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The MOSDEF-LRIS Survey: The Interplay Between Massive Stars and Ionized Gas in High-Redshift Star-Forming Galaxies1
Monthly Notices of the Royal Astronomical Society ( IF 4.8 ) Pub Date : 2020-05-23 , DOI: 10.1093/mnras/staa1410
Michael W Topping 1 , Alice E Shapley 1 , Naveen A Reddy 2 , Ryan L Sanders 3 , Alison L Coil 4 , Mariska Kriek 5 , Bahram Mobasher 2 , Brian Siana 2
Affiliation  

We present a joint analysis of rest-UV and rest-optical spectra obtained using Keck/LRIS and Keck/MOSFIRE for a sample of 62 star-forming galaxies at $z\sim2.3$. We divide our sample into 2 bins based on their location in the [OIII]/Hb vs. [NII]/Ha BPT diagram, and perform the first differential study of the rest-UV properties of massive ionizing stars as a function of rest-optical emission-line ratios. Fitting BPASS stellar population synthesis models, including nebular continuum emission, to our rest-UV spectra, we find that high-redshift galaxies offset towards higher [OIII]/Hb and [NII]/Ha have younger ages (log(Age/yr)=$7.20^{+0.57}_{-0.20}$) and lower stellar metallicities ($Z_*=0.0010^{+0.0011}_{-0.0003}$) resulting in a harder ionizing spectrum, compared to the galaxies in our sample that lie on the local BPT star-forming sequence (log(Age/yr)=$8.57^{+0.88}_{-0.84}$, $Z_*=0.0019\pm0.0006$). Additionally, we find that the offset galaxies have an ionization parameter of log(U)=$-3.04^{+0.06}_{-0.11}$ and nebular metallicity of 12+log(O/H)=$8.40^{+0.06}_{-0.07}$, and the non-offset galaxies have log(U)=$-3.11\pm0.08$ and 12+log(O/H)=$8.30^{+0.05}_{-0.06}$. The stellar and nebular metallicities derived for our sample imply that the galaxies offset from the local BPT relation are more alpha-enhanced ($7.28^{+2.52}_{-2.82}$ O/Fe$_\odot$) compared to those consistent with the local sequence ($3.04^{+0.95}_{-0.54}$ O/Fe$_\odot$). However, even galaxies that are entirely consistent with the local nebular excitation sequence appear to be alpha-enhanced -- in contrast with typical local systems. Such differences must be considered when estimating gas-phase O/H at high redshift based on strong emission-line ratios. Specifically, a similarity in the location of high-redshift and local galaxies in the BPT diagram may not be indicative of a similarity in their physical properties.

中文翻译:

MOSDEF-LRIS 巡天:高红移恒星形成星系中大质量恒星与电离气体之间的相互作用1

我们在 $z\sim2.3$ 处对使用 Keck/LRIS 和 Keck/MOSFIRE 获得的 62 个恒星形成星系样本进行了静止紫外线和静止光学光谱的联合分析。我们根据样本在 [OIII]/Hb 与 [NII]/Ha BPT 图中的位置将样本分为 2 个区间,并对大质量电离星的静止紫外特性作为静止-光发射线比。将 BPASS 恒星群合成模型(包括星云连续谱发射)与我们的静止紫外光谱拟合,我们发现高红移星系向更高的 [OIII]/Hb 和 [NII]/Ha 偏移,其年龄更小(log(Age/yr) =$7.20^{+0.57}_{-0.20}$) 和较低的恒星金属丰度($Z_*=0.0010^{+0.0011}_{-0.0003}$)导致更难电离光谱,与我们样本中位于本地 BPT 恒星形成序列上的星系相比(log(Age/yr)=$8.57^{+0.88}_{-0.84}$, $Z_*=0.0019\pm0.0006$)。此外,我们发现偏移星系的电离参数为 log(U)=$-3.04^{+0.06}_{-0.11}$ 和星云金属丰度为 12+log(O/H)=$8.40^{+0.06 }_{-0.07}$,非偏移星系有 log(U)=$-3.11\pm0.08$ 和 12+log(O/H)=$8.30^{+0.05}_{-0.06}$ . 为我们的样本推导出的恒星和星云金属丰度意味着与那些一致的星系相比,从局部 BPT 关系偏移的星系更具有 alpha 增强($7.28^{+2.52}_{-2.82}$O/Fe$_\odot$)与本地序列($3.04^{+0.95}_{-0.54}$O/Fe$_\odot$)。然而,即使是与本地星云激发序列完全一致的星系,也似乎是 alpha 增强的——与典型的本地系统形成对比。在基于强发射线比估计高红移下的气相 O/H 时,必须考虑这种差异。具体而言,BPT 图中高红移星系和本地星系位置的相似性可能并不表示它们的物理特性相似。
更新日期:2020-05-23
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