Sarka WykesSarka WykesRadboud University NijmegenRadboud University Nijmegen // University of AmsterdamUniversity of Amsterdam

Minneapolis, 12th June 2015Minneapolis, 12th June 2015

Circular phenomena Circular phenomena inin

CentaurusCentaurus AA

Collaboration with Tom since 15th May 2013 Collaboration with Tom since 15th May 2013

(Turbulent eddies close to driving scale(Turbulent eddies close to driving scale // Bow shocks from Bow shocks from jet-stellar wind interactionsjet-stellar wind interactions // Circular polarisationCircular polarisation))

LongLong -- standing questionstanding question ▣▣ Composition of AGN jets at subparsec scalesComposition of AGN jets at subparsec scales : : 'light' (electron'light' (electron -- positron) or 'heavy' (electronpositron) or 'heavy' (electron -- proton)?proton)?

Infering jet compositionInfering jet composition : : power carried by the jetspower carried by the jets and their impact on surrounding and their impact on surrounding intraclusterintracluster // intragroup gasintragroup gas jet-launching mechanismjet-launching mechanism

Challenging enterpriseChallenging enterprise

▣▣ CP CP signal generally very smallsignal generally very small

▣▣ Instrumental effects largeInstrumental effects large (in particular for circularly- (in particular for circularly- polarised antenna feeds)polarised antenna feeds)

▣▣ Observations Observations not easily interpretablenot easily interpretable

Circular polarisation (CP)Circular polarisation (CP)

CentaurusCentaurus AA

Feain

+11

▣▣ NearestNearest (3.8 (3.8 ±± 0.10.1 Mpc)Mpc) radioradio galaxy galaxy FR FR I classI class ; 1''; 1'' ~~ 18.418.4 pcpc

outstanding testbed: e.g.outstanding testbed: e.g. models models of jet energetics, of jet energetics, BB-field structure, -field structure, particle content particle content andand acceleration, acceleration, jet formationjet formation

Parkes+ATCA 1.4 GHz

Chandra 0.4 - 2.5 keV 1.4 GHz contours

Hard

cast

le 1

5

KpcKpc jet jet

Jet speedJet speed : : ~~ 0.50.5cc ((100100 pc)pc)

Jet powerJet power :: PP

jj ~~ 11 xx 10104343 erg erg ss-1-1 (current jet) (current jet) (Croston+09; Wykes(Croston+09; Wykes++13)13)

PPjj ~~ 11 ̵̵ 55 xx 10104343 erg erg ss-1-1 (pre-(pre- existing) (existing) (WykesWykes++13; Neff13; Neff++15)15)

▣▣ Jet viewing angle 50Jet viewing angle 50°°

Goodger+10

Chandra 0.4 - 2.5 keV

VLA 8.4 GHz

ang. res. ~ 0.5''

0.8 x 0.2''

(Hardcastle(Hardcastle++03)03)

Goodger+10

Stationary andStationary andmoving knotsmoving knots

Jet opening Jet opening angle:angle:

~~ 1515°°

______

GCGC

Jet viewing angleJet viewing angle : : ~~ 5050°°

>>

Subparsec jet Subparsec jet

▣▣ Jet speed at subparsec scalesJet speed at subparsec scales vv

j j ~~ 0.10.1 -- 0.30.3c c (Tingay(Tingay++01;01; MMüüllerller++14)14)

- jet acceleration downstream- jet acceleration downstream oror - sampling disparate jet layers- sampling disparate jet layers

▣▣ JJstatstat

component atcomponent at 0.0540.054 pcpc projec.projec...: :

αα ~ –~ – 1.0 → 1.0 → optically thinoptically thin

▣▣ Dip in surface brightnessDip in surface brightness at 0.4at 0.4 pc projected ispc projected is stationary (2007stationary (2007 -- 2011)2011)

Local jet morphology indicatesLocal jet morphology indicates circumfluent behaviourcircumfluent behaviour

→ → standing discontinuitystanding discontinuity

Likely Likely distortion by a stardistortion by a star

Mülle

r+1

4

VLBI 8.4 GHz~ 3.3 x 0.8 mas

▄▄0.1 pc

AGN jetsAGN jets : Mass loading: Mass loading

▣▣ Internal entrainmentInternal entrainment: : ''injection' of mass lost injection' of mass lost from stars within jet from stars within jet boundaryboundary CentaurusCentaurus AA :: ~~ 2.2.33 xx 1100-3 -3

M M yryr -1-1

(Wykes+13; Wykes+15)(Wykes+13; Wykes+15) ▣▣ External entrainmentExternal entrainment: :

''ingestion' of ambient gas ingestion' of ambient gas from surrounding ISM via from surrounding ISM via turbulent boundary layerturbulent boundary layer CentaurusCentaurus AA :: ~~ 4.74.7 xx 1100-5 -5

M M yryr

-1-1

(Wykes+13)(Wykes+13)

▣▣ PrPrompt entrainment?ompt entrainment?: : mass entrapment of mass entrapment of ambient gas at leading ambient gas at leading end of the jetend of the jet

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☉

☉▣▣ Mass from Mass from accretion disc?accretion disc? ↘↘

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Origin of CP Origin of CP

▣▣ Intrinsically from synchrotron emission Intrinsically from synchrotron emission (Legg(Legg & & Westfold 1968)Westfold 1968)

▣▣ Through Through Faraday conversion of linear polarisation toFaraday conversion of linear polarisation to circularcircular. Propagation effect . Propagation effect (Jones (Jones & & O'Dell 1977; Jones 1988)O'Dell 1977; Jones 1988)

