Ultraviolet to optical diffuse sky emission as seen by the Hubble Space Telescope Faint Object Spectrograph

0.48 MB·

by  K. Kawara

Publ.Astron.Soc.Japan(2014)00(0),1–14 1
doi:10.1093/pasj/xxx000
ULTRAVIOLET TO OPTICAL DIFFUSE SKY
EMISSION AS SEEN BY THE HUBBLE SPACE
TELESCOPE FAINT OBJECT SPECTROGRAPH
7
1 K. KAWARA1, Y. MATSUOKA2, K. SANO3,4,*, T. BRANDT5, H. SAMESHIMA6, K.
0
2 TSUMURA7, S. OYABU8 and N. IENAKA1

n 1InstituteofAstronomy,UniversityofTokyo,2-21-1,Osawa,Mitaka,Tokyo181-0015,Japan
a
J 2NationalAstronomicalObservatoryofJapan,2-21-1Osawa,Mitaka,Tokyo181-8588,Japan
4 3DepartmentofAstronomy,GraduateSchoolofScience,TheUniversityofTokyo,Hongo
7-3-1,Bunkyo-ku,Tokyo113-0033,Japan

] 4InstituteofSpaceandAstronauticalScience,JapanAerospaceExplorationAgency,3-1-1
A
Yoshinodai,Chuo-ku,Sagamihara,Kanagawa252-5210,Japan
G
5InstituteforAdvancedStudy,EinsteinDr.,Princeton,NJ,USA
.
h 6KoyamaAstronomicalObservatory,KyotoSangyoUniversity,Motoyama,Kamigamo,Kita-ku,
p
Kyoto,603-8555,Japan
-
o 7FrontierResearchInstituteforInterdisciplinaryScience,TohokuUniversity,Sendai980-8578,
r
t Japan
s
a 8NagoyaUniversity,Furo-cho,Chikusa-ku,Nagoya464-8601,Japan
[
∗E-mail:[email protected]

1
v Received;Accepted
5
8 Abstract
8
0 Wepresentananalysisoftheblank skyspectraobservedwiththe FaintObjectSpectrograph
0 on board theHubbleSpaceTelescope. Westudy the diffuse skyemission fromultraviolet toop-
.
1 tical wavelengths, which is composed of the zodiacal light (ZL), diffuse Galactic light (DGL),
0 and residual emission. The observations were performed toward 54 fields distributed widely
7
1 overthesky,withthespectralcoveragefrom0.2to0.7µm. Inordertoavoidcontaminatinglight
: fromtheearthshine,weusethedatacollectedonlyinorbitalnighttime. Theobservedintensity
v
i isdecomposedintotheZL,DGL,andresidualemission,ineight photometric bandsspanning
X
ourspectralcoverage. WefoundthatthederivedZLreflectancespectrumisflatintheoptical,
r
a whichindicatesmajorcontributionofC-typeasteroidstotheinterplanetary dust(IPD).Inaddi-
tion,theZLreflectancespectrumhasanabsorptionfeatureat∼0.3µm. TheshapeoftheDGL
spectrumisconsistentwiththosefoundinearliermeasurementsandmodelpredictions. While
theresidualemissioncontainsacontributionfromtheextragalacticbackgroundlight,wefound
that the spectral shape of the residual looks similar to the ZL spectrum. Moreover, its optical
intensity is much higher than that measured from beyond the IPD cloud by Pioneer10/11, and
also than that of the integrated galaxy light. These findings may indicate the presence of an
isotropicZLcomponent,whichismissedintheconventionalZLmodels.
Key words: Earth — zodiacal dust — dust, extinction — galaxies: evolution — cosmic background
radiation
(cid:13)c 2014.AstronomicalSocietyofJapan.
2 PublicationsoftheAstronomicalSocietyofJapan,(2014),Vol.00,No.0
1 Dedication trainthiswavelengthrange(Bus&Binzel2002).Forexample,
C-typeasteroidshavemuchflatterreflectancespectrumthando
ThisworkwassuggestedandinitiatedbyKimiakiKawara,with
S-typeasteroids. However,therehasbeennoopticalmeasure-
theoriginalaimofmeasuringtheextragalacticbackgroundlight
mentsoftheZLreflectancespectrumpublishedtodate.
intheultraviolet tooptical wavelengths. Forseveralyears, he
made concentrated efforts with great patience and has written
upadraftofthispaper.Unfortunately,duetohisfailinghealth, 2.2 DiffuseGalacticLight
hewasnotabletoseeitspublicationbeforehepassedawayin
The DGL consists of starlight scattered by, and re-radiated as
January2015. Thispaperhassubsequentlybeencompletedby
thermalemissionfrom,theinterstellardustgrainsinthediffuse
hiscolleagues,andisherededicatedtohismemory.
interstellarmedium(ISM).Thescatteredcomponentdominates
intheUVtoopticalwavelengthrange.DGLmeasurementsare
usefultoconstrainthepropertiesoftheinterstellardust,suchas
2 Introduction
thesizedistributionandgrainalbedo. Interstellar100µmdust
Measurements of the diffuse sky emission are important for
emissionhasbeenusedasatraceroftheDGLemission,since
probing various astrophysical phenomena, such as interstellar
theintensities ofthesetwoemissionsareexpectedtocorrelate
dustemissionandextragalacticbackgroundlight(EBL),which
linearlyintheopticallythinlimit(Brandt&Draine2012).
complement observations of discrete sources (stars, galaxies,
Recent analyses have detected the DGL from the opti-
etc.) toshapeourunderstandingoftheuniverse. Fromultravi-
cal to near-IR wavelengths, using the data obtained with
olet(UV)toopticalwavelengths,thediffuseskyemissioncon-
Pioneer10/11(Matsuokaetal.2011),CIBER(Araietal.2015),
sistsoftheairglow,thezodiacallight(ZL),thediffuseGalactic
andtheDiffuseInfraredBackground Experiment (DIRBE)on
light(DGL),andtheresidualemissionincludingtheEBL.
boardCosmicBackgroundExplorer(COBE;Sanoetal.2015).
