The Prevalence of Bursty Star Formation in Low-mass Galaxies at <i>z</i> = 1–7 from H <i>α</i> -to-UV Diagnostics

Abstract We present an analysis of the bursty star formation histories (SFHs) of 346 star-forming galaxies at 1 ≲ z &lt; 7, selected from JWST/NIRSpec G395M and PRISM spectroscopy provided by the Cosmic Evolution Early Release Science (or CEERS) and Red Unknowns: Bright Infrared Extragalactic Survey (or RUBIES) surveys. We analyze the correlation of star formation rate versus stellar mass (the star-forming main sequence, SFMS) for our sample and find no significant difference between the intrinsic scatter in the H α -based SFMS and the UV-continuum-based SFMS. However, the diagnostic power of the SFMS is limited at high redshift and low stellar mass due to observational biases that exclude faint, quenched galaxies. To more directly probe star formation variability, we examine the dust-corrected H α -to-UV ratio, which is assumed to trace deviations from a constant SFH over the past ∼100 Myr. In our sample, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mn>73</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>4</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>4</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> % of galaxies exhibit H α -to-UV ratios inconsistent with a constant SFH. We do not observe any statistically significant evolution in the H α -to-UV ratio with redshift. Additionally, lower-mass galaxies (7 ≤ log( M * / M ⊙ ) &lt; 8.5) are 30% ± 1% more likely to lie above this equilibrium range—indicative of a recent (≲100 Myr) burst of star formation—compared to higher-mass systems (8.5 ≤ log( M * / M ⊙ ) ≤ 10.9). These results suggest that bursty SFHs are more common in low-mass galaxies at z ∼ 1–7, and that this remains relatively stable across ∼0.8–6 Gyr after the Big Bang.