Radio-loud AGN energetics
with LOFAR
Judith Croston
LOFAR Surveys Meeting
17/6/09
Understanding radio-galaxy physics is
important for galaxy feedback models!
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•
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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.
JC et al. 2003
Wise et al.
2007
Radio-galaxy energetics,
particle & field content
• k is unknown, and in general B and N0 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 ETOT (rule out equipartition in FRIs).
• Emin and shape of N(E) below observable radio region are
important: low-energy electrons dominate relativistic particle
population.
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: alow and gmin.
• See discussion in Harris (2004, astro-ph/0410485)
Figures from Harris (2004)
Inverse-Compton emission
from FRII radio lobes
• IC X-ray emission breaks the ne/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 0.006 – 2.
Colour: XMM IC
Contours: radio
JC et al. 2004
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 Xray observed 3C FRIIs
• Consistent with IC/CMB with B = (0.3 – 1.5) Beq
• > 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...
JC et al. 2005 ApJ 626 733
X-ray detected
lobes
Lower limits for
non-detected
lobes
Low-energy electron distribution
• Assume cut-off frequency, gmin = 10
– in hotspots, gmin ~ 100 – 1000 required (e.g. Carilli et al. 1991)
– adiabatic expansion => lower energy electrons in lobes
• Assume spectral index, alow = 0.5 (flattening)
– shock acceleration models predict d = 2 – 2.3 (a = 0.5 – 0.7)
– + hotspot observations (e.g. Carilli et al. 1991, Meisenheimer et al. 1997)
If alow = aobs:
• increase in Utot of up to
factor of 20
• Bobs/Beq decreases by up to
60%,
• IC/nuclear++
If gmin = 1000 (instead of 10):
• Utot and Bobs/Beq unchanged
• IC/nuclear -• conclusions not affected
If gmin = 1:
• increase in Utot by ~25%
• small decrease in Bobs/Beq
• IC/nuclear ++
Spatially resolved IC studies
• 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-n spectrum...
Isobe et al. 2002
Hardcastle & JC 2005
Goodger et al. 2008 & in prep.
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. 2008)
• Either radiating particles & field are NOT at equipartition or some other
particle population dominates the source energetics.
Worrall & Birkinshaw 2000 ApJ 530 719
Dunn & Fabian 2004 MNRAS 355 862
Combined X-ray & radio constraints
favour entrainment of ICM
• 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...
Hydra A:
“missing”
pressure as a
function of
distance
3C 31: required entrainment
rates
1.4 keV
5 keV
10 keV
50 keV
model
(1+r/rc) -2.0
(1 + r/rc) -1.0
r const.
Comparison with
theoretical
expectations
JC et al. in prep
Calibrating radio-loud (FRI) feedback
• X-ray cavities provide direct measurement of
energy input to ICM: Ekin >> Esynch
(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 lo-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-n radio spectrum promising for reducing
large scatter in cavity scaling relations (Bîrzan et
al. 2008)
Bîrzan et al. 2008
What LOFAR will do
• 10-200 MHz observations of large samples of radio-loud AGN will determine
distributions of low-n spectral index (& cut-off in some cases) for different
radio-loud AGN populations.
• Low-n spectra for large samples of FRIIs with X-ray coverage (100+ FRIIs):
– determine electron energy distribution for the energetically dominant population
below g ~ 105 via X-ray IC; constraints on particle acceleration
– remove factor ~ 20 uncertainty in ETOT , 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-n 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-n observations of cavity
samples at low-z
– Apply new calibrations to comprehensive FRI samples for tightly constrained AGN
feedback models