These results are marginally consistent with the DGL model
spectrapresentedbyBrandt&Draine(2012),whicharebased
2.1 ZodiacalLight
ontheinterstellardustmodelsofWeingartner&Draine(2001)
TheZListhebrightestemissioncomponentofthediffusesky
andZubkoetal.(2004).
brightness, when observations are made from the space. The
ZLconsistsofthesunlightscatteredbytheinterplanetarydust
2.3 ResidualEmission
(IPD)andthermalemissionfromtheIPD.FromtheUVtoop-
ticalwavelengths, theZLbrightness isdominated bythescat- Theresidualemission,whichisobtainedbysubtractingallthe
teredsunlight. foreground components (the ZL and DGL in the case of the
The IPD is expected to fall into the sun by the Poynting- space observations) from the observed diffuse sky emission,
Robertson drag and also leave the solar system by the radia- contains the EBL. The EBL is the cumulative emission from
tionpressure,inatimescaleof103–107 years,whichismuch knownextragalacticsourcessuchasgalaxies,intergalacticmat-
shorter than the age of the solar system (Mann et al. 2006). ter,protogalaxies, andvarious pregalacticobjects. Itmayalso
Therefore,theIPDparticlesshouldbesuppliedcontinuouslyby contain radiation originating fromdecays of elementary parti-
asteroidsorcomets,thoughthecontributionofeachcomponent cles, such as sterile neutrinos (Mapelli & Ferrara2005). The
totheIPDisunclear. Comparisonofthereflectancespectrum EBL is thus a fundamental quantity to constrain the energy
oftheZLwiththoseofasteroidsandcometsisausefulwayto emitted from theentire cosmological objects, which is impor-
identifytheIPDsupplier(s). Bycombining theZLreflectance tanttounderstandtheevolutionoftheUniverseandgalaxies.
measurementswiththeLowResolutionSpectrometer(LRS)on The contribution of known galaxies to the EBL can be es-
board Cosmic Infrared Background Experiment (CIBER) and timated by integrating the galaxy luminosity function; this is
thosewith InfraredTelescope inSpace (IRTS),Tsumuraetal. particularly true in the UV to near-IR wavelengths, thanks to
(2010)suggestedthatthespectralshapeoftheZLreflectanceis deepgalaxycountsobtainedfromspaceandground-basedob-
similartothatofaS-typeasteroid(Binzeletal.2001)fromthe servations (e.g.,Gardneretal. 2000; Madau&Pozzetti2000;
red-optical to near-infrared (IR) wavelengths (∼0.8–2.5µm). Totani et al. 2001; Dom`ınguez et al. 2011). Diffuse emis-
Ontheotherhand, YangandIshiguro(2015)reportedthatthe sionmeasurementshavesometimesreportedtheresidualemis-
ZL reflectance spectrum in the near-IR is similar to that of sionmuchstrongerthantheintegratedgalaxylight(IGL);such
comets,whichareclassifiedasD-typeasteroids(Bus&Binzel measurementsincludethosebyBernstein(2007)usingtheopti-
2002). calimagingdatatakenwiththeWideFieldPlanetaryCamera2
MeasurementsoftheZLreflectancespectrumintheoptical (WFPC2)onboardHubbleSpaceTelescope(HST),Matsumoto
areakeytounderstandingtheoriginoftheIPD,sincedifferent etal. (2005; 2015) usingthenear-IRspectroscopicdatataken
typesofasteroidshavesignificantly differentreflectancespec- withtheNear-InfraredSpectrometer(NIRS)onboardIRTS,and
PublicationsoftheAstronomicalSocietyofJapan,(2014),Vol.00,No.0 3
Gorjian, Wright, andChary(2000), WrightandReese(2000), 2.4 OutlineofthePresentPaper
Wright(2001),Cambre´syetal. (2001),Levenson,Wright,and
ThispaperpresentsanewanalysisoftheZL,DGL,andresid-
Johnson(2007),andSanoetal.(2015;2016a)usingthenear-IR
ual emission from the UV to optical wavelengths (0.2 - 0.7
imagingmapstakenwithCOBE/DIRBE.Suchastrongresidual
µm),usingtheblankskyspectraobtainedwiththeFaintObject
emissionis,ifinterpretedastheEBL,inconflictwiththeEBL
Spectrograph(FOS)onboardHST.Wesuccessfullydetermine
upper limit obtained by means of the intergalactic attenuation
the spectral shapes of the individual components, and discuss
of γ-rays photons from distant blazers (e.g., Aharonian et al.
their implications. In particular, we derive the ZL reflectance
2006; Albert et al. 2008; Abramowski et al. 2013). An up-
spectrumintheabovewavelength rangeforthefirsttime,and
to-dateresultofthegalaxy numbercounts fromthefar-UVto
alsodemonstratethatthespectralshapeoftheresidualemission
far-IRwavelengths hasbeenpresentedbyDriveretal. (2016).
isverysimilartothatoftheknownZL.
TheresultantIGLbrightnessisconsistentwithearliermeasure-
Thispaperisorganizedasfollows: Section3describesthe
ments, and is much fainter than the residual diffuse emission
HST/FOSobservations,thearchivaldata,andtheimpactofthe
obtained by most of the direct measurements in the optical to
earthshine on the observed sky brightness. In Section 4, we
near-IR.
decomposetheobservedintensityintotheZL,DGL,andresid-
ualemission,usingmodelsoftheindividualcomponents. This
As pointed out by Dwek, Arendt, and Krennrich (2005),
calculationisperformedineightphotometricbands,whosecen-
thespectrumofthenear-IRresidualemissionderivedfromthe
tralwavelengthsrangefrom0.23to0.65µm. Afterthederived
IRTS/NIRS (Matsumoto et al. 2005) measurements is similar
ZLandDGLarediscussedinSection5and6,respectively,we
tothatoftheZL.Thismayimplyapossibilitythattheresidual
showthespectrumoftheresidualemissionincomparisonwith
emissioncontainsanadditionalZLcomponent,whichismissed
earlier measurements and the ZL spectrum in Section 7. The
inthecommonlyusedDIRBEZLmodel(Kelsalletal. 1998).
summaryandconclusionappearinSection8.
TheDIRBE ZLmodel was developed by fitting only thetime
The FOS data are stored in specific intensity I
λ
variation of the sky brightness observed with COBE/DIRBE,
(ergscm−2s−1A˚−1arcsec−2), while IR and UV data
inordertodeterminethephysicalparametersoftheIPDstruc-
are frequently presented in Iν (MJysr−1) and Nλ
tures. Therefore,itcannotuniquelydeterminethetrueZLsig-
(photonscm−2s−1A˚−1sr−1), respectively. In this paper,
nal,becauseanarbitraryamountofanisotropicZLcomponent
most of the results are presented in νIν (nWm−2sr−1). The
couldbeadded(Hauseretal.1998).
conversionformulabetweentheseunitsare:
Strongresidualemissionhasalsobeenreportedintheopti- νIν(nWm−2sr−1)=[3000/λ(µm)]Iν(MJysr−1),
calwavelength,byBernstein(2007).However,sincetheresults νIν(nWm−2sr−1)=0.02Nλ(photonscm−2s−1A˚−1sr−1),
were obtained in only three wide photometric bands at 0.300, Iν(MJysr−1)=1.42×109λ(A˚)2Iλ(ergscm−2s−1A˚−1arcsec−2).
