High-redshift Quasars Research Articles

Photometric redshift estimation of BASS DR3 quasars by machine learning

ABSTRACT Correlating Beijing–Arizona Sky Survey (BASS) data release 3 (DR3) catalogue with the ALLWISE data base, the data from optical and infrared information are obtained. The quasars from Sloan Digital Sky Survey are taken as training and test samples while those from LAMOST are considered as external test sample. We propose two schemes to construct the redshift estimation models with XGBoost, CatBoost, and Random Forest. One scheme (namely one-step model) is to predict photometric redshifts directly based on the optimal models created by these three algorithms; the other scheme (namely two-step model) is to first classify the data into low- and high-redshift data sets, and then predict photometric redshifts of these two data sets separately. For one-step model, the performance of these three algorithms on photometric redshift estimation is compared with different training samples, and CatBoost is superior to XGBoost and Random Forest. For two-step model, the performances of these three algorithms on the classification of low and high redshift subsamples are compared, and CatBoost still shows the best performance. Therefore, CatBoost is regarded as the core algorithm of classification and regression in two-step model. In contrast to one-step model, two-step model is optimal when predicting photometric redshift of quasars, especially for high-redshift quasars. Finally, the two models are applied to predict photometric redshifts of all quasar candidates of BASS DR3. The number of high-redshift quasar candidates is 3938 (redshift ≥3.5) and 121 (redshift ≥4.5) by two-step model. The predicted result will be helpful for quasar research and follow-up observation of high-redshift quasars.

Estimating the Effective Lifetime of the z ∼ 6 Quasar Population from the Composite Proximity Zone Profile

Abstract The lifetime of quasars can be estimated by means of their proximity zone sizes, which are regions of enhanced flux blueward of the Lyα emission line observed in the rest-frame UV spectra of high-redshift quasars because the intergalactic gas has a finite response time to the quasars’ radiation. We estimate the effective lifetime of the high-redshift quasar population from the composite transmitted flux profile within the proximity zone region of a sample of 15 quasars at 5.8 ≤ z ≤ 6.6 with precise systemic redshifts, and similar luminosities, i.e., −27.6 ≤ M 1450 ≤ −26.4, and thus a similar instantaneous ionizing power. We develop a Bayesian method to infer the effective lifetime from the composite spectrum, including robust estimates of various sources of uncertainty on the spectrum. We estimate an effective lifetime of the quasar population as a whole of log 10 ( t Q / yr ) = 5.7 − 0.3 ( − 0.5 ) + 0.5 ( + 0.8 ) given by the median and 68th (95th) percentile of the posterior probability distribution. While our result is consistent with previous quasar lifetime studies, it poses significant challenges on the current model for the growth of supermassive black holes located in the center of the quasars’ host galaxies, which requires that quasar lifetimes are more than an order of magnitude longer.

The search for the farthest quasar: consequences for black hole growth and seed models

ABSTRACT The quest for high-redshift quasars has led to a series of record-breaking sources, with the current record holder at z = 7.642. Here, we show how future detections of z > 8 quasars impact the constraints on the parameters for black hole growth and seed models. Using broad flat priors on the growth parameters (Eddington ratio $\, {f_{\rm Edd}}$, duty cycle ${\cal D}$, seed mass M•,seed, and radiative efficiency ϵ), we show that the large uncertainties in their determination decrease by a factor ∼5 when a quasar’s detection redshift goes from z = 9 to z = 12. In this high-redshift regime, ϵ tends to the lowest value allowed, and the distribution for M•,seed peaks well inside the heavy seed domain. Remarkably, two quasars detected at z > 7 with low accretion rates (J1243+0100 and J0313–1806) already tighten the available parameter space, requiring $M_{\rm \bullet , seed} \gt 10^{3.5} \, {\rm M_\odot }$ and ϵ < 0.1. The radiative efficiency is a crucial unknown, with factor ∼2 changes able to modify the predicted mass by ∼3 orders of magnitude already at z ∼ 9. The competing roles of inefficient accretion (decreasing ϵ) and black hole spin-up (increasing ϵ) significantly impact growth models. Finally, we suggest that yields currently predicted by upcoming quasar surveys (e.g. Euclid) will be instrumental for determining the most-likely seed mass regime. For example, assuming thin-disc accretion, a detection of a quasar with $M_\bullet \sim 10^{10} \, {\rm M_\odot }$ by z ∼ 9–10 would exclude the entire parameter space available for light seeds and dramatically reduce the one for heavy seeds.

