An I-band calibration of the SBF method at blue colours(10)

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

10S.Mieskeetal.:SBFcalibrationatblue

colours

Fig.7.ApparentVmagnitudeplottedvs.SBFdistancemodu-lusforthefourdi erentassumedcalibrationrelationsA)toD),asde nedinSect.2andTable2.Filledcirclesaregalaxieswithseeingbetterthan0.85′′.Theirmeandistanceandits1σerrorareindicatedbytheverticalsolidanddashedlines.Thegreenticksat(m M)=31.39magmarkthereferenceFornaxclusterdistancefromFreedmanetal.(2001)anditserror.Opencirclesaredatapointsforgalaxieswithseeingworsethan0.85′′.Theywerenotincludedincalculatingthemeandistanceandareonlyshownforthesakeof

completeness.

Fig.8.(V-I)0colourplottedvs.SBFdistancemodulusforfourdi erentassumedcalibrationrelations,seeFig.7.

Fig.9.ApparentSBFmagnitude

mIfainterthanthelineforcaseB),while

applyingthesolidandlongdashedlinetotheothergalaxies.

ThezeropointerrorincludesourstatisticalerrorsandtheerroroftheFornaxclusterreferencedistance.Thestatisticalerrorsofthezeropointandtheslopewerederivedfromthedisper-sionofthedatapointsaroundthelinearrelation,notfromthesinglemeasurementerrors(thiswouldhavedecreasedtheer-rorsby50%).TheformaldisagreementwiththeTonryslopeof4.5is1.1σ,whilethedisagreementwitha0slopeis1.3σ.Forthefullsampleofgalaxies,thezero-pointchangesto 2.31±0.15,andtheslopeto2.80±1.36.Notsurprisingly,thisagreeswiththe ndingfromthesemi-empiricalapproachthatcaseC)isrejectedathighersigni canceforthefullsample.

Toquantifytheintrinsicscatter–the“cosmicvariance”–oftheempirical“goodseeing”calibration,wesubtractinquadraturethemeansinglemeasurementuncertaintyin

MItobemoresensitivetodi erentage/metallicity

combinationsinthebluethaninthered(seeleftpanelofFig.1).NotethatthetwotheoreticalpredictionsbyLiuetal.andWortheyetal.intherightpanelofFig.1thatpredictacolourindependent