Post on 15-Jan-2016
The Radio-IR Correlation:
Coupling of Thermal and Non-Thermal
Processes
The Radio-IR Correlation:
Coupling of Thermal and Non-Thermal
Processes
Amy KimballGeneral Exam
February 28, 2007
Radio
Radio
Gamma
X-ray
Optical
Near-IR
Mid-IR
Infrared
H2
H
www.astro.cornell.edu/research/projects/us-kaz/
OutlineOutline
• 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)Typical spectrum (M82)
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Black-body (dust)
Synchrotron
(SNe, AGN)
ThermalBremsstrahlung(HII regions)
(Radio regime) (Far-infrared regime)Condon 1992
Radio emissionRadio emission
• Accelerating charged particles emit radiation– In astrophysics: e-
– Electric field: Bremsstrahlung
– Magnetic field: synchrotron
Radiation from Accelerating, Charged
Particles
Radiation from Accelerating, Charged
Particles
change in electric field due to motion of particle; integrate power radiated in all directions
Larmor’s Radiation Formula:
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Thermal Bremsstrahlung:
accelerating e- in E-field
Thermal Bremsstrahlung:
accelerating e- in E-field
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e-e-
h
b
Ze+
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Electron velocity distribution:
Thermal Bremsstrahlung:
Spectrum
Thermal Bremsstrahlung:
Spectrum
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Log (frequency)
Log (intensity)
Thermal cutoffIe(-h/
kT)
Bremsstrahlung self-absorption
I 2
Thermal radio: HII regions
Thermal radio: HII regions
• O and B stars ionize their surrounding hydrogen
Synchrotron spectrum:
accelerating e- in B-field
Synchrotron spectrum:
accelerating e- in B-field
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Frequency (/c)
Relative intensity
Spectrum from a single
electron
Synchrotron emission: Ensemble of electrons
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
Cosmic ray energy spectrum
• Empirically known to have a power-law energy distribution
Where x ~ 2.2-3
• 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)
Synchrotron spectrum: power-law e- ensemble
(non-thermal)
Summary: Radio emission in galaxies
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
Blackbody (Thermal) Radiation
Thermal equilibrium:
Stars heat dust in galaxies
Stars heat dust in galaxies
http://www.arcetri.astro.it/~irasita/cosmodust/DUST.html
Dust particularly opaque to UV;Transparent to IR
FIR: Dust ReprocessingFIR: Dust
Reprocessing
Summary: Infrared emission in galaxiesSummary: 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 DataInfrared Data
• Infrared Astronomical Satellite (IRAS)
• First major infrared space telescope
• Covered 96% of sky• 20,000 galaxies
– Late-type (spirals)– ULIRGs– AGNs
http://irsa.ipac.caltech.edu/Missions/iras.html
IRASIRAS
LFIR (infrared)LFIR (infrared)
• LFIR: Estimate of 40-120m emission
Wavelength (m)
Filter response
function
12 m 24 m 60 m
100 m
http://irsa.ipac.caltech.edu/
• Radio emission at a single frequency: usually 1.4 GHz (20cm)– Westerbork or NVSS
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ThermalBremss.
Condon 1992
S (radio)
S (radio)
Subtract thermal radio to obtain pure non-thermal radio
First strong detection:
Disk galaxies; no AGN
First strong detection:
Disk galaxies; no AGN
– Virgo cluster spirals
– “field” spirals– “starburst nuclei” (HII region-like spectra)
Helou, Soifer, & Rowan-Robinson 1985
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Far-infrared: Log LFIR (W/m2)
Radio: Log S6.3GHz (mJy)
Spirals, Irregulars, Blue compact dwarfsSpirals, Irregulars, Blue compact dwarfs
Wunderlich, Klein, & Wielebinski 1987
24
23
22
21
20
19
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34 35 36 37 38
Radio: Log S6.3cm [W/Hz]
Infrared: Log LFIR [W]
IRAS + NVSS (more radio matches)
IRAS + NVSS (more radio matches)
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Log
Log
All IRAS galaxies with NVSS match (many more radio matches!)
Yun, Reddy, & Condon 2001
Q: ratio LFIR/SQ: ratio LFIR/S
Yun, Reddy, & Condon 2001
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Log Q
Log L60m (L)
AGN do not share relation
AGN do not share relation
Sopp & Alexander 1991
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Ellipticals
SO
Radio galaxies
Late-typespirals
Log (LFIR/L)
Log (Lradio/L
)
What is the connection?What is the connection?
