Large-scale variations of the dust optical properties in the Galaxy

0.38 MB·English

by  L. Cambresy

Astronomy&Astrophysicsmanuscriptno.1767 February2,2008
(DOI:willbeinsertedbyhandlater)
Large-scale variations of the dust optical properties in the Galaxy
L.Cambre´sy1,T.H.Jarrett2,andC.A.Beichman3
1 ObservatoireAstronomiquedeStrasbourg,F-67000Strasbourg,France
e-mail:[email protected]
2 InfraredProcessingandAnalysisCenter,100-22,CaliforniaInstituteofTechnology,CA91125
5
e-mail:[email protected]
0
3 MichelsonScienceCenter,InfraredProcessingandAnalysisCenter,100-22,CaliforniaInstituteofTechnology,CA91125
0
2 e-mail:[email protected]

n
Received;accepted
a
J
Abstract.Wepresentananalysisofthedustopticalpropertiesatlargescale,forthewholegalacticanticenterhemisphere.We
0
2 usedthe2MASSExtendedSourceCatalogtoobtainthetotalreddeningoneachgalaxylineofsightandwecomparedthisvalue
totheIRAS100µmsurfacebrightnessconvertedtoextinctionbySchlegeletal.(1998).Weperformedacarefulexamination

1 andcorrectionofthepossiblesystematiceffectsresultingfromforegroundstarcontamination,redshiftcontributionandgalaxy
v selectionbias.Wealsoevaluatedthecontributionofdusttemperaturevariationsandinterstellarclumpinesstoourmethod.The
4 correlationofthenear–infraredextinctiontothefar–infraredopticaldepthshowsadiscrepancy forvisualextinctiongreater
4 than 1 mag with a ratio A (FIR)/A (gal) = 1.31 0.06. We attribute this result to the presence of fluffy/composite grains
4 V V ±
characterizedbyanenhancedfar–infraredemissivity.Ouranalysis,appliedtohalfofthesky,providesnewinsightsonthedust
1
grainsnaturesuggestingfluffygrainsarefoundnotonlyinsomeveryspecificregionsbutinalldirectionsforwhichthevisual
0
extinctionreachesabout1mag.
5
0
/ Keywords.ISM:dust,extinction–Galaxies:photometry–Infrared:ISM
h
p
-
o 1. Introduction (Johnstoneetal.,2003).TheinitialresultsfromIRAS,DIRBE,
r or ISO, which were mainly focused on dust abundances and
st Large scale dust studies started with IRAS which revealed temperature, reveal much more if re-examined in light of
a the whole sky in four far–infrared (FIR) wavelength bands.
theserecentdustopticalpropertyresultsobtainedfromlonger
:
v It led to the current view of the interstellar dust seen as a 3
wavelengths, notably in the submm. Cambre´syetal. (2001)
i components grain population (De´sertetal., 1990): polycyclic
X have quantified the emissivity enhancement at 100 µm com-
aromatic hydrocarbons (PAH), very small grains (VSG)
r paring COBE/DIRBE with an extinction map of the Polaris
a and big grains (BG). The color analysis of IRAS surface Flare. Their results have been confirmed by delBurgoetal.
brightnesses provides insight to the variations of the 3 dust
(2003)whofoundsimilarbehaviorfortheFIRdustemissivity
componentabundances with respect to various environments.
in eight nearby interstellar regions observed by ISOPHOT.
BGdominateinmolecularcloudswhereasVSGaremorerep-
Dale&Helou (2002) studied a sample of galaxy spectral
resentativeofthediffuseinterstellarmediumusuallyassociated
energy distributions with ISO and IRAS, proposing a change
with the atomic hydrogen. Our understanding of interstellar
in the dust emissivity at wavelengths longer than 100 µm to
medium (ISM) dust has further evolved with advances in
match SCUBA brightnesses. ISOPHOT sources discoveredat
FIR and submillimeter (submm) instrumentation in the last
200 µm (but not detected at 100 µm) suggest a change in the
decade. Not only is the dust composed of different grain size
grainproperties(Lehtinenetal.,2003).
components but the optical properties of the dust change
with the environment. The PRONAOS balloon observations
reveal the complexity of the dust properties with emissivity Althoughitisnowestablishedthatthedustopticalproper-
variations (Bernardetal., 1999; Stepniketal., 2003) and tiesvarywiththeenvironment,likelyduetofluffygrainpopu-
spectral index variations (Dupacetal., 2002). More difficult lation,thespatialextentinthesevariationsisstillcontroversial
becauseoftheatmosphereabsorptionbutstillpossible,ground andthedensitythresholdforthetransitionfromregulartothe
based observations in the submillimeter range also find evi- enhancedemissivitydustpopulationremainsunknown.Inthis
denceofdustemissivityvariationswithSCUBA observations paperweaddressthesequestionsusingalargescaleanalysisof
thewholegalacticanticenterhemisphere.Weproposetocom-
Sendoffprintrequeststo:L.Cambre´sy paredatafromIRASandDIRBE,convertedtodustextinction
2 L.Cambre´syetal.:Large-scalevariationsofthedustopticalpropertiesintheGalaxy
bySchlegeletal.(1998)(hereafterSFD98),withthereddening
of2MASSgalaxies.Ourgoalistoinvestigatethedustproper-
tiesthroughtheapparentdiscrepancybetweenthesetwoquan-
tities.The2MASSextendedsourcecatalogcharacteristicsare
presentedinSect.2followedbyadetailedanalysisofthebiases
in the galaxycolorexcessesin Sect. 3. Section4 is dedicated
to the analysis of the correlation between the galaxy redden-
ingandtheFIRdustopacity,the conclusionsofthepaperare
presentedinSect.5.
