Modeling the Infrared Emission in Cygnus A

0.54 MB·English

by  G. C. Privon

Draftversion January17,2012
PreprinttypesetusingLATEXstyleemulateapjv.5/2/11
MODELING THE INFRARED EMISSION IN CYGNUS A
G. C. Privon1,2, S. A. Baum1,3, C. P. O’Dea4,5, J. Gallimore6,7, J Noel-Storr1, D. J. Axon3,8, and A. Robinson3
Draft version January 17, 2012
ABSTRACT
We present new Spitzer IRS spectroscopy of Cygnus A, one of the most luminous radio sources
in the local universe. Data on the inner 20′′ are combined with new reductions of MIPS and IRAC
2
photometry as well as data from the literature to form a radio through mid-infrared spectral energy
1
distribution (SED). This SED is then modeled as a combination of torus reprocessed active galactic
0
nucleus (AGN) radiation, dust enshrouded starburst, and a synchrotron jet. This combination of
2
physically motivated components successfully reproduces the observed emission over almost 5 dex in

n frequency. The bolometric AGN luminosity is found to be 1012 L⊙ (90% of LIR), with a clumpy
a AGN-heated dust medium extending to ∼ 130 pc from the supermassive black hole. Evidence is
J
seen for a break or cutoff in the core synchrotron emission. The associated population of relativistic

6 electrons could in principle be responsible for some of the observed X-ray emission though the syn-
1 chrotron self-Compton mechanism. The SED requires a cool dust component, consistent with dust
reprocessed radiation from ongoing star formation. Star formation contributes at least 6×1010 L⊙
O] to the bolometric output of Cygnus A, corresponding to a star formation rate of ∼10 M⊙ yr−1.
Subject headings: galaxies: active – galaxies: individual (Cygnus A) – galaxies: jets
C
.
h
p 1. INTRODUCTION SMBH mass of 2.5±0.7×109 M⊙. This measurement
is consistent with black-hole-mass–host-galaxy relations
- As oneofthe nearestpowerfulradio-loudactivegalac-
o (e.g., Magorrianet al. 1998; Ferrarese & Merritt 2000;
tic nuclei (AGN), Cygnus A provides an excellent lab-
r Gebhardt et al. 2000; Gu¨ltekin et al. 2009).
t oratory to study the environment and activity of pow-
s erful AGN. The luminosity of the AGN in Cygnus A The large scale radio morphology shows prominent
a hotspots, lobes, and a radio core. Both a jet and
(whichispredominantlyexpressedintheinfrared)ishigh
[ counterjet are visible in Very Large Array (VLA) and
enough to classify it as quasar (e.g., Djorgovski et al.
Very Long Baseline Array (VLBA) observations (e.g.,
1 1991). While not seen in total intensity, broad Hα is
v seen in polarized light (Ogle et al. 1997), lending sup- Sorathia et al. 1996). Due to the edge-brightened mor-
9 port for the existence of an obscured broad-line region phologyitisclassifiedasanFRIIsource(Fanaroff-Riley
1 (BLR). The polarized broad lines were detected within Type II; Fanaroff & Riley 1974).
3 the ionization cone seen by Jackson et al. (1996), giving X-ray observations of Cygnus A are consistent with
3 the presence of a hidden quasar. Chandra ACIS obser-
them a possible scattering origin in the NLR.
. vations from 0.7 to 9 keV by Young et al. (2002) show
1 The host galaxy of Cygnus A is a cD elliptical near
thehardX-rayfluxtobepeakedatalocationconsistent
0 the center of a cluster which appears to be undergo-
withthatoftheradiocoreandunresolved(lessthan0.′′4
2 ing a merger with cluster of similar size (Ledlow et al.
in size, determined by comparison with a model PSF).
1 2005). Tadhunter et al. (2003) obtained Hubble Space
Higher energy INTEGRAL observations show emission
: Telescope (HST) Space Telescope Imaging Spectrograph
v between 20 and 100 keV (Beckmann et al. 2006), al-
(STIS) spectra of the nuclear region. Stepping the slit
i though there may be some contamination from intra-
X across the nucleus, a velocity gradient indicative of ro-
cluster gas. This hard X-ray emission is likely due to
tation around the radio axis was observed. Model-
r
accretiondiskemissionComptonizedintheAGNcorona,
a ing the velocity as due to the potential of a super-
although the UV/optical emission is obscured.
massive black hole (SMBH) and stellar mass distribu-
InadditiontotheAGNactivity,HSTimaginghasalso
tion (measured from a 1.6µm NICMOS image) gives a
revealed star formation in the central region of Cygnus
A, which began < 1 Gyr ago (Jackson et al. 1998). It
1Chester F. Carlson Center for Imaging Science, Rochester is located in a 4 kpc ring around the nucleus, oriented
Institute ofTechnology, Rochester, NY14623
2Department of Astronomy, University of Virginia, Char- orthogonalto the radio axis.
lottesville,VA22904 Based on adaptive optics observations showing a sec-
3Radcliffe Institute for Advanced Study. Cambridge, MA ondary point source near the nucleus, Canalizo et al.
02138 (2003)suggestthatCygnusAmaybeinthelatestagesof
4Department of Physics, Rochester Institute of Technology,
a mergerevent. This mergereventand relatedaccretion
Rochester, NY14623
5Harvard-Smithsonian Center for Astrophysics. Cambridge, may be related to the current epoch of nuclear activity.
