1996

The AGN relation AGN have radio power coming from lobes and jets… unrelated to massive stars. AGN have IR emission from dusty torus… unrelated to massive stars. Same source directly powers radio and IR in AGN.

Individual parts of galaxies Paladino et al. –Test of leaky box model? P. 856 fig. 10 Murphy et al –Modeling CR diffusion w/ radio & IR maps –Figs. 1 and 2

Bremsstrahlung: Single electron in E-field

Synchrotron: Single electron

(Thermal radio/Thermal IR) O and B stars ionize surrounding gas and at the same time heat surrounding dust

Relation is real Apparent magnitude vs. apparent magnitude: 20cm vs 60micron Mobric et al. 2006