2. The2MASSextendedsourcecatalog
2.1.Presentation
The2MASSAll-SkyDataReleaseforextendedsources(XSC)
containspositionsandphotometryinJHK for1.6millionob-
s
jects(Jarrettetal.,2000a).About97%aregalaxiesbutthecat-
alog also contains galactic resolved sources such as HII re-
gions,planetarynebulae,reflectionnebulae,youngstellar ob-
jects,etc.Galaxieswereidentifiedusingadecisiontreemethod
which uses several parameters including size, shape, central
surfacebrightnessandcolor.Inordertoincreasethereliability
oftheobjectclassification,avisualclassificationhasalsobeen
performed. It is mainly based on the eye examination of the Fig.2. Magnitude distribution of galaxies at J (upper panel)
color-combinedimages, but also optical DSS images when it andK (lowerpanel)forlatitudesgreaterthan25degandlower
s
wasneeded.Roughly25-30%ofallXSCsourceshavebeenin- than5deg.Onlysourcesinthegalacticanticenterhemisphere
spected,withcloseto100%forbrightsources(K <12.5mag, (longitudefrom90to270deg)areconsidered.
s
Cutrietal.,2003).AllXSCsourceshavebeeninspectedinthe
galactic plane (b < 5deg) and in some otherregionsof spe-
cific interest (M|a|gellanic clouds, galaxy clusters like Virgo). level of completeness for the detection depends on the lati-
Visual inspection reveals that only a small fraction (less than tude as the stellar density increases in the plane. Magnitude
1%) of sources are artifacts of various origins such as bright distributionsfor b > 25degand b < 5deg are presentedin
| | | |
star neighborhood,airglow,meteorstreaksorimageedges.In Fig.2(Jarrettetal.,2000b).Thelowandhighlatitudesamples
total, among the 1.5 106 galaxies in the XSC, more than are quite similar. The latitude has only a small impact on the
3 105havebeen∼confir×medbyeye. value of the turn off in the distribution which represents the
× 2MASSsensitivitylimitforextendedobjects.Thelow-latitude
distribution is almost parallel to the high-latitude curve, in-
2.2.Catalogproperties
dicating the confusion noise reduces the completeness at all
magnitudes (except the very bright end, J < 9) without any
2.2.1. Sourcedensityandcompleteness
significantchangeintheobservedluminosityfunctionshapes.
Figure1showsthespatialdistributionoftheXSCgalaxies,fil- Consequently,we do not expectany significant color system-
tered from the Milky Way extended sources. The large scale aticeffectfromphotometrysignal-to-noise.
structuresoftheuniversedominatethesourcedistributionover
the local dust extinction. The confusion noise prevents the
2.2.2. Galaxycolors
galaxy identification in the very high stellar density regions
such as the Galactic Center (Jarrettetal., 2000b). The strik- The color distribution of galaxies is related to their morpho-
ingfilamentarystructurein thesourcedistributionreflectsthe logicaltype.Itreflectsvariouscharacteristicssuchasthestel-
largescalestructureofgalaxiesintheuniverse(seealsoJarrett, lar content (old/youngpopulations,star formingrate) and the
2004).ItisworthnotingthatifFig.1wererestrictedtoalongi- dust fraction. Jarrett (2000) and Jarrettetal. (2003) analyzed
tuderangeof90to270deg(i.e.theanticenterhemisphere),the 2MASS galaxy properties according to their morphological
Milky Way disk would barely be seen. We will therefore not types and found that ellipticals are slightly redder in near–
considerusingsourcecountsinthenear–infraredasanextinc- infraredcolorthanlenticularsandthatnormalbarredandtran-
tionestimatorasithasbeendonebyBurstein&Heiles(1982) sition barred spirals have similar colors. However the disper-
in the visible. Since the source counts are incomplete in the sionforspiralsis increasedbyafactorof2,goingfromearly
Galactic Center region,the analysis presentedin this paper is tolatetypeduetoimportantstarformationandamountofdust
restrictedtotheanticenterhemisphere,about8.5 105galaxies. material in the earliest types. For some morphologies, color
×
In the following, magnitudes always refer to the fiducial canbeclearlydifferentasforAGNwhichare0.3-0.4magred-
elliptical isophotal (K = 20 mag arcsec 2) magnitudes. The derthantheaverage,ordwarf,irregular/peculiarandcompact
s −
L.Cambre´syetal.:Large-scalevariationsofthedustopticalpropertiesintheGalaxy 3
Fig.1.Spatialdistributionofthe1.5 1062MASSgalaxiesprojectedinrectangularcoordinates.MilkyWayobjectshavebeen
×
removed.TheCartesianprojectionisingalacticcoordinatesandiscenteredattheanticenterdirection.Theabsenceofdetected
galaxiesnearthegalacticcenterresultsfromtheconfusionnoiseduetohighstellardensities.
galaxieswhicharebluerthanthenorm.Ellipticalsandspirals
dominatethegalaxypopulationandwefindJ K =1.0 0.1
s
− ±
forhigh-latitudegalaxies(noMilkyWayextinction)restricted
toJ <14.0andK <13.3.Thesemagnitudecutsareaddressed
s
laterinthepaperandcorrespondtoourhigh-latitudesampleas
shownFig.6.