MA02138 A near-infrared spectrum presented by
6Department ofPhysics,Bucknell University,Lewisburg,PA Bellamy & Tadhunter (2004) showed complicated
17837
7NationalRadioAstronomyObservatory,Charlottesville,VA, emission line properties suggesting an infalling molec-
22904 ular cloud, consistent with the Canalizo et al. (2003)
8SchoolofMathematicalandPhysicalSciences,Universityof picture of a minor merger. The H lines were seen in
2
Sussex,Falmer,BrightonBN19RH,UK4
2
λ (µm)
25 5
10 Table 1
Mid-infraredEmissionLineProperties
S III] O IV]Ne V] S III] Ne III]Ne V] Ne II] S IV] Ar III] Ne VI]Ar II]
[ [[ [ [[ [ [ [ [[
Line Flux FWHM
(×10−13 erg s−1 cm−2) (µm)
1 [ArII]6.9 1.93±0.08 0.11
[NeVI]7.6 1.37±0.09 0.11
F (Jy)ν [[[NSAerIVIIIII]1]018.25..98 121...651751±±±000...000334 000...011911
[NeV]14.3 1.51±0.04 0.10
0.1 [NeIII]15.6 4.13±0.04 0.15
[SIII]18.7 2.42±0.06 0.13
[NeV]24 2.51±0.06 0.37
[OIV]25.9 4.58±0.08 0.32
[SIII]33 3.33±0.08 0.31
0.01 10 100 HH22 SS((57)) 00..895434±±00..111320 00..0066
ν (THz) H2 S(3) 0.286±0.030 0.09
Figure 1. Spitzer IRS spectrum for Cygnus A. Extracted in a H2 S(2) 0.333±0.025 0.11
20′′ circularaperturefromspectralmappingdata. H2 S(0) 0.128±0.038 0.31
Note. —Onlydetections>3σarelisted. Errors
several components, both redshifted and blueshifted quotedfromPAHFIToutput.
relative to the systemic velocity, interpreted as emission
discussion in Section 5 and a summary in Section 6.
from a rotating torus. The observed near-infrared line
ratios are consistent with excitation by X-rays (from 2. DATA
the AGN), while likely ruling out shocks as a possible
New observations were combined with data compiled
excitation method.
from multiple archives and new reductions in order to
For a more detailed summary of Cygnus A, including
obtain a SED covering almost five orders of magnitude
propertiesofthelargerhostgalaxyandenvironment,see
in frequency. Our analysis is focused on the properties
the review by Carilli & Barthel (1996).
of the continuum emissionin this system. Below we dis-
While the bulk of the infrared emission is likely re-
cuss the reduction and analysis of the new Spitzer IRS
lated to the presence of an AGN, the star formation can
observations and the re-reduction of archival IRAC and
contribute to the infrared emission as well. Addition-
MIPS observations
ally, the bolometric luminosity of the AGN is uncertain.
Estimates have been made using template spectral en- 2.1. Spitzer IRS Observations
ergydistributions (SEDs) andX-rayobservations. How-
Cygnus A was observed (PI: Baum) in “mapping
ever, a significant portion of the bolometric luminosity
mode” using the low resolution mode of IRS on board
comes out in the infrared, via dust reprocessing of the
the Spitzer Space Telescope. Both the short- and long-
UV/optical continuum. An accurate determination of
wavelengthslits were stepped across the source in incre-
the bolometric luminosity then requires an understand-
mentsofhalftheslitwidth. Afterpipelinecalibrationat
ing of the contributions to the observed infrared lumi-
the Spitzer Science Center, the observations were com-
nosity.
binedintoaspectraldatacubeusingtheCubeBuilderfor
In this paper, we present modeling of the SED of
IRS Spectra Maps (CUBISM; Smith et al.2007a). From
CygnusA.Toaccomplishthiswecombinednewinfrared
this data cube, a spectrum was extracted in a 20′′ aper-
measurementsobtainedbytheSpitzerSpaceTelescope’s
ture (Figure 1).
InfraredSpectrograph(IRS;Werner et al.2004)withex-
istingSpitzerobservationswiththeImagingArrayCam-
2.1.1. Removal of Mid-infrared Emission Lines
era (IRAC; Fazio et al. 2004) and the Multiband Imag-
ing Photometer for SIRTF (MIPS; Rieke et al. 2004), TheIRSspectrumwasfitusingPAHFIT(Smith et al.
and measurements of the radio core from the literature 2007b) to measure and remove contributions from nar-
to construct a radio through infrared (∼ 2−105 GHz; row emissionlines. Integratedline fluxes and widths are
4−105 µm) SED of the inner regions of Cygnus A. The provided in Table 1. While a study of the emission line
resulting SED was subsequently modeled using compo- propertiesis beyondthe scopeofthis paper,wenote the
nents intended to replicate the physical processes likely detectionofmultiplehighionizationlinessuchas[OIV],
to produce the observed emission. Using the results of [NeV]and[NeVI].Theseareallconsistentwiththepres-
the fitting we have been able to decompose the infrared ence of an AGN (Genzel et al. 1998; Armus et al. 2007,
emissionanddetermine the bolometricluminosity ofthe and references therein). Of particular note are [Ne V]
AGNinCygnusA.Thisinturnalsoprovidesanestimate and [Ne VI] which require the presence of ionizing pho-
of the star formation rate. tons of at least 97.1 and 125.8 eV respectively. All the
Thepaperisorganizedasfollows: inSection2,wedis- measuredemissionlines inTable1 weresubtractedfrom
cuss the new Spitzer IRS observations, new reductions the IRS spectrum before fitting the SED.
of Spitzer IRAC and MIPS data, as well as the data
2.1.2. Dust Features
from the literature. In Section 3, we describe the com-
ponents used to model the SED, for which the results In addition to fitting the emission lines, we have also
are described in Section 4. We conclude with a general fit the mid-infrared dust features using PAHFIT. Table
3
Table 2 Table 4
Mid-infraredDustFeatures Fluxdensitiesfromtheliterature
λ Flux FWHM λ(µm) Fν (Jy) σ (Jy) Resolution(′′) Ref
(µm) (×10−13 erg s−1 cm−2) (µm)
450 0.34 0.06 8 1
6.2 1.76±0.23 0.19 800 0.56 0.08 13 2
6.7 5.99±0.49 0.47 850 0.53 0.05 14 1
7.4 22.3±0.8 0.94 1100 0.58 0.06 19 2
7.8 2.26±0.32 0.42 1300 0.59 0.07 11 3
8.3 5.50±0.23 0.42 3.3×103 (89GHz) 0.70 0.07 2 4
8.6 4.42±0.20 0.34 19.5×103 (15.4GHz) 1.22 0.20 Notprovidedinref 5
11.3 0.685±0.101 0.36 60.6×103 (5GHz) 0.97 0.20 2.0×3.1 5
12.0 2.71±0.12 0.54 109×103 (2.7GHz) 1.5 0.4 3.7×5.8 5
12.6 3.82±0.21 0.53
13.5 2.49±0.12 0.54 References. — 1 - Robsonetal. (1998), 2 - Ealesetal.