High-redshift quasars at z ≥ 3 – I. Radio spectra

ABSTRACT We present the radio properties of optically selected quasars with z ≥ 3. The complete sample consists of 102 quasars with a flux density level S1.4 ≥ 100 mJy in a declination range −35° ≤ Dec. ≤ +49°. The observations were obtained in 2017–2020 using the radio telescope RATAN-600. We measured flux densities at six frequencies 1.2, 2.3, 4.7, 8.2, 11.2, and 22 GHz quasi-simultaneously with uncertainties of 9–31 per cent. The detection rate is 100, 89, and 46 per cent at 4.7, 11.2, and 22 GHz, respectively. We have analysed the averaged radio spectra of the quasars based on the RATAN and literature data. We classify 46 per cent of radio spectra as peaked-spectrum, 24 per cent as flat, and none as ultra-steep spectra (α ≤ −1.1). The multifrequency data reveal that a peaked spectral shape (PS) is a common feature for bright high-redshift quasars. This indicates the dominance of bright compact core emission and the insignificant contribution of extended optically thin kpc-scale components in observed radio spectra. Using these new radio data, the radio loudness log R was estimated for 71 objects with a median value of 3.5, showing that the majority of the quasars are highly radio-loud with log R > 2.5. We have not found any significant correlation between z and α. Several new megahertz- peaked spectrum (MPS) and gigahertz- peaked spectrum (GPS) candidates are suggested. Further studies of their variability and additional low-frequency observations are needed to classify them precisely.

Kinematics and star formation of high-redshift hot dust-obscured quasars as seen by ALMA

Hot, dust-obscured galaxies (Hot DOGs) are a population of hyper-luminous obscured quasars identified by WISE. We present ALMA observations of the [C II] 158 μm fine-structure line and underlying dust continuum emission in a sample of seven of the most extremely luminous (EL; Lbol ≥ 1014 L⊙) Hot DOGs, at redshifts of z ≃ 3.0−4.6. The [C II] line is robustly detected in four objects, tentatively in one, and likely to have been red-shifted out of the spectral window in the remaining two, based on additional data. On average, [C II] is red-shifted by ≃780 km s−1 from rest-frame ultraviolet emission lines. EL Hot DOGs consistently exhibit very high [C II] surface densities, with Σ[CII] ≃ 1−2 × 109 L⊙ kpc−2, which is as high as the most extreme cases seen in other high-redshift quasars. As a population, EL Hot DOG hosts seem to be roughly centered on the main sequence of star-forming galaxies, but the uncertainties are substantial and individual sources can fall above and below. The average, intrinsic [C II] and dust continuum sizes (FWHMs) are ≃2.1 kpc and ≃1.6 kpc, respectively, with a very narrow range of line-to-continuum size ratios, 1.61 ± 0.10, suggesting they could be linearly proportional. The [C II] velocity fields of EL Hot DOGs are diverse: from barely rotating structures, to resolved hosts with ordered, circular motions, to complex, disturbed systems that are likely the result of ongoing mergers. In contrast, all sources display large line-velocity dispersions, FWHM[CII] ≳ 500 km s−1, which are, on average, larger than optically and IR-selected quasars at similar or higher redshifts. We argue that one possible hypothesis that explains the lack of a common velocity structure, the systematically large dispersion of the ionized gas, and the presence of nearby companion galaxies, may be that the EL Hot DOG phase could be recurrent, rather than a single event. The dynamical friction from the frequent in-fall of neighbor galaxies and gas clumps, along with the subsequent quasar feedback, would contribute to the high turbulence of the gas within the host in a process that could potentially trigger not only one continuous EL, obscured event –but instead a number of recurrent, shorter-lived episodes as long as external accretion continues.

