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

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 $z\gtrsim3$.