In addition, Lyα and Hα emission strengths are ex-
0.555, and 0.814µm, it is difficult to study the detailed spec-
pressed in units of Rayleighs (R), where 1R =
tral shape of the residual emission. If we observe an overall
1010/4πphotonsm−2s−1sr−1.
similarity in the spectral shapes of the residual and ZL emis-
sionsthroughtheopticaltonear-IRwavelengths,thenitwould
furtherstrengthen the hypothesis thatamissedZLcomponent 3 DATA
contributestheresidualemission.
3.1 Observations
Another constraint on the optical EBL was presented by WeusetheblankskyspectratakenwiththeFOSonboardthe
Matsuoka et al. (2011), who analyzed the data taken with HST.ThesedatawereobtainedinScienceVerification(SV)ob-
NASA’sPioneer10/11spacecrafts. Pioneer10/11arethefirst servationsin1991–1992, priortoinstallationoftheCorrective
spacecrafts to travel beyond the Asteroid Belt and explore the Optics Space Telescope Axial Replacement (COSTAR; in-
outersolarsystem.Matsuokaetal.(2011)madeuseoftheopti- stalledinDecember1993). Therelevantproposalinformation
calimagingdataat0.44and0.64µm,observedataheliocentric is summarized in Table 1. Lyons et al. (1992) and Lyons et
distance>∼3.26AU.Theyperformedaccuratestarlightsubtrac- al. (1993a)usedthesedatatodiscussdependencies ofthesky
tionanddecomposedtheobservedintensity intotheDGLand background ontelescopepointing directions. Weretrievedthe
theresidualemission. BecausetheZLisveryfaintandbelow calibrated data from the HST data archive, which were pro-
thedetectionlimitoftheinstrumentsbeyond3.26AU(Hanner cessedwiththeSpaceTelescopeScienceDataAnalysisSystem
et al 1974), the residual emission is free from ZL contamina- (STSDAS)calfostask.
tionandcanberegardedastheEBL.ThederivedEBLintensity TheFOSwasoneoftheoriginalHSTinstruments.Ithadtwo
is muchlower than the estimates of Bernstein (2007), and are digicon detectors with independent optical paths, which were
comparabletotheIGLintensity. photon counting detectors operated byaccelerating photoelec-
4 PublicationsoftheAstronomicalSocietyofJapan,(2014),Vol.00,No.0
Table1.FOSSVobservationsusedinthepresentanalysis
ProID Proposaltitle Nfld Nexp Tksec
SV2965 FOS-20SKYBACKGROUND-HIGHGALACTICLATITUDE 30 374 47.5
SV2966 FOS-21SKYBACKGROUND-LOWGALACTICLATITUDE 8 132 17.0
SV2967 FOS-22SKYBACKGROUND-LOWECLIPTICLATITUDE 16 222 28.5
Total 54 728 93.0
Note.—Col1:Proposalidentificationnumber.
Col2:Proposaltitle.
Col3:Totalnumberofobservedfields.
Col4:Totalnumberofexposures.
Col5:Totalexposuretimeinksec.
trons emitted fromatwo-dimensional transmissive photocath-
odeontoalineararrayof512silicondiodes. Thebluedigicon 1500 (a) 0.55µm
(BLUE)photocathodewassensitivefrom1150to5400A˚,while
the red digicon (RED) photocathode covered the wavelength 2m/sr) 1000
W/
4ra′·′n3g×e f4r′·o′3mw1a6s2u0setod.8S5i0n0ceA˚t.heTdhieodlaeragrersatyeenxtrteanndceedaopnerlytu1re′·′4o3f ν I (nν 500
inthedirectionperpendiculartothedispersion,theaperturehad
0
aneffectivecollectingarea1of4′·′3×1′·′43.Inthispaper,wean- 15 20 25 30 35 40
Limb angle(degrees)
alyzespectratakenwiththelowresolutiondispersers,namely,
G650L grating and PRISM, combined with the RED digicon. 1000
(b)
Thespectral coverages with these disperser configurations are 800
3540–7075A˚ and1850–8500A˚,respectively. 2m/sr) 600
W/
Theblankskyspectrawereobservedinastandardmanner. n
They were deflected by a quarter of a diode in the dispersion ν I (ν 400
200
direction(NXSTEPS=4)tobettersamplingandwasshiftedin
thedispersiondirectionsothatfiveseparateddiodescontributed 0
0.3 0.4 0.5 0.6 0.7 0.8
λ (µm)
toeachspectralpixel(OVERSCAN=5). Inthisway, the512
diode array produced readout at 2064 spectral positions. The 600
(c)
ACCUMmodewasused,wherespectrawerereadoutatspec- 500
ified intervals (typically a few minutes) and the accumulated 2m/sr) 340000
sum after each read was stored and recorded in consecutive W/
groupsinthestandardoutputdatafiles;eachconsecutivespec- ν I (nν 120000
trumwasmadeupofthesumofallpreviousintervalsofdatain
0
anACCUMobservation. -100
0.20 0.22 0.24 0.26 0.28 0.30 0.32 0.34
The exposures (i.e., integration per readout) were made in λ (µm)
the same way for all the relevant observations (see Lyons et
Fig.1.FOSskydatatakenattheparallelHDFfieldwiththePRISM/RED
al. 1992, for details). The telescope stayed still at the point-
configuration. Panel(a)plotstheskybrightnessat0.55µmasafunction
ing direction established prior to the firstexposure. All expo- oflimbangle;theorbitaldaytimeandtwilight/nightobservationsarerepre-
sures were taken in the sequence of G650L and then PRISM sentedbythecrossesandthedots,respectively. Thefollowingtwopanels
displaythemeandaytime(thedottedlines)andtwilight/nighttime(thesolid
at the same pointing direction. For each disperser, the first
lines)spectrafrom0.3to0.85µm(panelb)andfrom0.2to0.35µm(panel
ACCUMlated data file contains 2 exposures totaling 300 sec- c).
onds(i.e.,150secondsperexposure),thesecondcontains5ex-
posurestotaling600seconds,andthethirdcontains7exposures
3.2 Earthshine
totaling900seconds.