3. Extinctionfromgalaxycolors
TheextinctionestimationisbasedontheJ K galaxycolorex-
s
−
cessfromthenormaldistribution.Visualextinctionisderived
fromthecolorexcessE asfollows:
J−Ks
E = (J K ) (J K )
J−Ks − s obs− − s int
A = AJ AKs −1E
V AV − AV ! J−Ks
wheretheindicesobsandintstandforobservedandintrinsic, Fig.3. Density plot of the number of foreground stars from
respectively. (J K ) = 1.0 0.1 mag assuming zero red- a Monte Carlo simulation. The contamination probability de-
s int
− ±
shift.WeusetheextinctionlawfromRieke&Lebofsky(1985) pendson the latitude, the longitude(from90 to 270 deg)and
for which A /A = 0.282 and A /A = 0.112 yielding the the galaxy size which are taken from the XSC. The y-axis is
J V Ks V
color excess to visual extinction conversion formula A = inlogscaleandthebarscaleindicatesthenormalizeddensity
V
5.88 E .Althoughthedeterminationoftheextinctionlooks color coding.As an example,for a galaxy at b = 10 deg,the
straig×htfJo−rKwsard, systematic effects might occur and must be numberofforegroundstarsis between0.04and6(depending
overcome.Wedistinguishthreeindependentpossibleeffects: onlongitudeandgalaxysize)andthemostcommonnumberis
about0.2star(withinthe50%iso-contour).
– contaminationofthephotometrybyforegroundstars
– colorofagalaxyaffectedbyitsredshift.
– selectioneffectwiththegalacticlatitude.
The following section explores all the possible bias that may
contaminatethegalaxycolors.
4 L.Cambre´syetal.:Large-scalevariationsofthedustopticalpropertiesintheGalaxy
Fig.4. J Ks color contamination (upper panel) and K contamination (lower panel) of a typical galaxy (K = 12.5, with a
s s
−
12 diameter) by foregroundstars in the galactic plane for l = 90 deg (left) and l = 180 deg (right). In all panels, the cross
′′
representsthereferencegalaxyandthediamondsthebrightestforegroundstar.Solidlinesshowthecontaminationlevelversus
the magnitude of the foreground stars. For the upper panels, dotted lines give the probability of reaching the corresponding
contaminationlevel.
3.1.Biasesinthereddeningestimation – starluminosityfunctionslope(toextrapolatethenumberof
starfainterthanK =14):0.31
s
3.1.1. Foregroundstarcontamination
– J K from0.65magforstars athighgalactic latitudeto
s
−
The probability of having a star on a galaxy line of sight de- 0.98magatb = 0werediffuseextinctionisnotnegligible
pends on the galaxy size and the local stellar number den- andreddeningisappreciable.
sity.Thefirstparameterintroducesasystematiceffectbecause
Figure3showsthenumberofforegroundstarsforeachgalaxy
galaxiesof a givensize are not randomlyspread overthe sky
versus the galactic latitude. Using this number, the stellar lu-
butoftenconcentrateinclusterswhicharegroupsofsamedis-
minosityfunctionandthemagnitudelimitforstarsmentioned
tance, and thussimilar size, objects. The latter parameter,the
previously we removed the stellar component in the flux of
stellarnumberdensity,reliesmostlyonthegalacticlatitudeor
eachgalaxy.Thelevelofcontaminationisgloballysmallsince
the star distribution in the disk of the Milky Way. The fore-
lessthan1%ofthegalaxieshaveatleast1foregroundstar.For
groundcontaminationbiasesthegalaxyphotometrytowardthe
latitudehigherthan25degorlowerthan5degthefractionof
bluebecausestarsaregenerallybluerthangalaxieswithanav-
contaminatedgalaxiesis0.09%and1.5%,respectively.Anex-
erage J K color of 0.65 versus 1.0 (see Sect. 2.2.2).
s
− ∼ ∼ ampleofhowtheforegroundstarcontaminationtranslateinto
However,ifthestarisbrightenoughcomparedtothegalaxyit
magnitude and color bias is presented in Fig. 4. It shows the
is likelyidentifiedandits contributionto the totalfluxis sub-
contaminationofagalaxy,K = 12.5,witha12 diameter.To
tracted(Jarrettetal.,2000a).Thecontaminationarisesforstars s ′′
computetheprobabilityofpresenceofaforegroundstarweuse
morethan1magnitudefainterthanthegalaxy.AMonteCarlo
thestarcountmodeldescribedinJarrett(1992)andJarrettetal.
simulationiswelladaptedtoestimatethebias.Weusethefol-
(1994), with validation analysis presented in Cambre´syetal.
lowingparameters:
(2002). Themodelalso providesuswith the luminosityfunc-
– J(gal)+1 < J(star) < 19.0 and K (gal)+1 < tion at a given latitude and longitude. For the worst case, at
s
K(cid:2)s(star) < (cid:3)17.5. Fainter sources compri(cid:2)se the unif(cid:3)orm l = 90 and b = 0.5, the contamination could reach a few
backgroundlevel(i.e.backgroundfluctuations). percent of a magnitude in color (about 25% probability that
– galaxy size and local stellar number density for K < 14 ∆(J K )>0.02mag).Atthisstageanewcatalogisgenerated
s s
−
fromtheXSCcatalog. with the statistical correction of the foregroundstar contami-
L.Cambre´syetal.:Large-scalevariationsofthedustopticalpropertiesintheGalaxy 5
nature of the faint galaxies depends on the latitude. For in-
stancemorehighredshiftobjectsaredetectedathighlatitude.
