On the universality of star formation efficiency in galaxies

We analyze high-resolution hydrodynamics simulations of an isolated disk dwarf galaxy with an explicit model for unresolved turbulence and turbulence-based star formation prescription. We examine the characteristic values of the star formation efficiency per free-fall time, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:msub> <mml:mi>ϵ</mml:mi> <mml:mrow> <mml:mi mathvariant="normal">f</mml:mi> <mml:mi mathvariant="normal">f</mml:mi> </mml:mrow> </mml:msub> </mml:math> , and its variations with local environment properties, such as metallicity, UV flux, and surface density. We show that the star formation efficiency per free-fall time in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:mo>≈</mml:mo> <mml:mn>10</mml:mn> </mml:mrow> </mml:math> pc star-forming regions of the simulated disks has values in the range <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:msub> <mml:mi>ϵ</mml:mi> <mml:mrow> <mml:mi mathvariant="normal">f</mml:mi> <mml:mi mathvariant="normal">f</mml:mi> </mml:mrow> </mml:msub> <mml:mo>≈</mml:mo> <mml:mn>0.01</mml:mn> <mml:mo>−</mml:mo> <mml:mn>0.1</mml:mn> </mml:mrow> </mml:math> , similar to observational estimates, with no trend with metallicity and only a weak trend with the UV flux. Likewise, estimated using projected patches of 500 pc size does not vary with metallicity and shows only a weak trend with average UV flux and gas surface density. The characteristic values of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:msub> <mml:mi>ϵ</mml:mi> <mml:mrow> <mml:mi mathvariant="normal">f</mml:mi> <mml:mi mathvariant="normal">f</mml:mi> </mml:mrow> </mml:msub> <mml:mo>≈</mml:mo> <mml:mn>0.01</mml:mn> <mml:mo>−</mml:mo> <mml:mn>0.1</mml:mn> </mml:mrow> </mml:math> arise naturally in the simulations via the combined effect of dynamical gas compression and ensuing stellar feedback that injects thermal and turbulent energy. The compression and feedback regulate the virial parameter, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:msub> <mml:mi>α</mml:mi> <mml:mrow> <mml:mi mathvariant="normal">v</mml:mi> <mml:mi mathvariant="normal">i</mml:mi> <mml:mi mathvariant="normal">r</mml:mi> </mml:mrow> </mml:msub> </mml:math> , in star-forming regions, limiting it to <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:msub> <mml:mi>α</mml:mi> <mml:mrow> <mml:mi mathvariant="normal">v</mml:mi> <mml:mi mathvariant="normal">i</mml:mi> <mml:mi mathvariant="normal">r</mml:mi> </mml:mrow> </mml:msub> <mml:mo>≈</mml:mo> <mml:mn>3</mml:mn> <mml:mo>−</mml:mo> <mml:mn>10</mml:mn> </mml:mrow> </mml:math> . Turbulence plays an important role in the universality of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:msub> <mml:mi>ϵ</mml:mi> <mml:mrow> <mml:mi mathvariant="normal">f</mml:mi> <mml:mi mathvariant="normal">f</mml:mi> </mml:mrow> </mml:msub> </mml:math> because turbulent energy and its dissipation are not sensitive to metallicity and UV flux that affect thermal energy. Our results indicate that the universality of observational estimates of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:msub> <mml:mi>ϵ</mml:mi> <mml:mrow> <mml:mi mathvariant="normal">f</mml:mi> <mml:mi mathvariant="normal">f</mml:mi> </mml:mrow> </mml:msub> </mml:math> can be plausibly explained by the turbulence-driven and feedback-regulated properties of star-forming regions.