TheHSTaltitudeabovethesurfaceoftheEarthrangesfrom580
to630km. Theorbitisinclinedat28◦5fromtheequator. The
1The physical dimensions of the individual diodes of the digicons corre- ·
sponded tospacings 0′′35along the dispersion direction and height of telescopecompletesoneorbitinevery96minutes,passinginto
·
1′′43perpendiculartoit. theEarthshadowineachorbit,withthetimeinshadow(orbital
·
PublicationsoftheAstronomicalSocietyofJapan,(2014),Vol.00,No.0 5
night) varying from28 to 36 minutes. Because the equatorial rapidly asthetelescopeorbits, andtheorbital day/night status
radius of the Earth is approximately 6378km, the earth limb couldswitchfromnighttodayorviceversaduringanexposure.
islocatedatapproximately24◦ belowthehorizon. Datataken Weobtainedalistofday/nightandnight/daypassingtimesfor
during orbital days have significant contamination fromearth- our observations, with the aid ofthe SpaceTelescope Science
shine, namely, scattered sunlight in the upper atmosphere and Institute (STScI) Help Desk, who kindly extracted the neces-
geocoronal emissionlinessuchasLyαandOI(e.g.,Lyonset sarytelemetrydata.STSDASdeaccumtaskwasusedtounbun-
al.1993a;Shawetal.1998;Brownetal.2000). dle individual exposures from the original ACCUMlated data
Herewedemonstratetheeffectoftheearthshine, usingthe files. Exposure lengths ranged from120 to 150 seconds, dur-
high-quality sky data obtained in the parallel mode observa- ing which the sun - HST - target angle could change as large
tions (proposal ID = 6339) of the Hubble Deep Field (HDF). as 10◦. The coordinates of the telescope were computed by
Theseobservationswerecarriedoutin1995December18–28, theSTSDAShstpostask. Usingthislistofday/nighttransition
afterinstallationoftheCOSTAR,forengineeringpurpose.The times, weassigned daytime fraction(purenighttime =0, pure
pointing center was (l,b)=(126◦1,54◦9) in Galactic coordi- daytime = 1) to individual exposures, and extracted only the
· ·
nates and (λ,β)=(148◦3,57◦2) in ecliptic coordinates. The purenightexposures.
· ·
PRISM and the RED-digicon were used, with the integration Wesearchedforpossiblecontaminationbydiscretesources
times of 46 and 57ksec forthe day and night spectra, respec- (mostly Galactic stars) in two ways, and removed those con-
tively. ThesameapertureasinourSVobservations wasused, taminated exposures from the subsequent analyses. The first
buttheeffectivecollectingareawassmaller(3′′7×1′′3)dueto istoexaminecutouts oftheSloanDigital SkySurvey(SDSS)
· ·
theCOSTARinstallation. and/or Digital Sky Survey images, using the Infrared Science
ArchiveserviceattheInfraredProcessingandAnalysisCenter.
Figure1acomparesthedaytimeandtwilight/nighttime sky
brightnessat0.55µm,asafunctionoflimbangle,whichisthe Thisprocessrejecteddiscretesourcesbrighterthan22.5or20.8
angle between the target fieldand the Earth limb. We usethe Vega-magnitudes in the B or R band (Mickaelian 2004). The
criteria of the daytime fraction (see below) 100 % for orbital secondistolookatthezerothorderspectraofthegratingdata.
day and 0 –50 % for twilight/nighttime. Figure 1bcompares If a small discrete source affects the sky spectrum, its zeroth
the daytime and twilight/nighttime spectra (the sharp spectral order spectral feature should be sharp, because such an ob-
risebeyond0.7µmisanartifact;seeWelshetal. 1998). These jectdoesnotfilltheapertureuniformly. Intotal,weidentified
two panels clearly demonstrate that the daytime spectrum is 7 fields with visible discrete sources, and 4 fields with sharp
brighterandbluerthanthetwilight/nighttime spectrum. Figure zeroth order spectra. Finally, we inspected all the individual
1c displays the daytime and twilight/nighttime spectra in the spectrabyeye,andremovedthosewithapparentnoisespikes.
728exposuresin54fieldssurvivedtheabovecleaningprocess,
bluest part of our spectral coverage. An excess emission is
seenaround0.28µminthedaytime,whoseprofileissimilarto whosetotalexposuretimeamountsto93.0ksec;thesedataare
summarizedinTable1.
thoseofgeocoronalemissionlinessuchasLyαandOIλ1304;
theseemissionlinesfillthespectrographapertureandproduce Figure 2 displays the sky distribution of the above 54
muchbroaderlinewidthsthantheirintrinsicwidths(Eracleous fields,usingtheMollweide projectioninGalactic coordinates.
&Horne1996). ThefeatureinFigure1cmight bearesonant Figure3presentsthenumberofexposuresasafunctionof(a)
MgorMgIIline,thoughthescaleheightofMgatomsisroughly the Galactic latitude, (b) the ZL-subtracted 100µm intensity
100km lower than the HST altitude. We note that Lyons et (Schlegeletal.1998),(c)theGalacticHαintensity(Finkbeiner
al. (1993b) reported a similar “erratic” behavior of UV lines 2003),and(d)theZLintensityat1.25µmbasedontheDIRBE
in FOS sky spectra, including the one at 2802 A˚. They found ZLmodel(Kelsalletal.1998).
thattheseUVlinesoccurexclusivelyduringdaytimeexposures,
whilethelinesareabsentonmostofthedaytimespectrataken.
4 ANALYSIS
Thus, weconclude that theremight beapossible contribution
fromgeocoronalemissionlinestoadaytimeskyspectrum.
