The Effect of FIR Emission from SDSS Galaxies on the SFD Galactic Extinction Map

[摘要] WecomparethemostsuccessfulandwidelyusedmapofGalacticdustextinction,providedbySchlegel,Finkbeiner,andDavis(1998,hereafterSFD),tothegalaxynumbercountsintheSloanDigitalSkySurvey(SDSS)photometric/spectroscopicDR4sample.WedividetheSDSSsurveyareainto69disjointsubregionsaccordingtothedustextinctionprovidedbySFDandcomparethesurfacenumberdensityofgalaxiesineachsubregion.Asexpected,thegalaxysurfacenumberdensitydecreaseswithincreasingextinction,butonlyforSFDextinctionvaluesaboveabout0.1to0.2magnitudes(dependingontheband).AtlowervaluesoftheSFDextinction,wefindthattheskysurfacedensityofgalaxiesincreaseswithincreasingextinction,preciselytheoppositeoftheeffectexpectedfromGalacticdust.WealsofindthattheaveragecoloroftheSDSSphotometricgalaxysampleisslightlybluerathigherSFDextinctionsinthisregime,againtheoppositeoftheeffectexpectedfromGalacticdust.Eventhoughtheseanomaliesoccuronlyforsight-lineswithlowSFDextinctionvalues,theyaffectapproximately68%ofthehighgalacticlatitudeskyinwhichgalaxiesandtheirclusteringpropertiesarenormallystudied.AlthoughitwouldbepossibletoexplaintheseeffectswithamysteriouscomponentofGalacticdust,whichisanti-correlatedwiththe100$$mu$$mfluxonwhichtheSFDextinctionmapisbased,thismodelisnotphysicallyplausible.Moreover,wefindthatthesurfacenumberdensityofSDSSphotometricquasarsdoesnotshowanysimilareffect,aswouldbeexpectediftheexplanationwereanunknownGalacticdustcomponent.Consideringtheseresults,wesuggestthatthefarinfrared(FIR)brightnessoftheskyinregionsoftruelowdustextinctionissignificantly“contaminated”bytheFIRemissionfrombackgroundgalaxies.Weshowthatsuchanexplanationisbothqualitativelyandquantitativelyconsistentwiththeavailabledata.BasedonthisinterpretationweconcludethatsystematicerrorsintheSFDextinctionmapduetoextragalacticFIRemissionarequitesmall,ontheorderhundredthsofamagnitude,butneverthelessstatisticallydetectable.Unfortunately,however,theseerrorsarealsoentangledinacomplexwaywithasignalofgreatinteresttomany“precisioncosmology”applications,namelythelarge-scaleclusteringofgalaxies.

[学科分类] 天文学(综合)

[关键词] "large-scale structure of universe";"cosmology: observations";"dust;extinction"