Burstiness in Low Stellar Mass H <i>α</i> Emitters at <i>z</i>  ∼ 2 and <i>z</i>  ∼ 4–6 from JWST Medium-band Photometry in GOODS-S

Abstract We analyze a sample of ∼4500 photometrically selected H α emitter galaxies at redshifts z ∼ 2 and z ∼ 4–6 selected from James Webb Space Telescope Near-Infrared Camera medium-band images in the Great Observatories Origins Deep Survey South. The bulk (80%) of the galaxies in our sample have stellar masses lower than 10 8 M ⊙ , with a median stellar mass of ∼10 7.3 M ⊙ . We derive H α rest-frame equivalent widths (EW <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow/> <mml:mrow> <mml:mn>0</mml:mn> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">H</mml:mi> <mml:mi>α</mml:mi> </mml:mrow> </mml:msubsup> </mml:math> ), line fluxes, and star formation rates (SFRs) using a robust photometric excess technique tested against spectroscopic measurements, being sensitive to EW <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow/> <mml:mrow> <mml:mn>0</mml:mn> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">H</mml:mi> <mml:mi>α</mml:mi> </mml:mrow> </mml:msubsup> <mml:mo>&gt;</mml:mo> <mml:mn>75</mml:mn> <mml:mspace width="0.25em"/> </mml:math> Å. Both <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mi mathvariant="normal">EW</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>0</mml:mn> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">H</mml:mi> <mml:mi>α</mml:mi> </mml:mrow> </mml:msubsup> </mml:math> and sSFR(H α ) exhibit an anticorrelation with stellar mass. At fixed stellar mass, low-mass galaxies ( M ⋆ &lt; 10 8 M ⊙ ) display a steep increase in sSFR(H α ) with redshift, following a relation sSFR(H α ) ∝ (1 + z ) 2.4 ± 0.3 . In contrast, more massive galaxies ( M ⋆ &gt; 10 8 M ⊙ ) show a significantly flatter evolution, with sSFR(H α ) ∝ (1 + z ) 1.0 ± 0.2 . By comparing the H α and rest-frame UV-derived SFRs, we probe the star formation histories of our galaxies in the past 100 Myr. The fraction of low-mass galaxies with signs of bursty star formation from their SFR(H α )/SFR(UV) is ∼46%. It quickly drops to ∼25% for M ⋆ &gt; 10 8 M ⊙ . This is consistent with the results from sSFR(H α ), showing 72% and 10%, respectively. SFR(H α )/SFR(UV) is a stricter criterion than those based on the galaxy sSFR(H α ), as it only identifies starbursts that are at the initial phases of a bursty star formation episode.