Search

We report the first results from a deep near-infrared campaign with the Hubble Space Telescope to obtain late-epoch images of the Hubble Ultra Deep Field, 10–15 yr after the first epoch data were obtained. The main objectives are to search for faint active galactic nuclei (AGN) at high redshifts by virtue of their photometric variability and measure (or constrain) the comoving number density of supermassive black holes (SMBHs), n _SMBH , at early times. In this Letter, we present an overview of the program and preliminary results concerning eight objects. Three variables are supernovae, two of which are apparently hostless with indeterminable redshifts, although one has previously been recorded as a z ≈ 6 object precisely because of its transient nature. Two further objects are clear AGN at z = 2.0 and 3.2, based on morphology and/or infrared spectroscopy from JWST. Three variable targets are identified at z = 6–7 that are also likely AGN candidates. These sources provide a first measure of n _SMBH in the reionization epoch by photometric variability, which places a firm lower limit of 3 × 10 ^−4 cMpc ^−3 . After accounting for variability and luminosity incompleteness, we estimate n _SMBH ≳ 8 × 10 ^−3 cMpc ^−3 , which is the largest value so far reported at these redshifts. This SMBH abundance is also strikingly similar to estimates of n _SMBH in the local Universe. We discuss how these results test various theories for SMBH formation.

We present an analysis of JWST Ly α spectroscopy of z ≳ 6.5 galaxies, using observations in the public archive covering galaxies in four independent fields: Great Observatories Origins Deep Survey (GOODS)-N, GOODS-S, A2744, and the Extended Groth Strip (EGS). We measure the Ly α emission line properties for a sample of 210 z ≃ 6.5–13 galaxies, with redshifts confirmed independently of Ly α in all cases. We present three new detections of Ly α emission in JWST spectra, including a large equivalent width (EW; =143 Å) Ly α emitter (LAE) with strong C iv emission (EW = 21 Å) at z = 7.1 in GOODS-N. We measure the redshift-dependent Ly α EW distribution across our sample. We find that strong Ly α emission (EW > 25 Å) becomes increasingly rare at earlier epochs, suggesting that the transmission of Ly α photons decreases by 4× between z ≃ 5 and z ≃ 9. We describe potential implications for the intergalactic medium neutral fraction. There is significant field-to-field variance in the LAE fraction. In contrast to the three other fields, the EGS shows no evidence for reduced transmission of Ly α photons at z ≃ 7–8, suggesting a significantly ionized sight line may be present in the field. We use available NIRCam grism observations from the First Reionization Epoch Spectroscopically Complete Observations survey to characterize overdensities on large scales around known LAEs in the GOODS fields. The strongest overdensities appear linked with extremely strong Ly α detections (EW > 50 Å) in most cases. Future Ly α spectroscopy with JWST has the potential to constrain the size of ionized regions around early galaxy overdensities, providing a new probe of the reionization process.

JWST observations have recently begun delivering the first samples of Ly α velocity profile measurements at z > 6, opening a new window into the reionization process. Interpretation of z ≳ 6 line profiles is currently stunted by limitations in our knowledge of the intrinsic Ly α profile (before encountering the intergalactic medium (IGM)) of the galaxies that are common at z ≳ 6. To overcome this shortcoming, we have obtained resolved ( R ∼ 3900) Ly α spectroscopy of 42 galaxies at z = 2.1–3.4 with similar properties as are seen at z > 6. We quantify a variety of Ly α profile statistics as a function of [O iii ]+H β equivalent width (EW). Our spectra reveal a new population of z ≃ 2–3 galaxies with large [O iii ]+H β EWs (>1200 Å) and a large fraction of Ly α flux emerging near the systemic redshift (peak velocity ≃0 km s ^−1 ). These spectra indicate that low-density neutral hydrogen channels are able to form in a subset of low-mass galaxies (≲1 × 10 ^8 M _⊙ ) that experience a burst of star formation (sSFR > 100 Gyr ^−1 ). Other extreme [O iii ] emitters show weaker Ly α that is shifted to higher velocities (≃240 km s ^−1 ) with little emission near the line center. We investigate the impact the IGM is likely to have on these intrinsic line profiles in the reionization era, finding that the centrally peaked Ly α emitters should be strongly attenuated at z ≳ 5. We show that these line profiles are particularly sensitive to the impact of resonant scattering from infalling IGM and can be strongly attenuated even when the IGM is highly ionized at z ≃ 5. We compare these expectations against a new database of z ≳ 6.5 galaxies with robust velocity profiles measured with JWST/NIRSpec.

