| 其他摘要 | M33 (NGC598), with only about one tenth of Milk Way in terms of its baryonic mass, is the smallest disk galaxy in the Local Group. Due to its proximity, large angular size, and rather low inclination, M33 is very suitable to study its main properties of gas distribution and stellar population. Moreover, since M33 shows no signs of recent mergers and no presence of prominent bulge and bar component, detailed theoretical studies on this disk galaxy will help us to understand the formation and evolution of disk galaxies with small mass. Phenomenological model, which has been widely used in studies of the chemical evolution of Milky Way and other spiral galaxies in the Local Group, has achieved many important progress towards our understanding of main features of the evolution of these spiral galaxies. However, the previous models are not satisfactory, the main drawbacks of the previous model are as follows: (I) The evolution of the molecular and neutral gas are not calculated respectively. (II) Star formation rate (SFR), correlated with the total gas, is adopted. (III) The outflow process is not considered. However, these ingredients directly affect the evolution history of the galaxies. Firstly, the molecular gas and the neutral gas have great effect on the physical process in each galaxy. In recent years, high-quality, spatially resolved maps of the cold gas have become available for M33, so it is necessary to calculate the evolution of the molecular and neutral gas respectively to strictly constrain the model. Secondly, Since star formation process happen in giant molecular clouds, the SFR surface density should correlate better with the molecular surface density. At last, since M33 is less massive compared with MilkyWay, the lower potential well would very probably result in a significant outflow during the evolution of disk by supernova explosions and other perturbations. In order to solve above problems, we have reconstructed a chemical and spectrophotometric model for M33 disk which is similar to the former work on Milky Way, within which we consider both infall and outflow, calculate the molecular gas and neutral 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 the disk. Moreover, the gas outflow rate is assumed to proportional to the SFR surface density. Moreover, We include one simple prescription for molecular gas formation process in our model that the H2 fraction is determined by the pressure of the interstellar medium. Besides, apart from calculating the evolution of the molecular and neutral gas 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 parameters in the model, the infall timescale τ and the outflow efficiency bout. We use the model, modified by ourselves, to calculate the radial profiles of molecular hydrogen surface density, atomic hydrogen surface density, total gas surface density, SFR, surface brightness in FUV-band and K-band, oxygen abundance, as well as the FUV-K color in details. We explore the main properties of the star formation history of M33 via comparison between the model predictions and the observational data. The main conclusions of this work are as follws: (i) The star formation efficiency of M33 is higher than the average value derived by Leroy et al. (2008) on the basis of a large sample of galaxies, this is consistent with the previous observed results. (ii) The M33 disk 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 model adopting long τ results in blue colors, low metallicity, high H2 and Hi mass surface densities, high SFR surface density, while the gas outflow process mainly influences the metallicity. (iii) The model which adopts a mediate outflow rate and an inside-out formation scenario can be in good agreement with the most of observed constraints of M33. (iv) Comparing to the Kennicutt SF law, the H2-based SF law would be more suitable to describe the evolution of the galactic disk, especially for the radial distributions of both the cold gas and the stellar population. |
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