14.2 1.28±0.11 0.36 (1989), 3 - Salteretal. (1989), 4 - Wright&Birkinshaw (1984), 5 -
17.0 2.52±0.27 1.11 Alexanderetal.(1984).
17.4 0.397±0.065 0.21
17.9 0.802±0.099 0.29 µm. Their 24 µm flux is consistent with our IRS obser-
18.9 2.19±0.14 0.36 vations. Theslopebetweentheir70and160µmpointsis
33.1 3.54±0.51 1.66
steeperthanthatoftheRayleigh–Jeanstail. The70and
Note. — Onlydetections >3σ are listed. 160µm data were re-reducedfrom the BCD products in
Errorsquoted fromPAHFIToutput.
the Spitzer Science Center archive. Our measured flux
densities are givenintable 3. The core ofCygnus A was
unresolvedin both the 70 and 160µm bands. Apertures
Table 3 of30′′ and48′′ wereusedat70and160µm,respectively.
AdditionalInfraredFluxDensitiesfromSpitzer
Our 160 µm measurement has larger error bars, but is
broadly consistent with their value.
Instrument/Channel λ(µm) Fν (Jy) σ (Jy) Aperture(′′)
IRAC-2 4.5 0.010 0.003 12.2 2.3. Published Data
IRAC-4 8 0.054 0.013 12.2
MIPS-70 70 2.20 0.11 30 Cygnus A’s strong synchrotron emission at radio fre-
MIPS-160 160 0.668 0.033 48 quencies suggests that synchrotron may contribute to
the infrared as well. To anchor the synchrotron spec-
2 shows the integrated flux and profile FWHM basedon
trum we supplemented the Spitzer observations with
thePAHFIToutput. Incontrastwiththeemissionlines,
radio measurements from the literature. Core fluxes
thedustfeatureswerenotremovedastheyareexpressed
were obtained from Eales et al. (1989); Salter et al.
in the starburst models.
(1989); Alexander et al. (1984); Wright & Birkinshaw
Spoon et al.(2007)developedadiagnosticdiagramus-
(1984). Submillimeter nuclear fluxes were also taken
ing the 6.2 µm polycyclic aromatic hydrocarbon (PAH)
from Robson et al. (1998). Based on these archival data
and the strength of the 9.7 µm silicate feature (S ),
sil the unresolved radio core9 is flat spectrum (α = 0.18,
whichisseeninabsorptioninCygnusA.Thesetwospec-
F ∝ν−α), until ∼1 THz, where thermal emissionfrom
tralfeaturescanbeusedintandemtoclassifytherelative ν
dust begins to dominate the SED. The compiled radio
dominance of PAH emission, continuum emission, and
through infrared SED is shown in Figure 2 and the flux
silicate absorption. We follow their method of spline fit-
densities taken from the literature are providedin Table
ting to measure the depth of the silicate feature, finding
S ≈−0.8,whereS isthenegativeoftheapparentop- 4. The resolution of the observations is also provided.
sil sil
There is some uncertainty associated with the highest
ticaldepth. The EQWofthe 6.2 µmPAH is 0.0552µm,
frequencysub-mmobservations. The450µmobservation
placing Cygnus A on the border of region 1A and 2A in
suggeststhatthe synchrotronbreakmaybe occurringin
their diagnostic diagram, corresponding to objects dom-
this spectral regime. It is unclear if this is a genuine
inated by continuum emission in the mid-infrared. This
break or if the measurements are affected by variability.
is consistent with the presence of a strong AGN.
Additionalobservationsatthesefrequenciesmaybeable
2.2. Archival Spitzer IRAC+MIPS Data to clarify this issue.
A consideration of the apertures is important when
The nucleus and hotspots were imaged using IRAC at
assembling data across several orders of magnitude in
4.5 and 8.0 µm (see Stawarz et al. 2007, for a presen-
frequency. The resolution of the data used to assembled
tation of the data and analysis of hotspot properties).
the SED in Figure 2 varies by over a factor of 10, but in
As fluxes for the core were not presented, the archival
allcasesthe core componentis unresolved. At lowerfre-
datawereretrieved,re-reducedandcalibratedaccording
quencies the emission is due solely to synchrotron emis-
to the IRAC instrument manual. The images showed
sion from the flat spectrum radio core. There is no evi-
strong emission at the location of both radio hotspots
dence to suggestthat the largerscale steep spectrum jet
and the radio core in both channels. The emission was
will contribute emission in the infrared. Accordingly we
unresolvedinthecorecomponent,themeasuredfluxden-
use the unresolved VLA-scale core to anchor the syn-
sitiesforthiscomponentaregiveninTable3. Extraction
apertures of 12′′.2 were used for both channels. 9 Here“radiocore”refersto thecoreseenbytheVLA withon
Shi et al.(2005)presentedobservationofCygnusAus- kpc-scale resolution. This encompasses flux from the VLBI scale
ing the MIPS instrument on Spitzer at 24, 70, and 160 coreandjet.
4
Wavelength (µm) dust distributions (e.g. Baum et al. 2010). In order to
100000 10000 1000 100 10
10 reproduce the observed ratio of Type I/II AGNs, the
obscuring clouds collectively populate a rough toroidal
structure with some opening angle.
TheCLUMPYtorusmodelisspecifiedbymultiple ge-
1
ometricalparameters. Theouterradiusofthetorus(R )
o
is Y times the inner radius (R ), where the inner radius
d
is determined from a dust sublimation temperature of
F (Jy)ν 0.1 Tlum=in1o5s0i0tyKof(Rthde=A0G.4N×inL40u5.5niptsc.ofL14504i5s tehrge bs−o1lo)m. eTtrhice
geometryofthe modelisshowninFigure3. The clumps
have a Gaussian angular distribution, with σ parame-
0.01
terizing the width of the angular distribution from the
MIIRPSS mid-plane. The radial distribution is a power law with
RIRaAdiCo indexq: r−q. Theinclinationofthetorussymmetryaxis
SCUBA
0.001 to the line of sightis i andthe averagenumber ofclouds
0.001 0.01 0.1 1 10 100
along a given line of sight is N.
