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Many poststarburst galaxies at z ∼ 0.7 have been shown to retain substantial molecular gas reservoirs yet host low ongoing star formation, suggesting that the remaining gas may be inefficient at forming stars during the early postburst phase. We present new Atacama Large Millimeter/submillimeter Array CO(5-4) observations of nine gas-rich poststarburst galaxies at z ~ 0.7 from the Studying Quenching in Intermediate-z Galaxies: Gas, anguLar momentum, and Evolution (SQuIGGLE) survey, providing a view of the molecular gas excitation in these systems. Combined with existing CO(2-1) data, we detect CO(5─4) in eight out of nine targets and find that most have moderate CO excitation with r52 = L'CO(5−4)/L'CO(2−1) ~ 0.1−0.3 . These systems show no clear trend between r52 and either total or surface-density of star formation. Specifically, all objects have ΣSFR ∼ 0.01─1 M⊙ yr−1 kpc−2, consistent with compact, modest star formation, even when allowing for buried activity, as these galaxies decline from their peak. One object, J1448+1010, which has clear optical, mid-infrared, and radio indicators of an active galactic nucleus, is an outlier with r52 ~ 0.6; its elevated excitation likely requires significant nonstellar heating, with a contribution from potentially obscured star formation. Together, most gas-rich SQuIGGLE poststarbursts have moderately excited molecular gas alongside little to modest star-forming activity, indicating that the remaining gas hosts relatively suppressed star formation efficiencies instead of strong buried starburst activity. 

Abstract Supermassive black holes (SMBHs) are found in the centers of massive galaxies, and galaxy mergers should eventually lead to SMBH mergers. Quasar activity has long been associated with galaxy mergers, so here we investigate if supermassive black hole binaries (SMBHBs) are preferentially found in quasars. Our multimessenger investigation folds together a gravitational-wave background signal from NANOGrav, a sample of periodic active galactic nucleus candidates from the Catalina Real-Time Transient Survey, and a quasar mass function, to estimate an upper limit on the fraction of quasars which could host an SMBHB. We find at 95% confidence that quasars are at most 5 times as likely to host an SMBHB as a random galaxy. Pulsar timing arrays may therefore be more likely to find SMBHBs by prioritizing quasars over a random selection of galaxies in their searches. 

Observational and theoretical studies have long held that rapid gas consumption in starbursts is responsible for the formation of the most massive quiescent galaxies at high redshift. However, studies of recently quenched “post-starburst” galaxies have discovered that a number of them are surprisingly luminous in CO, challenging this assumption. We present deep Atacama Large Millimeter/submillimeter Array CO(2–1) observations of 50 massive (log(M⋆/M⊙)∼11.2) post-starburst galaxies from the SQuIGGLE sample at z ∼ 0.7. We detect a large fraction (27/50) of the galaxies in CO(2–1). We perform new spectral energy distribution (SED) fits incorporating mid- and far-IR photometry to measure the star formation rates (SFRs) and histories that can reproduce both the rest-optical and dust properties of these galaxies. We find that the CO luminosity correlates with the age of the recent starburst, suggesting a gas-removal timescale of ≲150 Myr, an order of magnitude shorter than is implied by their SFRs under standard birth cloud dust assumptions. We find that while allowing for significantly more attenuation in birth clouds can raise SFRs by ∼0.5 dex, for almost all galaxies, it is neither required to fit the observed IR SED, nor is it sufficient to explain the observed depletion trend. Even the combination of significant buried star formation and ULIRG-like α_CO is not enough to explain this decay in CO luminosity. Furthermore, there is no strong evidence to support either of those modifications to the depletion time. Therefore, it remains a distinct possibility that the age–CO luminosity trend should not be interpreted as an evolutionary sequence, and that gas-rich SQuIGGLE galaxies will soon rejuvenate. 

Abstract: Atacama Large Millimeter/submillimeter Array observations have shown that candidate “post-starburst” galaxies (PSBs) at z~0.6 can retain significant molecular gas reservoirs. These results would imply that—unlike many model predictions—galaxies can shut down their star formation before their cold gas reservoirs are depleted. However, these studies inferred star formation rates (SFRs) either from [OII] line fluxes or from spectral energy distribution (SED) modeling and could have missed large dust-obscured contributions to the SFRs. In this study, we present Keck/NIRES observations of 13 massive (M_* >= 10^11M_⊙) PSBs, which allow us to estimate Hα SFRs in these gas-rich PSBs. We confirm the previously inferred low SFRs for the majority of the sample: 11/13 targets show clear Hα absorption, with minimal infilling indicating dust-corrected SFRs of <4.1Msun/yr. These SFRs are notably low given the large H2 reservoirs (∼(1–5) × 10^10Msun) present in 5/13 of these galaxies, placing them significantly offset from star-forming galaxies on the Kennicutt–Schmidt relation for star-forming galaxies. The [NII]/Hα ratios of all 13 PSBs imply contributions from non-star-forming ionization mechanisms (e.g., active galactic nuclei, shocks, or hot evolved stars) to their Hα emission, suggesting that even these low ongoing SFRs may be overestimated. These low Hα SFRs, dust corrected using Av estimates from SED fitting, confirm that these galaxies are very likely quiescent and, thus, that galaxies can quench before their cold gas reservoirs are fully depleted. 

