Red synchrotron jets in Parkes quasars

16 Pages·0.49 MB·

by  Matthew T. Whiting

Mon.Not.R.Astron.Soc.000,000–000 (0000) Printed1February2008 (MNLATEXstylefilev1.4)
Red synchrotron jets in Parkes quasars
M.T.Whiting1, R.L.Webster1 and P.J.Francis2,3
1 Astrophysics group, School of Physics, Universityof Melbourne, Victoria 3010, Australia
2 Research School of Astronomy and Astrophysics, Australian National University,Canberra, ACT 0200, Australia
3 Joint Appointment withthe Department of Physicsand Theoretical Physics, Faculty of Science
1February2008
1
0
0
ABSTRACT
2
WepresentmodelfitstospectralenergydistributionsintheopticalandNIRof>100

n flat-spectrum radio quasars from the Parkes Half-Jansky Flat-spectrum Sample. We
a find that ∼ 40% of the sources have power-law SEDs, while a similar number show
J evidencefortwoprimarycomponents:abluepowerlawandopticalsynchrotronemis-

9 sion.Thebluepowerlawissimilartothedominantcomponentobservedinthespectra
2 of optically-selected QSOs. There is strong evidence that the synchrotron component
hasaturnoverinthe UV-opticalrestframeofthe spectrum.Inthe remainingsources
1
it is likely that the synchrotron peaks at longer wavelengths. This mixture of two
v
components is supported by optical polarisation measurements in a subgroup of the
2
sources.The sourceswith power lawSEDs showevidence for an excessnumber of red
0
powerlawslopescomparedtooptically-selectedquasars.Thereareadditionalspectral
5
1 componentsinsomeofthesources,suchasdustandtheunderlyinggalaxy,whichhave
0 not been considered here.
1
Keywords: quasars:general–BLLacertaeobjects:general–radiationmechanisms:
0
non-thermal
/
h
p
-
o
r 1 INTRODUCTION NIRwavelengthscomesfromboththevariabilityintheNIR,
t
s andfrom thestrongpolarisation of afew sources.Stickelet
a The spectral energy distributions of Active Galactic Nuclei al.(1996)foundthatvariabilitywasageneralcharacteristic
v: provideasum of theemission processes contributingto the ofoptically faint flat-spectrumradiosources, althoughthey
i energyoutputoftheAGN.Clearly,severalmechanismscon- did not specifically discuss synchrotron emission. Instead,
X
tributeatmostfrequencies.Ifwecandeterminethedifferent they suggest that reddening due to intervening galaxies or
r components as a function of frequency, then we can deter- thehost galaxy itself was thelikely cause of thered optical
a
mine the energy generation mechanisms which are impor- colours.
tant.
While radio jets are quite common, and have been
Flat-spectrumradioquasarsareknowntobedominated widelyobserved,opticalcounterpartstothesejetsarequite
intheradiospectrumbysynchrotronemission.Itislessclear rare. In fact, only 14 optical jets are currently known (see
how far these synchrotron spectra extend towards higher O’Deaetal.(1999)andreferencestherein).Thefeatures(i.e.
frequencies,orthemagnitudeofthecontributiontheymake emissionknots)intheopticaljetsmatchthepositionsoffea-
to the optical and near-infrared (NIR) spectrum – a region tures in the radio jets, indicating that the emission regions
dominated bythe Big Blue Bump. andmechanismsforthetwospectralregimesareassociated.
Thefar-IRemissionofflat-spectrumquasarsappearsto Optical jets can be used to put strong constraints on the
be dominated by synchrotron emission (Haas et al. 1998), particleenergetics,byprovidinglimitsonthemaximumen-
consistent with aDoppler-boosted component that swamps ergies,andontheaccelerationmechanisms,duetotheshort
the expected emission from dust components. Synchrotron lifetimes of particles at these energies (Meisenheimer et al.
emission in the optical has been considered for many years 1996).
tobenecessarytoexplaintheopticaltoNIRphotometryof The quasars considered in this paper are selected
opticallyfaintandredflat-spectrumradioquasars.Thishas fromtheParkesHalf-JanskyFlat-spectrumSample(PHFS)
mostlybeenonthebasisofthesteepnessoftheoptical–NIR (Drinkwater et al. 1997), which consists of 323 objects se-
spectra(Riekeetal.1979; Beichmann 1981;Bregman 1981; lected to be radio-loud (S >0.5 Jy), and have a flat
2.7 GHz
Riekeetal.1982).Thesteepnessofthesespectrasuggesteda radiospectrum(α > 0.5,S(ν) να).Thesequasars
2.7/5.0 − ∝
cutoff,orsharpbreak,intheelectronenergyspectrum.Sup- have been shown (Webster et al. 1995; Francis et al. 2000)
portingevidenceforthepresenceofsynchrotronemissionat to have a large spread in B K colours, with the reddest
−
(cid:13)c 0000RAS
2 M.T.Whiting, R.L.Webster and P.J.Francis
objectshavingB K >7.Masci,Webster&Francis(1998) redshift galaxies, which had a prominent 4000˚A break be-
−
showed that this spread could not be accounted for by the tween the B and V bands. These sources show strong ev-
emission from the host-galaxy. idence for flux from the underlying galaxy in their spectra
In this paper we test the idea that optical synchrotron (Masci et al. 1998), and thus need an additional galactic
emission causes the red optical-NIR colours of the PHFS component to be modelled accurately. Sources with z > 3
quasars. Models representing emission from both an opti- were also excluded,as these had strong Lyαbreaks present
cal synchrotron component and a blue optical power law betweenB andV.Thisreducedthenumberofsourcesfrom
(representing the continuum emission from an unreddened 157 to117.
