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In a multiwavelength survey of 13 quasars at 5.8 ≲ z ≲ 6.5, which were preselected to be potentially young, we find five objects with extremely small proximity zone sizes that may imply UV-luminous quasar lifetimes of ≲100,000 yr. Proximity zones are regions of enhanced transmitted flux in the vicinity of quasars that are sensitive to the quasars' lifetimes because the intergalactic gas has a finite response time to their radiation. We combine submillimeter observations from the Atacama Large Millimetre Array and the NOrthern Extended Millimeter Array, as well as deep optical and near-infrared spectra from the medium-resolution spectrograph on the Very Large Telescope and on the Keck telescopes, in order to identify and characterize these new young quasars, which provide valuable clues about the accretion behavior of supermassive black holes in the early universe and pose challenges on current black hole formation models to explain the rapid formation of billion-solar-mass black holes. We measure the quasars' systemic redshifts, black hole masses, Eddington ratios, emission-line luminosities, and star formation rates of their host galaxies. Combined with previous results, we estimate the fraction of young objects within the high-redshift quasar population at large to be 5% ≲ f young ≲ 10%. One of the young objects, PSO J158-14, shows a very bright dust continuum flux (F cont = 3.46 0.02 mJy), indicating a highly starbursting host galaxy with a star formation rate of approximately 1420 M o˙ yr-1.

The amplitude of the ionizing background that pervades the intergalactic medium (IGM) at the end of the epoch of reionization provides a valuable constraint on the emissivity of the sources that reionized the universe. While measurements of the ionizing background at lower redshifts rely on a simulation-calibrated mapping between the photoionization rate and the mean transmission of the Lyα forest, at z 6 the IGM becomes increasingly opaque and transmission arises solely in narrow spikes separated by saturated Gunn-Peterson troughs. In this regime, the traditional approach of measuring the average transmission over large ∼50 Mpc/h regions is less sensitive and suboptimal. In addition, the five times smaller oscillator strength of the Lyβ transition implies that the Lyβ forest is considerably more transparent at z6, even in the presence of contamination by foreground z ∼ 5 Lyα forest absorption. In this work we present a novel statistical approach to analyze the joint distribution of transmission spikes in the cospatial z ∼ 6 Lyα and Lyβ forests. Our method relies on approximate Bayesian computation (ABC), which circumvents the necessity of computing the intractable likelihood function describing the highly correlated Lyα and Lyβ transmission. We apply ABC to mock data generated from a large-volume hydrodynamical simulation combined with a state-of-the-art model of ionizing background fluctuations in the post-reionization IGM and show that it is sensitive to higher IGM neutral hydrogen fractions than previous techniques. As a proof of concept, we apply this methodology to a real spectrum of a z = 6.54 quasar and measure the ionizing background from 5.4 ≤ z ≤ 6.4 along this sightline with ∼0.2 dex statistical uncertainties.

We present a new Bayesian algorithm making use of Markov Chain Monte Carlo sampling that allows us to simultaneously estimate the unknown continuum level of each quasar in an ensemble of high-resolution spectra, as well as their common probability distribution function (PDF) for the transmitted Lyα forest flux. This fully automated PDF regulated continuum fitting method models the unknown quasar continuum with a linear principal component analysis (PCA) basis, with the PCA coefficients treated as nuisance parameters. The method allows one to estimate parameters governing the thermal state of the intergalactic medium (IGM), such as the slope of the temperature-density relation γ -1, while marginalizing out continuum uncertainties in a fully Bayesian way. Using realistic mock quasar spectra created from a simplified semi-numerical model of the IGM, we show that this method recovers the underlying quasar continua to a precision of ≃7% and ≃10% at z = 3 and z = 5, respectively. Given the number of principal component spectra, this is comparable to the underlying accuracy of the PCA model itself. Most importantly, we show that we can achieve a nearly unbiased estimate of the slope γ - 1 of the IGM temperature-density relation with a precision of ± 8.6% at z = 3 and ± 6.1% at z = 5, for an ensemble of ten mock high-resolution quasar spectra. Applying this method to real quasar spectra and comparing to a more realistic IGM model from hydrodynamical simulations would enable precise measurements of the thermal and cosmological parameters governing the IGM, albeit with somewhat larger uncertainties, given the increased flexibility of the model.

