The Negative Feedback Effects On Star Formation In The Early Universe

In the past decades,the Lambda Cold Dark Matter(ACDM) cosmological model and the so-called "bottom-up" hierarchical theory for large scale structure formation have got decisive supports from different probing projects(e.g. HST,WMAP,SDSS) and high-resolution numerical simulations(such as N-body, hydrodynamics etc.) However,many questions including the details of the galaxy evolution in halos(e.g.the gas cooling and condensation,the starformation history(SFH) in galaxies and the feedback from luminous objects) and the reionization history of the IGM are not fully understood yet owing to the complexity of the baryonic evolution in the radiative background and gravitational field.These uncertainties are linked to the early formed objects which are not observable at present.Nevertheless we can still do something in this field based on the current observation data.Recent detections of Gunn-Peterson trough[Gunn & Peterson 1965]in the spectra of QSOs with z(?)6 indicate less 50%neutral hydrogen at z(?)6.5[Wyithe et al.2005,Fan et al.2006].The ongoing observations of the WMAP satellite on Cosmic Microwave Background (CMB) and the highest redshift QSOs study put very tight constraints on the reionization history of the Universe.The CMB observation(WMAP 5 years data) manifests the optical depth to Thomson electron scattering,τ_e=0.084(?) [Komatsu et al.2008],which suggests that our Universe might be reionized during the period of redshift 9.4≤z_re≤12.2. It is well-known that the chemical elements heavier than lithium are produced exclusively through stellar nucleosynthesis.Some of the first generation stars(the so-called populationⅢstars,hereafter,PopⅢ) die as SNe explosions, which can expel the heavy elements into the intergalactic medium(IGM).When the metal elements in IGM are enriched to a certain threshold Z_crit,the populationⅡ/Ⅰ(hereafter PopⅡ/Ⅰ) stars will form and take the place of the first stars to light the universe.As the earliest distributors of heavy elements,the SNe from PopⅢstars determine the transition from PopⅢto PopⅡ/Ⅰ.The existence of PopⅢstars can help explaining the metal enrichment(from Z(?)10^-12-10^-10 to the lowest metallicity of PopⅡstars Z(?)10^-4-10^-3),the formation of massive black holes,the reionization of the universe and the starting engine for the formation of the first galaxies and the G-dwarf,and so on[Ciardi & Ferrara 2005].But it is hopeless to observe the first generation stars until the launch of the successor of the Hubble Space Telescope(HST),called James Webb Space Telescope (JWST)[Barkana & Loeb 2001].JWST will show us Pair Instability SNe(PISNe) from massive PopⅢstars if the current theory is reasonable[Wise & Abel 2005].These facts demand people to pay attention to the high redshift objects and study their evolution processes.In the absence of the observational data of very high redshift(z≥10) objects,people often seek help from numerical simulations. Some authors concentrated on the effects of the first generation SNe explosions[Yoshida et al.2003,Kitayama & Yoshida 2005,Greif et al.2007],some studied the strong stellar and galactic winds from PopⅢstars[Meynet et al.2006, Ricotti et al.2008].Based on their works,we can obtain a model to describe the global effects from the PopⅢstars at early time(like the SFR density,the IGM reionization and so on.)In this PhD thesis,we studied the negative feedback effect from both supernovae (SNe) explosions and the stellar radiation.And we find the radiative feedback is important for the early generation stars.It can suppress the star formation significantly.But the mechanical feedback from the SNe explosions is not able to affect the early star formation considerably.The radiative and mechanical feedback dominates the star formation rate of the second or third generation stars.The feedback from first generation stars is very strong and should not be neglected.However,the effect on reionization from early stars is not significant,which results in a little contribution on the Thomson electron scattering optical depth.The early star formation in small-halo objects is likely to be self-regulated.We introduce the ACDM cosmological model in the first chapter.In chapter 2,we discuss the evolution of baryons,including the formation, evolution and extinction of the first stars,cosmological reionization and all kinds of feedback effects.In chapter 3,we emphasize the negative feedback from SNe.The radiative feedback effect will be calculated in chapter 4.Finally,we present our discussion and conclusion in the last chapter.