MAGAZ3NE: Dust Deficiency in Ultramassive Quiescent Galaxies at $3<z<4$ with ALMA Observations

Abstract

A major challenge in identifying massive quiescent galaxies at $z>3$ is distinguishing truly passive systems from dust-obscured star-forming galaxies, as both populations exhibit similar red ultraviolet (UV)-to-near-infrared (NIR) colors. In this work, we present ALMA Band 7 dust-continuum observations of five ultramassive galaxies (UMGs; $\log (M_\star / M_\odot) > 11$) spectroscopically confirmed at $z_{\rm spec} > 3$ from the MAGAZ3NE survey. Our results reveal that only one galaxy shows a faint 870 \um\ dust continuum detection, while the remaining four UMGs are undetected down to the $3σ$ depth . By incorporating ALMA constraints into the spectral energy distribution analysis, we confirm that these UV-NIR-selected systems are truly quiescent UMGs, lying more than one dex below the star-forming main sequence with $\mathrm{\log (sSFR/Gyr^{-1}) < -1}$, thereby ruling out the possibility of obscured star formation. We then estimate dust masses using both spectral energy distribution modeling and modified blackbody fitting, with consistent results between the two methods. We find that three UMGs have evolved into extremely dust-poor quiescent galaxies, with $M_{\mathrm{dust}}/M_\star \lesssim 10^{-4}$, while the ALMA-detected galaxy has a comparatively higher dust reservoir with $M_{\mathrm{dust}}/M_\star \sim 10^{-3}$. Our results present the most massive and extremely dust-poor spectroscopically confirmed quiescent galaxies known at $3 < z < 4$, providing valuable observational constraints on rapid dust removal and quenching processes in the early universe. Future molecular line observations will be essential to directly measure the gas content and verify the efficiency of the depletion process.

Figures (11)

• Figure 1: From left to right: ALMA Band 7 (870 $\mu$m) primary-beam-corrected continuum image of XMM-VID3-2457; the two-dimensional Gaussian model; the residual map after subtraction of the model, shown in units of the RMS; and the stacked continuum image of four non-detected galaxies. The 0.28$\times$ 0.22 synthesized beam of XMM-VID3-2457 is represented by the white ellipse at the bottom left corners.
• Figure 2: ALMA band 7 (872 $\mu$m) observation parameters. Flux density is estimated from pb-corrected images using imfit task.
• Figure 3: Rest-frame (RF) UVJ color-color diagram for 16 spectroscopically-confirmed UMG candidates (open cyan circles) presented in Forrest2020b and 12 spectroscopically-confirmed S-UMGs candidates (open orange circles) presented in Forrest2024b. The five UMGs from Forrest2020b observed with ALMA in this work are shown by filled cyan circles (Galaxy 1: XMM-VID3-2457; 2: XMM-VID3-1120; 3: XMM-VID1-2075; 4: COS-DR3-201999; 5: COS-DR3-202019), where cross symbols indicate non-detections in ALMA dust continuum with upper limits only. Two additional ALMA-detected UMGs analyzed previously in Chang2025arXiv are shown as filled salmon circles. The RF colors were estimated at the spectroscopic redshift using FAST++ and UV-to-8$\mu$m photometry only. The dashed line denotes the division between "dusty" and "non-dusty" star-forming regions Schreiber2018b_UVJ. The gray data points corresponds to massive galaxies with $\log (M_{\star}/M_{\odot})$$> 10.5$ at $3 < z_{\rm phot} < 4$ in the COSMOS2020 catalog Weaver2022, with their density distribution overplotted as gray contours.
• Figure 4: $K_s$-band images, ALMA band 7 (872$\mu$m) primary-beam-corrected continuum maps, and UV-to-millimeter/radio SED modeling of the five UMGs. Imaging cutouts are centered on the spectroscopic positions of the galaxies. White ellipses in the lower-left corners of the ALMA maps show the synthesized beam. The SED panels on the right show the best-fit model (top) and relative residuals (bottom). Dashed lines indicate the individual CIGALE SED components: unattenuated stellar emission (blue), dust emission (red), and AGN emission (cyan). For UMGs COS-DR3-201999 and COS-DR3-202019, an additional synchrotron radio emission component is included (brown). Observed photometry is shown as purple circles, with downward arrows indicating upper limits.
• Figure 5: The SFR versus stellar mass. Blue filled circles show the CIGALE-derived measurements for the five UMGs in this work, while open circles mark UMGs with ALMA upper limits only. Gray symbol represents the spectroscopically confirmed quiescent galaxy at $3<z<4$ observed with ALMA and fitted with CIGALE in Kubo2022. The gray line shows the star-forming main sequence from Tomczak2016. Shaded regions denote 1 dex below the SFMS, corresponding to the threshold for classifying galaxies as quiescent comparing to the main sequence. Orange filled circles represent two MAGAZ3NE UMGs analyzed using UV-to-FIR CIGALE SED with ALMA observations in Chang2025arXiv.