利用NOEMA开展的旋涡星系角秒级巡天（SWAN）- I. 绘制HCN与N$_2$H$^+$ 3mm谱线分布图
Surveying the Whirlpool at Arcseconds with NOEMA (SWAN)- I. Mapping the HCN and N$_2$H$^+$ 3mm lines
AI总结：
本研究依托NOEMA大型SWAN项目，首次获取近邻旋涡星系M 51的云尺度N₂H⁺(1-0)分布图，结合HCN、CO谱线分析致密气体示踪规律，验证HCN/CO的密度示踪能力并揭示二阶物理化学过程的影响。
摘要：
我们展示了“利用NOEMA开展的旋涡星系角秒级巡天”（Surveying the Whirlpool at Arcseconds with NOEMA, SWAN）的首批成果，这是一项IRAM北方扩展毫米波阵列（NOEMA）+30m望远镜大型项目，以云尺度分辨率（$\boldsymbol{\rm \boldsymbol{\text{∼}3\farcs=125 pc}}$）绘制了近邻标志性宏象旋涡星系M 51中90 GHz与110 GHz波段若干分子谱线的发射分布。作为本研究的一部分，我们获取了首个正常恒星形成星系内区约$\boldsymbol{\rm 5×7 kpc}$范围内的高灵敏度云尺度N$_2$H$^+$(1-0)分布图，并将其与HCN(1-0)、CO(1-0)发射进行对比，以检验它们追踪致密恒星形成气体的能力。我们的整个视场（FoV）内N$_2$H$^+$与HCN的平均谱线比为$\boldsymbol{\rm 0.20±0.09}$，整个星系盘（包括西南旋臂与星系中心）存在幅度达$\boldsymbol{\rm \text{≳}2}$倍的强区域变化。中心约1 kpc区域的HCN发射相较于N$_2$H$^+$偏高，这可能由活动星系核（AGN）驱动的激发效应导致。我们发现HCN与N$_2$H$^+$的强度呈强超线性相关（斯皮尔曼相关系数$\boldsymbol{\rm ρ_{Sp}\text{∼}0.8}$），在$\boldsymbol{\rm \text{≳}1.5}$ dex的强度跨度内，平均离散度约为$\boldsymbol{\rm 0.14}$ dex。排除中心区域后，数据最符合指数为$\boldsymbol{\rm 1.2}$的幂律关系，表明在更亮的区域中，单位HCN对应的N$_2$H$^+$含量更高。我们的观测证明，HCN与CO的谱线比是气体密度的灵敏示踪物，这与近期利用N$_2$H$^+$开展的银河系研究结果一致。AGN附近存在的特殊谱线比，以及星系盘内幂律拟合的离散性表明，除了与气体密度的一阶相关性外，二阶物理过程（如光深、气体温度）或化学过程（丰度变化）也体现在N$_2$H$^+$/CO、HCN/CO与N$_2$H$^+$/HCN的比值中。
英文摘要：
We present the first results from "Surveying the Whirlpool at Arcseconds with NOEMA" (SWAN), an IRAM Northern Extended Millimetre Array (NOEMA)+30m large program that maps emission from several molecular lines at 90 and 110 GHz in the iconic nearby grand-design spiral galaxy M~51 at cloud-scale resolution ($\sim$3\arcsec=125\,pc). As part of this work, we have obtained the first sensitive cloud-scale map of N$_2$H$^+$(1-0) of the inner $\sim5\,\times 7\,$kpc of a normal star-forming galaxy, which we compare to HCN(1-0) and CO(1-0) emission to test their ability in tracing dense, star-forming gas. The average N$_2$H$^+$-to-HCN line ratio of our total FoV is $0.20\pm0.09$, with strong regional variations of a factor of $\gtrsim 2$ throughout the disk, including the south-western spiral arm and the center. The central $\sim1\,$kpc exhibits elevated HCN emission compared to N$_2$H$^+$, probably caused by AGN-driven excitation effects. We find that HCN and N$_2$H$^+$ are strongly super-linearily correlated in intensity ($ρ_\mathrm{Sp}\sim 0.8$), with an average scatter of $\sim0.14\,$dex over a span of $\gtrsim 1.5\,$dex in intensity. When excluding the central region, the data is best described by a power-law of exponent $1.2$, indicating that there is more N$_2$H$^+$ per unit HCN in brighter regions. Our observations demonstrate that the HCN-to-CO line ratio is a sensitive tracer of gas density in agreement with findings of recent Galactic studies which utilize N$_2$H$^+$. The peculiar line ratios present near the AGN and the scatter of the power-law fit in the disk suggest that in addition to a first-order correlation with gas density, second-order physics (such as optical depth, gas temperature) or chemistry (abundance variations) are encoded in the N$_2$H$^+$/CO, HCN/CO and N$_2$H$^+$/HCN ratios.
机构、总结、中文摘要均由AI分析而来，可能存在错误，请以论文正文为准。