The active suppression of star formation in galaxies is critical for preventing the growth of overly massive systems and explaining the formation of present-day elliptical galaxies. We present a high-resolution, spatially resolved, multiwavelength study of two z ∼ 0.7 massive post-starburst galaxies, SDSS J1448+1010 and SDSS J2258+2313, from the Studying Quenching in Intermediate-z Galaxies: Gas, anguLar momentum, and Evolution (SQuIGGLE) survey, providing new insights into the role of mergers in driving quenching. Atacama Large Millimeter/submillimeter Array CO(2–1) observations show that both galaxies removed ∼50% of their molecular gas into extended tidal tails, spanning up to 65 kpc, following recent mergers. Hubble Space Telescope Wide Field Camera 3 imaging and grism spectroscopy show that while SDSS J1448+1010 exhibits Hα emission in its northern tidal tail consistent with ongoing star formation, SDSS J2258+2313 lacks detectable star-forming activity outside the central galaxy. Very Large Array 6 GHz continuum data reveal compact radio emission in SDSS J2258+2313, while SDSS J1448+1010 hosts small radio jets indicative of active galactic nucleus activity. Both galaxies retain substantial molecular gas reservoirs in their central regions, which appear more turbulent than “normal” star-forming galaxies, likely contributing to the observed low star formation rates in the hosts. Despite the similarities in their cold gas content and tidal features, the galaxies are distinct from each other in their star formation, gas–star alignment, and radio morphology, highlighting the complexity of tidal gas removal as a quenching mechanism at intermediate redshifts. 

Abstract Active galactic nuclei (AGN) are the signposts of black hole growth, and likely play an important role in galaxy evolution. An outstanding question is whether AGN of different spectral types indicate different evolutionary stages in the coevolution of black holes and galaxies. We present the angular correlation function between an AGN sample selected from Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP) optical photometry and Wide-field Infrared Survey Explorer mid-IR photometry and a luminous red galaxy (LRG) sample from HSC-SSP. We investigate AGN clustering strength as a function of luminosity and spectral features across three independent HSC fields totaling ∼600 deg², forz∈ 0.6 −1.2 and AGN withL_6μm> 3 × 10⁴⁴erg s⁻¹. There are ∼28,500 AGN and ∼1.5 million LRGs in our primary analysis. We determine the average halo mass for the full AGN sample (M_h≈ 10^12.9h⁻¹M_⊙), and note that it does not evolve significantly as a function of redshift (over this narrow range) or luminosity. We find that, on average, unobscured AGN (M_h≈ 10^13.3h⁻¹M_⊙) occupy ∼4.5× more massive halos than obscured AGN (M_h≈ 10^12.6h⁻¹M_⊙), at 5σstatistical significance using 1D uncertainties, and at 3σusing the full covariance matrix, suggesting a physical difference between unobscured and obscured AGN, beyond the line-of-sight viewing angle. Furthermore, we find evidence for a halo mass dependence on reddening level within the Type I AGN population, which could support the existence of a dust-obscured phase. However, we also find that quite small systematic shifts in the redshift distributions of the AGN sample could explain current and previously observed differences inM_h. 

Abstract Drawing from the Chandra archive and using a carefully selected set of nearby dwarf galaxies, we present a calibrated high-mass X-ray binary (HMXB) luminosity function in the low-mass galaxy regime and search for an already hinted at dependence on metallicity. Our study introduces a new sample of local dwarf galaxies (D< 12.5 Mpc andM_*< 5 × 10⁹M_⊙), expanding the specific star formation rates (sSFR) and gas-phase metallicities probed in previous investigations. Our analysis of the observed X-ray luminosity function indicates a shallower power-law slope for the dwarf galaxy HMXB population. In our study, we focus on dwarf galaxies that are more representative in terms of sSFR compared to prior work. In this regime, the HMXB luminosity function exhibits significant stochastic sampling at high luminosities. This likely accounts for the pronounced scatter observed in the galaxy-integrated HMXB population’sL_X/SFR versus metallicity for our galaxy sample. Our calibration is necessary to understand the active galactic nuclei content of low-mass galaxies identified in current and future X-ray survey fields and has implications for binary population synthesis models, as well as X-ray-driven cosmic heating in the early Universe. 