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Black-body (dust)
Synchrotron
(SNe, AGN)
ThermalBremsstrahlung(HII regions)
Condon 1992
(Radio regime) (Far-infrared regime)
Answer is….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 120m (M > 5M)
• Their supernova remnants accelerate free electrons which escape into the galaxy, and emit synchrotron (M > 8M)
• Not the complete answer…
LFIR SFR(star formation rate)
LFIR SFR(star formation rate)
From models:(assume starburst history, adopt IMF)
(Kennicutt 1998)
From observations: M81
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Gordon et al. 2004
UV H R Infrared Radio
Non-thermal Radio SFR
Non-thermal Radio SFR
• SNRs produce Lsynch too low by factor of 10
• Cosmic ray electrons escape from SNR and emit (~107 yrs) long after SNR is gone (~105 yrs)
• Simple model: use empirical Galactic Lsync fSN to infer Lsync SFR
Dispersion in QDispersion in Q
Yun, Reddy, & Condon 2001
Obric et al. 2006
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2.8 2.6 2.4 2.2 2.0 1.8 1.6
Log Q (60m/20cm)
=0.11
Radio + IR
Radio + IR + optical
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Log L60m (L)
Log Q60
Galaxies have different star
formation histories
Galaxies have different star
formation histories
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Kennicutt 1998
Galaxies have different
metallicities
Galaxies have different
metallicities
Tremonti et al. 2004
53,000 star-forming galaxies
Galaxies have different dustGalaxies have different dust
Dumke, Krause, & Wielebinski 2004
Models fit to spectrum of NGC 4631
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Relative flux
Galaxies have different magnetic
fields
Galaxies have different magnetic
fields
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Hummel et al. 1988
Enter SPITZEREnter 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 L60m to L20cm decreases with radiusRatio of L60m to L20cm decreases with radius
• Implies IR disk emission has smaller scale length than radio disk emission
Marsh & Helou 1995
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Radial distance [kpc]
Even stronger relation to IR
surface brightness
Even stronger relation to IR
surface brightness
Ratio tied to star formation regions rather than radius?
Infrared: Log f70m [Jy]
Log Q70
Line of constant radio surface brightness
Murphy et al. 2006
Q maps spiral arms!Q maps spiral arms!
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Gordon et al. 2004
M81
Log Q
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arcsec
arcsec
log
Implications of a small global Q
dispersion
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 LFIR (< 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
Putting the correlation to use
What we learnedWhat 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!Cheers!
• Committee: Željko, Eric, Scott, Marina
• Comments: Chris, Daryl, Jillian, Mirela, Lucianne, Nick, Oliver, Peter, Stephanie
• Outfit: UWAWA
Yun, Reddy, & Condon 2001
Yun, Reddy, & Condon 2001
• Possible trends:– Higher dispersion at higher luminosity?
– Steeper at low luminosity?
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Condon, Anderson, & Helou 1991
Condon, Anderson, & Helou 1991
• Optically selected spiral and irregular galaxies
– Steepens, higher dispersion at lower LFIR
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Synchrotron: Single electron
Synchrotron: Single electron
Related correlations?
Related correlations?
• What other correlations should we see– CO-FIR (ref. Devereux&Young1991)
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Murgia et al. 2005
Radio-CO relation
And other galaxies?And other galaxies?
Wrobel & Heeschen 1991
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AGN
E/SO
Wrobel & Heeschen 1988
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Radio core or jet/lobe dominated
Radio-loud galaxiesRadio-loud galaxies
• Radio emission comes from lobes/jets: decoupled from infrared emission
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Sanders & Mirabel 1996
The AGN relationThe 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
Individual parts of galaxies
• Paladino et al.– Test of leaky box model? P. 856 fig. 10
• Murphy et al. 2006– Modeling CR diffusion w/ radio & IR maps
– Figs. 1 and 2
Bremsstrahlung:Single electron in
E-field
Bremsstrahlung:Single electron in
E-field
Synchrotron: Single electron
Synchrotron: Single electron
Synchrotron: Single electron
Synchrotron: Single electron
Synchrotron: Single electron
Synchrotron: Single electron
(Thermal radio/Thermal IR)
(Thermal radio/Thermal IR)O and B stars ionize surrounding gas and at the same time heat surrounding dust
Relation is realRelation is real
• Apparent magnitude vs. apparent magnitude: 20cm vs 60micron
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Mobric et al. 2006