quasar) arefittedtobroad-bandoptical-NIR spectra.Stan- Twenty-one of the reddest sources (B K >5) in the
−
dard goodness-of-fit techniquesare used todetermine mod- subsample were amongst those excluded, as they have only
els which are consistent with the observations. upper limits on B, V and possibly R, or no redshift. The
Thedataset wascompiled byFrancis,Whiting&Web- sources with upper limits are both red and optically faint.
ster(2000)(FWWhereafter),andcomprisesbroadbandop- Sincethefluxofthesesourcestypicallydecreasesrapidlyin
ticalandNIRphotometryinthebandsB,V,R,I,J,H and the blue (i.e. from V to B), there are three possible expla-
K. This photometry is quasi-simultaneous, meaning all ob- nations for them. They are either dust-reddened (causing
servations for agiven source weremade within several days thebluedecreaseinflux),highredshift(andhenceLyαab-
(six at most) of one another. This minimises the effects of sorbed), or they are dominated by synchrotron that turns
source variability. The source selection used for this paper overrapidly.Futureanalysisofthespectralenergydistribu-
is explained in Section 2. The simultaneity, as well as the tions (SEDs) of these sources will only strengthen our final
breadthand densityof thespectral coverage of thisdataset conclusions.
providesanexcellentbasistomodelthebroadbandemission Those sources excluded purely because of their lack of
from a large numberof flat-spectrum quasars. measured redshift generally have high B K colours (B
− −
The dataset is described in Section 2. In Section 3 we K > 5), since the most likely reason they have no redshift
test thehypothesisthattheemission in theoptical-NIR re- isthattheyarefaintintheoptical(andinparticularinthe
gion is well-fitted by a single power law. In Section 4 we blue). Further discussion of the sources excluded from the
describethemoresophisticatedmodelthatwasfittedtothe sample is given in Section 10.
data–representingtheaccretiondiskandsynchrotronemis-
sion–andthemethodthatwasusedtoperformthefitting.
The results of this fitting are described in Sections 5 & 6, 2.2 Errors on photometry
while further tests of the model fits are examined in Sec-
ThephotometrygiveninFWWquotederrorbars,wherethe
tion 7, using polarisation and emission line measurements.
estimated error comprised two parts: a random error com-
Apossiblerivalmodel–thatofblack-bodyemission byhot
ponent and an assumed 5% error in the photometric zero
dust–isconsideredinSection8.Theeffectofemissionlines
points, which were added together in quadrature. The pho-
on the photometry and the resulting fits is investigated in
tometric zero point errors were estimated from the scatter
Section 9. Finally, Section 10 contains discussion of the re-
inzeropointsbetweendifferentstandardstarmeasurements
sults and their implications.
in an individual night: Francis et al. adopted a value of 5%
to account for this error.
However, thiszero point error ignores a numberof fac-
2 DATA
tors that we believe may be important for the analysis in
thispaper. These factors are as follows:
The data which have been fitted by these models are de-
scribed in detail in FWW. A total of 157 sources from the
The optical zero magnitude fluxes were taken from
PHFS were observed, with quasi-simultaneous broadband •
Bessell,Castelli&Plez(1998).Thesefluxeswerederivedfor
photometryobservationsinthebandsB,V,R,I,J,H and
anA0star,andsowillbeslightlyincorrectforaquasarspec-
K.Thesemagnitudeswereconvertedintobroadbandfluxes
trum. This has the effect of introducing small colour terms
using thezero points given in the same paper.
into the photometry, the size of which will depend on the
spectralindexof theobject beingobserved.Asimilar effect
will of course be present in the NIR.Bersanelli, Bouchet &
2.1 Excluded sources
Falomo (1991) found that spectral shape differences could
Not all sources in FWW were used. Firstly, we only con- produce systematic errors of at least a few percent in the
sidered those sources with complete contemporaneous pho- NIRflux.
tometry in all bands, or those with only one observation The zero point fluxes in the optical are taken from
•
missing (either not observed or an upper limit). This was a different reference (Bessell et al. 1998) to those in the
to ensure that the number of degrees of freedom in the NIR,whichwerecalculated byP.McGregor (CASPIRman-
modelfittingwasgreaterthanzero.Thosesourceswithmore ual,MSSSO,ANU),assumingthatVegaiswellrepresented
than one band missing were not included in the analysis. in the NIR by a black body of temperature 11200 K, and
Sources without a measured redshift were also excluded, normalisation F (555nm) = 3.44 10−12W cm−2 µm−1
λ
×
since the redshift is needed to obtain the correct shape (Bersanelli etal.1991). Thesedifferentzeropointsmaynot
of the observed synchrotron spectrum. One of the sources be exactly equivalent, which will produce small offsets be-
that had an unknown redshift (z = 0) in Drinkwater et al. tween the optical and NIRparts of the SED.
