Proper Motions of Jets on the Kiloparsec Scale: New Results with HST

3 MB·

by  Eileen T. Meyer

galaxies
Article
Proper Motions of Jets on the Kiloparsec Scale:
New Results with HST
EileenT.Meyer1,*,WilliamB.Sparks2,MarkosGeorganopoulos1,RoelandvanderMarel2,
JayAnderson2,SangmoT.Sohn2,JohnBiretta2,ColinNorman2,MarcoChiaberge2
andEricPerlman3
1 DepartmentofPhysics,UniversityofMaryland,BaltimoreCounty,Baltimore,MD20742,USA;
[email protected]
2 SpaceTelescopeScienceInstitute,Baltimore,MD21218,USA;[email protected](W.B.S.);[email protected]
7 (R.v.d.M.);[email protected](J.A.);[email protected](S.T.S);[email protected](J.B.);
1 [email protected](C.N.);[email protected](M.C.)
0 3 FloridaInstituteofTechnology,Melbourne,FL32901,USA;eperlman@fit.edu
2
* Correspondence:[email protected]

n
a AcademicEditors:JoseL.Gómez,AlanP.MarscherandSvetlanaG.Jorstad
J Received:13September2016;Accepted:16January2017;Published:date

0
2 Abstract: TheHubbleSpaceTelescoperecentlycelebrated25yearsofoperation. Someofthefirst
imagesofextragalacticopticaljetsweretakenbyHSTinthemid-1990s;withtimebaselinesonthe

]
A orderof20yearsandstate-of-the-artastrometrytechniques, wearenowabletoreachaccuracies
G inproper-motionmeasurementsontheorderofatenthofamilliarcsecondperyear. Wepresent
the results of a recent HST program to measure the kiloparsec-scale proper motions of eleven
.
h
nearbyopticaljetswithHubble,thefirstsampleofitskind. WhenpairedwithVLBIproper-motion
p
- measurementsontheparsecscale,wearenowabletomapthefullvelocityprofileofthesejetsfrom
o
near the black hole to the final deceleration as they extend out into and beyond the host galaxy.
r
t Weseeconvincingevidencethatweak-flavorjets(i.e.,FRIs)haveaslowlyincreasingjetspeedupto
s
a 100pcfromthecore,wheresuperluminalcomponentsarefirstseen.
[

1 Keywords: propermotions;extragalacticjets;HubbleSpaceTelescope
v
6
4
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5 1. Introduction
0
1. Many aspects of the physics of relativistic jets from super-massive black holes are not well
0 understood, including the particle makeup of the jet, their lifetimes, and the speed profile of the
7
plasmaasitextendsoutofthehostgalaxyandintotheintergalacticmedium. Proper-motionstudies
1
: allowustodirectlyobservetheapparentspeedsofthesejets,resultinginconstraintsontheirLorentz
v
factors(Γ). Hundredsofobservationsofjetswithverylongbaselineinterferometry(VLBI)intheradio
i
X
havedetectedpropermotionsofjetsonparsecandsub-parsecscales,relativelyneartotheblackhole
r engine(e.g.,[1–5]). Becausethesejetsarehighlyrelativistic,apparentsuperluminalmotionscanresult
a
fromvelocitiesnearthespeedoflightcoupledwithrelativelysmallviewingangles. Westartwith
somedefinitions: thedimensionlessobservedapparentvelocity β isrelatedtotherealvelocity
app
β = v/c(wherecisthespeedoflight)andviewingangleθthroughthewell-knownDopplerformula
β = βsinθ/(1−βcosθ). Ameasurementofβ impliesbothalowerlimitontheLorentzfactor
app app
(Γ ≈ β )andanupperlimitontheviewingangle. Theseconstraintsaredifficulttoderiveusing
min app
othermeanssuchasspectralfitting,duetotheinherentdegeneracybetweenintrinsicpower,angle,
andspeedintroducedbyDopplerboostingoftheobservedflux.
Whiletherearelargesamplesofproper-motionmeasurementsforjetsonparsecscales,direct
observationsofjetmotionsonmuchlargerscales(kpc–Mpc)withhigh-resolutiontelescopessuch
as the Very Large Array (VLA) or Hubble Space Telescope (HST) are rare. However, the much
Galaxies2017,5,8;doi:10.3390/galaxies5010008 www.mdpi.com/journal/galaxies
Galaxies2017,5,8 2of9
lower-than-VLBI resolution necessarily also limits potential observations of apparent motions to
sourcesintheverylocalUniverse,andoftenrequireyearsorevendecadesofrepeatedobservations.
