Radio-loud AGN energetics with LOFAR Judith Croston LOFAR Surveys Meeting 17/6/09

Understanding radio-galaxy physics is important for galaxy feedback models! X-ray cavity measurements show energy is available to balance cooling in cluster cores, but timescales uncertain + various detection biases. When central AGN switches off, up to ¾ of available energy still contained within radio lobes – subsequent evolution of lobe contents & impact on the cluster depend on cavity particle & B content. FRIs (typical cluster centre sources) and powerful FRIIs have different energetics and particle/field content (e.g. JC et al. 2004, 2005, 2008; Dunn et al. 2004, 2005; Kataoka & Stawarz 2005): understanding the origins of this difference is crucial for relationship between accretion mode, jet production and feedback. Wise et al JC et al. 2003

 is unknown, and in general B and N 0 can’t be disentangled: common to assume minimum energy/equipartition. The main exception is when inverse-Compton emission from the same electron population can be detected: typically true for FRII radio galaxies and quasars. Measurements of external pressure/X-ray cavity detections can also constrain E TOT (rule out equipartition in FRIs). E min and shape of N(E) below observable radio region are important: low-energy electrons dominate relativistic particle population. Radio-galaxy energetics, particle & field content

The low-energy electron population Most of the energy density in extragalactic radio sources is at energies below currently observable radio region. Radio-source properties depend strongly on assumed spectrum below ~ 300 MHz:  low and  min. See discussion in Harris (2004, astro-ph/ ) Figures from Harris (2004)

Inverse-Compton emission from FRII radio lobes IC X-ray emission breaks the n e /B degeneracy of radio synchrotron => direct probe of low-energy electron spectrum and of lobe energetics. IC useful in jets & hotspots too, but for lobes beaming & other X-ray emission processes unimportant. In most cases CMB photon field dominates over nuclear photons (e.g. Brunetti et al. 1997) & SSC. Can now routinely detect IC emission from the lobes of FRII radio galaxies and RL quasars: ~ 30 X-ray detections spanning redshifts of – 2. JC et al Colour: XMM IC Contours: radio

Comastri et al. 2003, Hardcastle et al. 2002, Brunetti et al. 2002, Isobe et al. 2002, Hardcastle & JC 2005, JC et al. 2004

IC/CMB from FRII lobes: results for large samples X-ray detection in at least one lobe in 70% of X- ray observed 3C FRIIs Consistent with IC/CMB with B = (0.3 – 1.5) B eq > 75% of sources at equipartition or slightly electron dominated => magnetic domination must occur rarely, if at all. Unlikely that relativistic protons dominate source energetics. Total internal energy in FRII radio sources is typically within a factor of 2 of minimum energy (see also Kataoka & Stawarz 2005) But assumptions about the low-energy electron population introduce significant uncertainty in these results... X-ray detected lobes Lower limits for non-detected lobes JC et al ApJ

Low-energy electron distribution Assume cut-off frequency,  min = 10 –in hotspots,  min ~ 100 – 1000 required (e.g. Carilli et al. 1991) –adiabatic expansion => lower energy electrons in lobes Assume spectral index,  low = 0.5 (flattening) –shock acceleration models predict  = 2 – 2.3 (  = 0.5 – 0.7) –+ hotspot observations (e.g. Carilli et al. 1991, Meisenheimer et al. 1997) If  min = 1000 (instead of 10): U tot and B obs /B eq unchanged IC/nuclear -- conclusions not affected If  min = 1: increase in U tot by ~25% small decrease in B obs /B eq IC/nuclear ++ If  low =  obs : increase in U tot of up to factor of 20 B obs /B eq decreases by up to 60%, IC/nuclear++

Spatially resolved IC studies Isobe et al Hardcastle & JC 2005Goodger et al & in prep. Chandra & XMM allow us to investigate spatial variation of N(E) and B in lobes. Lack of correlation between radio and X-ray structure indicates N(E) changes alone can’t explain radio structure; changes in B alone can’t explain relation to radio spectral structure => both are required. Also relies heavily on assumptions about low- spectrum...

X-ray environments & cluster cavities FRI radio lobes at equipartition are under-pressured relative to their environments ( e.g. Morganti 1988, Killeen et al. 1988, Feretti et al. 1990, Taylor et al. 1990, Böhringer et al. 1993, Worrall et al. 1995, Hardcastle et al. 1998, Worrall & Birkinshaw 2000, JC et al. 2003, Dunn & Fabian 2004, JC et al. 2008, Birzan et al ) Either radiating particles & field are NOT at equipartition or some other particle population dominates the source energetics. Worrall & Birkinshaw 2000 ApJ Dunn & Fabian 2004 MNRAS

Combined X-ray & radio constraints favour entrainment of ICM Hydra A: “missing” pressure as a function of distance Fraction of energy in radiating particles decreases dramatically with distance:. These constraints rules out relativistic proton domination, electron dominance and simple B-dominated models (e.g. Nakamura et al. 2006, Diehl et al. 2008) Consistent with entrained, heated ICM dominating radio-lobe energetics. Good constraints for models of FRI entrainment, but this relies on assumptions about low-energy electron population keV 5 keV 10 keV 50 keV 3C 31: required entrainment rates Comparison with theoretical expectations model (1+r/r c ) -2.0 (1 + r/r c ) -1.0 r const. JC et al. in prep

Calibrating radio-loud (FRI) feedback X-ray cavities provide direct measurement of energy input to ICM: E kin >> E synch (e.g. Bîrzan et al. 2004, Dunn & Fabian 2004, Dunn & Fabian 2008) Cavity detection only possible for modest sample sizes at low/moderate z and is subject to incompleteness problems: depends on angle to l- o-s, X-ray data quality, cluster luminosity, etc. Feedback models require radio surveys of FRIs to high z to relate direct measurements of energy input to RL AGN population statistics. Low- radio spectrum promising for reducing large scatter in cavity scaling relations (Bîrzan et al. 2008) Bîrzan et al. 2008

What LOFAR will do MHz observations of large samples of radio-loud AGN will determine distributions of low- spectral index (& cut-off in some cases) for different radio-loud AGN populations. Low- spectra for large samples of FRIIs with X-ray coverage (100+ FRIIs): –determine electron energy distribution for the energetically dominant population below  ~ 10 5 via X-ray IC; constraints on particle acceleration –remove factor ~ 20 uncertainty in E TOT, factor ~2 uncertainty in B assuming CMB dominates IC photon seed field in most cases, and uncertainty about the role of nuclear IC scattering Low- spectra for very large samples of FRIs, including cavity sources, will: –Remove > order of magnitude uncertainty in energetics of radiating particles & field in FRIs/cluster cavities: important to determine entrainment and heating rates. –Allow detailed calibration of AGN heating relations via low- observations of cavity samples at low-z –Apply new calibrations to comprehensive FRI samples for tightly constrained AGN feedback models