(1997),0829+046,hasapublishedredshiftvalueofz=0.18 IftheskyconditionsatSidingSpringObservatorywere
•
(Falomo 1991), which is used here. Also excluded were low not completely photometric for all observations for a given
(cid:13)c 0000RAS,MNRAS000,000–000
Red synchrotron jets in Parkes quasars 3
source(particularlyifthetransparencychangedbetweendif-
ferentbands),thenthemeasuredphotometrywillhavesmall
band-to-banderrors present.
The observations for each source were taken quasi-
•
simultaneously (meaningall observationsweremadewithin
atmostasix-dayperiod),tominimisetheeffectsofvariabil-
ity.However,anumberofthesourcesinthePHFShavebeen
foundtoexhibitintra-dayvariabilityintheoptical(Romero
et al. 1999; Heidt & Wagner1996), and a larger numberno
doubthavesimilarpropertiestothese.Therefore,variability
on timescales of the order of those separating our observa-
tions is likely for some of the sources. Such variability can
beup to 0.1 mag over theperiod of a night.
Finally, the presence of strong emission lines in the
•
quasars’ spectra could boost the flux of a band above the
levelofthecontinuum.Wehavetriedtoquantifythiseffect
in Section 9, although this is not possible to do for many
sources,duetothelackofbothphotometryandaspectrum.
Some additional effects may be due to line blends (such as
Feiiblends), but this is probably not as strong an effect as
you would find in a radio-quiet sample. Figure 1. Histogram of fitted power law indices. Hatched his-
togram indicates good fits, while open histogram indicates all
Hence, to take account of all these factors, we have in- fits.
creasedthesystematicerrorinthephotometryto10%.The
random error is kept at the same level as that presented in
FWW, and is added in quadratureto thesystematic error. distributionofresultingpowerlawindices,bothforthegood
fitsand for all sources, is shown in Fig. 1.
Thespectralindicesofthesegoodfitsspanawiderange
of values. At one extreme there are the sources with rela-
3 POWER LAW MODEL
tively blue SEDs (α . 1.4). These sources are charac-
−
The object of our analysis is to find physical models to ex- terised by their blue continuum, the presence of moderate
plaintheopticalandNIRemission.FWWfoundthatabout tostrongemission lines,andgenerally lowX-rayflux(most
90% of thePHFS haveapproximately powerlaw SEDs.We were not detected byROSAT (Siebert et al. 1998)).
firstwishtotestthismorerigorously.Asastartingpoint,we AttheotherextremearethesourceswithredderSEDs
choose to fit (naively) a simple power law, with an uncon- (that is, flatter in f ), with 1.α. 0. These sources are
λ
−
strained spectral index. This will separate out the sources blazar typeobjects, with high optical polarisation (Wills et
that have power law SEDs from those that show some cur- al.1992)aswellasrelativelyweak(orevenabsent)emission
vaturein theirspectrum. lines – in fact, all the BL Lac objects in our sample are in
thisregion.
3.1 Fits to data
The model we choose to fit is f (λ) = cλα, so that the 3.2 Interpretation of power laws
PL
normalisation c and the spectral index α vary. (Note that
this model implies f ν−2−α.) So,wehavefitapowerlawtoalargemajorityofthesources
ν ∝ in our sample, spanning a wide range of spectral indices.
This model is fit to the data using a least-squares
Arethephysicalprocesses that generatethispower law the
method. This generates a χ2 value:
sameforallsources?Thatis,doesthepowerlawintheblue
7 (y f (λ ))2 sources havethe same origin as that in thered sources?
χ2 = i− σP2L i , Thefirst class of sources –thebluesources –are being
Xi=1 i fit by a blue power law, which has similar colours to the
which indicatesthegoodness of fit.A fittoasourcewill be blue power law emission seen in optically-selected quasars
deemed to be “good” when the value of χ2 is less than the (Francis 1996). This is likely to be the optical part of the
cut-off value corresponding to the 99% confidence level of continuum emission from the accretion disk (often termed
the χ2 distribution. For a source with 5 degrees of freedom thebig bluebump).
(asisthecaseformost sourceswiththispowerlaw model), However,thepowerlawbeingfittothereddersourcesis
this cutoff level is 15.09. (Note that increasing the value of mostlikelyofdifferentorigintothatseeninthebluesources.
this cutoff is equivalent to increasing the confidence level These objects exhibit characteristics commonly associated
– for example, the 99.5% cutoff is 16.75 for 5 degrees of with optical synchrotron emission (such as high optical po-
freedom.) If the χ2 value is greater than this cutoff level, larisation and lack of prominent emission lines), and so we
thenwerejectthenullhypothesisthatthepowerlawmodel postulatethatthisemissionis,atleastinpart,someformof
fitsthe data. synchrotron emission. The slope of the power law can then
When we fit this power law model to thedata, we find beusedtodeterminep,thepowerlaw indexoftheelectron
that 83 sources (or 71% of the total) have good fits. The energydistribution(i.e.definedsuchthatN(E) E−p:see
∝
(cid:13)c 0000RAS,MNRAS000,000–000
4 M.T.Whiting, R.L.Webster and P.J.Francis
Section 4.2). Using the values shown in Fig. 1, we obtain Over the wavelength of our observations, the BBB is
2<p<6 (using 1.5<α<0.5 and p=2α+5). modelledasasimplepowerlaw, f λαB orf ν−2−αB.