For many years, there were only two measured proper motions of jets on kpc scales, both taken
with the VLA. These were the famous result of β up to 6c measured by [6] for the jet in M87
app
(z=0.004,d=22Mpc),andaspeedof≈4cforaknotinthejetin3C120(z=0.033,d=130Mpc)by[7],
thoughthiswaslatercontradictedbyadditionalVLAandMerlinobservations[8,9]. In1999,thefirst
measurementofpropermotionsintheopticalwasaccomplishedby[10], usingfouryearsofHST
FaintObjectCamera(FOC)imagingtoconfirmthefastsuperluminalspeedsintheinnerjetofM87.
Inthispaper,wepresentsomerecentproper-motionsresultsforthreenearbyopticaljetsobtained
fromarchivalandnewHSTdata,enabledinmajorpartbythelongoperatinglifetimeofHSTandthe
resulting20-yearbaselinesfromtheoriginalsnapshotimagesofopticaljetsfirsttakeninthe1990s.
2. Methods
All three optical jets were observed with several of the Hubble Space Telescope imaging
instruments over a span of 10–20 years. The analysis of the new and archival data was similar
forallthreejets. WewillpresentfirstthemethodsforthejetinM87,andthenbrieflyremarkonthe
data used for 3C 264 and 3C 273, which was treated very similarly. The data used for all jets and
furtherdetailscanbefoundin[11–13].
2.1. M87
2.1.1. Astrometry
ForM87,thereferenceframeisbasedonhundredsofbrightglobularclustersassociatedwith
the host galaxy which are effectively stationary to proper motions over the time of this study.
ThereferenceframewasbuiltusingtheAdvancedCameraforSurveysWideFieldCamera(ACS/WFC)
exposures in the F606W filter, by first detecting the positions of the globular clusters in each
flat-fielded, CTE-corrected ACS/WFC image using a PSF-peak-fitting routine, then applying the
standard (filter-specific) geometric correction to those positions, and then finding the best linear
transformationforeachimagewhichmatchesthepositionsintheindividual(geometrically-corrected)
framestoamasterreferenceframe.ThislaststepisdonebyaroutinesimilartoMultiDrizzle,butbetter
optimizedforastrometry(see[14,15]). Theprocessoffindingtransformationsforallframesisiterated
sothatthemasterreferenceframe(super-sampledtoapixelscaleof0.025(cid:48)(cid:48)/pixel)effectivelygivesthe
averagepositionofeachreferencesourceinageometrically-correctedframe.
Thefinalreferencesystemconsistsofpositionsandmagnitudesforover1300globularclusters
within ∼100(cid:48)(cid:48) of the M87 core position. Among all clusters, the median one-dimensional RMS
residualrelativetothemeanpositionwas0.05(reference-frame)pixels(1.25mas),correspondingtoa
√
systematicastrometricaccuracy(×1/ 56)of0.17mas;overthe13.25yearbaselinethisisequivalent
to0.003c. AstrometricsolutionswerethenfoundforalltheACSHighResolutionCamera(ACS/HRC),
ACS/WFC,andWideFieldPlanetaryCamera2(WFPC2)imagesintheF814Wfilter. Typicalnumbers
ofglobularclustersusedtomatchtheframeswere∼200–500forACS/WFC,∼15–30forWFPC2images,
and∼15–25forACS/HRC;correspondingsystematicerrorswere∼0.006,∼0.03,and∼0.05pixels,
or0.003c,0.015c,0.025cover13.25years,respectively.
2.1.2. MeasuringtheKnotPositions
Within the M87 jet, we first identified the significant features (peaks) of interest using
stacked images. The image stacks include the 2006 ACS/WFC stack, four WFPC2 stacks
(epochs 1995–1996, 1998–1999, 2001, and 2007–2008), and three stacks of ACS/HRC exposures
(November 2002–December 2003, July 2004–September 2005, November 2005–November 2006).
The host galaxy was modeled using the ACS/WFC stacked image with the IRAF/STSDAS tasks
ellipseandbmodelandthensubtracted. Atwo-dimensionalcontinuousfunctionalrepresentationof
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theimagecutoutswasthencreatedusingtheCosineTransformfunction(FourierDCT)inMathematica,
whichallowedustofindprominentpeaks,aswellascontoursofconstantfluxlevelaroundthosepeaks.
