arXiv subjects

Feige Wang

Publications and source records attributed to Feige Wang.

At least 19 recordsLinked to original sources

arXiv2026

The Roman eXtreme Deep Field (RXDF)

The Roman eXtreme Deep Field (RXDF) program is one of the five General Astrophysics Survey (GAS) programs approved for observing time with the Nancy Grace Roman Space Telescope in Cycles 1 and 2. It has been allocated 386.41 hours to carry out an imaging survey to AB = 30 mag (5-sigma) over ~140x larger area than the Hubble eXtreme Deep Field (HXDF) full-depth area (ACS+WFC3/IR). The RXDF will cover the full Roman wavelength range with 7 bands, reaching AB = 30 mag in RZYJH, 29 mag in F, and 28 mag in K, over a full-depth area of 678.75 arcmin^2 embedded in a total area of 1,243 arcmin^2, and far exceeding the depths of the Roman Core Community Surveys (CCS). The RXDF is within the Euclid Ultra Deep Field (EUDF) near the North Ecliptic Pole (NEP), a strategic long-term field for generational space facilities, with a wealth of multi-wavelength data including extensive coverage from the James Webb Space Telescope (JWST) NEXUS Treasury program. The observations will cover 3 epochs at a 1-year cadence, each epoch divided into 3 sub-epochs ~10 days apart, enabling time-domain studies on time baselines from ~10 days to over ~2 years. The RXDF is uniquely positioned to address critical questions in reionization, large scale structure (LSS), growth of supermassive black holes (SMBHs), little red dots (LRDs), and high-z supernovae (SNe); the volumes probed by HST+JWST are too small at these extreme depths, and even the deepest CCS tiers are too shallow. In addition to our key objectives, a wealth of additional science will be enabled by engaging the community with our rapidly released datasets, revolutionizing a wide range of science for a lasting legacy. This short document, which is converted from the approved RXDF proposal, aims to provide the community with a summary of the program.

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arXiv2026

SHELLQs. Black Hole Mass and Eddington Ratio Distributions of Intermediate-Luminosity Quasars at 6<z<7

We present near-infrared spectroscopy of 21 quasars at 6.07 < z < 6.90 obtained with JWST/NIRSpec and Subaru/MOIRCS. These targets have absolute ultraviolet magnitudes of -25 < M_1450 < -22 and are drawn from a sample of z>6 quasars identified in the Subaru High-z Exploration of Low-Luminosity Quasars (SHELLQs) project. These quasars occupy the intermediate-luminosity regime between luminous quasars and the faint high-redshift AGNs uncovered by JWST. We detect broad Balmer emission lines and MgII 2798 with underlying continua from the NIRSpec and MOIRCS targets, respectively. The virial black hole masses of this sample span a wide range of 7.2 < log M_BH/M_sun < 9.4, with Eddington ratios of -1.3 < log L_bol/L_Edd < 0.4. Combining these measurements with our previous mass estimates for other SHELLQs quasars, we construct a sample of 27 quasars with M_BH estimates at 6 < z < 7 and derive the distributions of M_BH and L_bol/L_Edd. When compared with luminosity-matched quasars at z ~ 1.3, we find median offsets of delta log M_BH/M_sun = -0.4 and Delta log L_bol/L_Edd = 0.2 in the high-redshift sample. In addition, 11% of the high-redshift quasars are accreting near or above the Eddington limit. As a systematic check, adopting the Eddington-ratio-dependent calibration of single-epoch M_BH estimates increases the inferred super-Eddington fraction to 22%. These results indicate that a representative population of distant supermassive black holes undergo a rapid accretion phase, although not as extreme as that seen in the most luminous quasar population, consistent with the anti-hierarchical growth scenario of supermassive black holes.