This has been corrected in the redshift bias analysis, but one
canimagineotherselectioneffectsuchasalowsurfacebright-
nessgalaxyfractionoraface-ongalaxyfractionasafunction
ofthegalacticlatitude.Anotherpossibleeffectrelatedtofore-
ground extinction make the fainter outer parts of galaxies to
fall below the isophotal detection limit. These outer parts are
usuallybluerbecauseoftheageandmetallicitygradients.The
non-detectionof the outer parts would make the galaxycolor
toappearredderandthederivedextinctiontobeoverestimated.
Theresultingbiasdependsonthephotometricbandsused.For
J K ,Jarrettetal.(2003)provideradialprofileforabout500
s
−
large 2MASS galaxies showing the color gradient is actually
small (see their Fig. 15). The color change is about 0.1 mag
Fig.6. Latitude-dependentlimiting magnitude used to correct
from the galaxy center to the outer disk, where most of the
theselectioneffectofgalaxiesreddenedbytheMilkyWaydisk
change is happening well within the half-light radius. To re-
dust.Thecurvesrepresentthelossinsensitivityduetoconfu- duce the selection effects we evaluate the average extinction
sionnoise.
as a function of the latitude and we set a limiting magnitude
whichdependsonthegalacticlatitude.Figure6showsthatthe
magnitude cut is about 1 mag brighter at b > 30 deg than it
nationbias. Withthiscorrection,we expectcolorbiasestobe
isatb = 0deg.Thedifferencein K issmallerwithabout0.4
s
smallerthan1to2%.
mag.Itisastrongconstraintonthetotalgalaxynumberwhich
isreducedfrom8.5 105to1.2 105galaxies.
× ×
3.1.2. Redshiftcontribution
3.2.Extinctionmapping
Giventhe 2MASSpixelsize, galaxiesas small as 10 are re-
′′
solvedas extendedsources, reachingz 0.1 for the mostlu-
ItisclearfromFig.1thatthegalaxydensityvariationsfollow
∼
minousclustermembers(Jarrett, 2004). At thisredshiftlevel,
thelargescalestructureoftheuniverse(seealsoJarrett,2004).
more H-band stellar light is transfered into the K -band, red-
s The adaptive method described in Cambre´syetal. (2002) to
dening the J K galaxy color. If individual redshifts for
s derive the extinction is particularly well adapted in this case.
−
eachsourcewereknownthemethodwouldbetoapplytheK-
For each position in the map the extinction is obtained from
correctiontoobtainthephotometrythatwouldhavebeenmea-
themediancolorexcessofthe5closestneighbors.Thistech-
suredatz = 0.Howeverweareunabletomakethiscorrection
niqueoptimizesthespatialresolutionto thelocalgalaxyden-
as long as we do not have the individual redshift knowledge.
sity.Theresultis presentedin Fig.7.Theaveragespatialres-
Itisstillpossibletocorrectthebiasusingthecolor-magnitude
olution is 1 deg with variation from 30 in large scale struc-
′
diagram presentedin Fig. 5. The subsample selected to make
tures to about 4 deg for isolated galaxies. Such a low resolu-
thisdiagramcontainsonlytheobjectwithalineofsightchar-
tionmapshouldbeviewedwithcautionsincegalaxiesarenot
acterizedbyanIRAS100µmsurfacebrightnesslowerthan1.5
uniformly distributed in space. As mentioned Sect. 2.2.2 the
MJysr 1.Thelow100µmsurfacebrightnessensuresthatthe
− galaxycolorforanon-obscuredfieldis J K = 1.0 0.1.It
s
line of sight is free from local ISM reddening. Assuming the − ±
translatesintoastatisticaluncertaintyforthevisualextinction
fainterobjectsarethemostdistantobserved,theFig.5trendis of0.1 (A /A A /A ) 1/√5=0.3mag.Howeverthesys-
directlyrelatedtotheK-correction.Thediagramshowsared- × J V− Ks V −
tematic error resulting from the spatial non-uniformityof the
deningof J K 0.2 mag between the brightand the faint
s galaxy distribution probably dominates the error. The extinc-
− ∼
end of the magnitudedistribution which is in agreementwith
tion map obtained from galaxy color excess is not equivalent
thevalueexpectedforz 0.1.Thecolorexcessusedtoderive
to an average extinction at the given resolution. The extinc-
∼
theextinctionisthereforedefinedinthecolor-magnitudeplane
tion map presentedin Fig. 7 shouldactually be consideredas
asthedistancealongthereddeningvectorbetweeneachgalaxy
alowerlimitfortheextinctionsincegalaxiesarepreferentially
andthereferencelineofequationJ K =(16.5 K ) 1+0.81
s s − detectedinlowextinctionareas,i.e.betweendensecloudsthat
− −
(Fig.5).Thecolorexcessisdeterminedforeachgalaxyfollow-
blockthelightcomingfromlowsurfacebrightnessobjects.