ν (THz)
Figure 2. Radiothroughmid-infraredSEDwithnewSpitzerob- i to observer
servations and other data from the literature. See the text for
detailsandreferences.
chrotron emission in the nucleus, so the difference in
apertures should not affect the constructionof this SED
for the nucleus.
3. MODELING
σ
We aim to reproduce the major features in the SED:
the powerlaw emission at radio wavelengths, the strong
thermal emission at infrared wavelengths, and the over-
allcharacterofthesilicateabsorption. Extrapolatingthe
powerlawfromtheradiotothe infraredrequiresamodi-
ficationofthepowerlawspectrumtoavoidexceedingthe
observedmid-infraredflux. Theinfraredemissionisther-
malinnature,comingfromdustatavarietyoftempera- R R

turesrangingfromthecoldISM(T ∼20K)throughhot d o
dust near the AGN, up to the sublimation temperature
(T ∼1500 K). The power source for this dust heating is Figure 3. Geometry of theCLUMPY torus model. Figurefrom
Nenkovaetal. (2008), used with permission. See the text for an
a combination of star formation and AGN activity, with
explanationofthelabels.
an uncertain balance between the two.
As noted in the introduction previous studies of In addition to the geometrical parameters, the mod-
Cygnus A have found evidence for simultaneous ongo- els also vary the optical depth of each clump (τ ), us-
V
ing AGN activity and star formation, both of which can ing the Ossenkopf et al. (1992) dust composition and
contribute to the infrared emission. We model the con- Mathis et al. (1977) grain size distribution. The overall
tinuumemissionfrom∼2−105GHz(3×106−5µm;see scalingofthe modelflux is F , the bolometric flux of
AGN
Figure 2) using components to represent the AGN torus the AGN’s accretion disk component (treating the syn-
model,astarburst,andsynchrotronemission. Theselec- chrotronemission separately).
tionofmodelshas15freeparameters,andtheparticular The clumpy nature of the obscuration implies that
choices are discussed in the following sub-sections. there is a finite probability of a direct line of sight to
the BLR, even at high inclinations. However,as Cygnus
3.1. AGN Torus Model
Ashowsnoevidencefordirectlyobservedbroadlines,we
According to the unified scheme for radio loud AGN, have only fit models where the central regions are fully
the SMBH and accretion disk can be hidden from view obscured along our line of sight.
along some lines of sight by an obscuring torus. Along
3.2. Starburst
these obscured lines of sight the UV/optical radiation is
absorbed and re-radiated in the mid- and far-infrared. Star formation is represented using the
As noted above, Cygnus A shows evidence for a hidden Siebenmorgen & Kru¨gel (2007) models which assume
BLRthroughobservationsofHαinpolarizedlight. This spherical symmetry and an ISM with dust properties
suggests that Cygnus A harbors a “hidden” accretion characteristic of the Milky Way. Emission is broken
diskandaBLRwhichisobscuredalongourlineofsight. down into two components: an old stellar population
Nenkova et al.(2008) haveconstructeda modelfor an uniformly distributed through the volume and hot,
obscuringAGNtorus,wheretheobscurationisduetothe luminous O and B stars embedded in dusty hot spots.
presence of multiple clouds along the line of sight to the ThedensityofOBstarsiscentrallypeakedalthoughthe
AGN. We selectthis setofmodels because clumpy mod- model output is the emission integrated over the entire
elsseemtoprovidebetterfitstoSilfeaturesthansmooth starburst.
5
Thefreeparametersforthemodelare: starburstradius Forasimpleagingoftheelectronpopulation,thepost-
r, total luminosity L , ratio of luminosity in O and B break spectral index in Cygnus A would be α = 1.24.
SB 2
stars to total luminosity f , visual extinction from the As with Case I, we fit a dust screen in front of the syn-
OB
center to the edge of the nucleus A , and dust density chrotronemission (τ ).
V r
in hotspots around O and B stars n.
Owing to the spherical symmetry of this starburst 3.4. Stellar Contribution to the Mid-infrared
model and the known ring morphology of the star for- Starlightcanpotentiallycontributetothemid-infrared
mationinCygnus A,parameterssuchasAV andthe ra- flux, especially in a large aperture. Using the flux of the
dius r do not have straightforwardinterpretations here. stellar component from Jackson et al. (1998) and their
The assumption of optically thick star formation does brightnessprofile,wedeterminedthecontributiondueto
not have strong evidence (either for or against), given starlightina 20′′ aperture. The relativefluxes werecon-
the absence of high resolution far infrared observations. sistentwithanellipticalgalaxytemplatefromSilva et al.
(1998). Expectedfluxdensitiesat2.2,5,and10µmwere
3.3. Synchrotron computedusingthesametemplatespectrum. Aftersub-
tracting a nuclear point source, the K-band flux density
The strong radio emission in Cygnus A justifies the
in starlight is consistent with the flux in a 20′′ aperture
final model component. VLA core fluxes are consis-
asmeasuredusingTwoMicronAllSkySurvey(2MASS)
tent with powerlaw emission to the submillimeter where
data products (Skrutskie et al. 2006). At 5 and 10 µm,
thermal emissionbegins to dominate. Extrapolating the
the starlight contributes roughly 14% and 2% respec-
powerlaw to higher frequencies suggests that the syn-
tively,ofthefluxmeasuredbyIRS.Therefore,wedonot
chrotron spectrum must either be modified or subjected
includeanycontributiontothemid-infraredfluxfroman
to attenuation. The AGN is known to sit behind a
dust lane with significant extinction (A = 50 ± 30; old stellar population in our modeling.