The Impact of Powerful Jets on the Far-infrared Emission of an Extreme Radio Quasar at z ∼ 6

Abstract The interactions between radio jets and the interstellar medium play a defining role for the coevolution of central supermassive black holes and their host galaxies, but observational constraints on these feedback processes are still very limited at redshifts z > 2. We investigate the radio-loud quasar PSO J352.4034–15.3373 at z ∼ 6 at the edge of the Epoch of Reionization. This quasar is among the most powerful radio emitters and the first one with direct evidence of extended radio jets (∼1.6 kpc) at these high redshifts. We analyze NOrthern Extended Millimeter Array and Atacama Large Millimeter/submillimeter Array millimeter data targeting the CO (6–5) and [C ii] far-infrared (FIR) emission lines, respectively, and the underlying continuum. The broad 440 ± 80 km s−1 and marginally resolved [C ii] emission line yields a systemic redshift of z = 5.832 ± 0.001. Additionally, we report a strong 215 MHz radio continuum detection, 88 ± 7 mJy, using the Giant Metrewave Radio Telescope. This measurement significantly improves the constraints at the low-frequency end of the spectral energy distribution of this quasar. In contrast to what is typically observed in high-redshift radio-quiet quasars, we show that cold dust emission alone cannot reproduce the millimeter continuum measurements. This is evidence that the strong synchrotron emission from the quasar contributes substantially to the emission even at millimeter (FIR in the rest-frame) wavelengths. This quasar is an ideal system to probe the effects of radio jets during the formation of a massive galaxy within the first gigayear of the universe.

Quasar clustering at redshift 6

Context. Large-scale surveys over the last years have revealed about 300 quasi-stellar objects (QSOs) at redshifts above 6. Follow-up observations have identified surprising properties, such as the very high black hole (BH) masses, spatial correlations with surrounding cold gas of the host galaxy, and high CIV-MgII Velocity shifts. In particular, the discovery of luminous high-redshift quasars suggests that at least some BHs likely have high masses at birth and grow efficiently. Aims. Our aim is to quantify quasar pairs at high redshift for a large sample of objects. This provides a new key constraint on a combination of parameters related to the origin and assembly for the most massive BHs: formation efficiency and clustering, growth efficiency, and the relative contribution of BH mergers. Methods. We observed 116 spectroscopically confirmed QSOs around redshift 6 with the simultaneous seven-channel imager Gamma-ray Burst Optical/Near-infrared Detector in order to search for companions. Applying colour-colour cuts identical to those which led to the spectroscopically confirmed QSOs, we performed Le PHARE fits to the 26 best QSO pair candidates, and obtained spectroscopic observations for 11 of them. Results. We do not find any QSO pair with a companion brighter than M1450(AB) < −26 mag within our 0.1–3.3 h−1 cMpc search radius, in contrast to the serendipitous findings in the redshift range 4–5. However, a small fraction of such pairs at this luminosity and redshift is consistent with indications from present-day cosmological-scale galaxy evolution models. In turn, the incidence of L- and T-type brown dwarfs, which occupy a similar colour space to z ∼ 6 QSOs, is higher than expected, by a factor of 5 and 20, respectively.

The OBELISK simulation: Galaxies contribute more than AGN to H I reionization of protoclusters

We present the OBELISK project, a cosmological radiation-hydrodynamics simulation that follows the assembly and reionization of a protocluster progenitor during the first two billion years after the big bang, down to z = 3.5. The simulation resolves haloes down to the atomic cooling limit and tracks the contribution of different sources of ionization: stars, active galactic nuclei, and collisions. The OBELISK project is specifically designed to study the coevolution of high-redshift galaxies and quasars in an environment favouring black hole growth. In this paper, we establish the relative contribution of these two sources of radiation to reionization and their respective role in establishing and maintaining the high-redshift ionizing background. Our volume is typical of an overdense region of the Universe and displays star formation rate and black hole accretion rate densities similar to those of high-redshift protoclusters. We find that hydrogen reionization happens inside-out, is completed by z ∼ 6 in our overdensity, and is predominantly driven by galaxies, while accreting black holes only play a role at z ∼ 4.

