CEERS: Increasing Scatter along the Star-forming Main Sequence Indicates Early Galaxies Form in Bursts
Justin W. Cole, Casey Papovich, Steven L. Finkelstein, Micaela B. Bagley, Mark Dickinson, Kartheik G. Iyer, L. Y. Aaron Yung, L. Ciesla, R. Amorín, Pablo Arrabal Haro, Rachana Bhatawdekar, Antonello Calabrò, Nikko J. Cleri, Alexander de la Vega, Avishai Dekel, Ryan Endsley, Eric Gawiser, Mauro Giavalisco, Nimish P. Hathi, Michaela Hirschmann, Benne W. Holwerda, Jeyhan S. Kartaltepe, Anton M. Koekemoer, Ray A. Lucas, Sara Mascia, Bahram Mobasher, Pablo G. Pérez‐González, Giulia Rodighiero, Kaila Ronayne, Sandro Tacchella, Benjamin J. Weiner, Stephen M. Wilkins
Abstract
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.