Bar Formation, Gas Evolution, and Star Formation in Barred-Spiral Galaxies

Abstract

Bar structures are common in spiral galaxies. The understanding of the evolution of the bar galaxy is essential to the overall understanding of the disk galaxies. Many previous studies have done research on the evolution of bar galaxies, but there are still many parts that are not well understood, such as the temporal and spatial distributions of star formation in a nuclear ring, the formation and evolution of gaseous structures especially the ring size, and the effects of the gaseous component on a stellar bar. In this thesis, using both hydrodynamic simulations and self-consistent simulations including star formation and feedback, we try to understand unsolved problems for barred spiral galaxies.<br/> <br/> We first investigate the star formation in a nuclear ring without spiral arms. We use hydrodynamic simulations to study temporal and spatial behavior of star formation occurring in nuclear rings of barred galaxies where radial gas inflows are triggered solely by a bar potential. The star formation recipes include a density threshold, an efficiency, conversion of gas to star particles, and delayed momentum feedback via supernova explosions. We find that star formation rate (SFR) in a nuclear ring is roughly equal to the mass inflow rate to the ring, while it has a weak dependence on the total gas mass in the ring. The SFR typically exhibits a strong primary burst followed by weak secondary bursts before declining to very small values. The primary burst is associated with the rapid gas infall to the ring due to the bar growth, while the secondary bursts are caused by re-infall of the ejected gas from the primary burst. While star formation in observed rings persists episodically over a few Gyr, the duration of active star formation in our models lasts for only about a half of the bar growth time, suggesting that the bar potential alone is unlikely responsible for gas supply to the rings. When the SFR is low,<br/> most star formation occurs at the contact points between the ring and the dust lanes, leading to an azimuthal age gradient of young star clusters. When the SFR is large, on the other hand, star formation is randomly distributed over the whole circumference of the ring, resulting in no apparent azimuthal age gradient. Since the ring shrinks in size with time, star clusters also exhibit a radial age gradient, with younger clusters found<br/> closer to the ring. The cluster mass function is well described by a power law, with a slope depending on the SFR. <br/> <br/> We then investigate the effects of spiral arms on the ring star formation. We use hydrodynamic simulations to study the effect of spiral arms on the star formation rate (SFR) occurring in nuclear rings of barred-spiral galaxies. We _x000C_find that spiral arms can be an efficient means of gas transport from the outskirts to the central parts, provided that the arms are rotating slower than the bar. While the ring star formation in models with no-arm or corotating arms is active only during about the bar growth phase, arm-driven gas accretion makes the ring star formation both enhanced and prolonged significantly in models with slow-rotating arms. The arm-enhanced SFR is larger by a factor of _x0018_~3-20 than that in the no-arm model, with larger values corresponding to stronger and slower arms. Arm-induced mass inflows also make dust lanes stronger.<br/> Nuclear rings in slow-arm models are _x0018_ 45% larger than in the no-arm counterparts. Star clusters that form in a nuclear ring exhibit an age gradient in the azimuthal direction only when the SFR is small, whereas no noticeable age gradient is found in the radial direction for models with arm-induced star formation.<br/> <br/> As a final project, we run fully self-consistent simulations of galaxies similar to the Milky Way to study formation and evolution of a stellar bar in the presence of gas as well as a nuclear ring and star formation therein. The gas suffers radiative heating and cooling and is subject to star formation feedback. We consider two sets of models with cold or warm disks that differ in the Toomre stability parameter, and vary the gas fraction f_gas by fixing the total disk mass. We find that a bar forms earlier and more strongly as f_gas increases in the cold disk, while the bar formation is progressively delayed in the warm disks. The bar formation enhances the central mass concentration (CMC) which in turns makes the bars decay temporarily, after which the bars grow in size and strength again. Only the gas-free, warm-disk model undergoes buckling<br/> instability since rapid bar and CMC growth in models with gas excites the vertical stellar motions. The gas driven inward by the bar potential readily forms a star-forming nuclear ring. The ring is very small when it first forms and grows in size over time due to addition of gas with higher angular momentum. The ring star formation rate is episodic and bursty due to feedback, and well correlated with the bar strength. The bars and<br/> nuclear rings formed in our models have properties similar to those in the Milky Way.