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Spitzer Observations of 3C Quasars and Radio Galaxies:
Mid-Infrared Properties of Powerful Radio SourcesK. Cleary1, C.R. Lawrence1, J.A. Marshall2, L. Hao2, D. Meier1
1: JPL, California Institute of Technology2: Cornell University
• Why observe in infrared?
• Previous Work
• The Spitzer Sample
• Spectral Fitting
• Results
Summary
Why Observe in Infrared?
•Barthel (1989) - FR II RG are quasars with BH hidden behind obscuring dusty torus•Hidden quasar light is reprocessed and emitted at longer wavelengths•Signature of warm dust should be detectable in infrared
•FIR should be orientation independent
• Implies direct test of FRII/Quasar unification
• Quasars and Galaxies should have similar infrared luminosity
• Need to normalise by radio lobe luminosity to account for varying central engine power
Why Observe in Infrared?
Previous Work
• IRAS• Heckman et al. (1992), 6/117 RG and quasars, 3C z>0.3
– Quasars 3x more luminous (normalised) than galaxies
• Confirmed by Hes et al. (1995)– IRAS 60 um quasars systematically brighter than galaxies
– Beamed component may account for this difference
• Hoekstra et al (1997)– IRAS 60 um fluxes consistent with an orientation-based model
– Other processes such as optical depth also contribute
Previous Work
• ISO• van Bemmel et al. (2000)
– 4 3C Q/G pairs matched in redshift and radio power– Non-thermal contribution estimated at < 2%– Systematic excess found for quasars
• Meisenheimer et al. (2001)– 10 3C Q/G pairs– Dust luminosity distribution (normalised by radio power) similar for quasars and
galaxies
• Andreani et al (2002)– ISO photometry and mm data for sample of 3C quasars and galaxies– Quasar composite spectrum 3x brighter than galaxy spectrum in mm region
• Haas et al. (2004)– 3CR 17/51 galaxies, 17/24 quasars, – similar normalised restframe 70 micron luminosities
Previous Work
Both
Beamed synchrotron emission and
Dust extinctionModulate IR emission of quasars and galaxies to some degree.
• Spitzer provides additional constraints:– increased photometric sensitivity– MIR spectroscopic data
• Allows us to quantify these effects in orientation-unbiased sample
FRII SED
LOBE JET DUST ACCRETION DISKRadio Microwave
Sub-mmInfrared Visible
• Low-frequency radio emission from lobes is ISOTROPIC
• FRII radio sources uniquely useful in separating intrinsic from apparent differences
The Spitzer Sample
• 3CRR extremely powerful radio sources, selected for:– Radio-lobe rest luminosity L
> 1026 W/Hz/sr– Redshift, 0.4<z<1.2– Ecliptic latitude (for Spitzer
scheduling)
• =>16 Quasars, 18 Galaxies• Orientation-unbiased sample• IRS long low spectra, 15-37
m• MIPS photometry, 24, 70 &
160 m
IRS SpectraQuasars Galaxies
• Basic Morphology– Silicate
Emission– Silicate
Absorption– Emission Lines
Characteristic Luminosities
• Characteristic Luminosities (W/Hz/sr)
• 15 microns from IRS• 30 microns from MIPS
Origin of IR Emission
• Thermal– Dust heated by star-formation– Dust heated by “central engine”
• Non-thermal– Synchrotron from radio lobes– Synchrotron from radio jet
Spectral Fitting
• For all objects with IRS spectra, we fit the following components:– Warm dust + lobe synchrotron– Warm dust + lobe synchrotron + jet synchrotron– Warm dust + lobe synchrotron + cool dust– Warm dust + cool dust + lobe synchrotron + jet
synchrotron
• Combination with best chi-squared selected
Fit Parameters
• Synchrotron fitting functions• Dust model
– Temperature– Optical Depth
• Thermal fraction, ftherm = Ltherm/Ltotal
• Can correct observed MIR flux density for non-thermal emission
• At 15 microns, up to 90% non-thermal for some quasars
Testing Unification• Compare quasar and
galaxy luminosity• Normalise by radio
luminosity (Rdr = Ldust/Lradio)– Quasars 4 times
brighter than galaxies at 15 microns
• Correct for non-thermal emission
– Quasars on 2 times brighter than galaxies
• Correct for extinction– Quasars and galaxies
have same average brightness
Testing Unification• Compare quasar and
galaxy luminosity• Normalise by radio
luminosity (Rdr = Ldust/Lradio)– Quasars 4 times
brighter than galaxies at 15 microns
• Correct for non-thermal emission
– Quasars on 2 times brighter than galaxies
• Correct for extinction– Quasars and galaxies have
same average brightness
Role of Orientation
• Anticorrelation between optical depth and core dominance
• R<10-2, Median(tau)=1.1
• R>10-2, Median(tau)=0.4
• Infer equatorial distribution of dust
• Consistent with ‘dusty torus’ of unification schemes.
Summary
• We have observed an orientation-unbiased sample of extremely powerful 3CRR radio galaxies and quasars
• Detected powerful MIR emission (L24 > 1022.4 W/Hz/sr)
• IRS measurements provide powerful constraints on SED
• Allowed us to fit continuum synchrotron and dust components
Summary
• Non-thermal contribution to MIR up to 90% in some quasars
• At 15 microns, quasars are typically 4 times brighter than galaxies with same isotropic radio power
• Half of this difference is due to non-thermal emission present in quasars but not in galaxies
• Other half is due to absorption in galaxies but not in quasars