A galaxy-galaxy merger and the subsequent triggering of starburst activity are fundamental processes linked to the morphological transformation of galaxies and the evolution of star formation across the history of the Universe. Both nuclear and disk-wide starbursts are assumed to occur during the merger process. However, quantifying both nuclear and disk-wide star formation activity is non-trivial because the nuclear starburst is dusty in the most active merging starburst galaxies. This paper presents a new approach to this problem: combining hydrogen recombination lines in optical, millimeter, and free-free emission. Using NGC~3256 as a case study, H$\beta$, H40$\alpha$, and free-free emissions are investigated using the Multi Unit Spectroscopic Explorer at the Very Large Telescope of the European Southern Observatory (MUSE/VLT) and the Atacama Large Millimeter/submillimeter Array (ALMA). The H$\beta$ image obtained by MUSE identifies star-forming regions outside the nuclear regions, suggesting a disk-wide starburst. In contrast, the H40$\alpha$ image obtained by ALMA identifies a nuclear starburst where optical lines are undetected due to dust extinction ($A_{\rm V}\sim25$). Combining both MUSE and ALMA observations, we conclude that the total SFR is $49\pm2~M_{\odot}$~yr$^{-1}$ and the contributions from nuclear and disk-wide starbursts are $\sim34~\%$ and $\sim66~\%$, respectively. This suggests the dominance of disk-wide star formation in NGC~3256. In addition, pixel-by-pixel analyses for disk-wide star-forming regions suggest that shock gas tracers (e.g., CH$_3$OH) are enhanced where gas depletion time ($\tau_{\rm gas}$=$M_{\rm gas}/SFR$) is long. This possibly means that merger-induced shocks regulate disk-wide star formation activities.

中文翻译：

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由光学和毫米氢复合线以及尘埃合并星系 NGC 3256 中的自由发射追踪的恒星形成——MUSE/VLT 和 ALMA Synergy

星系-星系合并和随后引发的星暴活动是与星系形态转变和整个宇宙历史中恒星形成演化相关的基本过程。假设在合并过程中会发生核和磁盘范围内的星暴。然而，量化核和盘面恒星形成活动并非易事，因为在最活跃的合并星暴星系中，核星暴是尘土飞扬的。本文提出了解决这个问题的新方法：结合光学、毫米和自由发射中的氢复合线。以NGC~3256为例，H$\beta$, H40$\alpha$, 使用欧洲南方天文台甚大望远镜 (MUSE/VLT) 和阿塔卡马大型毫米波/亚毫米波阵列 (ALMA) 上的多单元光谱探测器对自由和自由排放进行了研究。MUSE 获得的 H$\beta$ 图像确定了核区域之外的恒星形成区域，表明存在盘面范围的星暴。相比之下，ALMA 获得的 H40$\alpha$ 图像识别出由于尘埃消光而无法检测到光线的核星暴（$A_{\rm V}\sim25$）。结合 MUSE 和 ALMA 的观测结果，我们得出结论，总恒星形成率是 $49\pm2~M_{\odot}$~yr$^{-1}$ 并且来自核和盘面星暴的贡献是 $\sim34~\% $ 和 $\sim66~\%$ 分别。这表明 NGC~3256 中盘面恒星形成占主导地位。此外，对盘面恒星形成区域的逐像素分析表明，在气体耗尽时间（$\tau_{\rm gas}$=$M_{\rm gas}/SFR$) 很长。这可能意味着合并引发的冲击调节了盘面范围内的恒星形成活动。