Passively evolving early-type galaxies at 1.4 ≲ z ≲ 2.5 in the Hubble Ultra Deep Field
Passively evolving early-type galaxies at 1.4 ≲ z ≲ 2.5 in the Hubble Ultra Deep Field
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DOI：
10.1086/430104
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发表时间：
2005-06-20
影响因子：
4.9
通讯作者：
Windhorst, RA
中科院分区：
文献类型：
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作者：
Daddi, E;Renzini, A;Windhorst, RA
我们报道了哈勃超深场(UDF)中七个发光的早期类型星系的完整样本，光谱红移在1.39和2.47之间，并到K-AB和lt；23。使用BZK选择标准，我们在UDF上预先选择了一组天体，这些天体满足z>；1.4的被动演化星系的光度学条件。这些天体的低分辨率光谱是由GRISM星系外科学计划(GRAPES)项目从哈勃太空望远镜(HST)的ACS GRISM数据中提取的。根据静止坐标系2640<；lambda<；2850埃的紫外线特征，已经确定了这七个星系的红移。这一特征主要是由于Fe II、镁I和镁II吸收的组合，这些吸收是由年龄大于0.5Gyr的恒星主导的恒星群的特征。这些星系的红移识别和被动演化的性质进一步得到了光度红移和总光谱能量分布(SED)的支持，超深的HST ACS NICMOS成像揭示了椭圆/早型星系典型的致密形态。从SED我们得到了大于或类似于10(11)M圆点的恒星质量和类似于1 Gyr的年龄。它们在<；z>；=1.7时的空间密度似乎大约是当地同行的2-3倍，进一步支持了这样一个观点，即这样的大质量和古老的星系在早期宇宙时代就已经无处不在。与局部大质量椭圆体相比，一些物体的有效半径要小得多，这可能是由于形态K校正、演化或中心点源的存在。核活动确实存在于星系的一个子集中，正如它们是硬X射线源这一事实所揭示的那样，这表明活跃的星系核(AGN)活动可能在停止恒星形成方面发挥了作用。
We report on a complete sample of seven luminous early-type galaxies in the Hubble Ultra Deep Field (UDF) with spectroscopic redshifts between 1.39 and 2.47, and to K-AB < 23. Using the BzK selection criterion, we have preselected a set of objects over the UDF, which fulfill the photometric conditions for being passively evolving galaxies at z > 1.4. Low-resolution spectra of these objects have been extracted from the Hubble Space Telescope (HST) ACS grism data taken over the UDF by the Grism ACS Program for Extragalactic Science (GRAPES) project. Redshifts for the seven galaxies have been identified based on the UV feature at rest frame 2640 < lambda < 2850 angstrom. This feature is mainly due to a combination of Fe II, Mg I, and Mg II absorptions, which are characteristic of stellar populations dominated by stars older than similar to 0.5 Gyr. The redshift identification and the passively evolving nature of these galaxies is further supported by the photometric redshifts and by the overall spectral energy distribution ( SED), with the ultradeep HST ACS NICMOS imaging revealing compact morphologies typical of elliptical/early-type galaxies. From the SED we derive stellar masses of greater than or similar to 10(11) M-circle dot and ages of similar to 1 Gyr. Their space density at < z > = 1.7 appears to be roughly a factor of 2-3 smaller than that of their local counterparts, further supporting the notion that such massive and old galaxies are already ubiquitous at early cosmic times. Much smaller effective radii are derived for some of the objects, compared to local massive ellipticals, which may be due to morphological K-corrections, evolution, or the presence of a central pointlike source. Nuclear activity is indeed present in a subset of the galaxies, as revealed by the fact that they are hard X-ray sources, which suggests that active galactic nucleus (AGN) activity may have played a role in discontinuing star formation.