We study quasar proximity zones in the redshift range 5.77 ≤ z ≤ 6.54 by homogeneously analyzing 34 mediumresolution spectra, encompassing both archival and newly obtained data, and exploiting recently updated systemic redshift and magnitude measurements. Whereas previous studies found strong evolution of proximity zone sizes with redshift and argued that this provides evidence for a rapidly evolving intergalactic medium (IGM) neutral fraction during reionization, we measure a much shallower trend ∝(1 + z)-1.44. We compare our measured proximity zone sizes to predictions from hydrodynamical simulations post-processed with one-dimensional radiative transfer and find good agreement between observations and theory, irrespective of the ionization state of the ambient IGM. This insensitivity to IGM ionization state has been previously noted, and results from the fact that the definition of proximity zone size as the first drop of the smoothed quasar spectrum below the 10% flux transmission level probes locations where the ionizing radiation from the quasar is an order of magnitude larger than the expected ultraviolet ionizing background that sets the neutral fraction of the IGM. Our analysis also uncovered three objects with exceptionally small proximity zones (two have Rp < 1 proper Mpc), which constitute outliers from the observed distribution and are challenging to explain with our radiative transfer simulations. We consider various explanations for their origin, such as strong absorption line systems associated with the quasar or patchy reionization, but find that the most compelling scenario is that these quasars have been shining for ≲105 years.

We present the first observations of foreground Lyα forest absorption from high-redshift galaxies, targeting 24 star-forming galaxies (SFGs) with z ∼ 2.3-2.8 within a 5′ × 14′ region of the COSMOS field. The transverse sightline separation is ∼2 h-1 Mpc comoving, allowing us to create a tomographic reconstruction of the three-dimensional (3D) Lyα forest absorption field over the redshift range 2.20 ≤ z ≤ 2.45. The resulting map covers 6 h-1 Mpc × 14 h-1 Mpc in the transverse plane and 230 h-1 Mpc along the line of sight with a spatial resolution of ≈3.5 h-1 Mpc, and is the first high-fidelity map of a large-scale structure on ∼ Mpc scales at z > 2. Our map reveals significant structures with ≳10 h-1 Mpc extent, including several spanning the entire transverse breadth, providing qualitative evidence for the filamentary structures predicted to exist in the high-redshift cosmic web. Simulated reconstructions with the same sightline sampling, spectral resolution, and signal-to-noise ratio recover the salient structures present in the underlying 3D absorption fields. Using data from other surveys, we identified 18 galaxies with known redshifts coeval with our map volume, enabling a direct comparison with our tomographic map. This shows that galaxies preferentially occupy high-density regions, in qualitative agreement with the same comparison applied to simulations. Our results establish the feasibility of the CLAMATO survey, which aims to obtain Lyα forest spectra for ∼1000 SFGs over ∼1deg2 of the COSMOS field, in order to map out the intergalactic medium large-scale structure at 〈z〉 ∼ 2.3 over a large volume (100 h-1 Mpc)3.

The detection of starlight from the host galaxies of quasars during the reionization epoch (z > 6) has been elusive, even with deep Hubble Space Telescope observations 1,2 . The current highest redshift quasar host detected 3 , at z = 4.5, required the magnifying effect of a foreground lensing galaxy. Low-luminosity quasars 4-6 from the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP) 7 mitigate the challenge of detecting their underlying, previously undetected host galaxies. Here we report rest-frame optical images and spectroscopy of two HSC-SSP quasars at z > 6 with the JWST. Using near-infrared camera imaging at 3.6 and 1.5 μm and subtracting the light from the unresolved quasars, we find that the host galaxies are massive (stellar masses of 13 × and 3.4 × 10 10  M ☉ , respectively), compact and disc-like. Near-infrared spectroscopy at medium resolution shows stellar absorption lines in the more massive quasar, confirming the detection of the host. Velocity-broadened gas in the vicinity of these quasars enables measurements of their black hole masses (1.4 × 10 9 and 2.0 × 10 8  M ☉ , respectively). Their location in the black hole mass-stellar mass plane is consistent with the distribution at low redshift, suggesting that the relation between black holes and their host galaxies was already in place less than a billion years after the Big Bang., (© 2023. The Author(s), under exclusive licence to Springer Nature Limited.)