V
Djorgovskiet al.1991). Thoughsomeattenuationofthe
3.5. Dust in the NLR
synchrotron flux is expected at higher frequencies, this
alone is not sufficient to explain why the synchrotron Warm dust in the NLR directly illuminated by
powerlaw does not continue through the mid-infrared. the AGN can also contribute to the mid- and
With extinction alone, the flux densities at 30 and 10 µ far-infrared emission (e.g., Groves et al. 2006).
m would be similar for the observed spectral index of Ramos Almeida et al. (2009) find that a significant
α = 0.18. For A = 50, the expected flux from the ex- contribution to the mid-infrared flux can come from
V
trapolated synchrotron emission would by itself exceed sources other than the AGN torus, particularly addi-
the measured infrared flux at 10µm. We conclude that tionalhotdust. InCygnusARadomski et al.(2002)find
theremustbe abreakinthepopulationofemitting elec- an extended component to the mid-infrared emission
trons. which is consistent with T ∼ 150 K dust. Some of this
Onepossible explanationis asimple cutoffinthe pop- extendedemissionisco-spatialwithsitesofpossiblestar
ulation of relativistic electrons at some energy (Case I). formation. Dust of this temperature is well reproduced
This would manifest itself as a cutoff in the synchrotron with our choice of starburst models (with the implicit
spectrum: assumption that this dust is heated by star formation).
Higher temperature dust is also present in the inner
Fν ∝ν−α1e−ννce−τr(ν) (1) regions of Cygnus A. However the resolved mid-infrared
images of the nuclear regions by Radomski et al. (2002)
whereν isthefrequencycorrespondingtothecutoffin areunabletodistinguishbetweendustinthe“torus”and
c
theenergydistributionoftheparticles,α isthespectral dustintheNLRheatedbytheAGN.Withoutstrongob-
1
index in the optically thin regime, and τ (ν) is the dust servationalmotivationfor anadditionaldustcomponent
r
screen between the synchrotron emitting region and the we opt not to include one. As will be shown in the next
observer. τ (ν) follows Draine & Lee (1984). section we are able to reproduce the observed emission
r
In Cygnus A the spectral index α is measured from without this additional component.
1
VLA core radio fluxes, and the amplitude of the pow-
3.6. Prior Constraints on Model Parameters
erlaw fixed from the same observations. The only free
parameters are the cutoff frequency νc and the optical Prior to fitting the models, the available parameter
depthτr. Theseparametersartpartlydegenerateinthat spacewasconstrainedusingresultsfrompreviousstudies
in trial runs we experienced a situation where τr and νc of Cygnus A. The opening angle and covering fraction
wouldbothincrease,effectivelyoffsetting eachother. To of the CLUMPY torus are determined from the σ and
combat this we limited τr such that AV does not exceed N parameters (see Equations (3) and (4) in Mor et al.
500 towards the radio source. 2009). We define the half opening angle of the torus as
An alternate model for the synchrotron emission at the angle at which the escape probability of a photon
higher frequencies is a broken powerlaw behind a dust drops below e−1:
screen (Case II). Aging of the population of relativistic
electrons results in a broken powerlaw whose spectral θ =90−σ lnN (3)
half 0
index increases for frequencies higher than a break fre- p
Tadhunter et al.(1999)measuredthe openingangleof
quency (Kardashev 1962). The functional form adopted
theionizationconeinCygnusAusingnearinfraredHST
is:
data, finding θ =(58±4)◦. We use this value as the
half
ν−α1 if ν <ν torus opening angle and limit the parameter range of σ
F ∝e−τr(ν)× break (2) and N according to Equation 3.
ν (cid:26)ν−α2 if ν ≥νbreak
6
Wavelength (µm) Wavelength (µm)
100000 10000 1000 100 10 1 100000 10000 1000 100 10 1
10 10
Sum Sum
AGSNta Trbourrusst AGN Torus
Synchrotron Starburst
1 1 Synchrotron
0.1 0.1
F (Jy)ν F (Jy)ν
0.01 0.01
0.001 0.001
0.0001 0.0001
0.001 0.01 0.1 1 10 100 1000 0.001 0.01 0.1 1 10 100 1000
ν (THz) ν (THz)
Figure 4. LEFT:CaseIfittotheCygnusASED.Theshadedregionshowsallacceptablemodelfitswithinthe95.4%confidenceinterval.
The lines are the components for the best-fit (lowest χ2). RIGHT: Same as left, but for Case II fit to the Cygnus A SED. Insets are a
zoomoftheregionaroundthe9.7µmSilabsorption.
11
CCaassee II
CCaassee IIII
00..88
00..66
ff
00..44
00..22
00
00 22 44 66 88 00 22 44 66 88 00 44 66 88 00 22 44 66 88 00
1.1. 1.1. 1.1. 1.1. 1.1. 2.2. 2.2. 2.2. 2.2. 2.2. 3.3. 0.0. 0.0. 0.0. 1.1. 1.1. 1.1. 1.1. 1.1. 2.2.
11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11
lloogg((LLAAGGNN//LLOO••)) lloogg((LLSSBB//LLOO••))
Figure 5. HistogramofbolometricAGNluminosityfrombestfittorusparameters (left)andbolometricstarburstluminosity(right)for
CasesI(blue,solidline)andII(red,dashedline).
The inclination range of the torus i was limited by used to determine the “acceptable” range of parameter
very long baseline interferometry (VLBI) observations values.
and modeling of the inner pc-scale jet (50◦ ≤ i ≤ 85◦, TheresultsofthefitsareshowninFigure4. Therange
Sorathia et al. 1996). Additionally, the available VLA of parameter values for fits within at 95.4% confidence
core radio fluxes were used to fix the synchrotron spec- interval are shown in Figures 5-9. Section 4.1 discusses
trum at radio frequencies (α , and the amplitude). the CaseI fits (exponentialcutoffin the relativisticelec-
1
tron population) and Section 4.2 discusses the Case II
4. RESULTS fits (broken powerlaw for the synchrotronemission).
TofittheobservedSED,the fluxfromeachmodelwas
summedineachwavelengthbinandcomparedtotheob- 4.1. Case I: Synchrotron Exponential Cutoff
servedfluxwithina20′′ aperture. Optimizationwasper-
The best fit combination of parameters for Case I has
formed using Levernberg–Marquardtleast-squares mini-
χ2/DOF = 1.35. The range of acceptable matches for
mization.
the 1641modelcombinationsis showninFigure 4 (left).