Imaging Sources in the Third Realization of the International Celestial Reference Frame

Abstract The third iteration of the International Celestial Reference Frame (ICRF3) is made up of 4536 quasars observed at S/X bands using very-long-baseline interferometry. These sources are high-redshift quasars, typically between 1 < z < 2, that are believed to host active galactic nuclei at their centers. The position of compact radio sources can be determined better than sources with large amounts of extended radio structure. Here we report information on a series of 20 observations from 2017 January through December 2017 that were designed for precise astrometry and to monitor the structure of sources included in the ICRF3. We targeted 3627 sources over the one year campaign and found the median flux density of 2697 detected sources at S band is 0.13 Jy, and the median flux density of 3209 sources detected at X band is 0.09 Jy. We find that 70% of detected sources in our campaign are considered compact at X band and ideal for use in the ICRF and 89% of the 2615 sources detected at both frequencies have a flat spectral index, α > 0.5

Detecting and Characterizing Young Quasars. II. Four Quasars at z ∼ 6 with Lifetimes < 104 Yr

Abstract The extents of proximity zones of high-redshift quasars enable constraints on the timescales of quasar activity, which are fundamental for understanding the growth of the supermassive black holes (SMBHs) that power the quasars’ emission. In this study, we obtain precise estimates for the ultraviolet (UV) luminous lifetimes of 10 quasars at 5.8 < z < 6.5. These objects were preselected to have short lifetimes based on preliminary measurements of their proximity zone sizes and were then targeted for high-quality follow-up submillimeter, optical, and infrared observations required to increase the measurements’ precision and securely determine their lifetimes. By comparing these proximity zone sizes to mock quasar spectra generated from radiative transfer simulations at a range of different lifetimes, we deduce extremely short lifetimes t Q < 104 yr for four objects in our sample, whereas the remaining quasars are consistent with longer lifetimes of t Q ≳ 105 yr. These young objects with small proximity zones represent ≲10% of the quasar population as a whole. We compare our results in detail to other studies on timescales of quasar activity, which point toward an average lifetime of t Q ∼ 106 yr for the quasar population. This is consistent with finding newly turned-on quasars approximately ∼1%–10% of the time. These young quasars represent a unique opportunity to study triggering and feedback mechanisms of SMBHs, since the onset of their UV-luminous quasar phase happened only recently, and therefore traces of this process might still be observable.

Evolution of High-redshift Quasar Hosts and Promotion of Massive Black Hole Seed Formation

Abstract High-redshift luminous quasars powered by accreting supermassive black holes (SMBHs) with mass ≳109 M ⊙ constrain their formation pathways. We investigate the formation of heavy seeds of SMBHs through gas collapse in the quasar host progenitors, using merger trees to trace the halo growth in highly biased, overdense regions of the universe. The progenitor halos are likely irradiated by intense H2-photodissociating radiation from nearby star-forming galaxies and heat the interior gas by successive mergers. The kinetic energy of the gas originating from mergers, as well as the baryonic streaming motion, prevents gas collapse and delays prior star formation. With a streaming velocity higher than the rms value, gas clouds in nearly all 104 realizations of merger trees enter the atomic-cooling stage and begin to collapse isothermally with T ≃ 8000 K via Lyα cooling. The fraction of trees that host isothermal gas collapse is 14% and increases with streaming velocity, while the rest form H2-cooled cores after short isothermal phases. If the collapsing gas is enriched to Z crit ∼ 2 × 10−3 Z ⊙, requiring efficient metal mixing, this fraction could be reduced by additional cooling via metal fine-structure lines. In the massive collapsing gas, the accretion rate onto a newly born protostar ranges between 3 × 10−3 M ⊙ yr−1 and 5 M ⊙ yr−1, among which a large fraction exceeds the critical rate suppressing stellar radiative feedback. As a result, we expect a distribution of stellar mass (presumably BH mass) ranging from several hundred to above 105 M ⊙, potentially forming massive BH binary mergers and yielding gravitational-wave events.

