The Evolution And Star Formation History Of Nearby Disk Galaxies

Galaxies are building blocks of large cosmological structures and they are families of the stars, the formation and evolution of them is one of the frontier subjects of twenty-first Century. Compared to large cosmological structure and stellar structure and evolution, there are many important issues on the formation and evolution of galaxies needed to be studied. In order to understand the formation and evolution of them, an increasing number of space and ground-based telescopes focus on them. It is well established now that the distribution and evolution history of galaxies are strongly affected by their morphologies, stellar mass and environment. However, the specific study on formation and evolution of galaxies is dependent on the development of observation level and technical level. Nearby galaxies are close enough to reveal the detailed structural properties, and they provide us with the most abundant observed constraints for the theoretical models of galaxy formation and evolution. Thus, Nearby galaxies are ideal’laboratories’to study star formation, metallicity enrichment, and stellar mass growth.In order to fully understand the evolution history of these galaxies, it is essential to understand the history of the individual galaxies. The simple phe-nomenological model, which adopts parameterized descriptions for some compli-cated physical processes, has already been proven to be a powerful tool to explore the formation and evolution of individual disk galaxies. At the same time, al-though the proportion of lower mass galaxies and interacting galaxies is very high in the Universe, we understand little about their formation and evolution histories. Thus we use the phenomenological model to study a simple isolated lower mass spiral galaxy NGC 300 and a relatively complicated interacting higher mass spiral galaxy M51a(NGC 5194). We use the best-fitting model predictions of NGC 300 and M51a to compare that of their counter-parts which have been studied in previous. By comparing the star formation histories of these typi-cal disk galaxies, we hope to have a better understanding of the formation and evolution of other disk galaxies, even the larger structure in the universe.First of all, we construct a simple chemical evolution model for NGC300 to build a bridge between its star formation history and its observed properties, including both the radial profiles and global observational constraints. By means of comparing the model predictions with the corresponding observations, it can be found that the model predictions are very sensitive to the adopted infall timescale. The outflow process plays an important role in shaping the gas-phase metallicity profiles along the disk of NGC 300, since it takes a large fraction of metals away from its disk during its evolution history. The Local Group spiral galaxy M33 is a near-optical twin of NGC 300. Although they are similar in appearance, M33 has a disk break at r≈8kpc, while NGC 300 has a pure exponential disk out to r≈14kpc. Furthermore, there is an HI bridge between M33 and M31, indicating the probability that they interacted with each other in the past, but NGC 300 is an isolated system. The aforementioned differences indicate that NGC 300 and M33 may have significantly different evolution histories and it should be interesting to compare the star formation history of NGC 300 with that of M33. Thus we compared the best-fitting, model-predicted evolution history of NGC 300 with that of M33. We find that the mean stellar age of NGC 300 is older than that of M33, there is a lack of primordial gas infall onto the disk of NGC 300, and a lower fraction of stars formed recently in NGC 300 than in M33. The comparative study of the two bulgeless systems also shows that local environmental difference may play an important part in the secular evolution of the galaxy disks.Secondly, we construct a simple evolution model for M51a to build a bridge between its star formation history and its observed properties, especially the radial distributions of cold gas surface density, metallicity, and the radial profiles of surface brightness in multibands. By comparing model predictions with the observed data, we can discuss the probable ranges for free parameters in the model and then know more about the main properties of the evolution and star formation history of M51a. We find that the model predictions are very sensitive to the free parameter and the model adopting a constant infall-peak time tp 7.0 Gyr can reproduce most of the observed constraints of M51a. Although our model does not assume the gas infall time-scale of the inner disc is shorter than that of the outer disc, our model predictions still show that the disk of M51a forms inside-out. By comparing the growth history of M51a predicted by the viable model with that of the Milky Way and UGC 8802, we find that the mean stellar age of M51a is younger than that of the Milky Way, but older than that of the gas-rich disc galaxy UGC 8802. In this work, we also introduce a’toy’ model to allow an additional cold gas infall occurred recently to imitate the influence of the interaction between M51a and its companion. Our results show that the current molecular gas surface density, the star formation rate and the UV-band surface brightness are important quantities to trace the effects of recent interaction on galactic star formation process.Finally, through comparing the metallicity gradients predicted by their re-spective best-fitting models, our results are consistent with the previous statisti-cal results. That is, with respect to the isolated galaxies, both inner and outer regions growing faster for more massive galaxies; the metallicity gradients in interacting spiral galaxies are shallower than that in isolated spiral galaxies.