λ ν
− ∝ ∝
If synchrotron emission is present in the spectra of at Francis (1996) found the median slope for a subsample of
least some of these quasars, then we can ask the question quasarsfromtheLargeBrightQuasarSurvey(LBQS),taken
“Isthesynchrotroncomponentbestmodelledbythispower from optical/NIR photometry, was f ν−0.35±0.3, and
ν
∝
law?” Thesynchrotron component will bepresent in one of notedthattheobservationswereconsistentwithanintrinsic
twoforms: apower law caused byan unbroken(power law) continuum slope of f ν−0.3 that is reddened by various
ν
∝
electron energy distribution, or a turning-over component amounts of dust. We therefore take our value of α to be
B
caused by a break or a cut-off in the electron energy distri- 1.7. The effects of allowing the value of α to vary are
B
−
bution. (Note that the power law can also be produced by considered in Section 10.
a synchrotron spectrum turning over at higher frequencies
than those observed.)
Bothoftheseformscanbetested.Thepowerlawmodel 4.2 Synchrotron emission
must produce power law indices that are consistent with
slopes of plausible energy distributions. The presence or All the PHFS sources are radio-loud flat-spectrum sources,
otherwise of a turnover can be evaluated by examining the and thus very likely have relativistic jets that emit syn-
sharpest possible turnover (caused by an abrupt cut-off in chrotron radiation, at least at radio frequencies. Could this
theenergydistributionatsomemaximumenergy).Thiswill synchrotron emission extend up to the optical/NIR part of
provide the maximum contrast with the power law, and is thespectrum?Ourpowerlaw fittingfrom theprevioussec-
consistent with modeling done by other authors (Meisen- tion provides circumstantial evidence for this: the redder
heimer et al. 1996, for example). sources in the optical tend to be the ones with higher po-
Many of the sources, while they have power law fits larisation (a good sign of synchrotron emission) and less
that we can not reject at the 99% confidence level, show prominent emission lines (possibly a sign that the emission
evidenceforcurvatureintheirSEDs.Thiscurvaturecanbe lines are being swamped by the presence of a synchrotron
‘n’-shaped (higher in the middle than at either end), ‘u’- component).Thesepiecesofevidenceareinvestigated more
shaped (lower in the middle) or perhaps take the form of deeply in Section 7.
aninflection(e.g.thefluxdecreases,levelsoffanddecreases As discussed above, the synchrotron spectrum could
again). (See Fig. 3 for illustrations of the different types.) take the form of either a power law, from a power law dis-
MostofthesourcesclassedasBLLacobjectsare‘n’-shaped, tribution of electron energies, or a power law with a break
and so we postulate that these sources are dominated by a orturn-over,duetoanelectronenergydistributionthatex-
synchrotron component that is turningover in the optical. hibitsabreakorevenacut-off.Thislattertypeofspectrum
Othersources,however,arebluerintheopticalthanin hasbeenseeninopticalsynchrotronjets(O’Deaetal.1999;
the NIR (i.e. show an inflection, without the turn-up seen Scarpaetal.1999),wheretheopticalspectrumislikeν−1.2–
in ‘u’-shaped sources). This is a possible indication of the ν−3.0,comparedtoaradio–opticalspectrumofν−0.6–ν−1.0.
presenceofexcessemissionintheNIR,inadditiontoablue Additionally, a synchrotron spectrum that has a turn-
power law. We propose that this excess emission is due to over will, when combined with the blue power law, be able
a synchrotron component that has turned over in the NIR, to reproduce an inflection-like SED. Such a spectrum, par-
andsodoesnotdominateintheoptical,wherethedominant ticularlyintheregionoftheturn-over,willalsobequitered,
emissionisinsteadabluepowerlawsimilartothatobserved thusaccounting for the red colours of many of theSEDs.
in thebluest sources.
4.2.1 Analytic modelling
4 PHYSICAL MODELS We consider here synchrotron emission from a population
of electrons with an energy distribution with the form of a
In response to this phenomenological classification, we pro-
power law up to some maximum energy and zero beyond
pose the following physical model. There are two compo-
this (i.e. an energy spectrum with an abrupt cutoff). This
nents in this model: one is a blue power law, representing
canbeexpressedasadistributionoftheLorentzfactorγ of
continuumemission from theaccretion disk region; and the
theradiating electrons:
second is synchrotron emission, representing emission from
the relativistic jet (that we know to be present due to the κγ−pdγ 1 γ γ
flat-spectrum radio emission seen in these objects). N(γ)dγ ≤ ≤ c
∝(0 γ >γc
ThisisconsistentwithmodellingdonebyMeisenheimer
4.1 Accretion Disk emission
et al. (1996) on thejet ofM87. Theyfound that theoverall
We find, from the simple power law fitting, that the bluest synchrotron spectrum of the brightest parts of the jet was
sourceshavepowerlawcontinua.Theslopesofthesepower best described by a spectrum that had a sharp cutoff at
lawsareconsistentwiththembeingthesamecomponentas ν 1015Hz, with an energy distribution of the form of a
c
that seen in optically-selected quasars, commonly termed stra≈ightpowerlawN(γ) γ−2.31,witharatherabrupthigh
∝
the “Big Blue Bump” (BBB). We take this component energy cutoff. We consider the effect of using a power law
to represent the underlying quasar continuum in the UV- synchrotron spectrum instead in Section 10.