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arXiv2026

NEXUS: Transient Searches and First Results from Year One Observations

We describe ongoing efforts of high-redshift transient searches using multi-epoch NIRCam imaging (F200W+F444W) and NIRSpec MSA/PRISM spectroscopy from the NEXUS JWST multi-cycle Treasury program targeting the north ecliptic pole region. The transient search area covers $\sim 61.5\,{\rm arcmin^2}$ between the reference epoch and each subsequent NEXUS-Deep epoch at a cadence of $\sim2$~months. In the first year of observations from NEXUS, we detect 68 robust transients, with the host photometric redshift distribution declining rapidly at $z>2$ but extending to $z_{\rm phot}\approx 6$. In addition, we obtained secure spectroscopic redshifts for 37 transients ($\sim54\%$) from NIRSpec/PRISM and NIRCam/WFSS, with seventeen at $1 < z < 2$, eight at $2 < z < 3$, two at $3 < z < 4$, and one tentative host association at z = 6.151. Overall, the NEXUS program recovers observed supernova (SN) rates broadly consistent with other SN search programs with JWST. While NEXUS achieves the highest transient detection efficiency, 6.6 SNe per imaging hr ($2.6\times$ COSMOS-SN and $26\times$ JADES-SN), the limited filter coverage (F200W+F444W only) limits robust identification and classification of high-$z$ SNe for deep spectroscopic follow-up. We describe the details of the data reduction and transient detection pipeline, and report the Year-1 transient sample along with their light curves, available MSA spectroscopy, host association, and the raw detection rate. We also describe and release a new PSF photometry package that properly accounts for correlated pixel noise from combining drizzled images.

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arXiv2026

To be lensed or not to be lensed: on the nature of the alleged high-z lensed quasars J0109-5424 and P170+20

We present a detailed multi-wavelength analysis of two gravitationally lensed z>6 quasar candidates: J0109-5424 and P170+20. Using JWST/NIRSpec near-infrared data from the Aether survey, we demonstrate that J0109-5424 is in fact a FeLoBAL quasar at z=2.07, exhibiting strong iron broad absorption lines that mimic the Lyman break. From the Pabeta emission line, we estimate a black hole mass of 3.9 x 10^8 Msun and a bolometric luminosity of Lbol = 7.7 x 10^45 erg/s for this quasar. Follow-up HST/ACS WFC imaging of P170+20 with the F555W filter, probing emission below the Lyman limit of the quasar, reveals a compact source located 0.09+/-0.04 arcsec from the quasar position. PSF modelling and subtraction yield no evidence for extended emission. This is inconsistent with a foreground galaxy acting as a lens. Furthermore, we obtain Gemini-South/Flamingos 2 near-infrared observations to complement the existing optical Gemini-North/GMOS spectrum of P170+20. Our spectral analysis shows that the observed spectral shape of P170+20 is consistent with a composite quasar and M-dwarf spectrum. However, this interpretation remains uncertain, as a chance alignment between an ultracool dwarf and a quasar within 0.1 arcsec would be unlikely. Overall, our findings establish J0109-5424 as an intermediate redshift source and do not favor the lensed nature of P170+20, although this possibility cannot be conclusively ruled out, highlighting the challenges of selecting lensed quasars at high redshift.

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arXiv2026

Quasar Impostors: Two Extremely UV-Bright ($M_{\rm UV}\approx-23.5$) Reionisation-Epoch Galaxies Powered by Very Massive Stars

The extreme bright end of the galaxy UV luminosity function during reionisation remains poorly constrained, particularly where the galaxy and quasar luminosity functions overlap and source classification becomes ambiguous. We present JWST/NIRSpec and ALMA Band-6 observations of J1450-0144 ($z=6.627$) and J1429-0104 ($z=6.796$), two $M_{\rm UV}\simeq-23.5$ sources originally classified as faint quasars by SHELLQs. NIRSpec reveals blue UV continua, strong P Cygni profiles in N V, Si IV, and C IV, broad He II $\lambda1640$ emission with rest-frame equivalent widths of $8.8\pm1.2$ and $3.7\pm1.1$ {\AA}, respectively, and narrow nebular lines, reclassifying both as extremely UV-luminous galaxies. Standard population-synthesis models cannot simultaneously reproduce the strong He II and wind features, whereas models incorporating very massive stars (VMS; $M\gtrsim100\,M_\odot$) with dedicated wind prescriptions can. These models favor a star-formation duration of 2-4 Myr for J1450-0144, with a broader allowed range for J1429-0104, stellar masses of $\log(M_\star/M_\odot)\approx9.2$-$9.9$, and star-formation rates of $\simeq300$-$540\,M_\odot\,{\rm yr}^{-1}$. Under the same VMS wind models, equivalent-width diagnostics imply $M_{\rm up}\gtrsim225\,M_\odot$ for J1429-0104, while J1450-0144 lies beyond even the $M_{\rm up}=475\,M_\odot$ grid. ALMA detects luminous [C II] 158 $\mu$m emission in both systems, with $L_{\rm [CII]}\approx0.8$ and $4.1\times10^{9}\,L_\odot$, respectively. J1429-0104 additionally shows bright dust continuum, with both [C II] and dust offset by $\sim5.4$ kpc from its UV emission. These sources demonstrate that VMS can power some of the most UV-luminous galaxies at cosmic dawn and show that source classifications, and hence the inferred demographics of both galaxies and quasars in the crossover regime, need revisiting.

