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

TL;DR

This study investigates the star-forming main sequence at high redshift (z~5) using JWST data, revealing a shallower slope than expected and exploring the scatter and potential causes for discrepancies with models.

Contribution

It provides new measurements of the SFMS slope and scatter at z~5 using Hα-selected galaxies, correcting for survey biases, and compares results with simulations and galaxy evolution expectations.

Findings

Measured SFMS slope of 0.45 before correction, steepening to 0.59 after bias correction.

Found evidence for decreasing scatter with increasing stellar mass.

Identified potential reasons for slope discrepancy, including calibration issues and galaxy quenching.

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 $\sim10^6$ 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 $z\sim4$-$5$ with $\log(\rm M_\star/M_\odot) = 6.4$ -$10.6$. 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 $\alpha = 0.59^{+0.10}_{-0.09}$, whereas current data on their own are inconclusive on the mass dependence of the scatter. These slopes differ significantly from the slope of $\approx1$ 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.