用NOEMA对旋涡星系进行角秒级巡天（SWAN）II：巡天设计与观测
Surveying the Whirlpool at Arcseconds with NOEMA (SWAN) II: Survey design and observations
AI总结：
本文介绍SWAN巡天，利用NOEMA和IRAM-30米望远镜对M51星系3毫米波段分子谱线进行高分辨率高灵敏度观测，探测到9条谱线，发现中心区域谱线比增强，揭示了小尺度过程的影响。
摘要：
我们介绍了用NOEMA对旋涡星系进行角秒级巡天（SWAN），这是一项高分辨率、高灵敏度的巡天，旨在绘制M51（旋涡星系）3毫米波段内的分子谱线。SWAN获得了迄今为止在外部星系中最大规模的高灵敏度N2H+发射图（5x7 kpc2），分辨率达到云尺度（3英寸，约125 pc）。我们描述了约214小时NOEMA干涉测量数据、约55小时IRAM-30米望远镜定制新观测以及NOEMA、新观测和约14小时存档30米观测数据相结合的观测与数据处理过程。我们探测到9条分子跃迁谱线的广泛发射。CO同位素13CO和C18O的J=1-0跃迁在整个观测视场（FoV）内均以高显著性被探测到。HCN、HNC、HCO+和N2H+（1-0）在星系中心、分子环和旋臂中被探测到，而激波示踪物HNCO（4-3）、（5-4）和光致离解区示踪物C2H（1-0）仅在中心约1 kpc和分子环中被探测到。对于我们探测到的大多数谱线，相对于CO的平均谱线比在中心1 kpc区域内（存在活动星系核及其低倾角外流）比盘面高出最多约3倍。在整个SWAN视场内，在每条谱线显著探测到的像素中，13CO、C18O、HCN、HNC、HCO+和N2H+分别比12CO暗8±2、29±6、17±3、37±5、26±5和63±38倍。尽管我们观察到M51中心与盘面等较大尺度环境之间谱线比的变化，但每个环境内部的离散也表明较小尺度过程的影响。能够测量这些效应，得益于SWAN数据集在多种环境中的高分辨率和高灵敏度。这提供了在外部星系中迄今所捕获的最大物理区域内这些分子跃迁的最清晰视图。
英文摘要：
We present Surveying the Whirlpool at Arcseconds with NOEMA (SWAN), a high-resolution, high-sensitivity survey to map molecular lines in the 3mm band in M51 (the Whirlpool galaxy). SWAN has obtained the largest high-sensitivity map (5x7 kpc2) of N2H+ emission at cloud-scale resolution (3" ~125 pc) in an external galaxy to date. We describe the observations and data reduction of ~214 hours of interferometric data from NOEMA, ~55 hours of tailored new observations with the IRAM-30m telescope and the combination of NOEMA, new and ~14 hours of archival 30m observations. We detect widespread emission from 9 molecular transition lines. The J=1-0 transitions of CO isotopologues 13CO and C18O are detected at high significance across the full observed field-of-view (FoV). HCN, HNC, HCO+, and N2H+(1-0) are detected in the center, molecular ring and spiral arms of the galaxy, while the shock tracer HNCO(4-3), (5-4) and PDR tracer C2H(1-0) are detected in the central ~1 kpc and molecular ring only. For most of the lines that we detect, average line ratios with respect to CO are increased by up to a factor of ~3 in the central 1 kpc, where an AGN and its low-inclination outflow are present, compared to the disk. Across the full SWAN FoV, 13CO, C18O, HCN, HNC, HCO+ and N2H+ are 8\pm2, 29\pm6, 17\pm3,37\pm5, 26\pm5 and 63\pm38 times fainter than 12CO, respectively, in pixels where each line is significantly detected. Although we observe variations in line ratios between larger-scale environments like the center and disk of M51, the scatter within each environment also indicates the influence of smaller-scale processes. The ability to measure these effects is only possible thanks to the high resolution and high sensitivity of the SWAN dataset across multiple environments. This provides the sharpest view of these molecular transitions over the largest physical area ever captured in an external galaxy.
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