Random Forests as a Viable Method to Select and Discover High-redshift Quasars

Abstract We present a method of selecting quasars up to redshift ≈6 with random forests, a supervised machine-learning method, applied to Pan-STARRS1 and WISE data. We find that, thanks to the increasing set of known quasars, we can assemble a training set that enables supervised machine-learning algorithms to become a competitive alternative to other methods up to this redshift. We present a candidate set for the redshift range 4.8–6.3, which includes the region around z = 5.5 where selecting quasars is difficult due to their photometric similarity to red and brown dwarfs. We demonstrate that, under our survey restrictions, we can reach a high completeness (66% ± 7% below redshift 5.6/ 83 − 9 + 6 % above redshift 5.6) while maintaining a high selection efficiency ( 78 − 8 + 10 % / 94 − 8 + 5 % ). Our selection efficiency is estimated via a novel method based on the different distributions of quasars and contaminants on the sky. The final catalog of 515 candidates includes 225 known quasars. We predict the candidate catalog to contain additional 148 − 33 + 41 new quasars below redshift 5.6 and 45 − 8 + 5 above, and we make the catalog publicly available. Spectroscopic follow-up observations of 37 candidates led us to discover 20 new high redshift quasars (18 at 4.6 ≤ z ≤ 5.5, 2 z ∼ 5.7). These observations are consistent with our predictions on efficiency. We argue that random forests can lead to higher completeness because our candidate set contains a number of objects that would be rejected by common color cuts, including one of the newly discovered redshift 5.7 quasars.

Host galaxies of high-redshift quasars: SMBH growth and feedback

ABSTRACT The properties of quasar-host galaxies might be determined by the growth and feedback of their supermassive black holes (SMBHs, 108−10 M⊙). We investigate such connection with a suite of cosmological simulations of massive (halo mass ≈1012 M⊙) galaxies at z ≃ 6 that include a detailed subgrid multiphase gas and accretion model. BH seeds of initial mass 105 M⊙ grow mostly by gas accretion, and become SMBH by z = 6 setting on the observed MBH−M⋆ relation without the need for a boost factor. Although quasar feedback crucially controls the SMBH growth, its impact on the properties of the host galaxy at z = 6 is negligible. In our model, quasar activity can both quench (via gas heating) or enhance (by interstellar medium overpressurization) star formation. However, we find that the star formation history is insensitive to such modulation as it is largely dominated, at least at z > 6, by cold gas accretion from the environment that cannot be hindered by the quasar energy deposition. Although quasar-driven outflows can achieve velocities $\gt 1000~\rm km~s^{-1}$, only ≈4 per cent of the outflowing gas mass can actually escape from the host galaxy. These findings are only loosely constrained by available data, but can guide observational campaigns searching for signatures of quasar feedback in early galaxies.

Taking a Long Look: A Two-decade Reverberation Mapping Study of High-luminosity Quasars

Abstract Reverberation mapping (RM) of active galactic nuclei (AGNs) has been used over the past three decades to determine AGN broad-line region (BLR) sizes and central black hole masses, and their relations with the AGN luminosity. Until recently, the sample of objects with RM data was limited to low-luminosity AGNs (L opt ≲ 1046 erg s−1) and low redshifts (z ≲ 0.5). Here we present results from an RM project of some of the most luminous and highest-redshift quasars that have been mapped to date. The study is based on almost 20 years of photometric monitoring of 11 quasars, 6 of which were monitored spectrophotometrically for 13 yr. This is the longest RM project carried out so far on this type of AGNs. We successfully measure a time lag between the C iv λ 1549 broad emission line and the quasar continuum in three objects, and measure a C iii] λ 1909 lag in one quasar. Together with recently published data on C iv RM, the BLR size is found to scale as the square root of the UV luminosity over eight orders of magnitude in AGN luminosity. There is a significant scatter in the relation, part of which may be intrinsic to the AGNs. Although the C iv line is probably less well suited than Balmer lines for determination of the mass of the black hole, virial masses are tentatively computed, and in spite of a large scatter, we find that the mass of the black hole scales as the square root of the UV luminosity.