optical part of the rest-frame spectrum – that is, the “un- Such a synchrotron spectrum is straightforward to
reddened”quasar continuum. model analytically. We use the “classical” synchrotron
(cid:13)c 0000RAS,MNRAS000,000–000
Red synchrotron jets in Parkes quasars 5
Figure2.Thesynchrotronmodelusedintheanalysis,inbothfλ andνfν units.Bothplotshavebeennormalisedsothatthepeakflux
infλ is1.Notethattheverticalscaleisdifferentinthetwoplots.
model, as first calculated by Schwinger (1949), and as de- We consider here a range of p values from p = 2.0 to
rivedbyanumberofauthors,particularlyPacholczyk(1970) p = 3.0, which gives a range of long-wavelength power law
and Longair (1994), whose derivation we follow. slopesofα = 0.5toα = 1.0.Thisrangecoversthedis-
S S
− −
The single particle luminosity for a radiating electron tribution of radio-to-optical slopes observed in optical syn-
(mass m , charge e, Lorentz factor γ = E/m c2, and pitch chrotronjets(Scarpa&Urry1999).Allowingptovarydoes
e e
angle,oranglebetweentrajectory andmagnetic fielddirec- not significantly alter the results of our analysis – see Sec-
tion, θ) in a uniform magnetic field B is given by tion 10 for further discussion.
We also note here that a value of p > 3 means that
√3e3Bsinθ
L1p(ω)= 8π2ǫ0cme F(x) FtheνFνν−fl(up−x1w)/i2llainncdresaoseνtFowardνs−lo(pn−g3e)r/2w)aavneldentghtihssr(essiunlctes
ν ν
∝ ∝
where ω=2πν is the angular frequency, in the radio flux being severely overestimated by the fitted
synchrotroncomponent,sincetheradioemissionalwayshas
2ωβm
x= e , a lower νFν flux than the optical. (This assumes that the
3γ2eBsinθ
samesynchrotroncomponentisresponsibleforboththeop-
and F(x)is defined in terms of theBessel function K5/3(z) tical andradio emission, which is anassumption commonly
by made, particularly for the modelling of optical synchrotron
∞ jets (Meisenheimer et al. 1996).)
F(x)=x K (z)dz.
5/3
Zx
We are interested in the luminosity of a population of
4.3 Model fitting
particles,soweneedtointegrateL (ω)oversuitabledistri-
1p
butionsofenergiesandpitchangles.Theenergydistribution These two components (the blue power law and the syn-
is that given above, while the pitch angle distribution that chrotroncomponent)arecombinedlinearlytoformamodel
weuseisanisotropicone,wheretheprobabilitydistribution f (λ) = aλ−1.7+bf (λ) that is fit to the data in the
C synch
is p(θ)dθ= 1sinθdθ. Thus,theintegrated luminosity from samewayasthepowerlawmodel(thatis,usingχ2 minimi-
2
such a population is sation).Thereducedχ2 value(thatis,χ2/ν)foreachofthe
two models (combined and power law) are compared, and
L(ω)= √3e3Bκ πsin2θ γcγ−pF(x)dγ dθ the model with the lowest χ2/ν is chosen to be the best fit
16π2ǫ cm
0 e Z0 (cid:18)Z1 (cid:19) model.
An example of such a spectrum is shown in Fig. 2, for Infittingthecombinedmodel, thelocation ofthepeak
γ = 104 and B = 10−4T = 1G (the value of κ has been wavelengthofthesynchrotronspectrum,λ ,wasallowedto
c p
takentobe1).Thepeakfrequencyν dependsonthesetwo vary. This variation was allowed to occur over a range of
c
values, and can be shown by simple arguments (Blandford rest frame wavelengths such thatthecurvatureofthespec-
1990) to be approximated by ν γ2B MHz (where B is trum caused by the turn-over affected the synchrotron flux
c ∼ c
measured in Gauss). in theregion of thedata points (in otherwords, wedid not
Theslope of thepower law tail(at frequencies ν ν ) wanttojustbefittingthepowerlawpartofthesynchrotron
c
≪
is related to the energy power law index by α =(p 5)/2 spectrum).Quantitatively,wetooktheminimumpeakwave-
S
(where f λαS). The energy distribution for the spe−ctrum lengthtobehalfadecadeshorterthantheBband(0.44µm)
inFig.2i∝stakentobeN(γ) γ−2.5 (i.e.p=2.5),givinga shifted to the rest frame. We then considered 20 λ values
p
power law of f λ−1.25. ∝ per decade (evenly spaced in log λ ), up to a maximum
λ ∝ 10 p
(cid:13)c 0000RAS,MNRAS000,000–000
6 M.T.Whiting, R.L.Webster and P.J.Francis
Figure 5. Distributions of the fitted power index α. The open
histogramshowstheresultsofthepowerlawfittoall117sources,
thehatched histogram showswhichsourceshave apower lawfit
accepted at 99% confidence, while the cross-hatched histogram
showswhichofthosesourceshavepowerlawfitsbetterthanthe
combinedfit.
