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Numerical simulation of a possible origin of the positive radial metallicity gradient of the thick disk

Research in Astronomy and Astrophysics 2014年第11期14卷 1406-1414页

作者： Awat Rahimi Kenneth Carrell Daisuke Kawata National Astronomical Observatories Chinese Academy of Sciences Mullard Space Science Laboratory University College London Holmbury St.Mary DorkingSurrey RH5 6NT UK

We analyze the radial and vertical metallicity and [α/Fe] gradients of the disk stars of a disk galaxy simulated in a fully cosmological setting with the chemo- dynamical galaxy evolution code GCD＋. We study how the radial abundance gradients vary as a function of height above the plane and find that the metallicity （[α/Fe]） gra- dient becomes more positive （negative） with increasing height, changing sign around 1.5 kpc above the plane. At the largest vertical height （2 〈丨z丨〈 3 kpc）, our simulated galaxy shows a positive radial metallicity gradient. We find that the positive metallicity gradient is caused by the age-metallicity and age-velocity dispersion relation, where younger stars have higher metallicity and lower velocity dispersion. Due to the age- velocity dispersion relation, a greater fraction of younger stars reaches 丨z丨〉 2 kpc at the outer region, because of the lower gravitational restoring force of the disk, i.e. flaring. As a result, the fraction of younger stars with higher metallicity due to the age-metallicity relation becomes higher at the outer radii, which makes the median metallicity higher at the outer radii. Combining this result with the recently observed age-metallicity and age-velocity dispersion relation for the Milky Way thick disk stars suggested by Haywood et al., we argue that the observed （small） positive radial metal- licity gradient at large heights of the Milky Way disk stars can be explained by flaring of the younger thick and/or thin disk stars.

关键词： Galaxy disk -- Galaxy kinematics and dynamics -- galaxies interac-tions- galaxies formation- galaxies evolution- galaxies abundances

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A Study of Binary Stellar Population Synthesis of Elliptical galaxies

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Chinese Journal of Astronomy and Astrophysics 2006年第6期6卷 669-679页

作者： Zhong-Mu Li Feng-Hui Zhang Zhan-Wen Han National Astronomical Observatories / Yunnan Observatory Chinese Academy of Sciences Kunming 650011 Graduate School of Chinese Academy of Sciences Beijing 100049

We determined the relative stellar ages and metallicities of about 80 elliptical galaxies in both low and high density environments using the latest binary stellar population （BSP） synthesis model and tested the predictions of a recent hierarchical formation model that adopted the new ACDM cosmology. The stellar ages and metallicities were estimated from two high-quality published spectra line indices, the Hβ and [MgFe] indices. The results show that the stellar populations of elliptical galaxies are older than 3.9 Gyr and more metal rich than 0.02. Most of our results are in agreement with the model predictions; （1） elliptical galaxies in denser environment are redder and have older populations than field galaxies; （2） elliptical galaxies with more massive stellar components are redder and have older and more metal rich populations than less massive ones; （3） the most massive galaxies have the oldest and most metal rich stars. However, some of our results differ from the model predictions on the metallicity distributions of low- and high-density elliptical galaxies and the dependence on the distance to the cluster center.

关键词： galaxies stellar content- galaxies formation- galaxies elliptical and lenticular, cD

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