Observing the host galaxies of high-redshift quasars with JWST: predictions from the BlueTides simulation

ABSTRACT The bright emission from high-redshift quasars completely conceals their host galaxies in the rest-frame ultraviolet/optical, with detection of the hosts in these wavelengths eluding even the Hubble Space Telescope (HST) using detailed point spread function (PSF) modelling techniques. In this study, we produce mock images of a sample of z = 7 quasars extracted from the BlueTides simulation, and apply Markov chain Monte Carlo-based PSF modelling to determine the detectability of their host galaxies with the James Webb Space Telescope (JWST). While no statistically significant detections are made with HST, we predict that at the same wavelengths and exposure times JWST NIRCam imaging will detect $\sim 50{{\ \rm per\ cent}}$ of quasar host galaxies. We investigate various observational strategies, and find that NIRCam wide-band imaging in the long-wavelength filters results in the highest fraction of successful quasar host detections, detecting $\gtrsim 80{{\ \rm per\ cent}}$ of the hosts of bright quasars in exposure times of 5 ks. Exposure times of ≳ 5 ks are required to detect the majority of host galaxies in the NIRCam wide-band filters, however, even 10 ks exposures with MIRI result in $\lesssim 30{{\ \rm per\ cent}}$ successful host detections. We find no significant trends between galaxy properties and their detectability. The PSF modelling can accurately recover the host magnitudes, radii, and spatial distribution of the larger scale emission, when accounting for the central core being contaminated by residual quasar flux. Care should be made when interpreting the host properties measured using PSF modelling.

The Deviation of the Size of the Broad-line Region between Reverberation Mapping and Spectroastrometry

Abstract The combination of the linear size from reverberation mapping (RM) and the angular distance of the broad-line region (BLR) from spectroastrometry (SA) in active galactic nuclei can be used as a “standard ruler” to measure the Hubble constant H 0. Recently, Wang et al. successfully employed this approach and estimated H 0 from 3C 273. However, there may be a systematic deviation between the response-weighted radius (RM measurement) and luminosity-weighted radius (SA measurement), especially when different broad lines are adopted for size indicators (e.g., Hβ for RM and Paα for SA). Here we evaluate the size deviations measured by six pairs of hydrogen lines (e.g., Hβ, Hα, and Paα) via the locally optimally emitting cloud (LOC) models of the BLR. We find that the radius ratios K (=R SA/R RM) of the same line deviated systematically from 1 (0.85–0.88) with dispersions between 0.063 and 0.083. Surprisingly, the K values from the Paα(SA)/Hβ(RM) and Hα(SA)/Hβ(RM) pairs not only are closest to 1 but also have considerably smaller uncertainty. Considering the current technology of infrared interferometry, the Paα(SA)/Hβ(RM) pair is the ideal choice for low-redshift objects in the SARM project. In the future, the Hα(SA)/Hβ(RM) pair could be used for high-redshift luminous quasars. These theoretical estimations of the SA/RM radius pave the way for future SARM measurements to further constrain the standard cosmological model.

Probing modified gravity theories with multiple measurements of high-redshift quasars

ABSTRACT In this paper, we quantify the ability of multiple measurements of high-redshift quasars to constrain several theories of modified gravity, including the Dvali–Gabadadze–Porrati braneworld scenario, generalized Chaplygin gas, f(T) modified gravity, and modified polytropic Cardassian model. Recently released sample of 1598 quasars with X-ray and ultraviolet flux measurements in the redshift range of 0.036 ≤ z ≤ 5.1003, as well as a compilation of 120 intermediate-luminosity radio quasars covering the redshift of 0.46 < z < 2.76 are respectively used as standard probes at higher redshifts. For all considered modified gravity theories, our results show that there is still some possibility that the standard Λ cold dark matter (ΛCDM) scenario might not be the best cosmological model preferred by the current quasar observations. In order to improve cosmological constraints, the quasar data are also combined with the latest observations of baryon acoustic oscillations, which strongly complement the constraints. Finally, we discuss the support given by the data to modified gravity theories, applying different information theoretic techniques like the Akaike information criterion, Bayesian information criterion, and Jensen–Shannon divergence.