Becausetheskybrightnessisverylowintheindividualspectral
elements,weperformthefollowinganalysiswiththesynthetic
photometry ineightbands from0.2to0.7µm,assummarized
3.3 NighttimeData
in Table 2. We first checked the consistency of the flux cali-
Inordertoavoid possiblecontamination oftheearthshine, we bration between the G650L and PRISM measurements in the
useonlythenighttimedatainthefollowinganalysis,although 0.42,0.47,0.55,and0.65µmbands,whetherthetwomeasure-
the contamination may be small in the twilight data as well ments are available. As a result, we found a significant con-
(see Figures 1 and 3 of Brown et al. 2000). When viewed flict between the G650L and PRISMbrightness, whose origin
fromtheEarth,theanglebetweentheHSTandthesunchanges isunknownatthemoment. Thuswedecidedtorecalibratethe
6 PublicationsoftheAstronomicalSocietyofJapan,(2014),Vol.00,No.0

90
1000
30
sr)
2m/
180 120 60 0 -60 -120 W/ 100
n
-30 ν I (ν

-90 10
0.2 0.3 0.4 0.5 0.6 0.7 0.8 1.0
λ (µm)
Fig.2.SkydistributionoftheFOSorbitalnighttimedatausedinthepresent
analysis,plottedwiththeMollweideprojectioninGalacticcoordinates. The Fig.4.FOSskyspectrumderivedbyintegratingallthenighttimespectrare-
solidlinerepresentstheeclipticplane. gardlessofthefields. ThesmallandgraylargedotsrepresenttheG650L
andPRISMdata,respectively.Thesolidlineandtheopencirclesrepresent
thesolarspectrumgivenbyColina,Bohlin,andCastelli(1996)andtheZL
MJy/sr brightnesspresentedbyLeinertetal. (1998),respectively;theywerearbi-
0.4 1 2 4 10 20 40
200 200 trarilyscaledforeaseofcomparingthespectralshapes.TheFOSspectrum
(a) (b)
150 150 ismuchsmootherthanthesolarspectrumbecauseofthelowerdisperser
Exposures100 Exposures100 resolution.
50 50
correct,andscaledowntheG650Lintensitybyauniformfac-
0 0
0 20 40 60 80 1.0 1.5 2.0 2.5 3.0 3.5
Galactic latitude (degrees) Log ν Iν(100 µm: nW/m2/sr) torof1.50.
MJy/sr Figure 4 presents the FOS sky spectrum compared with
0 0.5 1.0
200 200
(c) (d) the solar spectrum (Colina et al. 1996) and the ZL spectrum
150 150
Exposures100 Exposures100 (teLgerianteinrtgeatllalt.he1e9x9p8o).suTrehse, rsekgyarsdpleecstsruomf thweasfieolbdtacionoerddibnyatiens-.
50 50
These spectra are similar to each other in the optical wave-
0 0
-1.0 -0.5 0.0 0.5 1.0 1.5 2.0 0 1000 2000 3000 length,suggestingthattheZLdominatestheskyemission. On
Log Hα(R) ZL model ν Iν(1.25 µm: nW/m2/sr)
the other hand, the UV sky spectrum shows a significant ex-
Fig.3.Numberofexposuresasafunctionof(a)theGalacticlatitude,(b)the cess over the solar spectrum. This maybe due to strong time
ZL-subtracted100µmintensity,(Schlegeletal. 1998),(c)theGalacticHα
variationofthesolarspectralirradianceintheUV,asreported
intensity(Finkbeiner2003),and(d)theZLintensityat1.25µmbasedonthe
byErmollietal. (2013)fromtheSOlarRadiationandClimate
DIRBEZLmodel(Kelsalletal.1998).
Experiment.
G650Lmeasurementsasfollows,byreferringtotheearlierin-
dependentmeasurements. 4.1 ModelsoftheEmissionComponents
The FOS twilight/nighttime spectrum of the parallel HDF Here we decompose the observed intensity of the diffuse sky
field,whichwediscussedinSection3.2,hasthemean0.55µm emissionintothethreecomponents,i.e.,theZL,theDGL,and
brightness of 416nWm−2sr−1 (seeFigure 1b). On the other the residual emission, in each of the eight photometric bands.
hand,Leinertetal.(1998;theirtable16)presentstheZLbright- The EBL is included in the residual emission. The model
ness of 407nWm−2sr−1 at 0.5µm in the same field at the brightness,Iν,i(Model)atthei-thband,isdefinedas
time of our PRISM observations, based on the ground-based
measurementsbyLevasseur-RegourdandDumont(1980)with Iν,i(Model)=Iν,i(ZL)+Iν,i(DGL)+Iν,i(RSD), (1)
a slight update. These two independent measurements agree
with each other within 2%. Therefore, we concluded that the whereIν,i(ZL),Iν,i(DGL), andIν,i(RSD)arethebrightness
oftheZL,DGL,andresidualemission,respectively.
PRISM/REDmeasurementsarerobust. However,theG650in-
tensityislargerthanthePRISMintensitybyanalmostuniform Weminimizethefollowingχ2function:
factor,1.460−1.565,inthefouroverlappingbands. Although Neixp
thereasonisnotclear,thisdifferencemightbecausedbyflat- χ2i = X[Iν,i(Obs)−Iν,i(Model)]2/σν2,i (2)
field calibration. We thus assumethat the PRISMintensity is
j
PublicationsoftheAstronomicalSocietyofJapan,(2014),Vol.00,No.0 7
Neixp theZLforeground, itmaybecontributed byemissioncompo-
= X[Iν,i(Obs)−Iν,i(ZL)−Iν,i(DGL)−Iν,i(RSD)]2/σν2(,3i,)nents notassociatedwiththeISM(hereafternon-ISM100µm
j emission), e.g., the EBL at this wavelength. Equation (8) ac-
σ2 =σ2 (Obs)+σ2 (ZL)+σ2 (DGL), (4)counts for this non-ISM 100µm emission, which is assumed
ν,i ν,i ν,i ν,i
tobe0.8MJysr−1. Lagacheetal. (2000) estimateditsinten-
whereIν,i(Obs)istheobservedskybrightness, j referstothe sityof0.78±0.21MJysr−1,usingthedatacollectedwiththe
j-thexposure,andNi isthetotalnumberofexposuresinthe
exp DIRBEandFarInfraredAbsoluteSpectrophotometeronboard
i-thband. Thequantitiesσν,i(Obs),σν,i(ZL),andσν,i(DGL) the COBE. Matsuura et al. (2011) derived a similar non-ISM
are the uncertainty in the observed sky brightness, the ZL
intensityof0.67±0.19MJysr−1 at90µm,fromobservations
model, and the DGL model, respectively, and σν,i represents withthefar-infraredsurveyoronboardtheAKARIsatellite.The
theirsum. WedescribethemodelsoftheZLandDGLandthe
presence of non-ISM emission at this level (∼0.8MJysr−1)
relateduncertaintiesbelow.
