ALMaQUEST 巡天第十二篇：绿谷星系中由 HCN 和 HCO$^{+}$ 示踪的致密分子气体
The ALMaQUEST Survey XII: Dense Molecular Gas as traced by HCN and HCO$^{+}$ in Green Valley Galaxies
AI总结：
本文利用 ALMA 观测 HCN 和 HCO$^{+}$，研究绿谷星系恒星形成效率偏低的原因，发现其恒星形成受抑主要源于致密气体恒星形成效率降低，而非致密气体缺乏。
摘要：
我们展示了来自 ALMaQUEST 巡天中选取的三个绿谷星系和两个恒星形成主序星系的 ALMA 观测结果，观测对象为两种致密气体示踪物 HCN(1-0) 和 HCO$^{+}$(1-0)，这些星系由 CO(1-0) 发射示踪的分子气体比例相当。我们研究了导致这些绿谷星系低比恒星形成率的分子气体恒星形成效率（SFE$_{\ m mol}$）亏损是否源于致密气体的缺乏（以致密气体比例 $f_{\ m dense}$ 表征）还是致密气体的恒星形成效率（SFE$_{\ m dense}$）偏低。我们发现，当同时考虑恒星形成区和退休区像素时，由 CO 发射示踪的 SFE$_{\ m mol}$ 与 SFE$_{\ m dense}$ 紧密相关，并且仅微弱依赖于 $f_{\ m dense}$。在千秒差距尺度上，比恒星形成率（sSFR）主要由 SFE$_{\ m mol}$ 和 SFE$_{\ m dense}$ 驱动，其次依赖于 $f_{\ m mol}$，而与 $f_{\ m dense}$ 或致密气体与恒星质量比（$R_{\ m dense}$）的相关性最弱。与文献中其他工作相比，我们发现我们的绿谷样本在整体 SFE$_{\ m mol}$ 和 SFE$_{\ m dense}$ 上均低于恒星形成星系和星暴星系，同时表现出相似的致密气体比例。我们得出结论：这三个具有正常分子气体丰度的绿谷星系的恒星形成受到抑制，主要原因是 SFE$_{\ m dense}$ 的降低，而非致密气体的缺乏。
英文摘要：
We present ALMA observations of two dense gas tracers, HCN(1-0) and HCO$^{+}$(1-0), for three galaxies in the green valley and two galaxies on the star-forming main sequence with comparable molecular gas fractions as traced by the CO(1-0) emissions, selected from the ALMaQUEST survey. We investigate whether the deficit of molecular gas star formation efficiency (SFE$_{\rm mol}$) that leads to the low specific star formation rate in these green valley galaxies is due to a lack of dense gas (characterized by the dense gas fraction $f_{\rm dense}$) or the low star formation efficiency of dense gas (SFE$_{\rm dense}$). We find that SFE$_{\rm mol}$ as traced by the CO emissions, when considering both star-forming and retired spaxels together, is tightly correlated with SFE$_{\rm dense}$ and depends only weakly on $f_{\rm dense}$. The specific star formation rate (sSFR) on kpc scales is primarily driven by SFE$_{\rm mol}$ and SFE$_{\rm dense}$, followed by the dependence on $f_{\rm mol}$, and is least correlated with $f_{\rm dense}$ or the dense-to-stellar mass ratio ($R_{\rm dense}$). When compared with other works in the literature, we find that our green valley sample shows lower global SFE$_{\rm mol}$ as well as lower SFE$_{\rm dense}$ while exhibiting similar dense gas fractions when compared to star-forming and starburst galaxies. We conclude that the star formation of the 3 green valley galaxies with a normal abundance of molecular gas is suppressed mainly due to the reduced SFE$_{\rm dense}$ rather than the lack of dense gas.
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