CS, HC3N, and CH3CCH multi-line analyses toward starburst galaxies The evolution of cloud structures in the central regions of galaxies
CS, HC3N, and CH3CCH multi-line analyses toward starburst galaxies The evolution of cloud structures in the central regions of galaxies
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DOI：
10.1051/0004-6361/201014090
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发表时间：
2011-01-01
影响因子：
6.5
通讯作者：
Bayet, E.
中科院分区：
文献类型：
--
作者：
Aladro, R.;Martin-Pintado, J.;Bayet, E.
目标。我们的目标是研究致密分子气体的性质对内部的几个100 pc的四个附近的星暴星系占主导地位的光解离区（M 82）和大规模的冲击（NGC 253，IC 342，和Maffei 2），并连接的化学和物理性质的分子云的演化阶段的核星暴。对CS、HC 3 N（氰基乙炔）和CH 3CCH（甲基乙炔）三种稠密气体分子示踪剂进行了多次跃迁观测和分析，利用Boltzmann图确定了稠密气体的转动温度和柱密度，并利用大速度梯度模型计算了分子云中H-2的密度结构。CS和HC 3 N数据表明分子云中存在密度梯度。这两种分子显示出相似的激发条件，表明它们来自相同的气体组分。在M82中，CH 3CCH在河外源中具有最高的丰度分数（1.1 x 10（-8））。我们在所有星系中发现的密度和化学梯度都可以在星暴演化的框架中得到解释，这影响了星系核周围分子云的化学和结构。年轻的以激波为主的星暴星系，可能像马菲2，显示出相当均匀的密度和化学成分的云结构，这表明低星星形成活动。在演化的中间阶段，如NGC 253和IC 342的星系中的分子云，在分子云的密度较高的区域（核心）和密度较低的区域（晕）之间显示出高密度的对比（两个数量级），并且化学丰度相对恒定。最后，星系与最发达的星暴，M 82，有一个相当均匀的密度结构的云，大信封的原子/分子气体受到紫外光离解辐射的年轻星星集群，和非常不同的化学丰度的HC 3 N和CH 3CCH。
Aims. We aim to study the properties of the dense molecular gas towards the inner few 100 pc of four nearby starburst galaxies dominated both by photo dissociation regions (M 82) and large-scale shocks (NGC 253, IC 342, and Maffei 2), and to connect the chemical and physical properties of the molecular clouds with the evolutionary stage of the nuclear starbursts.Methods. We have carried out multi-transitional observations and analyses of three dense gas molecular tracers, CS, HC3N (cyanoacetylene), and CH3CCH (methyl acetylene), using Boltzmann diagrams in order to determine the rotational temperatures and column densities of the dense gas, and using a large velocity gradients model to calculate the H-2 density structure in the molecular clouds.Results. The CS and HC3N data indicate the presence of density gradients in the molecular clouds. These two molecules show similar excitation conditions, suggesting that they arise from the same gas components. In M82, CH3CCH has the highest fractional abundance determined in an extragalactic source (1.1 x 10(-8)).Conclusions. The density and the chemical gradients we found in all galaxies can be explained in the framework of the starburst evolution, which affects the chemistry and the structure of molecular clouds around the galactic nuclei. The young shock-dominated starburst galaxies, like presumably Maffei 2, show a cloud structure with a fairly uniform density and chemical composition that suggests low star formation activity. Molecular clouds in galaxies with starburst in an intermediate stage of evolution, such as NGC 253 and IC 342, show clouds with a high density contrast (two orders of magnitude) between the denser regions (cores) and the less dense regions (halos) of the molecular clouds and relatively constant chemical abundance. Finally, the galaxy with the most evolved starburst, M 82, has clouds with a fairly uniform density structure, large envelopes of atomic/molecular gas subjected to UV photodissociating radiation from young star clusters, and very different chemical abundances of HC3N and CH3CCH.