was also supported by Matsuoka et al. (2011), in their anal-
ysis of the correlation between the ZL-subtracted diffuse op-
4.1.1 ZodiacalLight
tical light and the SFD 100µm map. Planck Collaboration
Becausethere arenoZL modelfromtheUV tooptical wave-
XXX (2014) estimated the non-ISM 100µm emission to be
lengths, we estimate the ZL brightness by extrapolating the
0.44±0.03MJysr−1 basedontheirextendedhalomodel. We
near-IRDIRBEZLmodeltotheshorterwavelength:
note that, in principle, this non-ISM 100µm emission can in-
Iν,i(ZL)=aiIν,i(Sun)D1.25, (5) clude the (residual) ZL, EBL, and any other unknown com-
ponents. For example, Dole et al. (2006) suggested that the
σν,i(ZL)=0.02Iν,i(ZL), (6)
non-ISMemissionfoundbyLagacheetal. (2000)includesthe
where ai is the reflectance in the i-th band, and D1.25 refers residualZLof0.3MJysr−1.
tothebrightness oftheDIRBEZLmodelat1.25µm,inunits
In the optically thin case, Equation (7) should be ex-
of MJysr−1. Iν,i(Sun) is the i-th band intensity of the solar pressed as a linear correlation, Iν,i(DGL) = biIν,100. Thus
spectrum scaled to 1MJysr−1 at 1.25µm. Itis derived from
we call bi the slope coefficient or correlation slope, which
thesolarspectrumgivenbyColina,Bohlin,andCastelli(1996).
is also presented as νbi = [3000/λ(µm)]bi in units of
If the ZL spectrum is identical to the solar spectrum, then ai nWm−2sr−1/MJysr−1 below. However, our analysis needs
shouldbeunityinalltheeightbands.Thuswecallaithescaled to deal with optically thick regions with up to Iν,100 =
ZL reflectance, which is unity at 1.25µm by definition. The 50MJysr−1, corresponding to the visual extinction AV ∼ 5
factorof0.02inEquation(6)comesfromtheuncertaintyinthe if we adopt Iν,100/AV ∼ 8−15MJysr−1mag−1 (Ienaka et
DIRBE ZL model (Kelsall et al. 1998); here we assume that
al. 2013). With a compilation of the optical data of high-
thisvaluegivesthestatisticaluncertaintyoftheZL.
latitude clouds, Ienakaetal. (2013) foundthatthecorrelation
In the present analysis, the ZL spectrum is assumed to be
deviates from a linear function, i.e., the DGL intensity appar-
isotropic throughout the sky. Indeed, Tsumura et al. (2010)
entlysaturatestowardopticallythickregionswithhigh100µm
found such an isotropy of the ZL brightness at 0.8–1.25µm intensity. They suggested that a negative I2 term should
ν,100
fromtheCIBER/LRSmeasurements.TheZLissimilarlydomi-
be introduced to the DGL models (Equation 7) to account for
natedbythescatteredsunlightinboththeFOSandCIBER/LRS
thissaturationeffect. Atfar-UVwavelengthswheretheoptical
spectralcoverages. However, theaboveZLmodelmaybetoo
depthismuchlargerthanintheoptical,thecorrelationbecomes
simplistic to fully account for the wavelength-dependent scat-
flatter with alarger scatter toward optically thick regions with
tering phasefunction ofthe IPDgrains, andmayneed further Iν,100>5MJysr−1 (Haikalaetal. 1995; Sujathaetal. 2009;
improvementinfutureworks.
Murthyetal.2010;Sujathaetal.2010). Thislargescattermay
beexplainedbynotonlythesaturation, butalsothevariations
4.1.2 DiffuseGalacticLight
in the intensity and spectral shape of the interstellar radiation
TheDGLmodelbrightnessisdefinedas
field(ISRF);theISRFcanbequitedifferentfromthatinopti-
Iν,i(DGL)=biIν,100−ciIν2,100, (7) callythinregions,becauseopticallythickregionsmayhoststar
formationandhencemanyhotyoungstars.
Iν,100=Iν,SFD−0.8MJysr−1, (8)
OurDGLmodelwithaquadraticpolynomialfunctionmight
σν2,i(DGL)=[bi−2ciIν,100]2σν2,100, (9)
be too simple to fully account for the effects of the saturation
wherebiandciarefreeparameters.Iν,100isthe100µminten- andISRFvariation. Wecheckedthiswithanalternativemodel,
sityfromtheISM.Iν,SFDisthe100µmintensitytakenfromthe whichincludes anadditional cubicIν3,100 term,andfoundthat
diffuseemissionmapofSchlegeletal.(1998;SFDhereafter). boththequadraticandcubicfunctionmodelsfitreasonablywell
WhiletheSFD100µm maphas beenprocessedtoremove tothedata,i.e.,thereisnoadvantageofaddingthecubicterm.
8 PublicationsoftheAstronomicalSocietyofJapan,(2014),Vol.00,No.0
100
(a) DGL correlation slope
0.015
(a) 0.27 µm
MJy) Jy/sr) 0.010
2nW/m/ 10 00 -- 5300 MMJJyy//ssrr uals (M 00..000005
ν b (i 00 -- 2105 MMJJyy//ssrr Resid -0.005
0 - 10 MJy/sr -0.010
1 0 - 5 MJy/sr 0 200 400 600 800 1000
0.2 0.3 0.4 0.5 0.6 0.7 0.8 Night-depth at the exposure (seconds)
λ (µm)
2.0 0.12 (b) 0.55 µm
(b) Scaled ZL reflectance sr) 0.10
y/ 0.08
aled reflectance 01..80 Residuals (MJ 0000....00000246
Sc -0.02
0 200 400 600 800 1000
0.6 Night-depth at the exposure (seconds)

0.2 0.3 0.4 0.5 0.6 0.7 0.8
λ (µm) 25
(c) Lyα
100 R) 20
(c) Residuals k
sr) s ( 15
2m/ ual
W/ sid 10
n e
ν I (ν R 5
s 0
al 0 200 400 600 800 1000
u 10
d Night-depth at the exposure (seconds)
si
e
R
Fig.6.ResidualemissionbrightnessIν,i(RSD)asafunctionofthenight-
0.2 0.3 0.4 0.5 0.6 0.7 0.8
λ (µm) depthat0.27µm(panela),0.55µm(panelb),andforLyα(panelc). The
typicalerrorbarsarepresentedinthelowerrightofeachpanel.Weusethe
Fig.5.Fittingresultsofthesixsamples(Iν,100 <5,10,15,20,30,and fullsamplewithIν,100<50MJysr−1inthisfigure.