ingthisdefinition.Thestatisticaluncertaintyonthiscorrection
is negligible compared to the photometric uncertainty which
causesthescatterinFig.5. 4. ExtinctionandFIRdustopacitycomparison
4.1.Individuallineofsightanalysis
3.1.3. Selectioneffectwiththegalacticlatitude
Ourgoalisto comparetheextinctionmeasuredfrom2MASS
Galaxies detected through the Milky Way disk suffer from galaxycolorwiththedustopticaldepthintheFIR.IRASdata
more extinction than high-latitude objects. Consequently the providesthe100µmfluxdensity.Thetemperatureisneededto
6 L.Cambre´syetal.:Large-scalevariationsofthedustopticalpropertiesintheGalaxy
Fig.5. Left panel: color-magnitude diagram for galaxies non-reddened by the ISM (I < 1.5 MJy sr 1). The solid line
100µm −
indicates the magnitude-dependentreference for the extinction measurements. It is defined by the equation J K = (16.5
s
− −
K ) 1 +0.81.Rightpanel:K-correctionfor J Ks fromJarrett(2004).The colorvariationin ourgalaxysample is consistent
s −
−
withamaximumredshiftofabout0.1.
Rho Polaris Aquila
Ophiuchus flare rift
Taurus
Chamaeleon Orion
Av
Fig.7.Extinctionmapderivedfromthemediancolorinadaptivecellscontaining5galaxieseach.Thescaleislabeledinvisual
magnitudeunits.Thestatistical uncertaintyis0.3mag.AsforFig.1nogalaxyisdetectednearthegalacticcenterduetohigh
stellardensity.
translatethethermaldustemissiontoopticaldepth.Thework SFD98pixelextinctionmap.Dutraetal.(2003a)usedasimi-
hasbeendonebySFD98usingIRASdatamodifiedtotakeinto lar strategyfocusingon 20early typegalaxiesfor whichthey
accountthetemperaturevariationsderivedfromlowerresolu- couldderivetheextinctionfromtheirindividualspectra.Inthe
tionDIRBEdata.Theyhavecarriedoutthenextstepwhichis individualcomparisonof galaxieswith the SFD98 pixels, the
theconversionoftheopticaldepthintovisualextinction.Since spatialresolutionproblemisreversedandsinglegalaxieshave
they performed a careful calibration at high galactic latitude, thehigherresolutionwithatypicalresolutionofabout10 (the
′′
weusetheirdustmap. sizeofagalaxy)wheretheIRASbeamsizeisonly 5.
′
∼
The previous section pointed out that the extinction map
presentedinFig.7isnotequivalenttoanaverageextinctionat 4.1.1. Temperaturebias
thecellresolution.ThesituationisdifferentfortheFIRsurface
brightnessmapswhichprovidethetruearithmeticmeanvalue The existence of several dust temperature components has
atthebeamresolution.Thedirectcomparisonofthetwomaps been revealed by IRAS through the analysis of the corre-
isthereforenotappropriate.Asanalternativetoamaptomap lation between the 100 µm and 60 µm surface brightnesses
comparisonweproposeastrategywhichconsistsincomparing (Laureijsetal., 1991; Abergeletal., 1994). These results are
directly the individualgalaxy reddening to the corresponding actuallybasedontheobservationofchangesinthesmallgrain
L.Cambre´syetal.:Large-scalevariationsofthedustopticalpropertiesintheGalaxy 7
Fig.8.FigurefromCambre´syetal.(2001)representingtheef-
fective to total optical depth ratio as a function of the warm
componentfraction.Coldandwarmcomponentsaredescribed
by the modified Planck function P = B (T) (λ/λ ) 2 with
ν × 0 − Fig.9. ISM clumpiness bias derived from a synthetic molec-
T =13.5KandT =17.5K.
c w ular clouds (Padoanetal., 1998). The solid lines are the iso-
contours for A /A (gal) and the dashed lines correspond to
V V
the σ(A )/A ratio isocontours labeled in percentage. For
V V
abundances (Boulangeretal., 1990). In Lagacheetal. (1998)
σ(A )/A 25%, the bias in the extinction determination
V V
thetemperaturevariationsaredirectlymeasuredusingFIRAS ≈
(A A (gal))/A (gal)islessthan10%.
V V V
spectra and DIRBE maps. They found a dust temperature of −
about 17.5 K for the diffuse ISM and a colder component at
around15 K or less towardsmolecularclouds.In SFD98,the it is rather smooth compared to the gas. On small scale they
FIR optical depth is derived from the IRAS 100 µm bright- propose an upper limit for the σ(A )/A ratio of about 25%.
