Near-Infrared and Millimeter Constraints on the Nuclear Energy Source of the Infrared-luminous Galaxy NGC 4418
Near-Infrared and Millimeter Constraints on the Nuclear Energy Source of the Infrared-luminous Galaxy NGC 4418
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DOI：
10.1086/424620
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发表时间：
2004-07
期刊：
影响因子：
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通讯作者：
M. Imanishi;K. Nakanishi;N. Kuno;K. Kohno
中科院分区：
文献类型：
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作者：
M. Imanishi;K. Nakanishi;N. Kuno;K. Kohno
我们提出了近红外和毫米调查的红外发光星系NGC 4418，以前的观测表明，拥有一个强大的埋藏的活动星系核（AGN）的核。主要结果如下：（1）红外K波段光谱在2.3-2.4 μm处有CO的吸收特征，并有很强的H_2发射谱线。H2发射线的光度比暗示了热起源，H2 1-0 S（1）线的等效宽度是迄今为止在外部星系中观测到的第二大，仅次于被充分研究的强H2发射星系NGC 6240。(2)红外L波段光谱在3.3 μm处显示出清晰的多环芳烃（PAH）发射特征，这通常在恒星形成星系中发现。估计星星形成的光度从观测到的PAH排放只能占一小部分的红外光度。(3)毫米波干涉仪观测表明HCN（1-0）与HCO+（1-0）的光度比高达1.8，与以前在纯活动星系核中发现的一样。(4)用单碟毫米望远镜测量HCN（1-0）光度表明，HCN（1-0）与红外光度之比略大于其它红外发光星系的平均值，但在散射范围内。所有这些结果都可以用这样的假设来解释，即除了在原子核表面能量不显著、被微弱遮蔽的星星形成外，还存在一个深埋在尘埃和高密度分子气体中的强大的X射线发射活动星系核。
We present near-infrared and millimeter investigations of the nucleus of the infrared-luminous galaxy NGC 4418, which previous observations suggest possesses a powerful buried active galactic nucleus (AGN). We find the following main results: (1) The infrared K-band spectrum shows CO absorption features at 2.3–2.4 μm from stars and very strong H2 emission lines. The luminosity ratios of H2 emission lines are suggestive of a thermal origin, and the equivalent width of the H2 1–0 S(1) line is the second largest observed to date in an external galaxy, after the well-studied strong H2-emitting galaxy NGC 6240. (2) The infrared L-band spectrum shows a clear polycyclic aromatic hydrocarbon (PAH) emission feature at 3.3 μm, which is usually found in star-forming galaxies. The estimated star formation luminosity from the observed PAH emission can account for only a small fraction of the infrared luminosity. (3) Millimeter interferometric observations of the nucleus reveal a high HCN (1–0) to HCO+ (1–0) luminosity ratio of ∼1.8, as has been previously found in pure AGNs. (4) The measurements of HCN (1–0) luminosity using a single-dish millimeter telescope show that the HCN (1–0) to infrared luminosity ratio is slightly larger than the average, but within the scattered range, for other infrared-luminous galaxies. All of these results can be explained by the scenario in which, in addition to energetically insignificant, weakly obscured star formation at the surface of the nucleus, a powerful X-ray–emitting AGN deeply buried in dust and high-density molecular gas is present.