50MJysr−1),representedbythedifferenttypesoflinesasindicatedinthe
lower right ofpanel (a). Theopen circlesrepresent theinverse-variance
weightedmeanofthefoursamples,Iν,100<15,20,30,and50MJysr−1 allthesamples.Inthefollowingdiscussion,weusetheinverse-
(seetext). Panels(a),(b),and(c)presenttheDGLcorrelationslopeνbi= varianceweightedmeanofthebest-fitparametervaluesofthe
b[3ri0g0h0tn/eλs(sµνmI)ν],bii(,RtSheDs),carelesdpeZcLtivreelfly.ectanceai,andtheresidualemission foursamples,Iν,100<15,20,30,and50MJysr−1,asreported
inFigure5andTable2. Thequoteduncertaintiesaretwicethe
errorvalues obtained by a simple propagation of the errors of
WethusconcludethatthequadraticfunctionmodelinEquation the individual samples, which takes into account the fact that
(7)givesanadequaterepresentationoftheDGLforthepresent thefoursamplesarenotindependentofeachother.
purpose.
4.1.4 ContributionfromOtherEmissionComponents
4.1.3 FittingResults Figures6aand6bpresentthebrightnessofthederivedresidual
Weperformourfittinganalysisinthesixsampleswiththedif- emissionat0.27and0.55µm,asafunctionofthenightdepth.
ferentmaximum100µmintensity, namely, Iν,100<5,10,15, Thenightdepthisthetimeintervalbetweenday-to-nighttran-
20,30,and50MJysr−1,inordertoexaminetheeffectofDGL sition and the beginning of the exposure, or that between the
saturation in optically thick regions (see above). The results end of the exposure and night-to-day transition, whichever is
arepresented inFigure 5. TheDGL correlation slope νbi be- shorter. Thenight-timedurationistypically ∼2,000seconds,
comesnegativeinsomephotometricbandsinthetwosamples, so the night depth of ∼ 1,000 seconds corresponds to mid-
Iν,100<5and10MJysr−1(thenegativepointsarerepresented night. The figures exhibit no correlation, which confirms that
bythemissingpartsofthecurvesinFigure5a),whichsuggests theearthshinemakeslittlecontributiontoouranalysis.
thatourDGLmodelfailedtofitthedata. Thisislikelycaused We further examined dependency of the residual emission
bythelimitedfittingrangeofIν,100 inthesesmallestsamples. brightness on various quantities, which include the ZL model
Theotherfittingparametersweredeterminedreasonablywellin brightness, 100µm intensity, Galactic latitude and longitude,
PublicationsoftheAstronomicalSocietyofJapan,(2014),Vol.00,No.0 9
zenith angle, limb angle, moon phase, and the solar MgII in-
0.015
dex that is related to the solar activity. No correlation is
fisooutnrodp,iwc.hicIhnipnadritcicautelasrt,hnaotocuorrrreelsaidtiuoanlwemithisstihoenGIνa,lia(cRtiScDl)atiis- MJy/sr) 00..000150 (a) 0.27 µm
tudesuggeststhatGalacticstarsaresuccessfullyremoved(see als (
du 0.000
Section 3.3) and thatthe residual emissioncontains little con- esi
R
-0.005
tributionfromtheirradiation.
-0.010
TheFOSSVobservations usedinthis workalsomeasured 0.1 1.0 10.0 100.0
H α (R)
Lyα emission, using the BLUE digicon and the G150L grat-
ing. It provides an useful measure to examine the earthshine 0.12 (b) 0.55 µm
0.10
contributiontofar-UVdiffuseskybrightness,becausethesolar
Lyαcandiffuseinto thenight skythrough resonantscattering MJy/sr) 00..0068
byhydrogenatomsintheexosphereoftheEarth(Eracleous& uals ( 0.04
d
Horne1996). WedecomposedtheobservedLyαintensityinto esi 0.02
R
theZL,DGLandresidualemissioninthesamewayasabove, 0.00
and found that the Lyα intensity is dominated by the residual -0.02
0.1 1.0 10.0 100.0
emission, i.e., it has little correlation with the ZL or the SFD H α (R)
100µmbrightness. Figure6cpresentsthederivedLyαresidual 25
(c) Lyα
brightnessasafunctionofthenight-depth. TheLyαbrightness
20
decreases toward midnight, which suggests that the solar Lyα R)
emissionindeeddiffusesintothenightskyasdescribedabove. als (k 15
u
Similarcorrelationshavebeenfoundinthefar-UVcountrates esid 10
R
at the 0.153 and 0.231µm bands from the Galaxy Evolution 5
Explorer(GALEX)observations(Sujathaetal2010.). 0
0.1 1.0 10.0 100.0
Figure7displaystheresidualemissionbrightnessasafunc- H α (R)
tionoftheGalacticHαintensity,takenfromFinkbeiner(2003).
Noclearcorrelationisfound,whichisconsistentwiththeidea Fig.7.Residual emission brightness Iν,i(RSD) as a function of the
GalacticHαintensity,at0.27µm(panela),0.55µm(panelb),andforLyα
thattheLyαemissionisofgeocoronalorigin. (panel c). Thetypicalerrorbarsarepresentedinthelowerrightofeach
panel.WeusethefullsamplewithIν,100<50MJysr−1inthisfigure.
5 ZODIACAL LIGHT
flectance spectrum in this wavelength range. A similar trend
Figure 8a presents the derived ZL spectrum, which is a prod- wasreportedbyLummeandBowell(1985),whosuggestedthat
uct of the solar spectrum and the ZL reflectance ai (linearly theZLcolorcloselyresemblesthecolorsofC-typeasteroids.In
interpolated in wavelength). All the spectra in this panel are addition, some CM chondrites reportedly have flat reflectance
scaled to 1MJysr−1 = 2,400nWm−2sr−1 at 1.25µm, for spectraintheoptical(Vernazzaetal. 2015). Theseresultsmay
ease of mutual comparison. Our result is in good agreement alsosupporttheC-typeasteroidoriginoftheIPDparticles.
withtheZLspectrumpresentedbyLeinertetal. (1998,toward Yang and Ishiguro (2015) suggested that the most of the
λ−λ⊙=90◦,β=0◦).WealsoshowtheIRresultstakenfrom IPDparticlesoriginatefromcomets(D-typeasteroids),bycom-
theCIBER/LRS measurements(Tsumuraetal. 2010) andthe paring optical properties (i.e., albedo andspectralgradient) of
IRTS/NIRS measurements (Matsumoto et al. 1996). The ZL various asteroids (Bus & Binzel 2002) with those of the ZL
spectrumisredderthanthesolarspectrumat>1.5µm,which (Ishiguroetal. 2013). Thisresultisconsistentwithanumeri-
maypointtomajorcontributionoflargeIPDparticles(>1µm) calsimulation,whichtakesintoaccountkinematicanddynam-
tothenear-IRZL(Matsuuraetal.1995). ical processes of the IPD (Nesvorny´ et al. 2010). However,
Figure8bpresentsthederivedZLreflectance,alongwiththe wefoundthattheZLspectrumdifferssignificantlyfromthatof
IRresults presentedbyTsumuraetal. (2010)andMatsumoto D-type asteroids (seeFigure 8b), which is in conflict with the
etal. (1996). Inthenear-IR,Tsumuraetal. (2010)suggested cometaryoriginoftheIPDresponsiblefortheopticalZL.