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arXiv2026

NEXUS: Spectral Variability of Little Red Dots and Blue Active Galactic Nuclei at $2 \lesssim z \lesssim 6$

We present spectral measurements for 17 Little Red Dots (LRDs) and 14 blue broad-line active galactic nuclei (AGNs) at $2\lesssim z \lesssim 6$ using multi-epoch JWST NIRSpec MSA spectra from the NEXUS program, sampling rest-frame timescales of $\sim 1-3$ months. Overall, the LRD population shows significantly enhanced Balmer decrement compared with both blue JWST AGNs at similar redshifts and 56 low-redshift broad-line AGNs matched in H$\rm\alpha$ luminosity. The rest-optical continua of LRDs show little ensemble variability (rms $\lesssim 3\%$), and the total H$\rm\alpha$ emission also shows weaker ensemble variability compared with low-redshift AGNs matched in H$\rm\alpha$ luminosity and rest-frame timescales. Based on the flux uncertainties, we constrain the intrinsic H$\rm\alpha$ rms variability to be $\lesssim 4\%$ for the LRD population over these timescales. Combining our results with recent broad-line variability measurements of LRDs over yearly to decade timescales reveals a low-level white-noise pattern across all timescales, in stark contrast to the variability amplitude ($\sim 6\%$ over monthly timescales) and red-noise pattern observed in normal AGNs. These results add to the growing observational studies that suggest population-wise, LRDs have weak variability both in optical continuum and broad-line emission. Furthermore, the distinct white-noise broad-line variability pattern suggests different production mechanisms of broad-line emission in LRDs as opposed to normal AGNs, and/or different properties of the driving ionizing flux from the central engine.

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arXiv2026

A quasar hatching from a buried red phase at z = 3.7

We present JADES-GS 209777, previously cataloged as CANDELS J033238.02-274626.2, hereafter "the Hatchling," a red quasar at $z=3.711$. While the source has been reported in earlier deep-field catalogs, our multiwavelength analysis reveals a visible active nucleus still embedded in a dense gas- and dust-rich environment. Red quasar continua are often attributed to dust attenuation, including non-standard extinction curves, but the highly comprehensive multiwavelength data for this source provide direct constraints on the material being cleared. Using JWST/NIRSpec, NIRCam, MIRI, HST, MUSE, Chandra, ALMA, and VLA data, we detect broad emission lines and strong X-ray emission, showing that the active nucleus is at least partially exposed. We also detect H$\alpha$ and He I absorption, indicating dense gas close to the nucleus. Kinematically disturbed O I, Mg II, Na D, and [O III] features, together with extended Ly$\alpha$ emission over $\gtrsim 20$ kpc, further show that multiphase gas is being accelerated from the nuclear region into the host-galaxy environment. The ALMA detection reveals strong dust emission, with the inferred infrared luminosity placing the system in the ULIRG regime. The continuum is red and sharply declining toward the rest-frame UV, resembling compact red AGNs, and may reflect extreme dust attenuation, gas reprocessing, possible BAL-like absorption, or a combination of these effects. Regardless of which mechanism dominates the continuum shape, the line diagnostics show that the visible nucleus remains partially obscured by nearby material. The Hatchling therefore represents a unique opportunity to explore a poorly known transition phase in which feedback is likely clearing an enshrouded quasar and allowing it to emerge toward a more unobscured active nucleus.

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arXiv2026

First-star imprints in a metal-poor galaxy overdensity near the end of reionization

The first generation of stars, known as Population III (Pop III), formed from primordial gas consisting solely of hydrogen and helium and is believed to have emerged only a few hundred million years after the Big Bang. Detecting the chemical enrichment of metal-poor circumgalactic gas offers a promising way to trace the enrichment signature of Pop III stars. Along the sightline to the quasar SDSS J0100+2802, a metal absorber at $z = 5.945$, showing over-abundant carbon and silicon compared to solar, has been reported to be consistent with the enrichment pattern of Pop III stars. With the James Webb Space Telescope, we report the discovery of an unusually metal-poor galaxy overdensity of 17 members (mean metallicity $\approx 3\%$ solar) near this metal absorber, which is $\sim 0.4$ dex more metal-poor than coeval galaxies in similarly overdense environments. This less chemically evolved system may have provided favorable conditions for preserving the absorption signatures of Pop III enrichment. We infer a minimum dark matter halo of $\log(M_{\mathrm{h,min}}/M_{\odot})=10.68^{+0.93}_{-1.72}$, supporting late-time Pop III formation at the outskirts of atomic hydrogen cooling halos. Our findings open a promising observational pathway to identify the chemical imprints of the first stars and constrain the conditions for their formation.

