A systematic study of silicate absorption features in heavily obscured AGNs observed by Spitzer/IRS
A systematic study of silicate absorption features in heavily obscured AGNs observed by Spitzer/IRS
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对 Spitzer/IRS 观测到的严重遮挡 AGN 中硅酸盐吸收特征的系统研究
DOI：
10.1051/0004-6361/202140483
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发表时间：
2021
影响因子：
6.5
通讯作者：
Toba Y.
中科院分区：
文献类型：
--
作者：
Tsuchikawa T.;Kaneda H.;Oyabu S.;Kokusho T.;Kobayashi H.;Yamagishi M.;Toba Y.
背景严重遮挡的活动星系核(AGN)在10-20μm的中红外(Mid-IR)波长范围内显示出较深的硅酸盐吸收特征。这些特征的详细剖面反映了硅酸盐尘埃的性质，目的通过从中红外光谱分析得到的硅酸盐粉尘特征，揭示大量粉尘对活动星系核活动的遮蔽作用。方法选取115个SPITZER/IRS观测到的重度遮挡活动星系核的中红外光谱，利用10μm非晶态和23μm晶态谱带进行光谱拟合，系统地分析了硅酸盐粉尘的组成。结果发现，被大量粉尘遮盖的活动星系核的主要成分是无定形橄榄石，其质量柱密度中值比无定形辉石和结晶镁橄榄岩的次要组分高一个数量级。在银河系中，无定形辉石的质量分数中位数∼为2%，显著低于弥散性星际介质尘埃，而结晶镁橄榄岩的质量分数中位数∼为6%，高于弥散性星际介质尘埃。无定形辉石的质量分数与结晶镁橄榄岩的质量分数呈正相关。结论无定形辉石的低质量分数表明暗色硅酸盐尘埃是一种新形成的尘埃，起源于星暴活动。结晶镁橄榄岩相对较高的质量分数意味着硅酸盐尘埃在靠近核的高温环境中被处理，并通过分子外流被输送到较冷的外部区域。考虑到无定形辉石是通过离子轰击从结晶镁橄榄岩转变而来的，质量分数之间的正相关可以自然地解释。我们还发现，高H2O冰吸收与硅酸盐质量柱密度之比的光谱倾向于指示低质量分数的无定形辉石和结晶镁橄榄石，这与靠近原子核的热尘埃过程的情景相一致。
ContextHeavily obscured active galactic nuclei (AGNs) are known to show deep silicate absorption features in the mid-infrared (mid-IR) wavelength range of 10–20 μm. The detailed profiles of the features reflect the properties of silicate dust, which are likely to include information on AGN activities obscured by large amounts of dust.AimsWe reveal AGN activities obscured by large amounts of dust through the silicate dust properties obtained from the mid-IR spectral bands.MethodsWe selected 115 mid-IR spectra of heavily obscured AGNs observed bySpitzer/IRS and systematically analyzed the composition of silicate dust by spectral fitting using the 10 μm amorphous and 23 μm crystalline bands.ResultsWe find that the main component of the silicate dust obscuring AGNs is amorphous olivine, the median mass column density of which is one order of magnitude higher than those of the minor components of amorphous pyroxene and crystalline forsterite. The median mass fraction of the amorphous pyroxene, ∼2%, is significantly lower than that of the diffuse interstellar medium (ISM) dust in our Galaxy, while the median mass fraction of the crystalline forsterite, ∼6%, is higher than that of the diffuse ISM dust. We also find that the mass fractions of the amorphous pyroxene and the crystalline forsterite positively correlate with each other.ConclusionsThe low mass fraction of the amorphous pyroxene suggests that the obscuring silicate dust is newly formed, originating from starburst activities. The relatively high mass fraction of crystalline forsterite implies that the silicate dust is processed in the high temperature environment close to the nucleus and transported to outer cooler regions by molecular outflows. The positive correlation between the mass fractions can be naturally explained considering that amorphous pyroxene is transformed from crystalline forsterite by ion bombardments. We also find that spectra with high ratios of the H2O ice absorption to silicate mass column density tend to indicate low mass fractions of amorphous pyroxene and crystalline forsterite, which is consistent with the scenario of the thermal dust processing close to the nucleus.
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10.1111/j.1365-2966.2011.18204.x
发表时间：
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影响因子：
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