An I-band calibration of the SBF method at blue colours

S.Mieskeetal.:SBFcalibrationatbluecolours

9

Fig.4.ExamplethumbnailsoftheSBFmeasurementproce-durefortwoFornaxclusterdEs.LeftFCC194,rightLSB6-4.Thumbnailsizeis12.5′′.Imagesequencefromtoptobottom:1.Originalgalaxyimage.2.Galaxylightmodelsubtractedfromformerimage.3.Formerimagedividedbysquarerootofgalaxymodel,withcontaminatingsources/ tresidualsmaskedandwiththemeasurementregionrestrictedtoacirculararea.4.AzimuthallyaveragedpowerspectrumoftheformerSBFimage,withthe tindicatedbythedashedline.

Wenotethatforthefullsample,caseC)isrejectedatevenhighersigni canceduetoalowermeandistanceandstrongercorrelationbetween(m M)and(V I

).

Fig.5.DistancemodulusforcaseA)(seetext)plottedvs.S/NoftheSBFmeasurement(left)andtheamountofback-ground uctuationsBGcontributingtothemeasuredSBFsig-nal(right).ThethreesourcesmarkedwithlargecirclesintherightpanelarethosebelowS/N=3intheleftpanel.Filledcirclesareforgalaxieswithseeingbelow0.85′′,opencirclesforseeingabove0.85′′,seeFig.6and

text.

Fig.6.The uctuationmagnitude

mIisinsigni cant.

4.2.Theempiricalapproach

Wealsogivethebest tresultofapurelyempiricalcalibrationvalidinthecolourrange0.85<(V I)0<1.10mag,assuming(m M)=31.39±0.12mag: