The slope and scatter of the star-forming main sequence at z ∼ 5: Reconciling observations with simulations

¹ Institute of Science and Technology Austria (ISTA) Am Campus 1 3400 Klosterneuburg, Austria
² MIT Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology Cambridge MA 02139, USA
³ Universite Claude Bernard Lyon 1, CRAL UMR5574, ENS de Lyon, CNRS Villeurbanne F-69622, France
⁴ Institute for Computational & Data Sciences, The Pennsylvania State University University Park PA 16802, USA
⁵ Institute for Gravitation and the Cosmos, The Pennsylvania State University University Park PA 16802, USA
⁶ Department of Astronomy, Oskar Klein Centre, Stockholm University, AlbaNova University Center SE-106 91 Stockholm, Sweden
⁷ Physics Department, Ben-Gurion University of the Negev PO Box 653 Beer-Sheva 8410501, Israel
⁸ Niels Bohr Institute, University of Copenhagen Jagtvej 128 2200 Copenhagen N, Denmark
⁹ Department of Astronomy, University of Geneva Chemin Pegasi 51 1290 Versoix, Switzerland
¹⁰ Laboratory of Astrophysics, École Polytechnique Fédérale de Lausanne (EPFL), Observatoire de Sauverny 1290 Versoix, Switzerland
¹¹ Cosmic Dawn Center (DAWN) Copenhagen, Denmark
¹² Max-Planck-Institut für Radioastronomie Auf dem Hügel 69 53121 Bonn, Germany
¹³ INAF – Osservatorio Astronomico di PAdova Vicolo dell’Osservatorio 5 I-35122 Padova, Italy
¹⁴ Centro de Astrobiología (CAB), CSIC-INTA, Carretera de Ajalvir km 4 Torrejón de Ardoz E-28850 Madrid, Spain
¹⁵ Kavli Institute for Cosmology, University of Cambridge Madingley Road Cambridge CB3 0HA, UK
¹⁶ Cavendish Laboratory, University of Cambridge JJ Thomson Avenue Cambridge CB3 0HE, UK

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 21 October 2025
Accepted: 6 January 2026

Abstract

Galaxies exhibit a tight correlation between their star formation rate (SFR) and stellar mass over a wide redshift range known as the star-forming main sequence (SFMS). With JWST, the SFMS can now be investigated at high redshifts down to masses of ∼10⁶ M_⊙, using sensitive star formation rate tracers such as the Hα emission, which allow us to probe the variability in the star formation histories. We present inferences of the SFMS based on 316 Hα-selected galaxies at z ∼ 4 − 5 with log(M_★/M_⊙) = 6.4 − 10.6. These galaxies were identified behind the Abell 2744 lensing cluster with NIRCam grism spectroscopy from the survey All the Little Things (ALT). At face value, our data suggest a shallow slope in the SFMS (SFR ∝ M_★^α, with α = 0.45). After we corrected this for the Hα-flux limited nature of our survey using a Bayesian framework, the slope steepened to α = 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 ∼1 that is expected from the observed evolution of the galaxy stellar mass function and from simulations. When we fixed the slope to α = 1, we found evidence for a decreasing intrinsic scatter with stellar mass (from ∼0.5 dex at M_★ = 10⁸ M_⊙ to 0.4 dex at M_★ = 10¹⁰ M_⊙). This difference might be explained by a (combination of) luminosity-dependent SFR(Hα) calibration, a population of (mini)-quenched low-mass galaxies, or underestimated dust attenuation in high-mass galaxies. Future deep observations with different facilities can quantify these processes, which will enable us to achieve better insights into the variability of the star formation histories.