M ilky W ay vsAndrom eda:a tale oftwo disks

Aim s. W e study the chem icalevolution ofthe disks ofthe M ilky W ay (M W ) and ofAndrom eda (M 31),in order to revealcom m on pointsand dierencesbetween thetwo m ajorgalaxiesofthe Localgroup. M ethods. W e use a large setofobservationaldata forM 31,including recentobservationsofthe StarForm ation Rate (SFR)and gasproles,aswellasstellarm etallicity distributionsalong itsdisk.W eshow that,when expressed in term s ofthe corresponding disk scale lengths,the observed radialproles ofM W and M 31 exhibit interesting sim ilarities, suggesting thepossibility ofa description within a com m on fram ework. Results. W end thattheprolesofstars,gasfraction and m etallicity ofthetwo galaxies,aswellasm ostoftheirglobal properties,arewelldescribed by ourm odel,provided thestarform ation eciency in M 31 disk istwiceaslargeasin the M W.W eshow thatthestarform ation rateproleofM 31 cannotbetted with any form oftheK ennicutt-Schm idtlaw (K S Law)forstarform ation.W e attribute those discrepanciesto the factthatM 31 hasundergone a m ore active star form ation history,even in the recentpast,assuggested by observationsofa \head-on" collision with the neighboring M 32 galaxy about200 M yrago. Conclusions. The M W hasm ostprobably undergone a quiescentsecularevolution,m aking possible a fairly successful description with a sim ple m odel.IfM 31 is m ore typicalofspiralgalaxies,as recently suggested by Ham m er et al. (2007),m ore com plex m odels,involving galaxy interactions,willberequired forthedescription ofspirals.