While many sources have been fitted better with the
Figure 4. Histograms of the reduced χ2 values, for all sources
combined model, a large number are still preferentially fit
andforeach ofthe best-fit models.Thedotted histogram shows
with the power law. If we plot a histogram (Fig. 5) of the
thosesourceswhosefitsarerejectedatthe99%confidence level.
powerlawindicesofthosestillfitbythepowerlawmodel,we
can see that those sources that are preferentially fit by the
peak of 10µm. For each of these synchrotron functions, a power law model are the bluer sources, while the majority
best fit to the data was found, and then the best of these of the sources with indices α > 1 are fit better by the
−
was chosen, giving thebest fit λ valuefor that source. combined model.
p
5 RESULTS FOR PHYSICAL MODELS 6 FITTED SYNCHROTRON COMPONENTS
Thiscombinedmodelwasfittedtothephotometry,andcom- Thepropertiesofthesynchrotroncomponentsthatarefitted
paredtothepowerlawfits.Thebestfittingmodelwascho- aspartofthecombinedmodelareofparticularinterest.As
senonthebasisofthelowestreducedχ2 value,asdescribed canbeseeninFig6,thepeakwavelengthsarerestrictedtoa
above.Forthedefaultvaluesoftheparameters(p=2.5and relativelynarrowrangeofvalues(approximatelyadecadein
α = 1.7), we find that 93 sources (or 79% of the total) wavelength).However,thisislikelytobelargelyareflection
B
−
are well fit by one of the models. Of these, 48 are best fit of the distribution of the wavelengths of the photometric
by the power law model, and 45 by the combined model. points.
How these numbers change with different parameter values Thestrengthofthefittedsynchrotroncomponentvaries
isdiscussedinSection10.4.Aselectionofthefitsareshown considerably from source to source. In Fig. 7, the ratio of
in Fig. 3, for a range of power law slopes and synchrotron the synchrotron and power law components at a rest-frame
peak wavelengths. wavelength of 0.5µm is shown for all sources best fit by the
We note here that although there are 48 sources best combined model. The main bulk of this distribution spans
fit by the power law model, many of these have combined nearly four ordersof magnitude. This large range of values,
fitsthatareonlyslightlyworsethanthepowerlawfit.This which is also seen in the normalisations of the individual
indicatesthatthereisnotagreatdealofdifferencebetween components, indicates that we are seeing a continuum of
the fits of the two models. This is not the case with the variationsofthesecomponents,probablyduetovariationsin
combined model sources, as for most of thesethe combined thestrengthsoftheinnerjetandemissionfromtheaccretion
model fit is a lot betterthan the power law fit. disk and/or surrounding regions.
Ahistogramofχ2/ν valuesisshowninFig.4,withthe Wealso notethat asmall numberof thesources at the
distributions for thetwo different models shown separately. high-ratioendofthedistributionarefaint,redsources,that
The distribution for the sources best fit by the combined arelikelytobesignificantlydust-reddened.Theyarethusfit
model is noticeably broader than that for the power law withadominantsynchrotroncomponent,asthesynchrotron
sources, with more sources havingvery low χ2 values. spectrumhas theapproximateform of apowerlaw with an
(cid:13)c 0000RAS,MNRAS000,000–000
Red synchrotron jets in Parkes quasars 7
Figure3.Examplesofthevarioustypesofphotometry,andthefitstothem.Thehorizontalaxisoneachplotiswavelength,inµm,and
theverticalaxisisfλ inWm−2µm−1.Thelocationsofnotableemissionlinesareshown,basedontheredshiftofeachquasar.Forthe
combinedfits,theindividualcomponents–thepowerlaw(αB =−1.7)andthesynchrotroncomponent(p=2.5)–areshownasdotted
lines.The λp value, where given, is for the quasar restframe. A fullcompilation of the fits to all the quasars inthe sample isgiven in
Whiting(2000).
exponential cut-off (which is the same as a power law with ationisitshighdegreeofpolarisation. Ifthereissignificant
dust extinction). synchrotron emission at optical and NIRwavelengths, then
one would expect to be able to detect a corresponding po-
larisation. Indeed, this has been used as a way to confirm
the presence of synchrotron emission in optical jets (Baade
7 TESTING THE FITS
(1956)providedthefirstexampleofthisforthejetofM87.)
7.1 Polarisation
In our combined model, the synchrotron component is
We have shown that a large fraction of the sources in the the only polarised component, as we assume that the BBB
PHFSshowevidenceforthepresenceofopticalsynchrotron component,whichisessentially emission fromtheaccretion
emission,wheretheamountofsynchrotronemissionpresent disk,isunpolarised(P <1%fortheBBB(Antonucci1988)).
in the spectrum changes with wavelength. How else can we Thus,theamountofpolarisationwilldependonthepropor-
testthismodel?Oneofthekeyfeaturesofsynchrotronradi- tionofthetotalfluxthatisduetothesynchrotronemission.
(cid:13)c 0000RAS,MNRAS000,000–000
8 M.T.Whiting, R.L.Webster and P.J.Francis
Figure 8. Polarisation from Wills et al. (1992) as a function of
the proportion of the total flux made up by synchrotron. The
sources are given different symbols according to the nature of
theirbestfitmodel.
