一个z=1.82的局部超大质量椭圆星系的模拟
A z=1.82 Analog of Local Ultra-massive Elliptical Galaxies
摘要：
我们目前在z=1.82的一个非常巨大的星系的观测表明，它的形态，大小，速度色散和恒星星族的属性，是完全一致的被动演变的祖先今天的巨型椭圆。这些发现是基于斯巴鲁望远镜上的多目标红外照相机和摄谱仪（MOIRCS）获得的来自COSMOS场的高z被动星系候选者（pBzK）的深光学静止框架光谱，我们精确测量了它的红移z=1.8230，并获得了其速度色散σ_star<326 km/s的上限。通过详细的恒星星族光谱计算，得到了恒星形成加权的年龄和形成红移分别为tsf ~1-2 Gyr和zform ~2.5-4，恒星质量为M_star~（3-4）× 10 ^{11} M_sun。这与从测量的sigma_star值和恒星半光半径导出的维里质量极限M_vir<7 × 10 ^{11}M_sun一致，也与基于Jeans方程的动力学质量极限一致。与先前报道的z~2处的超致密被动星系相比，当前z=1.82处的星系似乎具有与具有类似恒星质量的本地宇宙中的早型星系相似的尺寸和速度色散。这表明z~2质量被动星系可能表现出广泛的性质，从z~2到z=0可能遵循完全不同的演化历史。
英文摘要：
We present observations of a very massive galaxy at z=1.82 which show that its morphology, size, velocity dispersion and stellar population properties that are fully consistent with those expected for passively evolving progenitors of today's giant ellipticals. These findings are based on a deep optical rest-frame spectrum obtained with the Multi-Object InfraRed Camera and Spectrograph (MOIRCS) on the Subaru telescope of a high-z passive galaxy candidate (pBzK) from the COSMOS field, for which we accurately measure its redshift of z=1.8230 and obtain an upper limit on its velocity dispersion sigma_star<326 km/s. By detailed stellar population modeling of both the galaxy broad-band SED and the rest-frame optical spectrum we derive a star-formation-weighted age and formation redshift of t_sf~1-2 Gyr and z_form~2.5-4, and a stellar mass of M_star~(3-4)x10^{11} M_sun. This is in agreement with a virial mass limit of M_vir<7x10^{11}M_sun, derived from the measured sigma_star value and stellar half-light radius, as well as with the dynamical mass limit based on the Jeans equations. In contrast with previously reported super-dense passive galaxies at z~2, the present galaxy at z=1.82 appears to have both size and velocity dispersion similar to early-type galaxies in the local Universe with similar stellar mass. This suggests that z~2 massive and passive galaxies may exhibit a wide range of properties, then possibly following quite different evolutionary histories from z~2 to z=0.
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