Ingeneral,thenumberofinterestingpeakswaschosenbyhand,andthecontourlinelevelswereat50%
ofthefluxofthepeak,aftera“background”levelwassubtracted. Thepositionsdefiningthecontour
werereverse-transformedfrommasterframecoordinatesintoeachdistorted,galaxy-subtractedimage
sothatanintensity-weightedcentroidpositionfromthepixelswithinthecontourcouldbecalculated.
Thesefinalpositionswerethentransformedbackintothecommonreferenceframe,sothateachpeak
ofinterestwasmeasuredineveryexposure,resultinginover400positionmeasurementsspreadover
the13yearbaseline.
Tomeasuretheapparentmotionofthejetcomponentsovertime,wedefinethex-directionasthe
linefromthecorethroughthecenterofknotI(PAof290◦),withy-directionperpendicular. Datawere
binnedinto26timebins(shownontherightinFigure2),andalinearmodelwasfitusingstandard
weightedleastsquares. Theweightsweretakentobetheinverseofthevarianceforeachtimebin,
asmeasuredfrom10nearbyglobularclusterreferencesources.
2.2. 3C264
WeobtaineddeepV-bandimagingof3C264withtheACSinMayof2014inordertocompare
nearly20yearsofimagestakenbyHSTforevidenceofpropermotionsoftheseknots. Inaverysimilar
approachaswithM87,welocalizedover100globularclustersinthehostgalaxyasareferencesystem
onwhichtoregisterpreviousimagestakenwithWFPC2in1994,1996,and2002. Thesystematicerror
intheregistrationoftheWFPC2imagesisgenerallyontheorderof5milliarcseconds(mas)orless.
Tomeasuretheirapparentspeeds,thepositionofeachknotwasmeasuredwithacentroiding
technique. InthecaseofknotsBandC,wealsomodelledthejetasaconstant-densityconicaljetwith
superimposedresolvedknots,inordertobettermeasuretheirfluxesandpositions,particularlyinthe
finalepochwhentheyappeartooverlap.Weplottedthepositionofeachknotalongthedirectionofthe
jetaxisversustime,andfittedthedatawithaleast-squareslinearmodel. Aslopesignificantlylarger
thanzeroindicatessignificantpropermotions,andweusedtheconversionfactor1.442cyear/masto
convertangularspeeds(µ )tounitsofc.
app
2.3. 3C273
WeobtainedfourorbitsofACS/WFCimaginginF606WinMayof2014. Thesewerestackedinto
ameanreferenceimage(withcosmic-rayrejection)onasuper-sampledscalewith0.025(cid:48)(cid:48) pixels. The
registrationoftheeightindividualexposuresutilizedafull6-parameterlineartransformationbased
onthedistortion-correctedpositionsof15–20point-likesources. Themedianone-dimensionalrms
residualrelativetothemeanpositionwas0.07reference-framepixels,or1.75mas,correspondingtoa
√
systematicerrorontheregistration(×1/ 16)of0.44mas,orabouttwohundredthsofapixel.
2.3.1. BackgroundSourceRegistration
UnlikewithM87and3C264,theglobularclustersof3C273arenotdetectableintheolderWFPC2
imagingandthuscannotbeusedasareferenceframe. Instead,weidentified15backgroundgalaxies
basedonthecriteriathattheycanbeseenbyeyeabovethenoiseintheindividualWFPC2exposures.
ThesereferencegalaxiesarehighlightedinFigure1. Notethatgalaxies4,5,and6havebeenpreviously
identifiedasunrelatedtothejetbytheirlackofradioemission,andthebrightpointsourcenearthejet
isactuallyaforegroundstar(andthusunsuitableforregisteringimagesduetolikelypropermotions).
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Figure1.Theimmediateenvironmentof3C273asseenintheACS/WFCreferenceimagefrom2014.
Thebackgroundgalaxiesusedforimageregistrationareboxed.Thelargeroutlineisapproximately
theimagefootprintoftheearlier1995WFPC2imaging. (NotethattheimageisinthenativeCCD
orientation,andNorthisapproximatelytotheright.)
To match the archival images to a common reference frame, our general strategy was to use
the shape and light distribution of the galaxy to assist in matching their locations in each image.
