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1. CO Emission, Molecular Gas, and Metallicity in Main-sequence Star-forming Galaxies at z ∼ 2.3*

   Sanders, Ryan L.; Shapley, Alice E.; Jones, Tucker; Shivaei, Irene; Popping, Gergö; Reddy, Naveen A.; Davé, Romeel; Price, Sedona H.; Mobasher, Bahram; Kriek, Mariska; et al (January 2023, The Astrophysical Journal)

   Abstract We present observations of CO(3−2) in 13 main-sequence z = 2.0–2.5 star-forming galaxies at log ( M * / M ⊙ ) = 10.2 – 10.6 that span a wide range in metallicity (O/H) based on rest-optical spectroscopy. We find that L CO ( 3 − 2 ) ′ /SFR decreases with decreasing metallicity, implying that the CO luminosity per unit gas mass is lower in low-metallicity galaxies at z ∼ 2. We constrain the CO-to-H 2 conversion factor ( α CO ) and find that α CO inversely correlates with metallicity at z ∼ 2. We derive molecular gas masses ( M mol ) and characterize the relations among M * , SFR, M mol , and metallicity. At z ∼ 2, M mol increases and the molecular gas fraction ( M mol / M * ) decreases with increasing M * , with a significant secondary dependence on SFR. Galaxies at z ∼ 2 lie on a near-linear molecular KS law that is well-described by a constant depletion time of 700 Myr. We find that the scatter about the mean SFR− M * , O/H− M * , and M mol − M * relations is correlated such that, at fixed M * , z ∼ 2 galaxies with larger M mol have higher SFR and lower O/H. We thus confirm the existence of a fundamental metallicity relation at z ∼ 2, where O/H is inversely correlated with both SFR and M mol at fixed M * . These results suggest that the scatter of the z ∼ 2 star-forming main sequence, mass–metallicity relation, and M mol – M * relation are primarily driven by stochastic variations in gas inflow rates. We place constraints on the mass loading of galactic outflows and perform a metal budget analysis, finding that massive z ∼ 2 star-forming galaxies retain only 30% of metals produced, implying that a large mass of metals resides in the circumgalactic medium. 

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2. A Breakdown of the Black Hole–Bulge Mass Relation in Local Active Galaxies

   https://doi.org/10.3847/1538-4357/ad55e6  

   Sturm, Megan R; Reines, Amy E (August 2024, The Astrophysical Journal)

   Abstract We investigate the relation between black hole (BH) mass and bulge stellar mass for a sample of 117 local (z∼ 0) galaxies hosting low-luminosity, broad-line active galactic nuclei (AGNs). Our sample comes from Reines & Volonteri, who found that, for a given total stellar mass, these AGNs have BH masses more than an order of magnitude smaller than those in early-type galaxies with quiescent BHs. Here, we aim to determine whether or not this AGN sample falls on the canonical BH-to-bulge mass relation by utilizing bulge–disk decompositions and determining bulge stellar masses using color-dependent mass-to-light ratios. We find that our AGN sample remains offset by more than an order of magnitude from theM_BH–M_bulgerelation defined by early-type galaxies with dynamically detected BHs. We caution that using canonical BH-to-bulge mass relations for galaxies other than ellipticals and bulge-dominated systems may lead to highly biased interpretations. This work bears directly on predictions for gravitational-wave detections and cosmological simulations that are tied to the local BH-to-bulge mass relations. 

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3. The dust-to-gas mass ratio of luminous galaxies as a function of their metallicity at cosmic noon

   Popping, Gergö; Shivaei, Irene; Sanders, Ryan L.; Jones, Tucker; Pope, Alexandra; Reddy, Naveen A.; Shapley, Alice E.; Coil, Alison L.; Kriek, Mariska (February 2023, Astronomy & Astrophysics)