CP: polarised fraction CP: polarised fraction mmcc = |V| / I = |V| / I

Origin of CP Origin of CP Intrinsic CPIntrinsic CP

▣▣ Generated directly by radiating particle speciesGenerated directly by radiating particle species

▣▣ Requires significant charge imbalance in radiating Requires significant charge imbalance in radiating particles (electronparticles (electron -- proton jet) leading to vast amounts of proton jet) leading to vast amounts of internal Faraday rotation, unless cutoff in relativistic particle internal Faraday rotation, unless cutoff in relativistic particle energy spectrum rather high, energy spectrum rather high, γγ

minmin ≳≳ 100 100

▣ ▣ Predicted CP spectrum in optically thin, homogeneous Predicted CP spectrum in optically thin, homogeneous modelsmodels : : mm

cc ∝ ν∝ ν−0.5−0.5 (S (S

νν ∝ ν∝ ν+α+α ) )

Faraday conversionFaraday conversion

▣▣ Demands large number of low-energy relativistic particles in Demands large number of low-energy relativistic particles in the jet, a relatively modest cutoff in the particle energy the jet, a relatively modest cutoff in the particle energy spectrum, spectrum, γγ

minmin ≲≲ 20 20

▣ ▣ Predicted CP spectrum in optically thin, homogeneous Predicted CP spectrum in optically thin, homogeneous modelsmodels : considerably steeper: considerably steeper

CP observations: general CP observations: general ▣▣ Detected in large number of extragalactic sourcesDetected in large number of extragalactic sources

▣▣ Degree of radio CP typically 0.2Degree of radio CP typically 0.2 –– 11 percentpercent

▣ ▣ Strongest signal associated with the innermost regions Strongest signal associated with the innermost regions of AGN jetsof AGN jets

▣ ▣ Results for some sources show higher radio CP signal Results for some sources show higher radio CP signal towards higher frequencies: e.g.towards higher frequencies: e.g. association withassociation with intrinsic inhomogeneity in the jet may be responsible for intrinsic inhomogeneity in the jet may be responsible for increase in increase in mm

cc towards shorter wavelengths towards shorter wavelengths

▣ ▣ Direct observational evidence preferring one CP generationDirect observational evidence preferring one CP generation mechanism over another remains scarcemechanism over another remains scarce ;; only Wardleonly Wardle++98 (quasar 3C98 (quasar 3C 279) and Homan279) and Homan && WardleWardle 0404 (FR(FR I radio galaxy 3CI radio galaxy 3C 84) strongly favouring Faraday 84) strongly favouring Faraday conversion → initially light jetconversion → initially light jet

CP observations: CentaurusCP observations: Centaurus A A ▣▣ VLBAVLBA : utilised only the southern seven antennas: utilised only the southern seven antennas 6-h run6-h run : 4.5: 4.5 h on target, 1.5h on target, 1.5 h on calibration sources h on calibration sources

88 GHz GHz

total intensity imagetotal intensity image

peak surface brightnesspeak surface brightness == 0.650.65 JyJy beambeam−1−1

rmsrms == 4.94.9 mJymJy beambeam−1 −1

presence of distinct presence of distinct subcomponentssubcomponents

unusual feature ‘hanging’ unusual feature ‘hanging’ to the south at ∼to the south at ∼ 88 mas mas (0.147(0.147 pc projected), not pc projected), not discernible in older 8discernible in older 8 GHz GHz imagesimages

feature from an intruder?feature from an intruder?

VLBA~ 2.4 x 0.7 mas

Wykes, Reynolds+15

PRELIMINARYPRELIMINARY

CP observations: CentaurusCP observations: Centaurus A A

1515 GHz GHz

total intensity imagetotal intensity image

peak surface brightnesspeak surface brightness == 0.94 Jy0.94 Jy beambeam−1−1

rmsrms == 2.02.0 mJymJy beambeam−1−1

presence of distinct presence of distinct subcomponentssubcomponents

unusual feature at unusual feature at ∼ ∼ 88 mas mas (0.147(0.147 pc projected) pc projected) present, though less present, though less prominentprominent

PRELIMINARYPRELIMINARY

Wykes, Reynolds+15

VLBA~ 2.4 x 0.7 mas

CP observations: CentaurusCP observations: Centaurus A A

▣ ▣ No significant LP or CP at 8 and 15No significant LP or CP at 8 and 15 GHzGHz

▣ ▣ 8 GHz: 1σ upper limit on 8 GHz: 1σ upper limit on P P ofof 1 1 mJymJy beambeam−1−1, , on CP of 2on CP of 2 mJymJy beambeam−1−1

▣▣ Shortfall of significant LP most readily explained by Shortfall of significant LP most readily explained by external depolarisation in thermal Faraday screenexternal depolarisation in thermal Faraday screen ; ; however, this not expected to affect the CP levels however, this not expected to affect the CP levels

▣ ▣ Lack of CP probably due to optical thicknessLack of CP probably due to optical thickness

▣ ▣ 2222 GHz results to follow. Non-detection at 8 and 15GHz results to follow. Non-detection at 8 and 15 GHz GHz does not preclude detection at higher frequenciesdoes not preclude detection at higher frequencies

▣ ▣ Even a clear signal suggesting electronEven a clear signal suggesting electron -- proton composition proton composition on the smallest scales will not detract from the total amount on the smallest scales will not detract from the total amount of baryon mass (derived by Wykes+13 and Wykes+15) of baryon mass (derived by Wykes+13 and Wykes+15) incorporated into the jetincorporated into the jet

Many thanks, Tom! Many thanks, Tom!