Furthermore,iftherelativeamountofsynchrotronemission
changeswithwavelength,asitdoeswiththemodelswehave
fitted, then the amount of polarisation should also change
with wavelength.
Suchdependencieshavebeeninvestigatedpreviouslyby
anumberofdifferentauthors,forsamplesthatincludesome
Figure6.Histogramofpeakwavelengths,showingallsourcesfit
sources considered here. Wills et al. (1992) studied a large
best with the combined model, in a) the rest frame, and b) the
sampleofbright,flat-spectrumcore-dominantquasars,mea-
observed frame. The solid histograms show all sources accepted
at 99% confidence, while the hatched histograms show the BL suringtheiropticalpolarisation.Aninterestingresultisthat
Lacobjects.Thelocationsofthreeprominentemissionlinesand they found that the fraction of quasars with P > 3% in a
photometricbandsaregivenforreference. fixed observed passband decreased with increasing z, pos-
sibly indicating that the percentage polarisation decreases
towardsshorterrestframewavelengths.Thiswould becon-
sistent with thepresenceof asynchrotron component turn-
ing overin theoptical region.
Impey&Tapia(1990)presentradioandopticaldatafor
a slightly larger sample of radio-selected quasars, including
optical polarisation measurements. They find strong statis-
ticallinksbetweenstrongopticalpolarisationandproperties
such as compact radio structure, superluminal motion and
weak emission lines. They explain this by requiring the op-
tical emission, as well as the compact radio emission, to be
relativistically beamed.
Smithetal.(1988)obtainedmulticolour(UBVRI)po-
larisation measurementsof11highlypolarised quasars,and
found that three of these exhibited decreasing polarisation
towardshorterwavelengths,whichtheymodelled asacom-
bination of polarised synchrotron emission, and two unpo-
larised components, from the broad-line region and the ac-
cretiondisk.Noneofthesesources,however,arepartofour
sample.
Figure 7. Histogram of ratios of the synchrotron component
7.1.1 Optical polarisation
to the power law component of the combined model, for those
sourcesbestfitbythecombinedmodel.Theratioiscalculatedat Firstly,wewishtocompareourmodelpredictionswithpub-
arest-framewavelength of0.5µm.Thehatched histogramshows
lished optical polarisation measurements. We use the large
theBLLacobjects.
catalogue of measurements compiled by Wills et al. (1992).
Wewanttocomparethesemeasurementswiththefractional
(cid:13)c 0000RAS,MNRAS000,000–000
Red synchrotron jets in Parkes quasars 9
amount of flux due to the fitted synchrotron component. ness,wealsoshowalineofconstantpolarisation,normalised
However, since the observations of Wills et al. were made totheK-band point.All thesesources are best fit with the
withoutafilter,wehavedeterminedtheaveragesynchrotron combined model, with the exception of 1101 325, which is
−
fraction byintegrating over therange 0.3µm – 1µm. fit best by the power law model (its SED taking the form
WehavethenplottedinFig.8thepercentagepolarisa- of a blue power law, λ−1.81). We have used the combined
tion as a function of this average synchrotron fraction. For model fit to it for the purposesof Fig. 9.
those sources that were best fit by the power law model, Wehavealso included polarisation measurementsfrom
we have calculated the fraction from the combined model Wills et al. (1992) where they exist. Again, these are put
fit, and indicated these sources by a different symbol. All at an observed wavelength of 0.5µm. Note, of course, that
the power law sources with polarisation measurements had thesepoints are not simultaneous with the NIRpoints.
relatively red power law indices (i.e. α> 1.3). The fitted synchrotron component generally replicates
−
The spread in polarisation measurements at large syn- well the wavelength dependence of the polarisation, al-
chrotronfractions ismuchgreaterthanat low fractions, in- though for some sources, such as 1313 333, the points are
−
dicating that the high polarisation sources generally have equally well given by a constant polarisation component.
large amounts of synchrotron fitted to them (at the wave- Thisiswhatyouwouldexpectfromapuresynchrotroncom-
lengthsatwhichthepolarisationismeasured).Additionally, ponent,andthesesourcesaretypicallyBLLacobjects,from
allbutoneofthesourcesatzero(ornear-zero)synchrotron which you would expect to see a synchrotron-dominated
fraction havelow optical polarisation. SED.Anotableexceptiontothisis0537 441,whosepolar-
−
The two exceptions to this picture are 0202 172 and isation isnotfitwellbyaconstantsynchrotroncomponent,
−
1020 103. Firstly, 1020 103 has a very high synchrotron but is fit better by a combination of a synchrotron and a
− −
fraction ( 95%), but has very little optical polarisation significant power law component. However, it is apparent
∼
(P =0.58%). In this case, the source has a power law con- from these plots that having simultaneous optical polarisa-
tinuumwithaslightcurvature,whichisfitwellbyanalmost tion measurements would better help discriminate between
puresynchrotroncurve.Itmaybethatthiscurvatureisdue thecombined model and a constant polarisation model.
