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 Components

Lobe

Jet

Dust

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

Spectral FitsGalaxy 3C 184

SED IRS Spectrum

Spectral FitsQuasar 3C 138

SED IRS Spectrum

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

Thermal fraction

Non-thermal correction

Non-thermal correction

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

Conclusion

• We have addressed a long-standing question in AGN unification

• Quasars are more luminous IR emitters than galaxies because of:– Doppler boosted synchrotron in quasars– Extinction from dusty torus in galaxies– Both orientation-dependent effects