大麥哲倫雲中繞恆星塵對光譜能量分佈的影響

Abstract

距離我們遙遠的星系通常是不可解析的,因此,我們只能透過光譜 能量分佈來研究他們的物理性質。然而,現今的光譜能量分佈模型只 考慮了來自恆星以及星系間介質的貢獻,並沒有納入圍繞在初期恆星 體或是晚期恆星(漸近巨星分支)的繞恆星塵的貢獻。對於晚期恆星 只占一小部分的星系而言,這樣的模型並不會造成太大的差異,但對 於擁有不可忽略數量的晚期恆星的星系來說,這樣的模型可能無法精 確解釋星系中的物理現象。 因此,為了研究繞恆星塵對於整個星系光譜能量分佈的影響,我們 利用Harris & Zaritsky (2009) 提供的大麥哲倫雲的恆星形成歷史,以及 星系光譜模擬程式 Pegase(Fioc & Rocca-Volmerange 1997) 去計算大麥 哲倫雲的光譜能量分佈,並且只針對不被繞恆星塵影響的紫外光、可 見光波段做擬合。 我們發現,就算是擬合地最好的光譜能量分佈,在近紅外波段,也 和觀測到的光譜能量分佈有著明顯的差距。這表示大麥哲倫雲中的繞 恆星塵對於光譜能量分佈的影響是不可忽略的。因此,我們建議未來 的光譜能量分佈模型應該要考慮將繞恆星塵的貢獻考慮進去。

Unresolved distant galaxies can typically only be studied through their spectral energy distributions (SEDs). An SED model of a galaxy usually con- sists of contributions due to starlight (in the UV/optical and near-infrared) and thermal emission of interstellar dust (in the far-infrared). However, present-day SED models typically do not accounted for dusty envelopes of AGB stars. This might be acceptable for starburst galaxies with a relatively large population of young stars, but it might not accurately repre- sent galaxies with a significant population of dusty Asymptotic Giant Branch (AGB) stars. In order to estimate the contribution from circumstellar dust, we first gen- erate model stellar SEDs with the population synthesis code Pegase and the star formation history of the LMC from Harris & Zaritsky (2009), which we use to fit the observed SED from previous works at wavelengths less than 1 μm. Then we subtract the best-fit model SED and the interstellar medium (ISM) SED constructed by Bernard et al. (2008) from the observed SED. We found that there is a significant discrepancy (20 − 40%) between our best-fit SED and the observed SED minus the ISM SED. This result indicates the importance of the contribution from circumstellar dust in the near-infrared SED. Thus, we suggest that an improved stellar spectral library that includes dusty AGB spectra and an improved set of stellar evolutionary tracks that considers dust formalism in stellar winds is needed.