Instead of identifying a single location associated with each galaxy in the deep reference image,
weinsteadsamplethegalaxyinagridpattern,resultinginalistofpositionsalongwiththefluxat
eachpoint,samplingacrosseachgalaxy. Aninitialastrometrictransformationsolutionwasfoundby
supplying≈10pairsofmatchedlocationsfoundbyhandbetweenthegeometrically-correctedraw
imageandtheACSreferenceimageandcalculatingthesixtransformationparameters(withoutmatch
evaluation/rejection). Theinitialtransformationsolutiondescribedaboveisusedasastartingpoint
to transform the x,y locations for each galaxy grid in the reference frame into x ,y location in
gc gc
the geometrically corrected image. The intensity can then be sampled at each location in the raw
image,tobecompareddirectlytothescaledcountsvaluepredictedbythescaledreferencepixelvalue.
Thisinformationisusedtocalculatetheoptimalpositionshiftwhichisusedtoupdatethelocationof
thegalaxyinthereferenceframe.
For each individual exposure, we then compile an updated list of position matches between
thereferenceframeandtheGCimagefromthemeanx,yvalueofthegalaxysamplinggridineach.
Ingeneral,weusedasubsetofthebackgroundgalaxieswhichwereidentifiablebyeyeandnotoverly
affectedbycosmicrayhits. Theprocessoffindingtheinitialx ,y values,followedbyfindingthe
gc gc
optimal δx,δy improvement on the mean position, was iterated until the positions of the galaxies
stoppedimproving.
2.3.2. MeasuringKnotPositionsin3C273
Asaconsistencycheck,wealsomeasuredtheshiftofeachknotusingasecondmethodwhichwe
refertoasthe“cross-correlation”method. Overagridwithsub-pixelspacingof0.2super-sampled
pixels(5mas),weshiftedthe1995and2003imagesofeachindividualknotrelativetothe2014image
(samecutoutarea)overa6 × 6pixelarea, evaluatingthesumofthesquareddifferencesbetween
interpolatedfluxovertheknotareaforeachx/yshiftcombination. Theresultingsum-of-squared
differences image in all cases clearly shows a smooth “depression” feature which is reasonably
well-fitbyatwo-dimensionalGaussianunderthetransformationg =1− f/max(f),where f isthe
originalsumofsquareddifferences. Takingtheminimum f locationasmeasuredbythepeakofthe
two-dimensionalGaussianfit,wemeasuretheoptimalshiftforeachknot.
To measure the approximate error on the positions measured, we repeated both of the above
methodsforsimulatedimagesofthejetateachepoch. Thesimulatedimageswerecreatedbytaking
the deep 2014 ACS image and adding a Gaussian noise component appropriately scaled from the
countsintheoriginalWFPC2exposures. Sincethe2014imageitselfhassomenoise,andalsoaslightly
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differentPSFfromtheWFPC2images,thismethodlikelyslightlyoverestimatestheerrors. Wetakethe
erroroneachknotmeasurementtobethestandarddeviationofthemeasurementsinthesimulated
images(10ineachepoch).
Finally,weplottedthepositionofeachfeaturerelativetothe2014position,versustime,tolook
forevidenceofpropermotions. Wehavetransformedfromthecoordinateframeofthealignedimages
(Northup)toonebasedonthejet,wherepositivexisintheoutflowdirectionalongthejet(takenas
positionangle(PA)42◦ southofeast)andpositiveyisorthogonalandtothenorthofthejet. Forboth
methods,theestimatederroronthemeasurementhasbeenconvolvedwiththesystematicerrorof
theregistration,whichis0.18,0.22,and0.02referencepixels(4.5,2.8,and0.5mas)forthe1995,2003,
and2014epochs,respectively.
3. ResultsandDiscussion
3.1. M87
ThemostnotablefindingsforM87werethefindingsofbothdecelerationandtransversemotions
for the first time. In Figure 2, we show the knot complex D, which consists of three sub-knots.
KnotDisthemostconsistentlybrightfeatureaftertheknotA/B/Ccomplex,andsignificantproper
motions were measured in previous studies. As suggested by the contours overlaid on Figure 2,
D-Middleisoneofthefastestcomponentswithaspeedof4.27±0.30calongthejet,whilethebrighter
D-West shows evidence of deceleration radially, slowing to a near stop by the final epoch in 2008,
while maintaining one of the largest transverse speeds of −0.59 ± 0.05c (right panel of Figure 2).
