Accelerated by Dark Matter: a High-redshift Pathway to Efficient Galaxy-scale Star Formation

Abstract

In the local Universe, star formation is typically inefficient both globally and when considered as the fraction of gas converted into stars per local free-fall time. An important exception to this inefficiency is regions of high gravitational accelerations $g$, or equivalently surface densities $\Sigma = g/(\pi\,G)$, where stellar feedback is insufficient to overcome the self-gravity of dense gas clouds. In this paper, I explore whether dark matter can play an analogous role in providing the requisite accelerations on the scale of entire galaxies in the early cosmos. The key insight is that characteristic accelerations in dark matter halos scale as $(1+z)^2$ at fixed halo mass. I show this is sufficient to make dark matter the source of intense accelerations that might induce efficient star formation on galactic scales at cosmic dawn in sufficiently massive halos. The mass characterizing this regime scales as $(1+z)^{-6}$ and corresponds to a relatively constant comoving number density of $n(>\!M_{\rm vir}) \approx 10^{-4}\,{\rm Mpc}^{-3}$ at $z \gtrsim 8$. For somewhat rarer halos, this model predicts stellar masses of $M_{\star} \sim 10^{9}\,M_{\odot}$ can form in regions that end up with sizes $\mathcal{O}(100\,{\rm pc})$ over $40\,{\rm Myr}$ time-scales at $z\approx 12-14$; these numbers compare well to measurements for some of the brightest galaxies at that epoch from James Webb Space Telescope (JWST) observations. Dark matter and standard cosmological evolution may therefore be crucial for explaining the surprisingly high levels of star formation in the early Universe revealed by JWST.