Low Star Formation Efficiency in Typical Galaxies at z = 5–6
Low Star Formation Efficiency in Typical Galaxies at z = 5–6
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DOI：
10.3847/1538-4357/ab3a46
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发表时间：
2018-11
期刊：
影响因子：
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通讯作者：
R. Pavesi;D. Riechers;A. Faisst;G. Stacey;P. Capak
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作者：
R. Pavesi;D. Riechers;A. Faisst;G. Stacey;P. Capak
利用甚大阵和阿尔马，我们获得了一个z = 5-6的“正常”星系样品中的CO（2-1），[C ii]和[N ii]线发射和多个尘埃连续谱测量。我们报告的最高红移检测低J CO排放的莱曼break星系，在z = 5.7。CO谱线的光度意味着一个巨大的分子气藏，为（1.3 ± 0.3）（αCO/4.5 M（K km s−1 pc 2）−1）× 1011 M，表明星星的形成效率很低，气体耗尽的时间尺度为101 Gyr。这个效率比传统上观测到的z 105星爆要低得多，这表明z 106主序星星系的恒星形成条件可能与迄今为止探测到的z 103的正常星系相当，但在整个红移范围内气体比例都在上升。我们还获得了一个主序星星系的深CO上限，z = 5.3，星星形成率大约低三倍，这可能意味着高αCO转换因子，通常在低金属丰度星系中发现。对于一个包含两个CO目标的样本，我们还发现，除了在z = 5-6的最红外发光的“正常”星系外，所有星系的[N ii] 205 μm发射相对于[C ii]都很微弱，这意味着电离气体中的辐射场相对于较低的红移更强或更硬。这些辐射特性表明，低金属丰度可能是常见的典型的1010 M的星系在z = 5-6。虽然主序星星在最初的10亿年中形成的一小部分可能发生在与较低红移相似的条件下，但较低的金属丰度可能会影响其余的人口。
Using the Very Large Array and ALMA, we have obtained CO(2–1), [C ii], and [N ii] line emission and multiple dust continuum measurements in a sample of “normal” galaxies at z = 5–6. We report the highest-redshift detection of low-J CO emission from a Lyman break galaxy, at z ∼ 5.7. The CO line luminosity implies a massive molecular gas reservoir of (1.3 ± 0.3)(αCO/4.5 M⊙ (K km s−1 pc2)−1) × 1011 M⊙, suggesting low star formation efficiency with a gas depletion timescale of order ∼1 Gyr. This efficiency is much lower than traditionally observed in z ≳ 5 starbursts, indicating that star-forming conditions in main-sequence galaxies at z ∼ 6 may be comparable to those of normal galaxies probed up to z ∼ 3 to date but with rising gas fractions across the entire redshift range. We also obtain a deep CO upper limit for a main-sequence galaxy at z ∼ 5.3 with an approximately three times lower star formation rate, perhaps implying a high αCO conversion factor, as typically found in low-metallicity galaxies. For a sample including both CO targets, we also find faint [N ii] 205 μm emission relative to [C ii] in all but the most IR-luminous “normal” galaxies at z = 5–6, implying more intense or harder radiation fields in the ionized gas relative to lower redshift. These radiation properties suggest that low metallicity may be common in typical ∼1010 M⊙ galaxies at z = 5–6. While a fraction of main-sequence star formation in the first billion yr may take place in conditions not dissimilar to lower redshift, lower metallicity may affect the remainder of the population.