Some degeneracy between parameters was seen in the
HistogramsofthebestfitparametersaregiveninFigures
model results. Comparison model runs with fewer prior
5(left)and6,wheref isthefractionofmodelswithinthe
constraints on the parametersresulted in similar χ2 val-
95.4%confidence interval in a given bin. Parametersfor
uestothosequotedbelowforregionsofparameterspace
thebestfitmodelaremarkedwithasmallbluehorizontal
which are unlikely to be physically reasonable matches
bar.
to Cygnus A (e.g., CLUMPY torus fits with covering
fractions of unity and θ = 0◦). Prior constraints
half 4.1.1. AGN/Torus Properties and Contribution
from other observations are thus important in eliminat-
ingregionsofparameterspacewhichmaybestatistically The fits favor a bolometric accretion disk luminosity
reasonable but physically unrealistic. of log(LAGN/L⊙) ∼ 11.8−12.0 (median of 11.82 with
Afterthemodelswerefit,confidenceintervalswerede- an interquartile spread of 0.09). The fits clearly favor
terminedseparatelyforCaseIandCaseIIthroughboot- an extended torus (Y = 200) with low opacity clouds
strapping with replacement (Efron 1981). These were (A ∼ 10−30). For the median bolometric luminosity,
V
7
11
Case I
Case II
00..88
00..66

00..44
00..22
00
5500 6600 7700 8800 33 44 55 66 77 88 11001122114411661188220022222244 1100 2200 3300 4400 6600 110000
IInncclliinnaattiioonn ((ddeegg)) NN CClloouudd AAVV
11
00..88
00..66
ff
00..44
00..22
00
2200 2255 3300 2200 220000 00..00 11..00
σσ ((ddeegg)) YY qq
11
00..88
00..66

00..44
00..22
00
00..0000..1100..2200..3300..4400..5500..6600..7700..8800..9911..00 00 00 00 00 00 00 00 00 00 00 00 00 00 0 00 2200 4400 6600 8800 110000112200114400116600118800200220
22 44 66 88 00 22 44 66 88 00 22 44 6
TToorruuss CCoovveerriinngg FFrraaccttiioonn 11 11 11 11 11 22 22 22 2 LLiinnee ooff SSiigghhtt AAVV
IInntteeggrraatteedd EEqquuaattoorriiaall AA
VV
Figure 6. Histograms of torus parameters for acceptable fits within the 95.4% confidence interval for Cases I (blue, solid line) and II
(red,dashedline). Thesmallhorizontalbarsdenotetheparametervalueforthebest-fitmodel. Fromlefttoright,toptobottom: viewing
angle of the torus (90 meaning the torus is viewed edge-on), N - average number of clouds along an equatorial line of sight, AV - for an
individual cloud, σ - angular width of the distribution of torus clouds, Y - ratioof inner to outer radii, q- power law index of the radial
distributionoftorusclouds,coveringfractionofthetorus(equivalenttotheescapeprobabilityofoptical/UVphotons,assumingoptically
thickclouds),integrated AV alonganequatorial lineofsight,integrated AV alongourlineofsight(calculated usingN,σ,thecloudAV,
andtheinclination). Parameters inthe firsttworowsarediscreteandthehistogramsrepresenttherelativenumberofmodelswiththose
specificvalues. Parameters inthebottom row arederived fromthe parameters inthetop tworowsas wellas the luminosityof theAGN
component (showninFigure5),andarecontinuous.
the inner radius of the torus is R = 0.6 pc, giving a and 120. However the A is poorly constrained beyond
d V
correspondingoutertorusedgeof∼125pc(forY=200). having a well-defined lower bound.
Therangeofvaluesforiwaslimitedbasedonprevious The torus covering fraction was computed using the
work in the radio regime. The fits prefer an inclination methodofMor et al.(2009,theirEquations(3)and(4)).
for the torus on the high end of the range, i ≈ 80◦. Bydesign,thederivedcoveringfractionsforthetorusare
N has a bimodal distribution, with 25% of fits having between 50% and 70%.
N ≤ 6 clouds along an equatorial line of sight and ∼
60% having N ≥ 20 clouds. Most of the fits within the 4.1.2. Starburst
95.4% confidence interval have a flat radial distribution ThemedianvalueofthestarburstluminosityforCaseI
of clouds (q =0). islog(LSB/L⊙)∼10.8,with atailupto ∼11.6contain-
Theopacityofindividualcloudsisanti-correlatedwith ingapproximately40%ofthefits(Figure5). Thiscovers
theradialextentofthetorus;smalltorussizesfavorhigh a range of star formation rates from 10 to 70 M⊙ yr−1,
AV values. High values of N (i.e., many clouds along as determined by the LIR calibration from Kennicutt
a given line of sight) are favored for small torus sizes, (1998b) (for a review of SFR estimates, see Kennicutt
however for large torus sizes, both small and large N 1998a).
values are acceptable.
We calculate the total extinction through the torus
SFR L L
along both an equatorial line of sight (Figure 6 middle, =4.5×10−44 FIR =1.72×10−10 FIR
bottom row), and along our line of sight to the torus (M⊙ yr−1) erg s−1 L⊙
(4)
(Figure 6 right, bottom row). The integrated equatorial
Roughly half the fits within this confidence interval
A spans a range between 100 and 250. When view-
V
have 40% of the starburst luminosity in the form of
ing angle is taken into consideration,the range narrows,
OB stars. The distribution of size parameters is rela-
with most fits showing the line of sight A between 80
V
tively flat, although even the largest sizes would be un-
8
11
Case I
Case II
00..88
00..66
00..44
00..22
00
00..4400 00..6600 00..9900 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 55 00 00
11 22 33 44 55 66 77 88 99 00 11 22 33 44 55 33 00 00
ff ffOOBB 11 11 11 11 11 11 0.0. 1.1. 3.3.
