CEERS: Increasing Scatter along the Star-forming Main Sequence Indicates Early Galaxies Form in Bursts

Abstract We present the star formation rate–stellar mass (SFR– M * ) relation for galaxies in the Cosmic Evolution Early Release Science survey at 4.5 ≤ z ≤ 12. We model the JWST and Hubble Space Telescope rest-UV and rest-optical photometry of galaxies with flexible star formation histories (SFHs) using BAGPIPES. We consider SFRs averaged from the SFHs over 10 Myr (SFR 10 ) and 100 Myr (SFR 100 ), where the photometry probes SFRs on these timescales, effectively tracing nebular emission lines in the rest-optical (on ~10 Myr timescales) and the UV/optical continuum (on ~100 Myr timescales). We measure the slope, normalization and intrinsic scatter of the SFR– M * relation, taking into account the uncertainty and the covariance of galaxy SFRs and M * . From z ~ 5 to 9 there is larger scatter in the SFR 10 – M * relation, with <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>σ</mml:mi> <mml:mo stretchy="false">(</mml:mo> <mml:mi>log</mml:mi> <mml:mi mathvariant="normal">S</mml:mi> <mml:mi mathvariant="normal">F</mml:mi> <mml:msub> <mml:mrow> <mml:mi mathvariant="normal">R</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>100</mml:mn> </mml:mrow> </mml:msub> <mml:mo stretchy="false">)</mml:mo> <mml:mo>=</mml:mo> <mml:mn>0.4</mml:mn> </mml:math> dex, compared to the SFR 100 – M * relation, with <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>σ</mml:mi> <mml:mo stretchy="false">(</mml:mo> <mml:mi>log</mml:mi> <mml:mi>S</mml:mi> <mml:mi>F</mml:mi> <mml:msub> <mml:mrow> <mml:mi>R</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>10</mml:mn> </mml:mrow> </mml:msub> <mml:mo stretchy="false">)</mml:mo> <mml:mo>=</mml:mo> <mml:mn>0.1</mml:mn> </mml:math> dex. This scatter increases with redshift and increasing stellar mass, at least out to z ~ 7. These results can be explained if galaxies at higher redshift experience an increase in star formation variability and form primarily in short, active periods, followed by a lull in star formation (i.e., “napping” phases). We see a significant trend in the ratio R SFR = SFR 10 /SFR 100 in which, on average, R SFR decreases with increasing stellar mass and increasing redshift. This yields a star formation “duty cycle” of ~40% for galaxies with <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>log</mml:mi> <mml:msub> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>*</mml:mo> </mml:mrow> </mml:msub> <mml:mo>/</mml:mo> <mml:msub> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>⊙</mml:mo> </mml:mrow> </mml:msub> <mml:mo>≥</mml:mo> <mml:mn>9.3</mml:mn> </mml:math> at z ~ 5, declining to ~20% at z ~ 9. Galaxies also experience longer lulls in star formation at higher redshift and at higher stellar mass, such that galaxies transition from periods of higher SFR variability at z ≳ 6 to smoother SFR evolution at z ≲ 4.5.