MAGAZ3NE: Far-IR and Radio Insights into the Nature and Properties of Ultramassive Galaxies at z ≳ 3

Abstract Deep and wide-field near-infrared (NIR) surveys have recently discovered and confirmed ultramassive galaxies (UMGs; log ( M ⋆ / M ⊙ ) > 11) spectroscopically at z ≳ 3. 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 10 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 spec = 3.255) and COS-DR1-209435 ( z 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 ⋆ / M ⊙ ) = 11.72 over the next 0.72 Gyr. We present multipronged evidence for AGN activity in both UMGs. Our findings suggest a scenario where AGN feedback in 195616 may have contributed to gas depletion during quenching, while 209435 remains actively star-forming despite hosting an obscured AGN. Our work shows the importance of FIR-to-radio observations for accurately inferring the nature and properties of galaxies at z ≳ 3.