M31's Star Formation History And Compare With The Milky Way

Andromeda galaxy (M31,NGC224) is the biggest spiral in the Local Group. Since it is not far from Galaxy, it provides us a unique candidate that the galaxy structure and morphology can be observed carefully. By studying the star formation history and chemical evolution of M31. and comparing with the Milky Way Galaxy, we are able to understand more about the formation and evolution of spiral galaxies.Based on the chemical evolution model of Milky Way disk. We have establishan infail model without IRA (Instantaneous Recycling Approximation) to studythe star formation history in M31 disk. Firstly, we adopt the same parametersthat well fit the Milky Way disk to the M31 disk evolution. We found thatthe obtained gas surface density and star formation rate(SFR) profiles do notfit the M31's observation. This demonstrates that the star formation history inthe M31 disk may totally different from that in the Milky Way disk. And if theSchmidt type SFR law is valid for both disks, the corresponding star formationefficiency(SFE) may differ. Then, we use the observed gas and SFR profiles ofM31 to determine the relation between SFR and gas mass surface density, mainlythe SFE in the M31 disk. It appears that SFE proportion to r^-1. In other words,the SFR is proportion to r^<sup>-2. Based on this new SFR. our best model shows thatthe total mass in disk should be 3 4 ×10¹⁰ (?) . and infail time scale should be0.2r+l Gyr. The best model can reproduce the gas and SFR profiles, and alsoroughly the abundance features along the disk. But there are some problems withthis model, such as high gas fraction comparing with Galaxy, high abundance inthe inner disk especially iron. All of these problems should be relate to the formhow SFR depend on radius. Finally, we discuss the effect of different parameterson the model predictions. Those parameters include: total disk mass M_tot. infailtime scale Υ(r), index in the relationship between SFR and radius. We find thatthe smaller the M_tot, the closer to observation the gas is;the bigger the M_tot,the lower present gas fraction is. The shorter and flatter the r(r), the closer toobservation the gas is;the longer the Υ(r). the lower the present abundance is.It is a common knowledge that the radius dependence of infail time scale is themain mechanism of abundance gradient. But through our work we find that the behavior of SFR profile (how it depends on radius) can also significantly influence the abundance gradient.Comparing with Galaxy, M31 has much lower gas and SFR profiles, but their total dynamic: mass are comparable. That means M31 needs faster star formation rate and infall rate (namely shorter infall time scale) in order to consume more gas . However, the low nietallicity and flat abundance gradient require lower star formation efficiency, and few metal poor stars means the infall time scale can not be very short. This contradiction is intrinsic. Comparing with observation, we can conclude that the chemical evolution model of Galaxy can not fit the M31's feature. They must have different star formation history. Indeed, there exists a lot of obvious evident that M31 had some interaction or accretion with companions in the past time. Those processes play an important role in the formation and evolution history of the M31 galaxy.Toy model which describe the influence on SFR caused by interaction show us qualitatively that the results of model can be improved if we take the interaction into account.