Nature of the emission line and star-formation activity in ID 588

Establish whether the Hα emission of the AGN-hosting Spiderweb protocluster galaxy ID 588 is powered by star formation or by its active galactic nucleus, and thereby determine its true star-formation rate and quiescent classification.

Background

ID 588 is an AGN-hosting Hα emitter whose SED-derived star-formation rate lies near the quiescence threshold, while its Hα-based star-formation rate is much higher. Its UVJ colors also place it just outside the passive region, making its classification uncertain.

The paper excludes ID 588 from the robust quiescent sample because the origin of its Hα emission is unresolved. Targeted ALMA observations and decomposition of the Hα line are identified as the observations needed to determine whether AGN activity or star formation dominates the measured emission.

References

On the other hand, because the nature of its Hα line remains uncertain, in what follows, we conservatively exclude this galaxy from our robust sample. Targeted ALMA observations and a decomposition of its Hα line would be needed to assess the impact of AGN feedback on its SFR measurement.

— Rapid quenching and early gas depletion in the core of a galaxy protocluster at z=2.2  (2609.05285 - D'Eugenio et al., 4 Sep 2026) in Section 3, “Sample description and UVJ colors”