Search

We report the identification of 64 new candidates of compact galaxies, potentially hosting faint quasars with bolometric luminosities of Lbol = 1043–1046 erg s−1, residing in the reionization epoch within the redshift range of 6 ≲ z ≲ 8. These candidates were selected by harnessing the rich multiband datasets provided by the emerging JWST-driven extragalactic surveys, focusing on COSMOS-Web, as well as JADES, UNCOVER, CEERS, and PRIMER. Our search strategy includes two stages: applying stringent photometric cuts to catalog-level data and detailed spectral energy distribution fitting. These techniques effectively isolate the quasar candidates while mitigating contamination from low-redshift interlopers, such as brown dwarfs and nearby galaxies. The selected candidates indicate physical traits compatible with low-luminosity active galactic nuclei, likely hosting ≈105–107M⊙ supermassive black holes (SMBHs) living in galaxies with stellar masses of ≈108–1010M⊙. The SMBHs selected in this study, on average, exhibit an elevated mass compared to their hosts, with the mass ratio distribution slightly higher than those of galaxies in the local Universe. As with other high-z studies, this is at least in part due to the selection method for these quasars. An extensive Monte Carlo analysis provides compelling evidence that heavy black hole seeds from the direct collapse scenario appear to be the preferred pathway to mature this specific subset of SMBHs by z ≈ 7. Notably, most of the selected candidates might have emerged from seeds with masses of ∼105M⊙, assuming a thin disk accretion with an average Eddington ratio of fEdd = 0.6 ± 0.3 and a radiative efficiency of ϵ = 0.2 ± 0.1. This work underscores the significance of further spectroscopic observations, as the quasar candidates presented here offer exceptional opportunities to delve into the nature of the earliest galaxies and SMBHs that formed during cosmic infancy. [ABSTRACT FROM AUTHOR]

We report the detection of cold dust in an apparently quiescent massive galaxy (log (M ⋆/M⊙) ≈ 11) at z ∼ 2 (G4). The source is identified as a serendipitous 2 mm continuum source in a deep ALMA observation within the field of Q2343-BX610, a z  = 2.21 massive star-forming disc galaxy. Available multiband photometry of G4 suggests redshift of z ∼ 2 and a low specific star formation rate (sSFR), log (SFR/ M ⋆)[yr−1] ≈ −10.2, corresponding to ≈1.2 dex below the z  = 2 main sequence (MS). G4 appears to be a peculiar dust-rich quiescent galaxy for its stellar mass (log (M dust/ M ⋆) = −2.71 ± 0.26), with its estimated mass-weighted age (∼1–2 Gyr). We compile z ≳ 1 quiescent galaxies in the literature and discuss their age–ΔMS and log (M dust/ M ⋆)–age relations to investigate passive evolution and dust depletion scale. A long dust depletion time and its morphology suggest morphological quenching along with less efficient feedback that could have acted on G4. The estimated dust yield for G4 further supports this idea, requiring efficient survival of dust and/or grain growth, and rejuvenation (or additional accretion). Follow-up observations probing the stellar light and cold dust peak are necessary to understand the implication of these findings in the broader context of galaxy evolutionary studies and quenching in the early universe. [ABSTRACT FROM AUTHOR]

We present the results of our Keck /DEep Imaging Multi-Object Spectrograph spectroscopic follow-up of candidate galaxies of i -band-dropout protocluster candidate galaxies at z ∼ 6 in the COSMOS field. We securely detect Lyman α emission lines in 14 of the 30 objects targeted, 10 of them being at z  = 6 with a signal-to-noise ratio of 5–20; the remaining galaxies are either non-detections or interlopers with redshift too different from z  = 6 to be part of the protocluster. The 10 galaxies at z ≈ 6 make the protocluster one of the riches at z > 5. The emission lines exhibit asymmetric profiles with high skewness values ranging from 2.87 to 31.75, with a median of 7.37. This asymmetry is consistent with them being Ly α, resulting in a redshift range of z  = 5.85–6.08. Using the spectroscopic redshifts, we recalculate the overdensity map for the COSMOS field and find the galaxies to be in a significant overdensity at the 4σ level, with a peak overdensity of δ = 11.8 (compared to the previous value of δ = 9.2). The protocluster galaxies have stellar masses derived from Bagpipes spectral energy distribution fits of |$10^{8.29}\!-\!10^{10.28} \rm \, M_{\rm \odot }$| and star formation rates of |$2\!-\!39\, \rm M_{\rm \odot }\rm \, yr^{-1}$|⁠ , placing them on the main sequence at this epoch. Using a stellar-to-halo-mass relationship, we estimate the dark matter halo mass of the most massive halo in the protocluster to be |$\sim 10^{12}\rm M_{\rm \odot }$|⁠. By comparison with halo mass evolution tracks from simulations, the protocluster is expected to evolve into a Virgo- or Coma-like cluster in the present day. [ABSTRACT FROM AUTHOR]