V V
nessusinganeffectivetemperatureforalineofsight.Itisnot The σ(A )/A ratio is a constraint on the dust distribution
V V
equivalenttothesumoftheopticaldepthsfromdifferenttem- but the scaling function is also needed to compute the bias.
peraturecomponents.Theimpactofthispointonthefinalop- Padoanetal.(1998)generatesyntheticmolecularcloudsfrom
ticaldepthisdiscussedinCambre´syetal.(2001)withamodel supersonicturbulence.Weuseasampleofunreddenedgalaxies
assuming the 100 µm emission comes from a mixture of two togetherwiththeirsimulationtocomputethebias.Themethod
distinct components. Assuming a ν2 emissivity law, a single consistsinapplyingtheextinctionfromthesimulationandthen
black body and a combination of two black bodies are fitted toapplyamagnitudecutwhichisequivalenttotheinstrumental
totheemission.TheresultispresentedinFig.8,whichshows sensitivitylimit.Thentheratioofthearithmeticmeanextinc-
theeffectiveopticaldepthisalwayssmallerthanthesumofthe tiontothemedianextinctionfromtheremaininggalaxiesgives
opticaldepthforthetwodustcomponents.Whentemperature thebias.Weappliedthismethodtovariousvaluesforthemin-
changesalongalineofsight,SFD98areexpectedtounderesti- imumandmaximumextinctionvaluesinthesimulatedcloud;
matetheamountofinterstellarmaterial.Ontheotherhand,the resultsarepresentedinFig. 9.For eachruntheσ(A )/A ra-
V V
temperaturevariationshaveabsolutelynoeffectonthegalaxy tio is computedand constrainsthe fluctuationsexpectedfrom
reddening.ThereisabiasintheSFD98opticaldepthdetermi- smallscales.Forσ(A )/A 25%thegalaxycolorexcessun-
V V
≈
nationwhichdependsonthefractionofwarmdustcomponent derestimates the average extinction by about 10% if the real
alonga line of sight. This fractionis hard to estimate andac- extinctiongoesfrom3 to 20 magwithin a single IRASpixel.
cordingto themodelingofFig.8a 5to 15%underestimation Forlowerextinctions,thesystematiceffectdecreases.Itissafe
intheSFD98extinctionispossible. toconsidera10%biasasanupperlimit.
4.1.2. ISMclumpinessbias 4.2.Results
Section3.2focusesontheroleofthegalaxydistributioninthe Oursamplecontains1.2 105galaxiescorrectedforallbiases
×
extinctionestimationandshowsanon-lineareffectresultsfrom describedinSect.3.1.To comparetheextinctionobtainedfor
thenon-uniformgalaxydistribution.Itisstillrelevantherebe- eachgalaxywiththeFIRextinctionmappixelsweproceedas
causewecompareeachgalaxytothecorrespondingIRASpixel follows:thetwoquantitiesareaveragedon1degwidestripsin
whichisabout5 large.Iftheinterstellarmediumisclumpyat latitude that cover the whole anticenter hemisphere longitude
′
the IRAS scale we expect a systematic effect since galaxies range (from 90 to 270 deg). Almost all the spatial informa-
will preferentiallybe detectedin the less obscuredpartof the tion is lost in this operationexceptthe latitude. Point sources
pixelwhereastheFIRextinctionreliesonthearithmeticmean contaminate the IRAS fluxes. To derive their extinction map
over the pixel. Ladaetal. (1999) and Thoravaletal. (1997) Schlegeletal. (1998) used the PSCz catalog (Saundersetal.,
have discussed the dust clumpiness and they both conclude 2000) to remove the point sources brighter than 0.6 Jy at 60
8 L.Cambre´syetal.:Large-scalevariationsofthedustopticalpropertiesintheGalaxy
Fig.10.Upperpanel:differenceofextinctionbetweenSFD98
and2MASSgalaxiesversusthegalaxyextinction,restrictedto
SFD98pixelsnotcontaminatedorcorrectedforpointsources
Fig.11.Extinctionfrom2MASSgalaxies(solidline)andfrom
(=coveredby the PSCz catalog). The deviationfor A > 1 is
V SFD98 (dashed line) versus the galactic latitude cosecant for
fittedbythedashedlineofequationA (FIR)=1.31 A (gal)
V V thenorthern(upperpanel)andthesouthern(lowerpanel)part
× −
0.24. Lower panel: same as upper panel but with all SFD98
oftheanticenterhemisphere.ThesampleisrestrictedtoSFD98
pixels,includingthoseforwhichthecontaminationcannotbe
pixelsnotcontaminatedor correctedfor pointsources(=cov-
assessed.ThedashedlineisA (FIR)=1.58 A (gal) 0.66
V V eredbythePSCzcatalog).
× −
µmovermostoftheskyat b >5degandoverpartsofthesky
| |
at lower latitudes. The low-latitudesky is criticalin our anal-
ysis and we adopt a conservative approach. Among the 1149
low-latitude IRAS pixels (0.5 < b < 1.5 deg) containing a
We chose to be conservative keeping only pixels covered by
2MASSgalaxythecontaminationcanbeassessedforonlyfor
thePSCzcatalogwhichrepresent23%ofourgalaxysampleat
259(=23%)ofthem.TheupperpanelofFig.10showstheex-
b < 5degandalmostthewholeskyat b > 5deg.However
tinction difference between SFD98 and 2MASS galaxies ver- | | | |
pixelsactuallycontaminatedinthelow-latituderegionsofthe
sus the galaxy extinction for pixels not contaminated or cor-
PSCzcatalogrepresentonly7%ofthepopulation.Itsuggests
rected for point sources, i.e. in the area coveredby the PSCz
we are probably underestimating the A (FIR)/A (gal) ratio
catalog. For extinction greater than 1 mag a linear fit gives V V
whichwouldbeintherangefrom1.31to1.58.