AAVV SSiizzee ((kkppcc))
11
00..88
00..66
00..44
00..22
00
22 33 44 55
lloogg((nn))
Figure 7. Histograms of starburst parameters for acceptable fits within the 95.4% confidence interval for Cases I (blue, solid line) and
II(red, dashed line). Thesmallhorizontal barsdenote theparameter value forthebest-fit model. Fromlefttoright,top tobottom: the
fractionofluminosityinOandBstars,integratedextinctionthroughthestarburst,sizeofthestarburst,dustdensityinhotspots around
OandBstars. fOB andthesizearediscreteandthehistogramsrepresenttherelativefrequencyofmodelswiththosespecificvalues. The
AV andnparametersaremorefinelysampledthanthehistogramrepresents;theplotsshowtherelativefractionofmodelswithparameters
inthenotedranges.
resolvedbyourSpitzerobservations. Thedistributionof spectrum would rely more heavily on fitting the wings,
dust density n peaks around 103 cm−3. The extinction which could prove difficult in a continuum dominated
through this starburst is relatively unconstrained. source such as Cygnus A.
The strengths of dust features in starburst models
falling within the 95.4% confidence interval were also 4.1.3. Synchrotron Radiation
measured using PAHFIT to compare with the intensi- The synchrotron emission amplitude was fixed using
ties of observeddust feature. Figure 8 shows histograms the non-thermal emission from the radio core, assuming
of the predicted intensities from our models divided by it to be a point source at all frequencies observed. The
the measured intensity from the IRS spectrum. Only synchrotron model, therefore, has only two parameters:
dust features which are detected in the IRS spectrum cutoff frequency (ν ) and extinction due to dust (τ ).
c r
have been plotted. In general the agreement is good The distribution of cutoff frequencies is somewhat
with most models matching the measured line intensi- broad,coveringtherangeofν ≈10−60THz(5−30µm)
ties. However for some dust features a significant num- (Figure9). Thefitsshowarangeofacceptableextinction
ber of the models within the confidence interval predict forthedustscreen,peakingintherangeofA ≈60−80,
V
emission in excess of what is observed. While a detailed slightlylowerthanthepredictedlineofsightA fromthe
V
comparison of the relative strengths of dust features is torus models. The integrated luminosity of the “core”
beyond the scope of the paper, we suggest the models synchrotronflux is log(Lsync/L⊙)∼11.2.
are able to generally reproduce the dust features seen in This break in the synchrotron spectrum is consistent
thespectrum. Thedustfeatureswhichshowthegreatest other powerful FR II radio sources, where an extrap-
discrepancybetweenobservedfluxesandthosepredicted olation of the radio synchrotron emission significantly
from the modeling (6.2, 7.8, and 11.3 µm) comprise 3 of exceeds the observed optical flux (e.g., Schwartz et al.
the 5 lowest signal-to-noise dust feature fits in the spec- 2000;Sambruna et al.2004;Mehta et al.2009). Inthese
trum. Thediscrepancymaythenbe due tothe difficulty cases, the similarity of the radio and X-ray spectral in-
of fitting low equivalent width dust features. dicessuggeststhattheX-raymaybeproducedbyinverse
PAHFIT also attempted to fit eight other dust fea- Compton (IC) scattering.
tures in the IRS spectrum. The upper limits for three With the modeled synchrotron spectrum, a magnetic
(5.7,14.0,and15.9µm)areconsistentwithexpectations field strength, and the assumption that each electron
from the starburst models. One dust feature (16.4 µm) emits at a single frequency, the energy distribution of
has a measured limit below that expected from the best relativistic electrons can be determined:
fitting starburst model, and so is discrepant. The other
four features (7.6, 10,7, 11.2, and 12.7 µm) also have
4πm cν
e
upper limits from PAHFIT which are lower than the ex- γ(ν)= (5)
r 3eB
pected value from the starburst models. However, these
are coincident with or near other spectral features (e.g., whereallconstantsandvaluesareinthe cgssystemof
narrow emission lines or the Sil absorption). Thus an units.
accurate measurement of these dust features in the IRS
9
-2.5-2.5-2.0-2.0-1.5-1.5-1.0-1.0-0.5-0.50.00.00.50.51.01.01.51.52.02.0 -2.5-2.5-2.0-2.0-1.5-1.5-1.0-1.0-0.5-0.50.00.00.50.51.01.01.51.52.02.0 -2.5-2.5-2.0-2.0-1.5-1.5-1.0-1.0-0.5-0.50.00.00.50.51.01.01.51.52.02.0 -2.5-2.5-2.0-2.0-1.5-1.5-1.0-1.0-0.5-0.50.00.00.50.51.01.01.51.52.02.0
11
00..88 66..22 µµmm 66..77 µµmm 77..44 µµmm 77..88 µµmm
00..66
00..44
00..22
00
11
00..88 88..33 µµmm 88..66 µµmm 1111..33 µµmm 1122..00 µµmm
00..66
00..44
00..22
00
11
ff
00..88 1122..66 µµmm 1133..55 µµmm 1144..22 µµmm 1177..00 µµmm
00..66
00..44
00..22
00
11
00..88 1177..44 µµmm 1177..99 µµmm 1188..99 µµmm 3333..11 µµmm
00..66
00..44
00..22
00
55005500550055005500 55005500550055005500 55005500550055005500 55005500550055005500
2.2.2.2.1.1.1.1.0.0.0.0.0.0.1.1.1.1.2.2. 2.2.2.2.1.1.1.1.0.0.0.0.0.0.1.1.1.1.2.2. 2.2.2.2.1.1.1.1.0.0.0.0.0.0.1.1.1.1.2.2. 2.2.2.2.1.1.1.1.0.0.0.0.0.0.1.1.1.1.2.2.
---------- ---------- ---------- ----------
lloogg ((PPrreeddiicctteedd FFlluuxx // OObbsseerrvveedd FFlluuxx))
Figure 8. Histogramsoflog(PredictedFlux/MeasuredFlux)fordustfeaturesdetectedintheIRSspectrumofCygnusA.Thepredicted
values were measured from the Siebenmorgen&Kru¨gel (2007) models using PAHFIT. The solid blue line denotes Case I while the red
dashedlinedenotes CaseII.(seeTable2forthemeasuredfluxvalues).