We present NOEMA CO (2–1) line and ALMA 870 µm continuum observations of a main-sequence galaxy at z = 1.38. The galaxy was initially deemed a "gas giant" based on the gas mass derived from sub-mm continuum (log(Mgas/M⊙) = 11.20 ± 0.20), however, the gas mass derived from CO (2−1) luminosity brings the gas mass value down to a level that is consistent with typical values for star-forming galaxies at that redshift (log(Mgas/M⊙) = 10.84 ± 0.03). Meanwhile, the dust-to-stellar mass ratio remains elevated above the scaling relations by a factor of 5. In this work, we explore the potential physical picture and consider an underestimated stellar mass and optically thick dust as possible explanations. Based on the updated gas-to-stellar mass ratio, we can rule out the former; while the latter may indeed contribute to the overestimation of the dust mass, it is not sufficient enough to explain the observed physical picture overall. Instead, other plausible explanations include enhanced HI reservoirs, an unusually high metallicity, or the presence of an optically dark, dusty contaminant. Using the ALMA data at 870 µm coupled with HST/ACS imaging, we find an extended morphology in dust continuum and clumpy star-formation in rest-frame UV in this galaxy. In addition, a tentative ~10 kpc dusty arm is found to be bridging the galaxy center and a clump in F814W image. The galaxy shows levels of dust obscuration similar to the so-called HST-dark galaxies at higher redshifts, thus falling into the optically faint and dark JWST color-color selection at z > 2. It is therefore possible that our object may stand as a low-z analog of the HST-dark populations. This galaxy serves as a caveat to the gas masses based on the continuum alone, with a larger sample required to unveil the full picture. [ABSTRACT FROM AUTHOR]

We report the detection of the CO(12–11) line emission toward G09-83808 (or H-ATLAS J090045.4+004125), a strongly-lensed submillimeter galaxy at z = 6.02, with Atacama Large Millimeter/submillimeter Array observations. Combining previously detected [O  iii ] 88 μm, [N  ii ] 205 μm, and dust continuum at 0.6 mm and 1.5 mm, we investigate the physical properties of the multi-phase interstellar medium in G09-83808. A source-plane reconstruction reveals that the region of the CO(12–11) emission is compact (⁠|$R_\mathrm{{e, CO}}=0.49^{+0.29}_{-0.19}\:\mbox{kpc}$|⁠) and roughly coincides with that of the dust continuum. Non-local thermodynamic equilibrium radiative transfer modeling of CO spectral-line energy distribution reveals that most of the CO(12–11) emission comes from a warm (kinetic temperature of T kin = 320 ± 170 K) and dense [log (n H2/cm−3) = 5.4 ± 0.6] gas, indicating that the warm and dense molecular gas is concentrated in the central 0.5 kpc region. The luminosity ratio in G09-83808 is estimated to be L CO(12-11)/ L CO(6-5) = 1.1 ± 0.2. The high ratio is consistent with those in local active galactic nuclei (AGNs) and 6 < z < 7 quasars, the fact of which implies that G09-83808 would be a good target to explore dust-obscured AGNs in the epoch of reionization. In the reconstructed [O  iii ] 88 μm and [N  ii ] 205 μm cubes, we also find that a monotonic velocity gradient is extending over the central starburst region by a factor of 2 and that star-forming sub-components exist. High-resolution observations of bright [C  ii ] 158 μm line emissions will enable us to characterize the kinematics of a possible rotating disk and the nature of the sub-components. [ABSTRACT FROM AUTHOR]

We present observations of [N  ii ] 205 μm, [O  iii ] 88 μm, and dust emission in a strongly-lensed, submillimeter galaxy (SMG) at z = 6.0, G09.83808, with the Atacama Large Millimeter/submillimeter Array (ALMA). Both [N  ii ] and [O  iii ] line emissions are detected at >12σ in the |${0{^{\prime \prime}_{.}}8}$| -resolution maps. Lens modeling indicates that the spatial distribution of the dust continuum emission is well characterized by a compact disk with an effective radius of 0.64 ± 0.02 kpc and a high infrared surface brightness of ΣIR = (1.8 ± 0.3) × 1012  L ⊙ kpc−2. This result supports that G09.83808 is the progenitor of compact quiescent galaxies at z ∼ 4, where the majority of its stars are expected to be formed through a strong and short burst of star formation. G09.83808 and other lensed SMGs show a decreasing trend in the [N  ii ] line to infrared luminosity ratio with increasing continuum flux density ratio between 63 and 158 μm, as seen in local luminous infrared galaxies (LIRGs). The decreasing trend can be reproduced by photoionization models with increasing ionization parameters. Furthermore, by combining the [N  ii ]/[O  iii ] luminosity ratio with far-infrared continuum flux density ratio in G09.83808, we infer that the gas phase metallicity is already Z ≈ 0.5–0.7  Z ⊙. G09.83808 is likely one of the earliest galaxies that has been chemically enriched at the end of reionization. [ABSTRACT FROM AUTHOR]