   Aims. We aim to quantify the relation between the dust-to-gas mass ratio (DTG) and gas-phase metallicity of z  = 2.1 − 2.5 luminous galaxies and contrast this high-redshift relation against analogous constraints at z  = 0. Methods. We present a sample of ten star-forming main-sequence galaxies in the redshift range 2.1 <  z  < 2.5 with rest-optical emission-line information available from the MOSDEF survey and with ALMA 1.2 millimetre and CO J  = 3 − 2 follow-up observations. The galaxies have stellar masses ranging from 10 10.3 to 10 10.6   M ⊙ and cover a range in star-formation rate from 35 to 145 M ⊙ yr −1 . We calculated the gas-phase oxygen abundance of these galaxies from rest-optical nebular emission lines (8.4 < 12 + log(O/H) < 8.8, corresponding to 0.5−1.25 Z ⊙ ). We estimated the dust and H 2 masses of the galaxies (using a metallicity-dependent CO-to-H 2 conversion factor) from the 1.2 mm and CO J  = 3 − 2 observations, respectively, from which we estimated a DTG. Results. We find that the galaxies in this sample follow the trends already observed between CO line luminosity and dust-continuum luminosity from z  = 0 to z  = 3, extending such trends to fainter galaxies at 2.1 <  z  < 2.5 than observed to date. We find no second-order metallicity dependence in the CO – dust-continuum luminosity relation for the galaxies presented in this work. The DTGs of main-sequence galaxies at 2.1 <  z  < 2.5 are consistent with an increase in the DTG with gas-phase metallicity. The metallicity dependence of the DTG is driven by the metallicity dependence of the CO-to-H 2 conversion factor. Galaxies at z  = 2.1 − 2.5 are furthermore consistent with the DTG-metallicity relation found at z  = 0 (i.e. with no significant evolution), providing relevant constraints for galaxy formation models. These results furthermore imply that the metallicity of galaxies should be taken into account when estimating cold-gas masses from dust-continuum emission, which is especially relevant when studying metal-poor low-mass or high-redshift galaxies. 

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4. Gas properties as a function of environment in the proto-supercluster Hyperion at z  ∼ 2.45

   https://doi.org/10.1051/0004-6361/202553926  

   Gururajan, G; Cucciati, O; Lemaux, B C; Talia, M; Zamorani, G; Pozzi, F; Decarli, R; Forrest, B; Shen, L; De_Lucia, G; et al (June 2025, Astronomy & Astrophysics)

   The cosmic star formation rate density, molecular gas density, and active galactic nucleus (AGN) activity of the Universe peak atz∼2–3, demonstrating the Universe is most active at this epoch. The nature of the galaxies at these redshifts and their properties as a function of their environment are particularly interesting with respect to our understanding of the mechanisms driving their star formation and quenching. Atz∼2.5, the massive (∼4.8×10¹⁵ M_⊙) proto-supercluster Hyperion, consisting of seven groups and peaks and extending over a co-moving volume of 60×60×150 Mpc³, is an excellent laboratory for probing the properties and evolution of galaxies as a function of their environments. We used a large compilation of photometric (optical to radio wavelengths, COSMOS2020, COSMOS-Super-deblended, and A³COSMOS), and spectroscopic (C3VO, HST-Hyperion, VUDS, zCOSMOS, DEIMOS10K, and MAGAZ3NE) data to assign membership and study the relation between the local environment and the molecular gas mass, the star-formation rate (SFR), gas depletion timescales, and quenching mechanisms. We find that the depletion timescales and the molecular gas fractions decrease and SFR increases in denser environments at the ∼2σlevel, suggesting accelerated evolution in the densest regions of this proto-supercluster resulting from gas stripping, over-consumption, and/or cessation of cold flows. Dedicated observations at sub-millimeter (sub-mm) wavelengths will enable further spectroscopic confirmations and better coverage at these wavelengths, thereby offering more conclusive results on the environmental implications on gas reservoirs of galaxies in Hyperion. 

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5. The Redshift Evolution of the M _BH – M _* Scaling Relation: New Insights from Cosmological Simulations and Semianalytic Models

   https://doi.org/10.3847/1538-4357/adbeef  

   Dattathri, Shashank; Natarajan, Priyamvada; Porras-Valverde, Antonio J; Burke, Colin J; Chen, Nianyi; Di_Matteo, Tiziana; Ni, Yueying (May 2025, The Astrophysical Journal)

   Abstract We study the coevolution of black holes (BHs) and their host galaxies in the ASTRIDandTNG300cosmological simulations and the DARKSAGEsemianalytic model (SAM), focusing on the evolution of the BH mass–stellar mass (M_BH–M_*) relation. Due to differences in the adopted subgrid modeling of BH seeding, dynamics, and feedback, the models differ in their predicted redshift evolution of theM_BH–M_*relation. We find that it is the interplay between the star formation rate (SFR) and the black hole accretion rate (BHAR) that drives the evolution of the mean relation. We define a quantity $\mathcal{R}$ , the ratio between the specific BHAR and SFR (i.e., $\mathcal{R}\equiv$ sBHAR/sSFR), and demonstrate that it is $\mathcal{R}$ that governs the evolution of individual sources in theM_BH–M_*plane. The efficiency of BH growth versus stellar mass growth in the sSFR–sBHAR plane reflects the partitioning of gas between fueling star formation versus BH accretion. This partitioning depends on the implementation of BH dynamics and the nature of how black hole feedback quenches galaxies. In the cosmological simulations (ASTRIDandTNG300), the BHAR and SFR are intrinsically linked, resulting in a tightM_BH–M_*correlation, while the DARKSAGESAM produces a significantly larger scatter. We discuss these results in the context of recently discovered overmassive BHs and massive quenched galaxies at high redshift by the James Webb Space Telescope. 

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