A (FIR)/A (gal) = 1.31 0.06. The lower panel is similar
V V
±
butincludesallpixels,whichmeanspixelsnotcontaminatedor Figure11presentsthesamedatainanotherform.Itshows
correctedfor pointsourcesand pixelsfor which the contami- A versusthelatitudecosecant.Thediscrepancyappearsmore
V
nationcannotbeassessed.TheratioA (FIR)/A (gal)becomes clearly in the south because extinction is more significant in
V V
1.58. The discrepancy with the previous value is significant. thisareawiththelargeenvelopesurroundingtheTauruscloud.
L.Cambre´syetal.:Large-scalevariationsofthedustopticalpropertiesintheGalaxy 9
4.3.Discussion whole line of sight, but they are totally independent. We
haveinvestigatedthegalacticanticenterhemisphereandfound
It is now understood that the SFD98 map overestimates
A (FIR)/A (gal) = 1.31 0.06fortheSFD98extinctionmap
V V
the extinction at least in some specific regions that have ±
restricted to pixels corrected for point source contamination.
been carefully examined by various authors. For instance
We correctforseveralbiases thatwould affectthe correlation
Arce&Goodman(1999)deducedanoverestimationfactorfor
analysis.Theforegroundstarcontaminationestimatedwiththe
SFD98 of 1.3 to 1.5 in the Taurus clouds, Chenetal. (1999)
stellardensitynumberandthegalaxysizehasbeenfoundneg-
obtained a 1.16 average factor towards 131 globular clusters
ligible with about 1-2% uncertainty. Redshifted galaxy pho-
at b > 2.5 deg. A factor of 2 was found by Cambre´syetal.
| | tometry has been corrected using a color-magnitudediagram.
(2001) in the Polaris Flare which is a peculiar regionwith an
ThecorrectionisconsistentwiththeexpectedK-correctionfor
unexpected low temperature for a cirrus. In the galactic cen-
z < 0.1.Thepresenceofseveraldusttemperaturecomponents
terdirectiona discrepancyfactorfrom1.31to1.45havebeen on∼a line sight implies a systematic effect in the FIR extinc-
measuredusing 2MASSdata (Dutraetal., 2003b, 2002). In a
tionmap.Thiseffectyieldsa5to15%underestimationofthe
recent paper Dutraetal. (2003a) studied the spectral proper-
FIRextinctionwhichreducesthediscrepancybetweenFIRand
ties of 20 early-type galaxies located at b < 25 deg to de-
| | galaxycolorextinction.Thedifferenceinthespatialresolution
rive the extinction along the line of sight. They found a ratio
oftheextinctionvalue(6.1 forSFD98and 10 for2MASS
′ ′′
A (FIR)/A (gal) 1.31for A > 0.75and an agreementfor ∼
V V ≈ V galaxies)roughlycompensatesthetemperaturesystematicsbe-
lower values. Although their sample containsonly 20 objects
causeoftheISMclumpiness.
they benefit from the high reliability of spectra. Our analysis
Inthispaperwehavegeneralizedpreviousstudiesthathad
yieldsaverysimilarconclusiononalargescalewithasample
already concluded the SFD98 map overestimates the extinc-
of1.2 105galaxies.Dutraetal.(2003a)andArce&Goodman
× tion for some specific directions to half of the sky. At large
(1999)suggesttheSFD98mapshouldberecalibrated,butthey
scalethediscrepancyappearsforA >1,suggestingthewhole
V
donotproposeanyphysicalreasonforthisadjustment. galacticdiskisaffected.Weattribute∼ourresulttothepresence
We support the idea of an enhanced emissivity of the
offluffy/compositegrainswhichhaveanenhancedemissivity
dust in the FIR due to fluffy grains (Cambre´syetal., 2001;
intheFIR.Ourlargescalestudysuggeststheyaremorecom-
Stepniketal., 2003). A grain growth through grain-grain co-
monthatpreviouslythoughtsincetheywouldbeformedeven
agulationoraccretionofgasspeciesisexpectedindensecold
atlowextinctionandnotonlyindensecoldclouds.Moreanal-
molecular clouds (Draine, 1985) and leads to porous grains.
ysisareneededtoconfirmthispointespeciallytorecovermore
Dwek (1997) studied the fluffy grain optical properties and
spatialinformation.ComplementaryFIRandsubmmdatafrom
showedtheemissivityintheFIRisincreasedforporousgrains.