We study the kinematics of the interstellar medium (ISM) viewed “down the barrel” in 20 gravitationally lensed galaxies during cosmic noon ( z = 1.5–3.5). We use moderate-resolution spectra ( R ∼ 4000) from Keck’s Echellette Spectrograph and Imager and Magellan/MagE to spectrally resolve the ISM absorption in these galaxies into ∼10 independent elements and use double Gaussian fits to quantify the velocity structure of the gas. We find that the bulk motion of gas in this galaxy sample is outflowing, with average velocity centroid $\left\langle {v}_{\mathrm{cent}}\right\rangle =-141$ km s ^−1 (±111 km s ^−1 scatter) measured with respect to the systemic redshift. A total of 16 out of the 20 galaxies exhibit a clear positive skewness, with a blueshifted tail extending to ∼ −500 km s ^−1 . We examine scaling relations in outflow velocities with galaxy stellar mass and star formation rate, finding correlations consistent with a momentum-driven wind scenario. Our measured outflow velocities are also comparable to those reported for FIRE-2 and TNG50 cosmological simulations at similar redshift and galaxy properties. We also consider implications for interpreting results from lower-resolution spectra. We demonstrate that while velocity centroids are accurately recovered, the skewness, velocity width, and probes of high-velocity gas (e.g., v _95 ) are subject to large scatter and biases at lower resolution. We find that R ≳ 1700 is required for accurate results for the gas kinematics of our sample. This work represents the largest available sample of well-resolved outflow velocity structure at z > 2 and highlights the need for good spectral resolution to recover accurate properties.

The 100% detection rate of Ly α emission in a sample of four luminous z ∼ 8 galaxies with red Spitzer/IRAC colors suggests objects with unusual ionizing capabilities that created early ionized bubbles in a neutral era. Whether such bubbles reflect enhanced ionizing properties (nature) or an overdense environment (nurture), however, remains unclear. Here we aim to distinguish between these hypotheses via a search for Ly α emission in five fainter galaxies drawn from the CANDELS-GOODS fields using a similar IRAC excess and UV magnitudes that should reflect reduced clustering effects. Using Keck/MOSFIRE we tentatively detect >4 σ line emission in only two targets at redshifts z _Ly _α = 7.1081 and 7.9622 with rest-frame EWs of 16–17 Å, ∼1.5× weaker compared to their brighter counterparts. Thus, we find a reduced rate for Ly α emission of ${0.40}_{-0.25}^{+0.30}$ compared to ${1.00}_{-0.44}^{+0.00}$ for more luminous examples. The lower rate agrees with predictions from simulations of a mostly neutral intergalactic medium and an intrinsic EW _0,Ly _α distribution for z ∼ 6 galaxies. However, even with an extreme EW _0,Ly _α model, it is challenging to match the detection rate for the luminous objects. Spectral energy distribution fitting of our fainter sample indicates young and star-forming systems, albeit with less extreme star formation rates and ionization parameters compared to their luminous counterparts. The enhanced Ly α rate in luminous galaxies is thus likely a byproduct of both extreme ionizing properties as well as environmental effects. Further studies with JWST may be required to resolve the physical nature of this puzzling population.

We present measurements of the CO luminosity functions (LFs) and the evolution of the cosmic molecular gas density out to z ∼ 6 based on an 8.5 arcmin ^2 spectral scan survey at 3 mm of the iconic Hubble Deep Field North (HDF-N) observed with the NOrthern Extended Millimeter Array (NOEMA). We use matched filtering to search for line emission from galaxies and determine their redshift probability distributions exploiting the extensive multiwavelength data for the HDF-N. We identify the seven highest-fidelity sources as CO emitters at 1 < z < 6, including the well-known submillimeter galaxy HDF 850.1 at z = 5.18. Four high-fidelity 3 mm continuum sources are found to be radio galaxies at z ≤ 1, plus HDF 850.1. We constrain the CO LFs in the HDF-N out to z ∼ 6, including a first measurement of the CO(5–4) LF at 〈 z 〉 = 5.0. The relatively large area and depth of the NOEMA HDF-N survey extends the existing LFs at 1 < z < 4 above the knee, yielding a somewhat lower density by 0.15–0.4 dex at the overlap region for the CO(2–1) and CO(3–2) transitions, attributed to cosmic variance. We perform a joint analysis of the CO LFs in the HDF-N and Hubble Ultra Deep Field from ASPECS, finding that they can be well described by a single Schechter function. The evolution of the cosmic molecular gas density from a joint analysis is in good agreement with earlier determinations. This implies that the impact of cosmic field-to-field variance on the measurements is consistent with previous estimates, adding to the challenges for simulations that model galaxies from first principles.