[论文解读] The MOSDEF Survey: Measurements of Balmer Decrements and the Dust Attenuation Curve at Redshifts z~1.4-2.6

Naveen A. Reddy, Mariska Kriek+8 more|arXiv (Cornell University)|Apr 10, 2015

Galaxies: Formation, Evolution, Phenomena被引用 9

一句话总结

本研究利用MOSDEF巡天的深近红外光谱，测量了224个高红移（z~1.4–2.6）星系的尘埃消光曲线。结果发现，在λ > 2500 Å波段，消光曲线与SMC曲线相似，且其归一化值比Calzetti（2000）星暴曲线低20%，导致应用该曲线时SFR估算值低20%，星体质量低0.16 dex，且在高SFR下，电离谱线相对于连续谱的尘埃消光程度更高。

ABSTRACT

We present results on the dust attenuation curve of z~2 galaxies using early observations from the MOSFIRE Deep Evolution Field (MOSDEF) survey. Our sample consists of 224 star-forming galaxies with nebular spectroscopic redshifts in the range z= 1.36-2.59 and high S/N measurements of, or upper limits on, the H-alpha and H-beta emission lines obtained with Keck/MOSFIRE. We construct composite SEDs of galaxies in bins of specific SFR and Balmer optical depth in order to directly constrain the dust attenuation curve from the UV through near-IR for typical star-forming galaxies at high redshift. Our results imply an attenuation curve that is very similar to the SMC extinction curve at wavelengths redward of 2500 Angstroms. At shorter wavelengths, the shape of the curve is identical to that of the Calzetti relation, but with a lower normalization (R_V). Hence, the new attenuation curve results in SFRs that are ~20% lower, and log stellar masses that are 0.16 dex lower, than those obtained with the Calzetti attenuation curve. Moreover, we find that the difference in the reddening---and the total attenuation---of the ionized gas and stellar continuum correlates strongly with SFR, such that for dust-corrected SFRs larger than 20 Msun/yr assuming a Chabrier IMF, the nebular emission lines suffer an increasing degree of obscuration relative to the continuum. A simple model that can account for these trends is one in which the UV through optical stellar continuum is dominated by a population of less reddened stars, while the nebular line and bolometric luminosities become increasingly dominated by dustier stellar populations for galaxies with large SFRs, as a result of the increased dust enrichment that accompanies such galaxies. Consequently, UV- and SED-based SFRs may underestimate the total SFR at even modest levels of ~20 Msun/yr. [Abridged]

研究动机与目标

• 利用多波段测光和近红外光谱，直接测量高红移星形成星系中的尘埃消光曲线。
• 通过考虑恒星连续谱与电离气体的尘埃消光差异，解决基于紫外和Hα的星形成率（SFR）估算之间的差异。
• 研究尘埃性质和几何结构如何随单位星体质量的星形成率（SFR/M*）和巴尔默光学深度变化。
• 通过使用更准确、基于实测约束的消光曲线，改进基于SED的星体质量和SFR估算。

提出的方法

• 利用深度多波段测光，按特定星形成率和巴尔默光学深度对星系构建复合谱谱分布函数（SEDs）。
• 利用MOSFIRE光谱的高信噪比Hα和Hβ发射线测量，推导出巴尔默减退量和电离气体的尘埃红化。
• 通过不同波段复合SED比率计算消光曲线，直接从紫外到近红外波段约束其形状和归一化值。
• 将推导出的消光曲线与标准模板（如Calzetti 2000、SMC）进行比较，评估其归一化值和形状的差异。
• 量化电离气体与恒星连续谱之间的色指数差异，以评估尘埃消光的差异性。
• 通过将新曲线应用于标准SED拟合流程，模拟其对SFR和星体质量估算的影响。

实验结果

研究问题

• RQ1 高红移星形成星系（z~1.4–2.6）的尘埃消光曲线与本地模板（如Calzetti 2000和SMC）相比如何？
• RQ2 直接从高红移星系复合SED推导出的消光曲线的归一化值和形状是什么？
• RQ3 电离发射线的尘埃红化与恒星连续谱相比如何？这种差异是否随SFR变化？
• RQ4 由于高红移星系中尘埃消光的差异性，基于紫外和Hα的SFR估算值在多大程度上出现分歧？
• RQ5 修改消光曲线对高红移星系研究中星体质量和SFR估算有何影响？

主要发现

• 在λ > 2500 Å波段，消光曲线的形状和归一化值与小麦哲伦云（SMC）消光曲线非常相似。
• 在2500 Å以下波长，该曲线与Calzetti等（2000）星暴消光曲线形状相同，但Δk(λ)的归一化值低1.63倍，意味着单位红化程度下的消光更弱。
• 应用新消光曲线后，SFR估算值比使用Calzetti（2000）曲线的结果低约20%。
• 使用新曲线估算的星体质量比使用Calzetti（2000）曲线的结果低Δlog(M*/M☉) ≈ 0.16 dex。
• 存在微弱趋势表明在2175 Å处有额外吸收，提示尘埃颗粒性质可能存在变化。
• 电离气体与恒星连续谱之间的红化差异随SFR增加而增大，当SFR > 20 M☉ yr⁻¹时，电离谱线相对于紫外连续谱的消光程度逐渐增强。