Spitzer,HerschelandPlanckareexpectedtosolvethisproblem
If thiseffectappliesto the presentanalysisitmeansthe grain
byprovidingdustspectraatlongwavelengths.
coagulationstarts at extinctionclose to 1 mag. We would ex-
pect variations from cloud to cloud depending on their star Acknowledgements. WeacknowledgeP.Padoanforprovidinguswith
formingactivity, the local interstellar radiation field and their the synthetic interstellar cloud data. This publication makes use of
geometry. The minimum extinction value at which the FIR dataproductsfromtheTwoMicronAllSkySurvey,whichisajoint
to extinctionratio changesis actually notthe correctquantity projectoftheUniversityofMassachusettsandtheInfraredProcessing
and Analysis Center/California Institute of Technology, funded by
to check since it relies on the column density rather than the
theNationalAeronauticsandSpaceAdministrationandtheNational
3D volume density. Several low density clouds on the same
ScienceFoundation.
line of sight and a single dense cloud may have the same
column density but their A (FIR)/A (gal) would be very dif-
V V
ferent. However, Fig. 11 points out the extinction values be- References
yond the threshold for the enhanced emissivity are generally
Abergel,A.,Boulanger,F.,Mizuno,A.,&Fukui,Y.1994,ApJ,
restricted to low-latitude regions with b < 8 deg. Our inter-
423,L59
| |
pretationforthegrainaggregateformationleadstotheconclu-
Arce,H.G.&Goodman,A.A.1999,ApJ,512,L135
sionthathigher-densitycloudsarepreferentiallylocatedinthe
Bernard, J. P., Abergel, A., Ristorcelli, I., et al. 1999, A&A,
plane, which is in total agreement with our knowledge about
347,640
theISMdistributionintheGalaxy.Acomparativeanalysisfor
Boulanger, F., Falgarone, E., Puget, J. L., & Helou, G. 1990,
various environment would confirm our hypothesis if clouds
ApJ,364,136
were provedto be differentin their FIR to extinction correla-
Burstein,D.&Heiles,C.1982,AJ,87,1165
tion.Unfortunatelythespatialinformationinouranalysis,the
Cambre´sy,L.,Beichman,C. A., Jarrett,T.H.,&Cutri,R. M.
galacticlatitude,doesnotallowustoconcludeonspatialvari-
2002,AJ,123,2559
ationoftheA (FIR)/A (gal)ratio.
V V Cambre´sy,L.,Boulanger,F.,Lagache,G.,&Stepnik,B.2001,
A&A,375,999
Chen,B.,Figueras,F.,Torra,J.,etal.1999,A&A,352,459
5. Conclusion
Cutri, R. M., Skrutskie, M. F., van Dyk, S., et al. 2003,
The 2MASS galaxy colors appears to be a very interesting Explanatory Supplement to the 2MASS All Sky Data
extinction estimator to compare with FIR extinction. Both Release
extinction estimators are sensitive to the absorption for the Dale,D.A.&Helou,G.2002,ApJ,576,159
10 L.Cambre´syetal.:Large-scalevariationsofthedustopticalpropertiesintheGalaxy
del Burgo, C., Laureijs, R. J., A´braha´m, P., & Kiss, C. 2003,
MNRAS,346,403
De´sert, F.-X., Boulanger,F., & Puget, J.-L. 1990,A&A, 237,
215
Draine,B.T.1985,inProtostarsandPlanetsII,621–640
Dupac, X., Giard, M., Bernard, J.-P., et al. 2002, A&A, 392,
691
Dutra,C.M.,Ahumada,A.V.,Claria,J.J.,etal.2003a,A&A,
408,287
Dutra,C.M.,Santiago,B.X.,&Bica,E.2002,A&A,381,219
Dutra, C. M., Santiago, B. X., Bica, E. L. D., & Barbuy, B.
2003b,MNRAS,338,253
Dwek,E.1997,ApJ,484,779
Jarrett,T.H.1992,PhDthesis,MassachusettsUniv.,Amherst.
Jarrett,T.H.2000,PASP,112,1008
Jarrett,T.H.2004,Publ.Astron.Soc.Aust.,21,396
Jarrett,T.-H.,Chester,T.,Cutri,R.,etal.2000a,AJ,120,298
Jarrett,T.H.,Chester,T.,Cutri,R.,etal.2000b,AJ,119,2498
Jarrett,T.H.,Chester,T.,Cutri,R.,Schneider,S.E.,&Huchra,
J.P.2003,AJ,125,525
Jarrett, T. H., Dickman, R. L., & Herbst, W. 1994, ApJ, 424,
852
Johnstone,D.,Fiege,J.D.,Redman,R.O.,Feldman,P.A.,&
Carey,S.J.2003,ApJ,588,L37
Lada,C.J.,Alves,J.,&Lada,E.A.1999,ApJ,512,250
Lagache,G.,Abergel,A., Boulanger,F., &Puget, J.-L.1998,
A&A,333,709
Laureijs,R.J.,Clark,F.O.,&Prusti,T.1991,ApJ,372,185
Lehtinen, K., Mattila, K., Lemke, D., et al. 2003, A&A, 398,
571
Padoan, P., Juvela, M., Bally, J., & Nordlund, A. 1998, ApJ,
504,300
Rieke,G.H.&Lebofsky,M.J.1985,ApJ,288,618
Saunders, W., Sutherland, W. J., Maddox, S. J., et al. 2000,
MNRAS,317,55
Schlegel,D.J.,Finkbeiner,D.P.,&Davis,M.1998,ApJ,500,
525
Stepnik,B.,Abergel,A.,Bernard,J.-P.,etal.2003,A&A,398,
551
Thoraval,S.,Boisse´,P.,&Duvert,G.1997,A&A,319,948