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arXiv2026

Probing Direct Contributions of Galaxies and AGN to Cosmic Reionization in a Quasar Field J0226+0302 with JWST NIRCam and NIRSpec

We present JWST Cycle 2 NIRCam and NIRSpec observations in a quasar field J0226+0302 at z=6.5412 to probe the direct connections between the intergalactic medium (IGM), galaxies, and AGN during reionization. This field was previously observed by the JWST ASPIRE program and eight [OIII]-emitting galaxies were detected at 5.3<z<6.4 with a single NIRCam pointing. Using new NIRCam and NIRSpec observations, we identify 65 additional line-emitting galaxies at 5.3<z<6.4. The IGM-galaxy cross-correlation function shows a ~2 sigma excess IGM transmission at ~10-40 cMpc from galaxies when compared with the average IGM transmission, suggesting a significant contribution from regions traced by star-forming galaxies to the local ionizing background during reionization. The IGM-galaxy cross-correlation function is consistent with THESAN simulations with an IGM neutral fraction of 5%-7% and an average ionizing photon escape fraction f_esc of 6% from galaxies. Among 49 line-emitting galaxies observed by NIRSpec, we identify four AGN through detection of broad H-alpha emission lines with an AGN fraction of (8+/-4)%. By measuring the IGM effective optical depth around the AGN and the IGM-AGN cross-correlation function, we find that the IGM transmission is higher within 5 cMpc/h of the AGN than around the majority of [OIII] emitters. We interpret the excess IGM transmission as resulting from the local radiation enhancement by the AGN, and estimate f_esc of 50%-100% of the AGN from the IGM-AGN cross-correlation function. Future JWST NIRSpec observations in quasar fields will yield a more constraining IGM-AGN cross-correlation function, providing further insights into the roles of galaxies and AGN in reionization.

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arXiv2026

High-Resolution ALMA Imaging for a Gravitationally-lensed Quasar at $z=6.5$: Constraining the AGN Contribution to Galactic-Scale Dust Heating

We present high-resolution (beam size $0\farcs076\times0\farcs040$) Atacama Large Millimeter/submillimeter Array (ALMA) observations of the far-infrared $(\lambda_\text{rest}=162.7\mu\rm{m})$ dust continuum of J0439+1634, a gravitationally lensed quasar at $z=6.52$. We perform pixelated lens modeling for the visibility data, finding that J0439+1634 is well-described by a singular isothermal ellipsoid plus an external shear lensing model. The best-fit lensing potential exhibits a naked-cusp configuration, confirming the finding in Fan et al. (2019). The reconstructed source plane continuum emission shows a compact bright core, with size $\lesssim200$ pc and peak brightness $\sim0.6 \text{ Jy arcsec}^{-2}$. The total continuum flux at 245 GHz is $3.36\pm0.02$ mJy. The flux magnification is {$4.63\pm0.03$}, indicating an average source-plane resolution of $0\farcs019$ (equivalent to 104 pc). The spatial resolution around the supermassive black hole reaches $\sim36$ pc. %Using the new lensing model, we re-fit the Hubble Space Telescope image for J0439+1634, and find that the position of the optical quasar is consistent with the brightest pixel in the dust continuum map. Leveraging the exceptional source-plane resolution, we build a radiative transfer model to describe the observed dust emission profile. The best-fit model indicates that heated dust from the active galactic nucleus (AGN) dominates the sub-millimeter emission at $r\lesssim100$ pc and that star-heated dust dominates the outer region of the host galaxy. AGN heating contributes {$\sim13\%$} to the observed sub-mm flux. Therefore, previous far-infrared-based star formation rate measurements for most high-redshift quasars are likely mildly overestimated.