toothereffectsthansynchrotron,whichexplainsthelackof
polarisation.Apossiblecandidateiscontaminationfromthe
7.2 Emission lines
very strong Hαline, which would boost thecontinuum level
in thecentreof theSED. Synchrotron flux emitted from the PHFS quasars is most
Secondly,0202 172hasameasuredpolarisationof5.15, likelytocomefromathin,relativisticjet,andtheradiation
−
butalso hasaverybluepowerlaw continuum(α= 1.85). will not be isotropic. This means that there will be very
−
This continuum slope is possibly too steep to attribute to little synchrotron radiation directed towards the emission
synchrotron(itimpliesavalueofp=1.3–inturnimplying lineclouds, which havea muchlarger covering angle. Thus,
a rather flat energy distribution ). However, the location of the ionisation of these clouds will be due to the continuum
the synchrotron peak may have shifted in the period since emission from the central accretion disk region – in other
the polarisation measurements were made (thepolarisation words, the Big Blue Bump.
measurements were taken 8 years prior to our photometry One might expect that adding a non-ionising syn-
observations). chrotron component to the continuum will have the effect
of reducing the equivalent width of the emission lines from
theBLR, dueto the fact that theflux in theemission lines
7.1.2 Near-infrared polarisation
is not changed but thecontinuum fluxis increased.
Totrytoavoidtheproblemofnon-simultaneityofthepho- To test this prediction, we compared the equivalent
tometry and polarisation observations, we obtained polari- widths of five emission lines (Civ 1549, Ciii] 1909, Mgii
sationmeasurementsof8quasarsintheNIR,usingtheIRIS 2798, Hβ, and the doublet [Oiii] 4959,5007) with the ratio
instrument on the Anglo Australian Telescope. The details of synchrotron to continuum fluxat theline wavelength, to
and results of the observations will be presented elsewhere see if some form of an anti-correlation is present. The de-
(Whiting et al., in preparation). These measurements are tailsoftheobservationswillpresentedelsewhere(Franciset
nearly simultaneous with the photometry measurements (a al. 2001). Objects that had spectra taken were essentially
differenceof 40days),andsocanbedirectlyrelatedtothe a random sample of the PHFS (subject to visibility during
∼
fittedcomponents.Thewavelengthdependenceofthepolar- theobserving run).
isationcanthenprovideanimportanttestonthemodelswe InFig.10,weshowtheresultsofthiscomparisonforthe
are fitting. Mgii and Hβ lines, which are the two broad lines with the
Since the synchrotron component is the only polarised longest wavelength (and hence the two lines most likely to
component, one would expect the percentage polarisation showareductioninequivalentwidth).Wehavealsoplotted
to be directly related to the amount of synchrotron flux those sources best fit with the power law model. The value
present,and,in fact, thepercentagepolarisation will bedi- oftheratiousedforthesesourceswastakenfromfittingthe
rectly proportional to the ratio of synchrotron flux to total combined model, and so are upperlimits to theratio.
flux. In Fig. 9, we have plotted the polarisation of each of The Mgii line does not show much relationship to the
thesequasarsasafunctionofwavelength,and,onthesame synchrotron ratio, while the Hβ line does show a reduction
plot, the synchrotron ratio normalised (arbitrarily, as it is in equivalent width with increasing amount of synchrotron.
thewavelengthdependenceweareinterestedin,notthepre- This lends some support to the hypothesis that excess syn-
cise normalisation) to the longest wavelength polarisation chrotron light is present. The difference in the two plots is
point (which is usually the K-band point). For complete- likelyduetothepresenceoftheturnoverinthesynchrotron
(cid:13)c 0000RAS,MNRAS000,000–000
10 M.T.Whiting, R.L.Webster and P.J.Francis
Figure 9. Near-infraredpolarisation of a selection of quasars as a function of wavelength. Alsoshown are optical measurements from
Wills et al. (1992). The solidlines show the fraction of the total flux made up by the synchrotron component (right-hand axis), while
thedashedlineshowsthisnormalised(seetext) totheK-banddatapoint,andisinpolarisationunits(left-handaxis).Thedottedline
showsconstantpolarisation,normalisedtotheK-banddatapointaswell.A∗ denotes theobjectisaBLLac.
flux, so that is has less effect at the shorter wavelength of other model be used instead of synchrotron? One possible
Mgii. There are, however, several sources that have high alternative is blackbody emission resulting from hot dust.
synchrotronratiostogetherwithhighequivalentwidths.The Totestthismodel,weusedablackbodyemission spec-
implications of theseresults are discussed in Section 10. trumduetodust at a temperatureof 1750 K(thesublima-
tion temperature characteristic of dust grains consisting of
graphite and silicates, e.g. Laor & Draine (1993)) emitted
in the quasars’ rest frame. A blackbody curve at this tem-
8 HOT DUST: AN ALTERNATIVE TO
perature would have its peak, in the quasar rest-frame, at
SYNCHROTRON?
1.66µm. This blackbody spectrum was combined with the
So far, we have shown that, for a number of sources, the same λ−1.7 power law used in the combined model to pro-
optical–NIR photometry is well fit with a power law plus ducea model that was fit to thedata.
a curved component, which we have assumed to be the The fits generated by this model were almost always
turnover of a synchrotron component. However could an- worsethanthoseofthesynchrotronmodel.Thiswasdueto
(cid:13)c 0000RAS,MNRAS000,000–000