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

4S.Mieskeetal.:SBFcalibrationatblue

colours

Fig.1.Theoretical“isochrones”inthe

a

a

M

I

=

2.00magandthecontinuationofequation(1)for(V I)0≤1.09.SeeSect.4formoredetails.

strumentalmagnitudesweremeasuredwiththeIRAFpackageAPPHOTinaperturesequaltothoseusedbyLandolt(1994).Then,asinglephotometricsolutionwasdeterminedfortheentire8k×8kimage.Chip-to-chipvariationsaroundthemeanphotometriczeropointweresmallerthan0.02mag.Tocor-rectforgalacticreddeningandabsorption,weusedtheval-uesfromSchlegeletal.(1998),whogiveAI=0.025andE(V I)=0.018forthecoordinatesoftheFornaxcluster.

3.2.SBFmeasurement

TheSBFmeasurementprocedurewassimilartothatalreadyoutlinedinMieskeetal.(2005)basedonVLTFORS1photom-etry.Somenuanceshavebeenmodi ed:the uctuationsfromtheskybackgroundwerenowderivedinthesameimageclosetotheinvestigateddEinsteadofinacomparison eld.Thosesky uctuationsconsistedofundetectedbackgroundgalaxiesandfringing.Theseeingwasnotconstantoveronechip,gen-erallyincreasingtowardstheedgesofthe eldofview.Thiscausedacorrespondinglyvaryingamountof uctuationfromundetectedbackgroundgalaxiesalongthechip.Alsothefring-ingvariedinsomecasessigni cantlyoveronechip,seeFig.2.Thesetwodependencesonpositionrequiredalocaldetermina-

tionofsky uctuations.NotethatfortheVLTFORSphotom-etrytherewasnosigni cantfringingnorstrongvariationsofseeingamongthedi erentexposures.

Inturnwedescribetherelevantmeasurementstepsfordeter-miningtheSBFmagnitude:

1.ModelmeangalaxylightwithELLIPSEusingasigmaclip-pingalgorithmtodisregardcontaminatingsources,subtractthemodel.

2.Detectandsubtractremainingcontaminatingobjectsfromoriginalimage.

3.Modelmeangalaxylightonthecleanedimage.4.Subtractmodeloforiginalimage.

5.Divideresultingimagebysquarerootofthemodel,cutoutcircularportionwithradiustypically20-25pixel(4 5′′),cor-respondingtoabout4-8seeingdiskdiameters.

6.Maskoutcontaminatingsourceslikeforegroundstarsandbackgroundgalaxies.

7.Calculatethepowerspectrum(PS)ofthecleanedimage.8.CalculatethePSoftheskybackgroundinthreedi erentblankimagesectionsclosetotheinvestigatedgalaxy.ThisskybackgroundPScontainsboth uctuationcontributionsfromundetectedbackgroundgalaxiesas uctuationscausedbyfringing,seeexplanationatbeginningofthissubsection.