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arXiv 0712.0582astro-ph
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在发光和超亮红外星系中增强的恒星形成效率的证据

Evidence of enhanced star formation efficiency in luminous and ultraluminous infrared galaxies

J. Gracia-Carpio, S. Garcia-Burillo, P. Planesas, A. Fuente, A. Usero

摘要:

本文介绍了利用IRAM 30 m望远镜对HCN和HCO^+的J=1-0和3-2谱线进行的新观测,这些谱线用于探测17个本地发光和超亮红外星系(LIRGs和ULIRGs)样本中的致密分子气体含量。这些观测结果使我们能够推导出描述本星系和高红移星系的FIR光度(L_FIR)和HCN(1-0)光度(L '_HCN(1-0))之间相关性的幂律的更新版本。我们首次提出了清晰的观测证据,即在LIRG和ULIRG中,稠密气体的恒星形成效率(SFE_dense)(以L_FIR/L '_HCN(1-0)的比值来衡量)明显高于正常星系,这一结果最近在高红移星系中也被发现。这可能意味着,在L_FIR >= 10^11 L_sun时,由稠密气体导出的肯尼卡特-施密特定律在统计学上有显著的上升。我们使用了一个单相大速度梯度(LVG)辐射转移代码来拟合三个独立的线比来自我们的观察。分析结果表明,在我们样本中大量的LIRG和ULIRG中,[HCN]/[HCO^+]丰度比可能比正常值高出一个数量级。在高L_FIR下HCN的过量意味着作为L_FIR的函数的L_FIR/L ′_HCN比率的所报道的趋势将低估SFE_dense的潜在的更显著的变化。用两相LVG模型得到的结果证实了在高LFIR时必须降低L 'HCN(1-0)-到-M_dense的转换因子。我们讨论了这些发现对使用HCN作为局域和高红移亮红外星系中致密分子气体示踪剂的影响。

英文摘要:

We present new observations made with the IRAM 30m telescope of the J=1-0 and 3-2 lines of HCN and HCO^+ used to probe the dense molecular gas content in a sample of 17 local luminous and ultraluminous infrared galaxies (LIRGs and ULIRGs). These observations have allowed us to derive an updated version of the power law describing the correlation between the FIR luminosity (L_FIR) and the HCN(1-0) luminosity (L'_HCN(1-0)) of local and high-redshift galaxies. We present the first clear observational evidence that the star formation efficiency of the dense gas (SFE_dense), measured as the L_FIR/L'_HCN(1-0) ratio, is significantly higher in LIRGs and ULIRGs than in normal galaxies, a result that has also been found recently in high-redshift galaxies. This may imply a statistically significant turn upward in the Kennicutt-Schmidt law derived for the dense gas at L_FIR >= 10^11 L_sun. We have used a one-phase Large Velocity Gradient (LVG) radiative transfer code to fit the three independent line ratios derived from our observations. The results of this analysis indicate that the [HCN]/[HCO^+] abundance ratios could be up to one order of magnitude higher than normal in a significant number of LIRGs and ULIRGs of our sample. An overabundance of HCN at high L_FIR implies that the reported trend in the L_FIR/L'_HCN ratio as a function of L_FIR would be underestimating a potentially more dramatic change of the SFE_dense. Results obtained with two-phase LVG models corroborate that the L'_HCN(1-0)-to-M_dense conversion factor must be lowered at high L_FIR. We discuss the implications of these findings for the use of HCN as a tracer of the dense molecular gas in local and high-redshift luminous infrared galaxies.

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