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arXiv2026

NEXUS: Abundance, Environments, and Spectral Diversity of Little Red Dots from the NIRSpec MSA Sample

We present a comprehensive study of Little Red Dots (LRDs) at 2.3 < z < 7.4 using NIRCam photometry and NIRSpec MSA/PRISM spectra from the ongoing NEXUS program. Photometric selection combining several commonly adopted methods yields a high completeness of about 85% for LRD selection over this redshift range and for a flux limit of F444W < 26. The overall purity is about 60%, with contamination from emission-line galaxies and normal active galactic nuclei (AGNs), as well as dwarf stars. Most (>90%) of the spectroscopically confirmed LRDs have robust broad-line detection. Our spectroscopic sample of 36 LRDs displays the full range of spectral diversity of LRDs. It includes objects with extreme Balmer breaks similar to the LRD "Cliff", as well as objects with moderately reddened rest-optical continua that can be fit with low-temperature blackbody components in the recent BH* model framework. The broad H$\alpha$ emission is correlated with the continuum emission at 5100 Angstrom, suggesting common origins for these emission components; the narrow [O III] emission, however, is poorly correlated with the optical continuum. We do not find evidence of redshift evolution in these spectral properties. The space density of LRDs declines toward z about 2, opposite to the trend for normal AGNs, although low-luminosity LRDs at z about 2-4 may be more abundant than currently probed by ground-based searches. The clustering of LRDs suggests that they live in dark matter halos of several times $10^{11}\ h^{-1}$ solar masses, albeit with large uncertainties. Overall, these results are consistent with recent observations of LRDs and with the emerging picture of accreting SMBHs enshrouded in dense gas envelopes as the origin of LRDs.

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arXiv2026

Aether-SHELLQs: JWST integral-field spectroscopy of candidate obscured quasars at z ~ 6

We present James Webb Space Telescope (JWST) NIRSpec integral field unit (IFU) observations of six galaxies at $z \sim 6$, obtained as part of the Aether project (General Observers program 5645). The targets were originally identified by the Subaru High-$z$ Exploration of Low-Luminosity Quasars (SHELLQs) survey, as candidate obscured quasars with luminous ($\gtrsim10^{43}$ erg s$^{-1}$) but narrow ($\lesssim500$ km s$^{-1}$) Ly$\alpha$ emission. Two objects exhibit a broad component in their Balmer lines (FWHM $>3000$ km s$^{-1}$), indicating the presence of active galactic nuclei (AGNs), while the remaining four show similar profiles in permitted and forbidden lines. Combining these data with similar SHELLQs objects reported previously, we find that the presence of broad lines is strongly correlated with Ly$\alpha$ luminosity ($L_{\rm Ly\alpha}$); the inferred AGN fraction is $>$77 % and $<$15 % above and below $L_{\rm Ly\alpha} =10^{44}$ erg s$^{-1}$, respectively. Dust-extinction corrections inferred from the Balmer decrement would imply unrealistically high Ly$\alpha$ luminosities, suggesting that the line-emitting gas consists of multiple zones. The IFU data reveal diverse spatial structures. The AGN hosts are compact, whereas the other galaxies show extended ionized gas on scales up to 10 kpc and star formation rates of 60 - 600 $M_\odot$ yr$^{-1}$. One of the extended objects exhibits a signature of rotation, while most of the others show little ordered kinematics, with velocity widths (FWHM) up to 200 - 300 km s$^{-1}$. These objects populate the intermediate luminosity regime between classical luminous quasars and the low-luminosity AGNs discovered by JWST, including Little Red Dots, potentially linking the two populations.

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arXiv2026

A $z \sim$ 6.2 Quasar on the Local M$_{\rm BH}$-$\sigma_{\rm \ast}$ Relation Quenching Its Host Galaxy from the Aether Survey

