The Kennicutt-Schmidt law and the main sequence of galaxies in Newtonian and Milgromian dynamics
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The Kennicutt-Schmidt law is an empirical relation between the star formation rate surface density (ΣSFR) and the gas surface density (Σgas) in disc galaxies. The relation has a power-law form ΣSFR ∝ Σgasn. Assuming that star formation results from gravitational collapse of the interstellar medium, ΣSFR can be determined by dividing Σgas by the local free-fall time tff. The formulation of tff yields the relation between ΣSFR and Σgas, assuming that a constant fraction (ΣSFE) of gas is converted into stars every tff. This is done here for the first time using Milgromian dynamics (MOND). Using linear stability analysis of a uniformly rotating thin disc, it is possible to determine the size of a collapsing perturbation within it. This lets us evaluate the sizes and masses of clouds (and their tff) as a function of Σgas and the rotation curve. We analytically derive the relation ΣSFR ∝ Σgasn both in Newtonian and Milgromian dynamics, finding that n = 1.4. The difference between the two cases is a change only to the constant pre-factor, resulting in increased ΣSFR of up to 25 per cent using MOND in the central regions of dwarf galaxies. Due to the enhanced role of disc self-gravity, star formation extends out to larger galactocentric radii than in Newtonian gravity, with the clouds being larger. In MOND, a nearly exact representation of the present-day main sequence of galaxies is obtained if ϵSFE = constant ≈ 1.1 per cent. We also show that empirically found correction terms to the Kennicutt-Schmidt law are included in the here presented relations. Furthermore, we determine that if star formation is possible, then the temperature only affects ΣSFR by at most a factor of √2.
Zonoozi , A H , Lieberz , P , Banik , I , Haghi , H & Kroupa , P 2021 , ' The Kennicutt-Schmidt law and the main sequence of galaxies in Newtonian and Milgromian dynamics ' , Monthly Notices of the Royal Astronomical Society , vol. 506 , no. 4 , pp. 5468-5478 . https://doi.org/10.1093/mnras/stab2068
Monthly Notices of the Royal Astronomical Society
Copyright © 2021 The Author(s). Published by Oxford University Press on behalf of Royal Astronomical Society. This work has been made available online in accordance with publisher policies or with permission. Permission for further reuse of this content should be sought from the publisher or the rights holder. This is the final published version of the work, which was originally published at https://doi.org/10.1093/mnras/stab2068
DescriptionFunding: Supported in part by Fondazione Cassa di Risparmio in Bologna, Project: “Biologia e clinica delle gravi insufficienze d’organo”, year 1999.
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