thattheZLisdominatedbyS-typeasteroidaldust,basedonthe ItisworthnotingadipintheZLreflectancespectrumseen
similarityoftheirreflectancespectraat>1.5µm.Ontheother at around 0.3µm. Sucha UV absorption featurehas been re-
hand, we found that the reflectance spectrum becomes much portedinearlierstudies,whichmeasuredreflectancespectrain
flatter intheoptical. This indicates alargecontribution tothe varioussamplesofasteroidsandmeteoritesinlaboratoryexper-
ZL dust fromC-type asteroids, which have asimilarly flatre- iments(e.g.,Hiroietal.1996;Yamadaetal.1999;Matsuokaet
10 PublicationsoftheAstronomicalSocietyofJapan,(2014),Vol.00,No.0
-1sr) 10
y
J
M
-1sr/
µm) 1100000000 (a) -2 Wm HPGCiSOAoTnLB eEE PeX rr S1e M0as/enu1nor1tt h e syMtt uaeadltt. ys a(u2l.o0 (k12a50 )1e0t )al. (2011)
5 n CIBER Arai et al. (2015)
at 1.2 νb (i MZWZDDBDAAM0001344/2B//BM CICMe00n3P3a 8 kB 3aBr a reBantnr daadtln t.& d&( 2tD 0&Dr1 arDa3ini)rnaeei n( 2(e20 0(121202)1)2)
-2 -1msr 11000000 1 0.2 0.3 0.4WZDDA00014./5M/BMC0P038. 63× 2B r aBnrad0nt .d&8t D&r Da1irnae.i0n (e2 0(21021)2)
W λ (µm)
n
0
40 Present study
o 2, 110000 LTesuinmerutr eat eatl .a (l.1 (929081)0) Fig.9.TheDGLcorrelationslopeνbiasafunctionofwavelength,compiled
ed t MSactasulemd oZtoL espt eacl.t r(u1m996) farlo.m20t1h1is;wopoerkn(cfiirllceldesc)i,rcthleesG),AthLeEXPimoneeaesru1r0e/m11enmtsea(Msuurretmhyeenttsal(.M2a0t1s0u;ookapeent
al Scaled solar spectrum
c diamonds), the CIBER measurements (Arai et al. 2015; open squares),
s
ν I (ν 1100 00..22 00..33 00..44 00..5500..66 00..88 11..00 22..00 33..00 tghreouDndIR-bBaEsemdemaseuarseumreemntesnt(sSatonwoaredt aal.hig2h01G5a;laocptiecnlattriitaundgelet)r,anasnlducethnet
λ (µm) cloud(Ienakaetal. 2013;asterisks). ThesyntheticDGLspectraofBrandt
andDraine(2012),towardahighGalacticlatitude(b=60◦),aredisplayed
with the four different lines: ZDA04/BC03 (solid), WD01/BC03 (dotted),
1.4
(b) ZDA04/MMP83 (dot-dashed), and WD01/MMP83 (dashed). The dot-dot-
Present study
Tsumura et al. (2010) dashedlinerepresentstheZDA04/BC03modelscaledtotheobservedval-
Matsumoto et al. (1996)
uesintheoptical.
1.2 ZL reflectance spectrum
S-type asteroid
C-type asteroid
ce D-type asteroid
an al.2015).DulayandLazarev(2004)proposedtheπ−π∗pras-
ct 1.0
e mon resonance in polycyclic aromatic hydrocarbon molecules
efl
R as an origin of this absorption, while Cloutis et al. (2008)
0.8 raised a possibility of Fe-O charge transfer transition in min-
erals,whichisexpectedtoproduceseveralabsorptionfeatures
at 0.2−0.4µm. The exact nature of this spectral dip will be
0.6
0.2 0.3 0.4 0.50.6 0.8 1.0 2.0 3.0 identified in future ZL measurements with much higher spec-
λ (µm) tralresolutionthanavailablenow.
Fig.8.Panel(a): TheZLbrightnessderivedinthiswork(filledcircles)and
thosemeasuredbyTsumuraetal. (2010;triangles)andMatsumotoetal.
6 DIFFUSE GALACTIC LIGHT
(1996;diamonds).TheZLandsolarspectraarerepresentedbythesolidand
d(λas−heλd⊙lin=es9,0r◦es,pβe=ctiv0e◦ly).,pTrheeseonpteendcbiryclLeesinsehrotwetthael.Z(L19b9ri8g)h.tnAelslsthteowdaartda Figure9presentsthederivedDGLcorrelationslopeνbi,along
arescaledto1MJysr−1=2,400nWm−2sr−1 at1.25µm. Panel(b): with the results from earlier measurements. As we discussed
TheZLreflectancederivedinthiswork(filledcircles)andthosemeasured in Section 4.1.2, the correlation slope νbi of our model (see
byTsumuraetal.(2010;triangles)andMatsumotoetal.(1996;diamonds). Equation 7) provides the DGL spectrum in optically thin re-
TheZLreflectancespectrum,whichconnectstheabovedatapoints,isrep- gions. Aclear4000A˚ breakisobservedintheDGLspectrum,
resentedbythesolidline. Thesespectraarenormalizedat1.25µm. The
whichwasalsofoundbyBrandtandDraine(2012)intheiranal-
arbitrarilyscaledreflectancespectrumofanS-typeasteroid25143-Itokawa
(Binzeletal. 2001), typicalC-type,andD-type(cometary)asteroids(Bus ysisofSDSSskyspectra.
&Binzel2002)arerepresentedbythedotted,dashed,anddot-dashedline,
Brandt and Draine (2012) studied the relation between the
respectively.
dust-scattered light and the 100µm emission in optically thin
ISM, with single-scattering radiative transfer calculations as-
suming a plane parallel galaxy. They used two models of the
local ISRF continua based on Mathis, Mezger, and Panagia
(1983;MMP83)andBruzualandCharlot(2003; BC03),com-
binedwithtwodustmodelsbasedonZubko,Dwek,andArendt
(2004; ZDA04) and Weingartner and Draine (2001; WD01).