被动早型星系中H $β$ Lick指数作为宇宙钟的稳健性
On the robustness of the H$β$ Lick index as a cosmic clock in passive early-type galaxies
摘要：
我们研究了$β$ Lick指数在$\sim 24000$质量（$\rm log(M/M_{\odot})>10.75$）和被动早型星系的样本中提取的SDSS在z<0.3，为了评估该指数的可靠性，以约束这些系统的形成和年龄演化的时代。我们进一步研究了利用这个指数作为“宇宙计时器”的可能性，即从它的微分演化与红移推导出哈勃参数，从而约束独立于其他探测器的宇宙学模型。我们发现H $β$强度随红移的增加而增加，正如被动演化模型中所预期的那样，并且在每个红移处，在更大质量的星系中显示出较弱的值。然而，观测到的指数与恒星星族合成模型的预测的详细比较突出了一个显着的紧张局势，观测到的指数系统低于预期。通过分析堆叠的光谱，我们发现了一个弱的[NII] $\lambda6584$发射线（在单光谱中检测不到），它与质量反相关，这可以解释为电离气体存在的暗示。我们估计的校正的H $β$指数的残余排放分量利用不同的方法，但发现它非常不确定和模型依赖。我们的结论是，虽然所观察到的H $β$ -z关系的定性趋势与预期的被动和缩小的情况是一致的，电离气体的可能存在，即使在最大的质量和被动的星系防止使用这个指数的年龄演化的定量估计和宇宙学的应用。
英文摘要：
We examine the H$β$ Lick index in a sample of $\sim 24000$ massive ($\rm log(M/M_{\odot})>10.75$) and passive early-type galaxies extracted from SDSS at z<0.3, in order to assess the reliability of this index to constrain the epoch of formation and age evolution of these systems. We further investigate the possibility of exploiting this index as "cosmic chronometer", i.e. to derive the Hubble parameter from its differential evolution with redshift, hence constraining cosmological models independently of other probes. We find that the H$β$ strength increases with redshift as expected in passive evolution models, and shows at each redshift weaker values in more massive galaxies. However, a detailed comparison of the observed index with the predictions of stellar population synthesis models highlights a significant tension, with the observed index being systematically lower than expected. By analyzing the stacked spectra, we find a weak [NII]$\lambda6584$ emission line (not detectable in the single spectra) which anti-correlates with the mass, that can be interpreted as a hint of the presence of ionized gas. We estimated the correction of the H$β$ index by the residual emission component exploiting different approaches, but find it very uncertain and model-dependent. We conclude that, while the qualitative trends of the observed H$β$-z relations are consistent with the expected passive and downsizing scenario, the possible presence of ionized gas even in the most massive and passive galaxies prevents to use this index for a quantitative estimate of the age evolution and for cosmological applications.
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