The Evolution And Star Formation History Of M33

M33(NGC598), with only about one tenth of Milk Way in terms of its baryonicmass, is the smallest disk galaxy in the Local Group. Due to its proximity, large angu-lar size, and rather low inclination, M33is very suitable to study its main properties ofgas distribution and stellar population. Moreover, since M33shows no signs of recentmergers and no presence of prominent bulge and bar component, detailed theoreticalstudies on this disk galaxy will help us to understand the formation and evolution ofdisk galaxies with small mass.Phenomenological model, which has been widely used in studies of the chemicalevolution of Milky Way and other spiral galaxies in the Local Group, has achievedmany important progress towards our understanding of main features of the evolutionof these spiral galaxies. However, the previous models are not satisfactory, the maindrawbacks of the previous model are as follows:(I) The evolution of the molecularand neutral gas are not calculated respectively.(II) Star formation rate (SFR), cor-related with the total gas, is adopted.(III) The outflow process is not considered.However, these ingredients directly afect the evolution history of the galaxies. Firstly,the molecular gas and the neutral gas have great efect on the physical process in eachgalaxy. In recent years, high-quality, spatially resolved maps of the cold gas havebecome available for M33, so it is necessary to calculate the evolution of the molec-ular and neutral gas respectively to strictly constrain the model. Secondly, Since starformation process happen in giant molecular clouds, the SFR surface density shouldcorrelate better with the molecular surface density. At last, since M33is less mas-sive compared with Milky Way, the lower potential well would very probably result ina significant outflow during the evolution of disk by supernova explosions and otherperturbations.In order to solve above problems, we have reconstructed a chemical and spectro-photometric model for M33disk which is similar to the former work on Milky Way,within which we consider both infall and outflow, calculate the molecular gas andneutral gas respectively, and use the star formation law which is correlated with the molecular gas. We adopt an exponential infall formalism to describe the growth of thedisk. Moreover, the gas outflow rate is assumed to proportional to the SFR surfacedensity. Moreover, We include one simple prescription for molecular gas formationprocess in our model that the H2fraction is determined by the pressure of the interstel-lar medium. Besides, apart from calculating the evolution of the molecular and neutralgas respectively, we use the ΣH2-based star formation law given by Leroy et al (2008)to describe how much cold gas turns into stellar mass. Using only two free parametersin the model, the infall timescale τ and the outflow efciency bout.We use the model, modified by ourselves, to calculate the radial profiles of molec-ular hydrogen surface density, atomic hydrogen surface density, total gas surface den-sity, SFR, surface brightness in FUV-band and K-band, oxygen abundance, as wellas the FUV-K color in details. We explore the main properties of the star formationhistory of M33via comparison between the model predictions and the observationaldata. The main conclusions of this work are as follws:(i) The star formation efciencyof M33is higher than the average value derived by Leroy et al.(2008) on the basisof a large sample of galaxies, this is consistent with the previous observed results.(ii)The M33disk is probably formed by means of a slow infalling of the primordial gas,and the model predictions are very sensitive to the infall timescale τ in that the modeladopting long τ results in blue colors, low metallicity, high H2and Himass surfacedensities, high SFR surface density, while the gas outflow process mainly influencesthe metallicity.(iii) The model which adopts a mediate outflow rate and an inside-outformation scenario can be in good agreement with the most of observed constraints ofM33.(iv) Comparing to the Kennicutt SF law, the H2-based SF law would be moresuitable to describe the evolution of the galactic disk, especially for the radial distri-butions of both the cold gas and the stellar population.