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

2S.Mieskeetal.:SBFcalibrationatbluecolours

surveytomeasuretheapparentI-bandSBFmagnitude

MIandthedereddenedcolour

(V I)0,determinedintherange1.05<(V I)0<1.3:

MRand(B R)in

the

range

1

.

<

(

B

R)<1.3magbytargetingnearbydwarfellipticalgalaxieswithknowndistances.TheseauthorsbasedtheircalibrationonthetheoreticalpredictionsofWorthey(1994)modelsandusedtheirobservationaldatatoadjustthezero-pointofthosemod-els.InJerjenetal.(2001)theyfoundthatthemodelspredictedtoofaintSBFamplitudesby0.13mag.Furthermore,theypro-posetwoseparatebranchesinthe

MRbecomesbrighterwithyounger

andmoremetal-richstellarpopulation.Thismeansthattheage-metallicitydegeneracyof

Mz

vs.(g z)for1.0<(g z)<1.3.Theyobtainedasigni cantslopeinthatcolourrange(4.5σ),butdidnot ndevidenceforabimodalityof

MF160Wand(V I)0intherange

1.05<(V I)0<1.24mag,whichwasbasedonlyongiantgalaxiesandyieldedacomparableslopetothatinequation(1).Forgroundbasedinfraredbands,Jensenetal.(1998)andLiuetal.(2002)presentedcalibrationsbetweenKand(V I)0,alsobasedpurelyongiantgalaxies.ThesmallsampleofJensenetal.intherange1.15<(V I)0<1.27magwasconsistentwithazeroslopebetween(V I)0and

MIareplottedvs.(V I)0for

asetofoldtointermediateagestellarpopulationswithawiderangeofmetallicities,takenfromdi erentliteraturesources:Worthey(1994),whouseshisownstellarpopulationmod-elsbasedonVandenBerg(1985)andYale(Greenetal.1987)isochrones;Liuetal.(2000),whousetheupdatedmodelsbyBruzual&Charlot(1993)plustheBertellietal.(1994)Padovaisochrones;Blakesleeetal.(2001)whousethestellarpopula-tionmodelsofVazdekisetal.(1996)andtheupdatedGirardietal.(2000)Padovaisochrones;andCantielloetal.(2003),whousethestellarpopulationsynthesiscodebyBrocatoetal.(1999,2000)andtheTeramo-Pisa-Romeisochrones(e.g.Castellanietal.1991,Castellanietal.1992andBonoetal.1997a,1997b).Equation(1)isalsoshowninFig.1.TheWortheymodelpredictionsaresplitupintotheoriginalpredic-