Tracking the cold molecular gas contents of galaxies is critical to understand the interplay between star formation and galaxy growth across cosmic time. Observations of the long-wavelength dust continuum, a proxy for the cold gas, are widely used in the high-redshift community because of their ease and efficiency. These measurements rely on the assumption of a molecular gas-to-dust mass ratio, typically taken to be δ_GDR ≈ 100 in massive, metal-rich systems. We present Atacama Large Millimeter/submillimeter Array observations of the 870μm dust continuum in a sample of five massive quiescent galaxies at z ∼ 1 with existing detections of CO(2–1). We find surprisingly weak dust emission, falling a factor of ≳0.4–0.8 dex below the typical correlation between CO and continuum luminosity. We interpret this dust deficiency as evidence for unusually high δ_GDR in these galaxies, which we calculate to range from 300 to at least 1200. Our results and other observations from the literature are generally compatible with predictions from the SIMBA cosmological simulation that dust is preferentially destroyed in quiescent galaxies. Ultimately, we conclude that the dust continuum is a highly unreliable tracer of the molecular gas in high-redshift quiescent galaxies. As a consequence, we may know much less about the cold gas contents of this population than previously thought. 

Abstract The recent Chandra-JWST discovery of a quasar in thez≈ 10.1 galaxy UHZ1 reveals that accreting supermassive black holes were already in place 470 million years after the Big Bang. The Chandra X-ray source detected in UHZ1 is a Compton-thick quasar with a bolometric luminosity ofL_bol∼ 5 × 10⁴⁵erg s⁻¹, which corresponds to an estimated black hole (BH) mass of ∼4 × 10⁷M_⊙, assuming accretion at the Eddington rate. JWST NIRCAM and NIRSpec data yield a stellar mass estimate for UHZ1 comparable to its BH mass. These characteristics are in excellent agreement with prior theoretical predictions for a unique class of transient, high-redshift objects, overmassive black hole galaxies (OBGs) by Natarajan et al., that harbor a heavy initial black hole seed that likely formed from the direct collapse of the gas. Given the excellent agreement between the observed multiwavelength properties of UHZ1 and theoretical model template predictions, we suggest that UHZ1 is the first detected OBG candidate. Our assertion rests on multiple lines of concordant evidence between model predictions and the following observed properties of UHZ1: its X-ray detection and the estimated ratio of the X-ray flux to the IR flux, which is consistent with theoretical expectations for a heavy initial BH seed; its high measured redshift ofz≈ 10.1, as predicted for the transient OBG stage (9 <z< 12); the amplitude and shape of the detected JWST spectral energy distribution (SED) between 1 and 5μm, which is in very good agreement with simulated template SEDs for OBGs; and the extended JWST morphology of UHZ1, which is suggestive of a recent merge and is also expected for the formation of transient OBGs. As the first OBG candidate, UHZ1 provides compelling evidence for the formation of heavy initial seeds from direct collapse in the early Universe. 

Abstract Quantifying the connection between galaxies and their host dark matter halos has been key for testing cosmological models on various scales. BelowM_⋆∼ 10⁹M_⊙, such studies have primarily relied on the satellite galaxy population orbiting the Milky Way (MW). Here we present new constraints on the connection between satellite galaxies and their host dark matter subhalos using the largest sample of satellite galaxies in the Local Volume (D≲ 12 Mpc) to date. We use 250 confirmed and 71 candidate dwarf satellites around 27 MW-like hosts from the Exploration of Local VolumE Satellites (ELVES) Survey and use the semianalyticalSatGenmodel for predicting the population of dark matter subhalos expected in the same volume. Through a Bayesian model comparison of the observed and the forward-modeled satellite stellar mass functions (SSMFs), we infer the satellite stellar-to-halo mass relation. We find that the observed SSMF is best reproduced when subhalos at the low-mass end are populated by a relation of the form $M_{\star }\propto M_{\mathrm{peak}}^{\alpha }$, with a moderate slope of ${\alpha }_{\mathrm{const}}=2.10\pm 0.01$and a low scatter, constant as a function of the peak halo mass, of ${\sigma }_{\mathrm{const}}={0.06}_{-0.05}^{+0.07}$. A model with a steeper slope (α_grow= 2.39 ± 0.06) and a scatter that grows with decreasingM_peakis also consistent with the observed SSMF but is not required. Our new model for the satellite–subhalo connection, based on hundreds of Local Volume satellite galaxies, is in line with what was previously derived using only MW satellites.