MAGAZ3NE: Far-IR and Radio Insights into the Nature and Properties of Ultramassive Galaxies at $z\gtrsim3$

Wenjun Chang, Gillian Wilson, Ben Forrest, Ian McConachie, Tracy Webb, Allison G. Noble, Adam Muzzin, Michael C. Cooper, Danilo Marchesini, Gabriela Canalizo, A. J. Battisti, Aurélien Le Bail, Percy L. Gomez, Stephanie M. Urbano Stawinski, Marie E. Wisz

First listed 2025-08-11 | Last updated 2025-08-11

Abstract

Deep and wide-field near-infrared (NIR) surveys have recently discovered and confirmed ultramassive galaxies (UMGs; $\log (M_{\star}/M_{\odot})>11$) spectroscopically at high redshift. However, most are characterized using only ultraviolet (UV)-to-NIR photometry, offering limited insight into obscured star formation and active galactic nucleus (AGN) activity. In this work, we add ten far-infrared (FIR)-to-radio passbands to the existing UV-to-NIR catalogs for two spectroscopically confirmed UMGs from the MAGAZ3NE survey, COS-DR3-195616 ($z_{\rm spec} = 3.255$) and COS-DR1-209435 ($z_{\rm spec} = 2.481$). Utilizing the full UV-to-radio photometry, we revise our earlier UV-NIR-based interpretation of the nature of these galaxies. While both were previously identified as quiescent, our analysis reveals that 195616 is an unobscured galaxy undergoing quenching, and 209435 is a heavily obscured, actively star-forming UMG. We find that 195616 has already depleted most of its molecular gas and is expected to experience minimal future stellar mass growth. In contrast, 209435 contains a substantial molecular gas reservoir and has a prolonged depletion timescale. It is anticipated to increase 0.34 dex in stellar mass, reaching a stellar mass of $\log (M_{\star}/M_{\odot})$ = 11.72 over the next 0.72 Gyr. We present multi-pronged evidence for AGN activity in both UMGs. Our findings support a scenario where AGN feedback in 195616 may have contributed to gas depletion during quenching, while 209435 continues to form stars despite hosting an obscured AGN, suggesting feedback has not yet suppressed star formation. Our work shows the importance of FIR-to-radio observations for accurately inferring the nature and properties of galaxies at $z\gtrsim3$.

Short digest

Adds ten FIR-to-radio passbands to the MAGAZ3NE UV–NIR catalogs and performs UV–radio SED fits for two ultramassive galaxies, COS-DR3-195616 (z=3.255) and COS-DR1-209435 (z=2.481), revising earlier classifications. The full SEDs show 195616 is unobscured and quenching with a largely depleted molecular-gas reservoir, while 209435 is a heavily obscured, actively star-forming UMG with a long depletion timescale. The authors forecast that 209435 will grow by 0.34 dex to log(M⋆/M⊙)=11.72 over 0.72 Gyr and present multi-pronged evidence for AGN in both systems, with feedback likely linked to gas depletion in 195616 but not yet suppressing star formation in 209435. The work demonstrates how FIR–radio constraints prevent UV–NIR-only misclassification and sharpen the AGN–quenching picture for ultramassive galaxies at early times.

Key figures to inspect

• Figure 1 (rest-frame UVJ): Check that 195616 sits in the quiescent wedge while 209435 lies in the dusty SF region—illustrating how UV–NIR colors alone could misclassify obscured systems.
• Figure 2 (MOSFIRE K-band spectra): Inspect continuum shape and the presence/weakness of marked emission lines that anchor the spectroscopic redshifts and inform the quiescent vs dusty-SF ambiguity for each object.
• Figure 3 (CIGALE UV–radio SED decompositions): Compare the dust peak, AGN component, and radio tail; 195616 requires little dust emission consistent with quenching, while 209435 shows strong FIR/radio with a non-negligible AGN contribution.
• Figure 4 (SFR–M⋆ plane): Contrast positions inferred from UV–NIR-only vs UV–radio fits (open vs filled symbols); 195616 falls below the main sequence, whereas 209435 shifts onto the star-forming locus when FIR–radio constraints are included.