Dense gas and star formation in individual Giant Molecular Clouds in M31

Abstract

Studies both of entire galaxies and of local Galactic star formation indicate a dependency of a molecular cloud's star formation rate (SFR) on its dense gas mass. In external galaxies, such measurements are derived from HCN(1-0) observations, usually encompassing many Giant Molecular Clouds (GMCs) at once. The Andromeda galaxy (M31) is a unique laboratory to study the relation of the SFR and HCN emission down to GMC scales at solar-like metallicities. In this work, we correlate our composite SFR determinations with archival HCN, HCO${}^{+}$, and CO observations, resulting in a sample of nine reasonably representative GMCs. We find that, at the scale of individual clouds, it is important to take into account both obscured and unobscured star formation to determine the SFR. When correlated against the dense-gas mass from HCN, we find that the SFR is low, in spite of these refinements. We nevertheless retrieve an SFR - dense-gas mass correlation, confirming that these SFR tracers are still meaningful on GMC scales. The correlation improves markedly when we consider the HCN/CO ratio instead of HCN by itself. This nominally indicates a dependency of the SFR on the dense-gas fraction, in contradiction to local studies. However, we hypothesize that this partly reflects the limited dynamic range in dense-gas mass, and partly that the ratio of single-pointing HCN and CO measurements may be less prone to systematics like sidelobes. In this case, the HCN/CO ratio would importantly be a better empirical measure of the dense-gas content itself.

9 pages, 6 figures, accepted for publication in MNRAS

References in corpus (14)

• The Critical Density and the Effective Excitation Density of Commonly Observed Molecular Dense Gas Tracers
• The anatomy of the Orion B Giant Molecular Cloud: A local template for studies of nearby galaxies
• Star formation rates and efficiencies in the Galactic Centre
• Molecular Line Emission as a Tool for Galaxy Observations (LEGO). I. HCN as a tracer of moderate gas densities in molecular clouds and galaxies
• Star Formation Relations in the Milky Way
• The Herschel Exploitation of Local Galaxy Andromeda (HELGA) VII: A SKIRT radiative transfer model and insights on dust heating
• Electron Excitation of High Dipole Moment Molecules Reexamined
• The Herschel exploitation of local galaxy Andromeda (HELGA) V: Strengthening the case for substantial interstellar grain growth
• Dense gas in low-metallicity galaxies
• Predicting HCN, HCO+, multi-transition CO, and dust emission of star-forming galaxies From local spiral and ultraluminous infrared galaxies to high-z star-forming and submillimeter galaxies
• Dense Gas in the Outer Spiral Arm of M51
• ALMA Multiple-Transition Molecular Line Observations of the Ultraluminous Infrared Galaxy IRAS 20551-4250: Different HCN, HCO+, HNC Excitation and Implications for Infrared Radiative Pumping
• Some like it cold: molecular emission and effective dust temperatures of dense cores in the Pipe Nebula
• HCN hyperfine ratio analysis of massive molecular clumps

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• The HASHTAG project I. A Survey of CO(3-2) Emission from the Star Forming Disc of M31
• A 260 pc resolution ALMA map of HCN(1-0) in the galaxy NGC 4321