Faint HCN in an Infrared Dark Cloud: Implications for Cosmic Star Formation

Extragalactic star formation research often has to make substantial assumptions, since star-forming sites in galaxies cannot be resolved well. To give an example, research often presumes that the mass of dense gas, M_dg, is proportional to the luminosity of the HCN emission, L_HCN. This can then, e.g., be used to study whether the star formation rate, Ṁ_star, is proportional to the mass of dense gas. A direct correlation between L_HCN, M_dg, and Ṁ_star is, for example, generally assumed to explain the so-called Gao & Solomon relation, i.e., Ṁ_star ∝ L_HCN for entire galaxies. Today this relation forms the foundation for much of our understanding of how stars and galaxies form over cosmic time. The exact relationship between, e.g., dense gas and line emission has, however, never been studied in detail in the Milky Way. The LEGO survey was therefore created with the goal of examining how line properties connect to physical properties of gas in molecular clouds. To do this, the LEGO survey collects data on various molecular clouds within the Milky Way. An analysis of one of these clouds — G11.11-0.12, a prototypical infrared dark cloud (IRDC) in our galaxy — is described in this presentation. We find that the HCN luminosity per unit mass of dense gas is by an order of magnitude fainter than typically assumed in extragalactic research, resulting in an unusually large mass-to-line ratio α_HCN = M_dg/L_HCN ≈ 100 M_sun/ (K km s^-1 pc²). We discuss implications for extragalactic research.