We report JWST/NIRSpec integral field unit (IFU) observations of the quasar J1512$+$4422 at $z \sim 6.2$ from the Aether survey. At $\sim$900 Myr after the Big Bang, this object already lies on the $M_{\rm BH}$-$\sigma_\ast$ relation found in the local universe, with an $M_{\rm BH} \simeq 8.9\times10^8\,M_\odot$ and a stellar velocity dispersion $\sigma_\ast \simeq 288$ km s$^{-1}$. We detect an outflow with a velocity of $\sim$478 km s$^{-1}$ in the nuclear region, which likely extends to $\sim$3.2 kpc in projection and has a median velocity of $\sim$352 km s$^{-1}$. The outflow dynamical time scale ($\sim$ 9 Myr) is consistent with the time scale of the current quenching process based on the star formation history as reported previously. The total mass outflow rate (92.6$^{+92.6}_{-74.1}$ M$_{\odot}$ yr$^{-1}$) is larger than the current star formation rate (0.9$^{+3.8}_{-0.8}$ or 4.3$^{+5.8}_{-3.7}$ M$_{\odot}$ yr$^{-1}$), and the total kinetic energy outflow rate (0.6$^{+0.6}_{-0.5}$\% of quasar luminosity) meets the threshold for negative quasar feedback as suggested by simulations. These results suggest that the outflow is capable of suppressing/quenching the star formation activity within the host galaxy. Furthermore, J1512$+$4422 exhibits $\sigma_\ast$, stellar mass and size similar to those of $z \gtrsim$ 3 quiescent/post-starburst galaxies, implying a link between the two. Overall, for objects like J1512$+$4422, the evolution of their SMBHs and host galaxies appears to be tightly coupled within the first billion years. The quasar feedback likely plays a critical role in both placing them on the $M_{\rm BH}$--$\sigma_\ast$ relation and quenching.

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arXiv2026

CO(7-6) and [C I](2-1) survey in z > 6 quasars

High-redshift ($z\gtrsim6$) quasars trace the earliest supermassive black holes and intense star formation, offering key laboratories for black hole-galaxy evolution at cosmic dawn. While far-infrared studies have revealed large dust reservoirs and strong [C II] emission, the physical conditions and molecular gas content of their ISM remain uncertain. We present ALMA Band 3 observations of the redshifted CO(7-6) and [C I](2-1) emission lines and dust continuum in a sample of 18 quasars at $z \sim 6$. We detected CO(7-6) in 15/18, [C I](2-1) in 6/18, and continuum in 13/18 sources. Line luminosities and continuum fluxes were used to estimate molecular gas masses from CO, [C I], and dust, and a hierarchical Bayesian cross-calibration of all four tracers yielded consistent per-source $M_{\rm H_2}$ estimates and conversion factors. Comparison with PDR and XDR model grids using the $L_{\rm [CII]}/L_{\rm [CI]}$ and $L_{\rm CO(7--6)}/L_{\rm TIR}$ ratios suggests gas densities of $n > 10^4$ cm$^{-3}$ and radiation fields of $G_0 \sim 10^3$--$10^4$ for sources consistent with PDR solutions, while many quasars fall outside the model parameter space. The $L'_{\rm CO(7-6)}/L'_{\rm [CI](2-1)}$ ratio indicates that a large fraction of the molecular gas resides in a warm and highly excited phase. Together these results suggest that classical PDR heating alone cannot explain the observed line ratios and that additional volumetric processes such as X-ray irradiation, turbulence and shocks, or enhanced cosmic-ray heating likely influence the excitation of the cold ISM. They demonstrate the power of multi-line diagnostics in revealing the excitation and structure of the cold ISM in early quasar host galaxies and highlight the need for joint analysis of CO, [C I], [C II], and dust emission to characterize star formation and AGN-driven heating at cosmic dawn.

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arXiv2026

Detection of an Extended Ly$\alpha$ Halo around a $\textit{z}=6.64$ Broad Absorption Line Quasar with the Keck Cosmic Web Imager

We present the first results from a program searching for extended Ly$\alpha$ halos around high redshift ($ z \gtrsim 6.5$) quasars using the red channel of the Keck Cosmic Web Imager (KCWI). Our observations reveal a Ly$\alpha$ halo extending to $\simeq11$ pkpc around the $z=6.64$ broad absorption line quasar J0910$-$0414. The Ly$\alpha$ velocity field displays a rotation-like gradient, and the gas velocity dispersion is consistent with gravitationally dominated motion ($\sigma_{\mathrm{Ly\alpha}}<300$ km s$^{-1}$). Comparison with the $[\mathrm{C\;II}]$ kinematics of the host galaxy core from ALMA observations shows that the Ly$\alpha$-emitting gas extends over a much larger region, shows distinct kinematics, and has a smaller velocity dispersion ($\sigma_{\mathrm{Ly\alpha}} \simeq 0.6\sigma_{\mathrm{[C\;II]}}$). The Ly$\alpha$ spectral region of the quasar is largely obscured by a deep $\mathrm{N\;V}$ absorption trough, and as a result, roughly $55\%$ of the total Ly$\alpha$ flux is from the extended halo. These observations demonstrate the potential of KCWI for probing the cool gas reservoir that fuels the growth of quasars and their hosts in the epoch of reionization.

