AN ULTRA-DEEP NEAR-INFRARED SPECTRUM OF A COMPACT QUIESCENT GALAXY AT z = 2.2
AN ULTRA-DEEP NEAR-INFRARED SPECTRUM OF A COMPACT QUIESCENT GALAXY AT z = 2.2
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DOI：
10.1088/0004-637x/700/1/221
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发表时间：
2009-05
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影响因子：
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通讯作者：
M. Kriek;P. V. van Dokkum;I. Labbé;M. Franx;G. Illingworth;D. Marchesini;R. Quadri
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文献类型：
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作者：
M. Kriek;P. V. van Dokkum;I. Labbé;M. Franx;G. Illingworth;D. Marchesini;R. Quadri
最近的几项研究表明，大约一半位于z ~ 2的大质量星系处于静止阶段。此外，这些星系通常被发现是超致密的，半光半径为1 kpc。我们用双子座近红外光谱仪获得了z = 2.1865处典型的静止超致密星系的~ 29小时光谱。光谱显示出强烈的光学断裂和几个吸收特征，这些特征以前在z bbb20静止星系中没有被检测到。光谱能量分布与恒星族合成模型的比较表明，恒星形成速率（SFR）较低，为1 - 3 M☉yr - 1，年龄为1.3-2.2 Gyr，恒星质量为~ 2 × 1011 M☉。我们发现了一些微弱的发射线，其发射在线比为[N ii]/Hα， [S ii]/Hα和[O ii]/[O iii]，是典型的低电离核发射在线区域。因此，无论是恒星连续体还是星云发射都不意味着恒星的活跃形成。当前的SFR小于过去平均SFR的1%。如果这个星系是z = 2以上紧凑型静止星系的代表，这意味着恒星形成的猝灭是非常有效的，也表明低光度活动星系核（agn）可能在这些天体中很常见。核发射是尺寸测量的一个潜在问题。然而，我们发现AGN对静帧光发射的贡献小于8%。可能的星暴后人口可能会更强烈地影响尺寸测量；虽然0.5 Gyr年龄的恒星群可以占到恒星总质量的10%，但它可以占到光学光的40%。尽管如此，这张光谱显示这个致密星系是由进化的恒星群主导的。
Several recent studies have shown that about half of the massive galaxies at z ∼ 2 are in a quiescent phase. Moreover, these galaxies are commonly found to be ultra-compact with half-light radii of ∼1 kpc. We have obtained a ∼29 hr spectrum of a typical quiescent, ultra-dense galaxy at z = 2.1865 with the Gemini Near-Infrared Spectrograph. The spectrum exhibits a strong optical break and several absorption features, which have not previously been detected in z > 2 quiescent galaxies. Comparison of the spectral energy distribution with stellar population synthesis models implies a low star formation rate (SFR) of 1–3 M☉ yr−1, an age of 1.3–2.2 Gyr, and a stellar mass of ∼2 × 1011 M☉. We detect several faint emission lines, with emission-line ratios of [N ii]/Hα, [S ii]/Hα, and [O ii]/[O iii] typical of low-ionization nuclear emission-line regions. Thus, neither the stellar continuum nor the nebular emission implies active star formation. The current SFR is <1% of the past average SFR. If this galaxy is representative of compact quiescent galaxies beyond z = 2, it implies that quenching of star formation is extremely efficient and also indicates that low luminosity active galactic nuclei (AGNs) could be common in these objects. Nuclear emission is a potential concern for the size measurement. However, we show that the AGN contributes ≲8% to the rest-frame optical emission. A possible post-starburst population may affect size measurements more strongly; although a 0.5 Gyr old stellar population can make up ≲10% of the total stellar mass, it could account for up to ∼40% of the optical light. Nevertheless, this spectrum shows that this compact galaxy is dominated by an evolved stellar population.