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991.
M.S. Oey 《Astrophysics and Space Science》2002,281(1-2):483-486
Two fundamental constraints on the earliest star formation conditions in the Galaxy are an apparent empirical low-metallicity threshold of-4 ? [Fe/H], an and upper limit to the fraction of Population III halo stars of F III < 4 × 10-4. How do these observed constraints compare with predictions of simple models? This is investigated within the framework of element dispersal from clustered core-collapse SNe. Simple arguments considering turbulent mixing within multi-phase ISM suggest that the observed low-metallicity threshold is consistent with rough expected values. However, the observed limit on F III is two orders of magnitude larger than predictions from this simple, one-zone inhomogeneous chemical evolution. 相似文献
992.
Ch. Skokos P. A. Patsis E. Athanassoula 《Monthly notices of the Royal Astronomical Society》2002,333(4):847-860
In this series of papers we investigate the orbital structure of three-dimensional (3D) models representing barred galaxies. In the present introductory paper we use a fiducial case to describe all families of periodic orbits that may play a role in the morphology of three-dimensional bars. We show that, in a 3D bar, the backbone of the orbital structure is not just the x1 family, as in two-dimensional (2D) models, but a tree of 2D and 3D families bifurcating from x1. Besides the main tree we have also found another group of families of lesser importance around the radial 3:1 resonance. The families of this group bifurcate from x1 and influence the dynamics of the system only locally. We also find that 3D orbits elongated along the bar minor axis can be formed by bifurcations of the planar x2 family. They can support 3D bar-like structures along the minor axis of the main bar. Banana-like orbits around the stable Lagrangian points build a forest of 2D and 3D families as well. The importance of the 3D x1-tree families at the outer parts of the bar depends critically on whether they are introduced in the system as bifurcations in z or in z˙ . 相似文献
993.
Zurab Berezhiani Paolo Ciarcelluti Santi Cassisi Adriano Pietrinferni 《Astroparticle Physics》2006,24(6):495-510
There can exist a hidden sector of the Universe in the form of parallel “mirror” world which has the same particle physics as the observable world and interacts with the latter only gravitationally. Big Bang nucleosynthesis bounds demand that the mirror sector should have a smaller temperature than the ordinary one. This implies that the mirror matter could play a role of dark matter, and in addition its chemical content should be dominated by helium. Here we study the evolutionary and structural properties of the mirror stars which essentially are similar to that of the ordinary stars but with higher helium contents. Being invisible in terms of photons, they could be observed only as MACHOs in the microlensing experiments. Using a numerical code, we compute evolution of stars with large helium abundances (Y = 0.30–0.80) and a wide range of masses, from 0.5 to 10 M. We found that helium dominated mirror star should have much faster evolutionary time (up to a factor 30) than the ordinary star with the same mass. In addition, we show the diagrams of luminosities, effective temperatures, central temperatures and densities, and compute the masses of the He-core at ignition and the minimum mass for carbon ignition, for different chemical compositions. The general conclusion is that mirror stars evolve faster as compared to ordinary ones, and explode earlier as type II supernovae, thus enriching the galactic halo of processed mirror gas with higher metallicity, with implications for MACHO observations and galaxy evolution. 相似文献
994.
Matt J. Jarvis Ross J. McLure 《Monthly notices of the Royal Astronomical Society》2006,369(1):182-188
We have identified two new galaxies with gas counter-rotation (NGC 1596 and 3203) and have confirmed similar behaviour in another one (NGC 128), this using results from separate studies of the ionized-gas and stellar kinematics of a well-defined sample of 30 edge-on disc galaxies. Gas counter-rotators thus represent 10 ± 5 per cent of our sample, but the fraction climbs to 21 ± 11 per cent when only lenticular (S0) galaxies are considered and to 27 ± 13 per cent for S0 galaxies with detected ionized gas only. Those fractions are consistent with but slightly higher than previous studies. A compilation from well-defined studies of S0 galaxies in the literature yields fractions of 15 ± 4 and 23 ± 5 per cent, respectively. Although mainly based on circumstantial evidence, we argue that the counter-rotating gas originates primarily from minor mergers and tidally induced transfer of material from nearby objects. Assuming isotropic accretion, twice those fractions of objects must have undergone similar processes, underlining the importance of (minor) accretion for galaxy evolution. Applications of gas counter-rotators to barred galaxy dynamics are also discussed. 相似文献
995.
基于目前流行的冷暗物质等级成团理论的星系形成的半解析模型取得了很大进展.在引入一系列合理的关于重子物质物理过程的假设,如气体冷却、恒星形成率、超新星能量反馈以后,结合N体数值模拟,星系形成的半解析模型一方面能较成功地预言许多与观测符合的近邻星系的性质,如光度函数、星系的两点相关函数、Thlly-Fisher关系、星系团中椭圆星系的颜色一星等关系等;另一方面还能较成功地预言宇宙的恒星形成历史,以及一部分高红移星系的性质,如Lyman-Break星系的数目随红移的分布.但是最近一些观测表明,半解析模型在高红移预言的大质量星系较少,且红星系数目也较观测少.重点讨论半解析模型成功与不足之处,并指出解决这些问题可能的途径。 相似文献
996.
997.
Jennifer A. Grimes Steve Rawlings Chris J. Willott 《Monthly notices of the Royal Astronomical Society》2005,359(4):1345-1355
We extend our previous analysis which used generalized luminosity functions (GLFs) to predict the number of quasars and galaxies in low-radio-frequency-selected samples as a function of redshift, radio luminosity, narrow-emission-line luminosity and type of unified scheme. Our extended analysis incorporates the observed submillimetre (850-μm) flux densities of radio sources, employs a new method which allows us to deal with non-detections, and focuses on the high-luminosity population. First, we conclude that the submillimetre luminosity L 850 of low-frequency-selected radio sources is correlated with the bolometric luminosity L bol of their quasar nuclei via an approximate scaling relation L 850 ∝ L 0.7±0.2 bol . Secondly, we conclude that there is quantitative evidence for a receding-torus-like physical process for the high-luminosity population within a two-population unified scheme for radio sources; this evidence comes from the fact that radio quasars are brighter in both narrow emission lines and submillimetre luminosity than radio galaxies matched in radio luminosity and redshift. Thirdly, we note that the combination of a receding-torus-like scheme and the assumption that the observed submillimetre emission is dominated by quasar-heated dust yields a scaling relation L 850 ∝ L 1/2 bol which is within the errors of that determined here for radio-selected quasars, and consistent with that inferred for radio-quiet quasars. 相似文献
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