PreviousresultsonknotD-Easthavebeenconflicting: B95foundthatitmovedinwardalongthejetat
0.23±0.12c(possiblyconsistentwithbeingstationary),whileB99foundalargeoutwardapparent
velocity of 3.12 ± 0.29c; our result of 0.28 ± 0.05 is more in line with B95. It is possible that there
isastationaryfeatureatD-East,throughwhichcomponentsemerge(analogoustowhatisseenin
HST-1). Inthatcase,thehigher-resolutionFOCwasperhapstrackinganemergingbrightcomponent,
whileoverlongerperiodstheglobalfeatureatD-Eastisstationary. Intheouterjet(fromknotA),
wefindapparentvelocitiesthatarestillsuperluminalandvelocityvectorsthatappeartolineupintoa
helical/side-to-sidepattern.
Figure2. SomeresultsfromtheM87study. Atupperleft,acloseviewofknotDcomplexoverfour
epochsfrom1995to2008. Asshown,theEasterncomponent(farleft)extendsbutdoesnotchange
thecorelocation,whilethetwosub-knotstotherightshowbothradialmotionanddeceleration.On
theright,thepositionversustimeoftheWesternmostcomponentofknotD.Inthelowerpanel,we
showthesuperimposedvelocityvectorsoneachcomponentintheM87jet.Lengthcorrespondstothe
magnitudeofthetotalspeed.
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On the left in Figure 3, we show the complete radial and transverse velocity field of M87 as
a function of distance along the jet, including previous work covering similar scales and radio
interferometryresults.
Figure3.ApparentspeedversusdistanceforM87(left)and3C273(right).ForM87,themostrecent
HSTresultsareindarkblue. Notethefastriseinspeedsonscaleslessthan100parsecs,reachinga
peakatHST-1beforegraduallyfallingoff.Inthecaseof3C264,ourresultssuggestagreatsimilarityto
thevelocitystructureseeninM87.Lowerpanelshowsthesignificanttransversevelocitiesdetectedin
ourstudy. Ontheright,wecomparetheVLBIscalemotionsfor3C273(left-half)fromtheMOJAVE
programtotheHSTpropermotions(right-half)onkpcscales(notethebreakinthedisplayedrangeat
thegreyverticalbar).
3.2. 3C264
Afteraligningallimagestoacommonreferenceframe,thefastpropermotionofknotBisclearly
visible,asshowninFigure4. WefoundthatknotsAandDhaveaβ consistentwithzero,whilethe
app
innerknotsBandChaveβ =7.0±0.8cand1.8±0.5c,respectively(Figure2). Thevalueforknot
app
BexceedsthefastestspeedsmeasuredinthejetinM87,theonlyothersourceforwhichspeedson
kpcscaleshavebeenmeasured[10,11]. ThedifferenceinspeedsbetweenknotsBandCputsthem
onacollisioncourse,aninteractionwhichhasalreadybeguninthefinalepochfrom2014,wherethe
knotsbegintooverlap(Figure2). Thisappearstobethefirstdirectobservationofcollidingknotsinan
extragalacticjet.
Our data show that in the final 2014 epoch, both knots B and C brighten at the same time by
approximately 40% over the mean flux level of the previous three epochs (Figure 4, right). This
suggeststhatfreshparticleaccelerationhasrecentlytakenplace,whichweattributetotheongoing
collisionbetweenthesecomponents. Underequipartition,thecoolinglengthfortheoptical-emitting
electronsforknotsBandCislongerthanthedistancetheytravelledinourobservations,consistent
withthelackofanydecayinfluxlevelsfortheseknotsoverthefirstthreeepochs. Thisisnotthe
caseforstationaryknotA,whichcanbeseentodecaywithatimescaleof∼70years. KnotAappears
analogoustoknotHST-1intheM87jet;thelatteristhoughttobeastationaryreconfinementshock
wherethejetpressuredropsbelowthatoftheexternalenvironment[16]. Theeventthatenergized
knotAmayhavebeenthepassageoffast-movingknotBcirca1971+8 , comparabletotheknotA
−17
decaytime.
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RadiodatatakeninOctober1983withtheVLAsuggestthatknotsBandCweremovingfaster
inthepastandmayhavedecelerated. InthecaseofknotB,aquadraticfittotheHST+VLAdata
suggeststhatinlate1983,theknothadaspeedof10.2c,slowingby0.16c/yeartoreach5.6catthe
beginningof2014. WhileaquadraticfitisverypoorforthecombineddataonknotC,alinearfit
betweenthe1983and1994epochssimilarlysuggestsaspeedof9.8c. Itappearsthatamuchmore
distinctdeceleration“event”occurredforknotC.