The physical conditions of the circumgalactic medium are investigated by means of intervening absorption-line systems in the spectrum of background quasi-stellar objects (QSOs) out to the epoch of cosmic reionization 1-4 . A correlation between the ionization state of the absorbing gas and the nature of the nearby galaxies has been suggested by the sources detected in either Lyα or [C II] 158 μm near to, respectively, highly ionized and neutral absorbers 5,6 . This is also probably linked to the global changes in the incidence of absorption systems of different types and the process of cosmic reionization 7-12 . Here we report the detection of two [C II]-emitting galaxies at redshift z ≈ 5.7 that are associated with a complex, high-ionization C IV absorption system. These objects are part of an overdensity of galaxies and have compact sizes (<2.4 kpc) and narrow linewidths (full width at half maximum (FWHM) ≈ 62-64 km s -1 ). Hydrodynamic simulations predict that similar narrow [C II] emission may arise from the heating of small (≲3 kpc) clumps of cold neutral medium or a compact photodissociation region 13,14 . The lack of counterparts in the rest-frame ultraviolet (UV) indicates severe obscuration of the sources that are exciting the [C II] emission. These results may suggest a connection between the properties of the [C II] emission, the rare overdensity of galaxies and the unusual high ionization state of the gas in this region., (© 2023. The Author(s), under exclusive licence to Springer Nature Limited.)

Planets with short orbital periods (roughly under 10 days) are common around stars like the Sun 1,2 . Stars expand as they evolve and thus we expect their close planetary companions to be engulfed, possibly powering luminous mass ejections from the host star 3-5 . However, this phase has never been directly observed. Here we report observations of ZTF SLRN-2020, a short-lived optical outburst in the Galactic disk accompanied by bright and long-lived infrared emission. The resulting light curve and spectra share striking similarities with those of red novae 6,7 -a class of eruptions now confirmed 8 to arise from mergers of binary stars. Its exceptionally low optical luminosity (approximately 10 35  erg s -1 ) and radiated energy (approximately 6.5 × 10 41  erg) point to the engulfment of a planet of fewer than roughly ten Jupiter masses by its Sun-like host star. We estimate the Galactic rate of such subluminous red novae to be roughly between 0.1 and several per year. Future Galactic plane surveys should routinely identify these, showing the demographics of planetary engulfment and the ultimate fate of planets in the inner Solar System., (© 2023. The Author(s), under exclusive licence to Springer Nature Limited.)

Bright quasars, powered by accretion onto billion-solar-mass black holes, already existed at the epoch of reionization, when the Universe was 0.5-1 billion years old 1 . How these black holes formed in such a short time is the subject of debate, particularly as they lie above the correlation between black-hole mass and galaxy dynamical mass 2,3 in the local Universe. What slowed down black-hole growth, leading towards the symbiotic growth observed in the local Universe, and when this process started, has hitherto not been known, although black-hole feedback is a likely driver 4 . Here we report optical and near-infrared observations of a sample of quasars at redshifts 5.8 ≲ z ≲ 6.6. About half of the quasar spectra reveal broad, blueshifted absorption line troughs, tracing black-hole-driven winds with extreme outflow velocities, up to 17% of the speed of light. The fraction of quasars with such outflow winds at z ≳ 5.8 is ≈2.4 times higher than at z ≈ 2-4. We infer that outflows at z ≳ 5.8 inject large amounts of energy into the interstellar medium and suppress nuclear gas accretion, slowing down black-hole growth. The outflow phase may then mark the beginning of substantial black-hole feedback. The red optical colours of outflow quasars at z ≳ 5.8 indeed suggest that these systems are dusty and may be caught during an initial quenching phase of obscured accretion 5 ., (© 2022. The Author(s), under exclusive licence to Springer Nature Limited.)