The slope and scatter of the star forming main sequence at z~5 : reconciling observations with simulations

Abstract

Galaxies exhibit a tight correlation between their star-formation rate and stellar mass over a wide redshift range known as the star-forming main sequence (SFMS). With JWST, we can now investigate the SFMS at high redshifts down to masses of $\sim 10^6$ M${}_{\odot }$, using sensitive star-formation rate tracers such as H$\alpha$ emission -- which allow us to probe the variability in star formation histories. We present inferences of the SFMS based on 316 H$\alpha$-selected galaxies at $z\sim 4$-$5$ with $\log ({\mathrm{M}}_{\star }/{\mathrm{M}}_{\odot })=6.4$ -$10.6$. These galaxies were identified behind the Abell 2744 lensing cluster with NIRCam grism spectroscopy from the ``All the Little Things'' (ALT) survey. At face value, our data suggest a shallow slope of the SFMS (SFR $\propto {\mathrm{M}}_{\star }^{\alpha }$, with $\alpha =0.45$). After correcting for the H$\alpha$-flux limited nature of our survey using a Bayesian framework, the slope steepens to $\alpha =0.59_{-0.09}^{+0.10}$, whereas current data on their own are inconclusive on the mass dependence of the scatter. These slopes differ significantly from the slope of $\approx 1$ expected from the observed evolution of the galaxy stellar mass function and from simulations. When fixing the slope to $\alpha =1$, we find evidence for a decreasing intrinsic scatter with stellar mass (from $\approx 0.5$ dex at M${}_{\star }=10^8$ M${}_{\odot }$ to $0.4$ dex at M${}_{\star }=10^{10}$ M${}_{\odot }$). This tension might be explained by a (combination of) luminosity-dependent SFR(H$\alpha$) calibration, a population of (mini)-quenched low-mass galaxies, or underestimated dust attenuation in high-mass galaxies. Future deep observations across facilities can quantify these processes, enabling better insights into the variability of star formation histories.

Main text 15 pages with 11 figures. Figure 7 shows the key measurements and Fig 8 model inference. Accepted for publication in A&A

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