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arXiv2026

Discovery of Quasar Variability and Early Accretion Disk Signatures at Cosmic Dawn

In the nearby universe, quasars are well known to exhibit variability in their brightness over time, offering a powerful tool to probe the physics of accretion onto the SMBH and directly measure the mass of the SMBH. However, detecting variability in early quasars remains challenging. Here, we report the detection of multi-wavelength infrared and X-ray variability in a quasar observed just 850 million years after the Big Bang. The infrared variability spans five filters, tracing rest-frame ultraviolet and optical emission from the accretion disk, while the X-ray variability probes the corona. The variable spectrum reveals that the accretion disk has a geometrically thin, optically thick structure. This provides observational constraints on the accretion disk structure at early times, when quasars are accreting at high Eddington ratios and reside in extreme environments. Our findings demonstrate the feasibility of characterizing accretion physics using variability in the early universe, laying the groundwork for studies exploiting upcoming facilities such as the Rubin Observatory and Roman Space Telescope. These facilities will discover large samples of variable high-redshift quasars, enabling population-level variability studies of accretion physics and black hole masses, filling key missing ingredients in understanding early SMBH growth.

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arXiv2026

A Close Quasar Pair in a Massive Galaxy Merger at $z=5.7$

Close quasar pairs are rare products of galaxy mergers in which both supermassive black holes (SMBHs) are actively accreting, offering strong constraints on merger-driven active galactic nuclei (AGN) evolution. Identifying close quasar pairs at $z\gtrsim4$ is challenging due to the declining quasar number density in the early Universe. Here we report the confirmation of a close quasar pair at $z=5.7$, J2037--4537, utilizing high-resolution Atacama Large Millimeter/submillimeter Array (ALMA) observations. The quasar host galaxies exhibit tidal disturbed features in both the far-infrared continuum emission and the {\cii} line emission, ruling out the doubly-imaged lensed quasar scenario. The two quasar hosts are massive $(M_\text{dyn}\gtrsim10^{10}M_\odot)$ and star-forming (SFR $\gtrsim500 M_\odot~ \mathrm{yr^{-1}}$). The confirmation of J2037--4537 puts a lower limit on the quasar pair fraction at $5.5 1.2\%$, which is much higher than the quasar pair fraction at $z\lesssim4$. J2037--4537 is expected to form a gravitationally-bound SMBH binary within $\lesssim2$ Gyr. The elevated quasar pair fraction at $z>5.5$, as indicated by J2037--4537, likely contributes to the high gravitational-wave background reported by recent Pulsar Timing Array experiments.

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arXiv2026

Three Hundred Quasars from the Couch: A first look at high-redshift quasar discovery with SPHEREx

Photometric selection of luminous high-redshift ($z\gtrsim4$) quasars is plagued by contamination from numerous low-mass Galactic stars, reddened lower-redshift quasars, as well as compact luminous red galaxies. Confirmation of these rare objects thus requires extensive spectroscopic campaigns on 4 and 8-meter-class telescopes with relatively low success rates. Here we demonstrate the utility of SPHEREx spectrophotometric survey data for quasar confirmation with no ground-based follow-up required, "from the couch," applied to candidates from a purposefully simplistic photometric and astrometric Gaia+WISE selection down to low Galactic latitudes ($|b|\geq8^\circ$). Primarily from the detection of their strong broad H$\alpha$ emission lines, we discover 87 new luminous $4.0 < z < 5.7$ quasars with median $M_\text{1450} = -27.5$, including 19 quasars at $z>5$, and recover 219 previously published quasars at $z>4$. We validate our SPHEREx selection with a 100% confirmation rate in ground-based spectroscopic follow-up of 29 of our new $z>4$ quasars, including 11 unpublished archival spectra. We also discover 203 additional lower-redshift quasars at $0.3 < z < 4$, consisting primarily of relatively rare highly-reddened and strong broad-absorption-line objects that are likely missed by traditional quasar surveys. Finally, we show that the Ly$\alpha$ absorption breaks and H$\alpha$ lines of luminous quasars are already detectable at redshifts $5.7\lesssim z\lesssim6.5$ after the completion of only the first of four all-sky surveys to be performed by SPHEREx during its planned two-year mission.

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