Intheinternalshockmodel,theefficiency(η)oftheconversionoftheliberatedkineticenergy
(E )intoradiationisgenerallyunknown,withvaluesassumedintheliteratureinordertomatch
kin
thesourceluminosity. Forthecollisionin3C264,wecanestimatetheenergyavailableforconversion
fromourobservationstobeatleast10−3,subjecttouncertaintiesinthetotalcoolingtime,andthe
futureevolutionoftheobservedflux.
3.3. 3C273
Inthecaseof3C273, allknotshavespeedsconsistentwithzerowithtypical1σ errorsonthe
order of 0.1−0.2 mas/year or 1.5c, and with 99% upper limit values ranging from 1−5c. Nearby
backgroundgalaxies,usedasacontrolcheck,showthattheselimitsareconsistentwithstationary
objectsinthesamefield(seeFigure3,right).
Theseresultssuggestthattheknotsinthekpc-scalejet,iftheyaremovingpacketsofplasma,
must be relatively slow, in agreement with previous studies based on jet-to-counterjet ratios in
radio-loudpopulations[17,18].Assumingthatthejeteitherremainsatthesamespeedordeceleratesas
youmovedownstream,the2σupperlimitspeedderivedfromallknotscombinedof1csuggeststhat
theentireopticaljetisatmostmildlyrelativistic,withamaximumLorentzfactorofΓ <2.9. However,
wecannotruleoutthepossibilitythattheknotsarestandingshockfeaturesintheflow,wherethebulk
plasmamovesthroughthefeatureswithahigherΓ. Thebestlimitsonthebulkplasmaspeedthus
remainthelimitsderivedfromthenon-detectionoftheIC/CMBcomponentingamma-raysby[19],
whereδ <7.8isimpliedassumingequipartitionmagneticfields.
Figure4. Resultsfor3C264. Previousradioobservationshaverevealedthattheinitiallynarrowly
collimatedjetbendsby∼10◦atthelocationmarkedbytheyellowcross[20],whichappearstoalign
wellwiththecentralaxisofthejetinourimaging,andwhichservesasourreferencepointforall
measured positions. On the left, the fast motion of knot B and subsequent collision knot C and
brighteningcanbeclearlyseenoverthe20yearstudy.Ontheright,weshowtheincreasingfluxofthe
twocollidingknots.
Galaxies2017,5,8 8of9
Finally,weshowthattheobservedupperlimitsonthepropermotionoftheknotsconfirmsthatthe
anear-equipartitionIC/CMBmodelfortheX-raysofthekpc-scaleknotsisruledout.Theequipartition
IC/CMBmodelrequiresthattheknotsareballisticpacketsofmovingplasmamovingatthebulkspeed
Γ ≈15−20whichwouldimplypropermotionsontheorderof10cor1.12mas/yearwhichcouldhave
beendetectedinourstudy;ourupperlimitseasilyrulethisoutatahighlevelofsignificance(>5σ).
Moving away from equipartition conditions, an IC/CMB model consistent with our observations
requiresajetpowerontheorderoffivetoseveralhundredtimestheEddingtonlimit,andisthus
energeticallydisfavored.
In comparison to other recent HST observations of lower-power optical jets M87 and 3C264,
wherehighlysuperluminalspeeds(6−7c)havebeenobservedintheopticalkpc-scalejet,ourfirst
proper-motionstudyofapowerfulquasarjetrevealsnosignificantpropermotions. Itremainstobe
seenwhetherthisisbecausethejethastrulydeceleratedandisonlymildlyrelativistic,orbecause
theknotfeaturesinsourcessuchasM87/3C264and3C273representverydifferentthings: moving
packetsofplasmainthefirstinstanceandstandingshocksinthesecond.
Acknowledgments:EileenT.MeyeracknowledgesHSTgrantsAR-11283,GO-12271,AR-12635,GO-13327
AuthorContributions:EileenT.Meyerreducedthedataandwrotethearticle. JayAnderson,TonySohn,and
RoelandvanderMarelassistedwiththegeometriccorrectionandastronomyportionoftheanalysis.Allauthors
contributedtotheinterpretationandhavereadandcommentedonthemanuscript.
ConflictsofInterest:Theauthorsdeclarenoconflictofinterest.
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