NuSTAR reveals the hidden nature of SS433

ABSTRACT SS433 is the only Galactic binary system known to persistently accrete at highly super-critical (or hyper-critical) rates, similar to those in tidal disruption events, and likely needed to explain the rapid growth of those very high redshift quasars containing massive SMBHs. Probing the inner regions of SS433 in the X-rays is crucial to understanding this system, and super-critical accretion in general, but is highly challenging due to obscuration by the surrounding wind, driven from the accretion flow. NuSTAR observed SS433 in the hard X-ray band across multiple phases of its 162 d superorbital precession period. Spectral-timing tools allow us to infer that the hard X-ray emission from the inner regions is likely being scattered towards us by the walls of the wind-cone. By comparing to numerical models, we determine an intrinsic X-ray luminosity of ≥ 2 × 1037 erg s−1 and that, if viewed face on, we would infer an apparent luminosity of >1 × 1039 erg s−1, confirming SS433’s long-suspected nature as an ultraluminous X-ray source (ULX). We present the discovery of a narrow, ∼100 s lag due to atomic processes occurring in outflowing material travelling at least 0.14–0.29c, which matches absorption lines seen in ULXs and – in the future – will allow us to map a super-critical outflow for the first time.

A hidden population of high-redshift double quasars unveiled by astrometry

Galaxy mergers occur frequently in the early universe and bring multiple supermassive black holes (SMBHs) into the nucleus, where they may eventually coalesce. Identifying post-merger-scale (i.e., <~a few kpc) dual SMBHs is a critical pathway to understanding their dynamical evolution and successive mergers. While serendipitously discovering kpc-scale dual SMBHs at z<1 is possible, such systems are elusive at z>2, but critical to constraining the progenitors of SMBH mergers. The redshift z~2 also marks the epoch of peak activity of luminous quasars, hence probing this spatial regime at high redshift is of particular significance in understanding the evolution of quasars. However, given stringent resolution requirements, there is currently no confirmed <10 kpc physical SMBH pair at z>2. Here we report two sub-arcsec double quasars at z>2 discovered from a targeted search with a novel astrometric technique, demonstrating a high success rate (~50%) in this systematic approach. These high-redshift double quasars could be the long-sought kpc-scale dual SMBHs, or sub-arcsec gravitationally-lensed quasar images. One of these double quasars (at z=2.95) was spatially resolved with optical spectroscopy, and slightly favors the scenario of a physical quasar pair with a projected separation of 3.5 kpc (0.46"). Follow-up observations of double quasars discovered by this targeted approach will be able to provide the first observational constraints on kpc-scale dual SMBHs at z>2.

The LOFAR LBA Sky Survey

Context. The LOw Frequency ARray (LOFAR) is the only radio telescope that is presently capable of high-sensitivity, high-resolution (i.e. &lt; 1 mJy beam−1 and &lt; 15″) observations at ultra-low frequencies (&lt; 100 MHz). To utilise these capabilities, the LOFAR Surveys Key Science Project is undertaking a large survey to cover the entire northern sky with Low Band Antenna (LBA) observations. Aims. The LOFAR LBA Sky Survey (LoLSS) aims to cover the entire northern sky with 3170 pointings in the frequency range between 42 − 66 MHz, at a resolution of 15″ and at a sensitivity of 1 mJy beam−1 (1σ). In this work, we outline the survey strategy, the observational status, and the calibration techniques. We also briefly describe several of our scientific motivations and present the preliminary public data release. Methods. The preliminary images were produced using a fully automated pipeline aimed at correcting all direction-independent effects in the data. Whilst the direction-dependent effects, such as those from the ionosphere, have not yet been corrected, the images presented in this work are still ten times more sensitive than previous available surveys at these low frequencies. Results. The preliminary data release covers 740 deg2 around the HETDEX spring field region at an angular resolution of 47″ with a median noise level of 5 mJy beam−1. The images and the catalogue of 25 247 sources have been publicly released. We demonstrate that the system is capable of reaching a root mean square (rms) noise of 1 mJy beam−1 and an angular resolution of 15″ once direction-dependent effects are accounted for. Conclusions. LoLSS will provide the ultra-low-frequency information for hundreds of thousands of radio sources, providing critical spectral information and producing a unique data set that can be used for a wide range of science topics, such as the search for high redshift galaxies and quasars, the study of the magnetosphere of exoplanets, and the detection of the oldest populations of cosmic-rays in galaxies, clusters of galaxies, as well as those produced by active galactic nuclei.