We report on the detection of the [C  ii ] 157.7 μ m emission from the Lyman break galaxy (LBG) MACS0416_Y1 at z  = 8.3113, by using the Atacama Large Millimeter/submillimeter Array (ALMA). The luminosity ratio of [O  iii ] 88 μ m (from previous campaigns) to [C  ii ] is 9.3 ± 2.6, indicative of hard interstellar radiation fields and/or a low covering fraction of photodissociation regions. The emission of [C  ii ] is cospatial to the 850 μ m dust emission (90 μ m rest frame, from previous campaigns), however the peak [C  ii ] emission does not agree with the peak [O  iii ] emission, suggesting that the lines originate from different conditions in the interstellar medium. We fail to detect continuum emission at 1.5 mm (160 μ m rest frame) down to 18 μ Jy (3σ). This non-detection places a strong limits on the dust spectrum, considering the 137 ± 26 μ Jy continuum emission at 850 μ m. This suggests an unusually warm dust component (T > 80 K, 90 per cent confidence limit), and/or a steep dust-emissivity index (βdust > 2), compared to galaxy-wide dust emission found at lower redshifts (typically T ∼ 30–50 K, βdust ∼ 1–2). If such temperatures are common, this would reduce the required dust mass and relax the dust production problem at the highest redshifts. We therefore warn against the use of only single-wavelength information to derive physical properties, recommend a more thorough examination of dust temperatures in the early Universe, and stress the need for instrumentation that probes the peak of warm dust in the Epoch of Reionization. [ABSTRACT FROM AUTHOR]

We present studies of a protocluster at z =2.5, an overdense region found close to a radio galaxy, 4C 23.56, using ALMA. We observed 1.1 mm continuum, two CO lines (CO (4–3) and CO (3–2)) and the lower atomic carbon line transition ([CI](3P1-3P0)) at a few kpc (0″.3-0″.9) resolution. The primary targets are 25 star-forming galaxies selected as Hα emitters (HAEs) that are identified with a narrow band filter. These are massive galaxies with stellar masses of > 1010Mʘ that are mostly on the galaxy main sequence at z =2.5. We measure the molecular gas mass from the independent gas tracers of 1.1 mm, CO (3–2) and [CI], and investigate the gas kinematics of galaxies from CO (4–3). Molecular gas masses from the different measurements are consistent with each other for detection, with a gas fraction (fgas = Mgas/(Mgas+ Mstar)) of ≃ 0.5 on average but with a caveat. On the other hand, the CO line widths of the protocluster galaxies are typically broader by ˜50% compared to field galaxies, which can be attributed to more frequent, unresolved gas-rich mergers and/or smaller sizes than field galaxies, supported by our high-resolution images and a kinematic model fit of one of the galaxies. We discuss the expected scenario of galaxy evolution in protoclusters at high redshift but future large surveys are needed to get a more general view. [ABSTRACT FROM AUTHOR]

We report the first detection of [NII] 122 μm line toward a QSO-SMG pair, BRI 1202-0725, at z = 4.7 using ALMA. Combining with [NII] 205 μm line detection and taking the line ratio of [NII]122/[NII]205, we constrain electron densities of both galaxies. The derived electron densities are ${26_{ - 11}^{ + 12}$ and ${134_{ - 39}^{ + 50}$ cm−3 for the SMG and the QSO, respectively, which are the first measurements for galaxies at z > 4. The electron density of the SMG is comparable to the Galactic plane and the average of local spiral galaxies, while the value for the QSO is comparable to local starbursts and optical-line based measurements for star-forming galaxies at z ∼ 2–3. Considering the similar star-formation rates (SFRs) of ≍ 1000 M⊙ yr−1 for both galaxies, our results suggest a large scatter of electron densities at fixed SFR and caution against using optical lines for dusty starbursts. The details of this report are presented in Lee et al. 2019 (submitted). [ABSTRACT FROM AUTHOR]

We report the first detection of [NII] 122 μm line toward a QSO-SMG pair, BRI 1202-0725, at z = 4.7 using ALMA. Combining with [NII] 205 μm line detection and taking the line ratio of [NII]122/[NII]205, we constrain electron densities of both galaxies. The derived electron densities are ${26_{ - 11}^{ + 12}$ and ${134_{ - 39}^{ + 50}$ cm−3 for the SMG and the QSO, respectively, which are the first measurements for galaxies at z > 4. The electron density of the SMG is comparable to the Galactic plane and the average of local spiral galaxies, while the value for the QSO is comparable to local starbursts and optical-line based measurements for star-forming galaxies at z ∼ 2–3. Considering the similar star-formation rates (SFRs) of ≍ 1000 M⊙ yr−1 for both galaxies, our results suggest a large scatter of electron densities at fixed SFR and caution against using optical lines for dusty starbursts. The details of this report are presented in Lee et al. 2019 (submitted). [ABSTRACT FROM AUTHOR]