10
The magnetic field strength has been estimated from 4.2.3. Synchrotron Properties and Contribution
VLBIobservationstoberoughly17mG(100mG)inthe
An alternate mechanism for limiting the influence of
jet (core) (Roland et al. 1988; Kellermann et al. 1981).
synchrotron emission at shorter wavelengths is for the
Using ν ≈ 30 THz and Equation 5 we find γ ∼ 2×104
spectrum to break at some frequency (see Section 3.3).
(8×103) for the jet (core).
Case II fits used a fixed pre-break spectral index of
In addition to radiating energy via synchrotron emis-
α = 0.18 and a post-break spectral index of α =1.24,
1 2
sion, relativistic electrons can also lose energy via IC
consistentwithagingoftherelativisticpopulation(with-
scattering. The ratio of the energy lost through syn-
out injection of additional particles; Kardashev 1962).
chrotron to the energy lost to IC is simply given by
As the flux density decreases in a slower fashion when
the ratio of the magnetic and radiationenergy densities,
compared to an exponential cutoff, the powerlaw must
with the cosmic microwave background (CMB) setting
break at lower frequencies to ensure that the observed
a minimum value for the radiation energy density. Us-
mid-infrared flux is not exceeded.
ing the magnetic field strength assumedabove (17 mG),
The break frequency is roughly 5 THz (60 µm).
and T = 2.73 K, synchrotron losses are dominant
CMB The unobscured synchrotron luminosity is found to be
by a factor > 108. A larger magnetic field would fur- log(Lsync/L⊙)∼11.1. Following the same arguments as
ther enhance this ratio, so IC losses from scattering of
CaseI,theelectronsemittingatthebreakfrequencyhave
CMB photons have a negligible effect on the population γ ∼7×103 (3×103) forjet (core)magnetic fieldvalues.
of relativistic particles.
Again, this is consistent with the observed properties of
4.2. Case II: Synchrotron Broken Power Law jets in other FR II radio sources.
The extinction of the dust screen in front of the ra-
The range of best fit models for Case II is shown in
dio source is lower than Case I fits, with A < 60, still
Figure 4 (right). The best fit model had χ2/DOF = V
within range of the extinction to the central source es-
1.03. Figure 4(right)showsthe rangeofthe 1580model
timated by Djorgovski et al. (1991), but lower than the
fits within the 95.4% confidence interval. Histograms of
computedlineofsightA fromthetorus. Thelargerdis-
V
the best fit parameters are given in Figures 5 (left) and
crepancy compared with Case I is due to the enhanced
6. A red horizontal bar marks the parameter for the
short wavelength emission of the broken power law at
parameters with the lowest χ2 value.
shorterwavelengths(whencomparedtotheCaseIexpo-
4.2.1. AGN/Torus nentialcutoff). TheequatorialtorusAV increasestopro-
videanoverallcoolertemperature,andthemodeleddust
In Case II fits the bolometric accretion disk luminos-
screen in front of the synchrotron component remains
ity is roughly log(LAGN/L⊙)∼11.8 (median 11.88 with small (in order to contribute sufficient flux at shorter
an interquartile range of 0.08). The distribution of lu-
wavelengths).
minosities is more strongly peaked than for Case I, with
over 80% of fits occupying the log(L/L⊙) = 11.8−12.0 5. DISCUSSIONOFGENERALSEDRESULTS
bin. A large torus (Y = 200) is exclusively preferred, 5.1. Luminosity and Kinetic Power in Cygnus A
correspondingto R ≈135 pc (using the median value
out
Our best estimate of the bolometric AGN luminos-
for the luminosity). Again, by design,the toruscovering
fractionisbetween0.5and0.7. Aninclinationofi=80◦ ity in Cygnus A (log(L/L⊙) ∼ 12, including the syn-
chrotron component and X-ray emission) is above the
is exclusively preferred.
Whysong & Antonucci (2004) estimate using Keck mid-
Thedistributionoftheaveragenumberofclumpsalong
infrared observations and a PG-quasar spectrum (3.9×
a line of sight N is again bimodal with either large (<
14) or small (< 8) numbers of clouds preferred. The 1011 L⊙), but is somewhat below the estimates of
number of clouds is anti-correlated with the extinction Tadhunter et al. (2003) who find Lbol = 12−55×1011
through individual clouds. The spread in equatorial AV L⊙ (althoughconsistentwiththelowendoftheirrange).
is somewhat large (120−260), but the line of sight A The bolometric AGN luminosity is the best constrained
V
is more constrained (100−160 for 80% of fits). parameter, and is insensitive to the synchrotron model
adopted.
4.2.2. Starburst The kinetic power in the expansion of the radio lobes
The starburst component in Case II fits show qual- in Cygnus A can be inferred from X-ray observations of
itatively similar behavior to Case I. The typical lumi- the cluster environment (Wilson et al. 2006). From this
nosity is comparable, but shows a smaller tail up to analysisofChandraobservationsCygnusAhasakinetic
log(LSB/L⊙) = 11.6 (∼ 30% of fits). Other parame- powerofLkin =1.2×1046ergs−1(log(Lkin/L⊙)=12.5),
tersaresimilartothoseinCaseIfits,withtheexception a factor of three larger than the bolometric AGN lumi-
ofthe sizewhichappearstohaveaveryslightpreference nosity inferred from the modeling. Given the uncertain-
for a larger size, suggesting an overall cooler dust tem- ties in the determinations of both the kinetic and bolo-
perature for a given luminosity. This may indicate a de- metricpower,itisunclearifthekineticpowerdominates
generacybetweenthestarburstandsynchrotronmodels. significantly over the power emitted as radiation.
The criticalfrequencyfor the synchrotronspectrum(see
5.2. Implications of the Modeling
thenextsection)isatlowerfrequencyforCaseII,result-
inginasmallercontributiontothefar-infraredflux. The The probable torus sizes from the SED modeling give
starburstmodelcompensateswithalargeroverallsizeto outer radii of R = 130 pc (≈ 0′′.2 at the distance of
o
provide additional cool dust emission (for a given star- CygnusA).Generally,the torusparametersproviderea-
burst luminosity). The dust feature strengths expressed sonableestimatesofthepropertiesoftheobscuringstruc-
in the models generally compare favorably with Case I. ture. This predicted outer radius is significantly larger