Green Flash: Residual Emissions Enshrouded in Low-mass Balmer-break Galaxies at $z\sim5$

Kosuke Takahashi, Takahiro Morishita, Tadayuki Kodama, Zhaoran Liu, Kazuki Daikuhara, Nuo Chen

First listed 2025-05-09 | Last updated 2025-05-09

Abstract

Recent James-Webb Space Telescope (JWST) observations have discovered galaxies that are already passively evolving at $z>4$, $\sim1.5$\,Gyr after the Big Bang. Remarkably, some of these galaxies exhibit strong emission lines such as \ha\ and \oiii\ while showing a strong continuum break at $\sim3650$\,Å i.e., Balmer break, giving us a unique insight into the physical mechanisms responsible for early galaxy quenching. In this study, we investigate the nature of four such galaxies at $z=5.10$--$5.78$ identified in the Abell~2744 field, using JWST/NIRCam and NIRSpec data. Our spectral energy distribution (SED) fitting analysis reveals that these galaxies have been mostly quiescent since $\sim100$\,Myr prior to the observed time. We find a higher dust attenuation in the nebular component than in the continuum in all cases. This suggests the presence of dusty star-forming regions or obscured AGN, which could be a {\it residual} signature of past quenching. For one of the galaxies with sufficient medium-band coverage, we derive the \hb+\oiii\ emission line map, finding that the line-emitting region is located in the center and is more compact ($R_e=0.7$\,kpc) than the stellar component ($R_e=0.9$\,kpc). For this specific galaxy, we discuss a scenario where quenching proceeds in the manner of ``outside-in", a stark contrast to the inside-out quenching commonly seen in massive galaxies at later cosmic times.

Short digest

JWST/NIRCam+NIRSpec data are used to isolate four low-mass Balmer-break galaxies in Abell 2744 at z=5.10–5.78 and fit their SEDs, showing they have been largely quiescent for ~100 Myr despite residual strong lines. All four require higher nebular than continuum attenuation, pointing to dusty star-forming pockets or obscured AGN as a lingering signature of quenching. For one galaxy with medium-band leverage, an Hβ+[O III] map shows centrally concentrated emission (Re≈0.7 kpc) that is more compact than the stellar body (Re≈0.9 kpc), favoring an outside-in quenching picture. Notably, one object (ID52153) sits on the z~5 main sequence even with a Balmer break, underscoring obscuration/bursty residual activity.

Key figures to inspect

• Figure 1 (color–color selection): Verify that the four spectroscopic BBGs occupy the Balmer-break locus and are separated from emission-line–mimic tracks; use the CIGALE evolutionary curves to gauge post-quenching ages and SF timescales.
• Figure 2 (RGB cutouts): Inspect central light concentration and morphology after PSF matching; compare blue (F115W) versus red (F356W/F444W) structure for hints of compact, older stellar cores versus enshrouded central activity.
• Figure 3 (NIRSpec PRISM spectra): Check placement of the Balmer break and the continuum-subtracted Hβ+[O III] and Hα residuals; assess line strengths versus the quiescent-looking continuum consistent with enhanced nebular extinction.
• Figure 4 (SFR–M⋆ at z~5): See which sources fall below the main sequence and the outlier ID52153 on it despite a Balmer break, highlighting obscured or bursty residual star formation.