Exploring the Mpc Environment of the Quasar ULAS J1342+0928 at z = 7.54

We use observations obtained with the ACS and Wide Field Camera 3 (WFC3) on board HST between 2018 June and 2019 June (PI: Bañados, Prog ID:1165). We obtained data in the F814W (ACS, 13 orbits) serving as the nondetection filter, and the F105W and F125W filters (WFC3, eight and four orbits each, respectively). To maximize the ACS surveyed area, WFC3 NIR imaging was observed in a 2 × 2 mosaic strategy. The final effective area covered to search for LBGs is computed based on the ACS/F814W image, as this area is covered by all three filters. The calculation masks out bad pixels in the weight map, resulting in an area of $12.28\,{\mathrm{arcmin}}^{2}$. All filter transmission curves are presented in Figure 1, with the rest-frame UV spectrum of ULAS J1342+0928 overlaid (Bañados et al. 2018a). The presented observations achieve 5σ limiting AB magnitudes of 28.20, 27.83, and 27.46 in the F814W, F105W, and F125W bands, respectively, as calculated with a 04 diameter circular aperture.

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We use the bias subtracted, flat-fielded, and cosmic-ray cleaned, reduced images provided by STScI, and implement an ad hoc method to ensure a good astrometric match between the different filters using DrizzlePac. ¹⁵ Indeed, such an alignment is nontrivial due to the small number of stars found in the field, which complicates standard reduction routines. We start by considering the HST WFC/F814W pipeline-reduced flc.fits files, downloaded from the MAST archive. ¹⁶ In order to create reference catalogs with enough sources, we run Source Extractor (Bertin & Arnouts 1996) on each image, after cleaning them from cosmic-ray contamination using the astrodrizzle routine with cosmic_ray_cr_clean = True. We use tweakreg to align the uncleaned, original flc.fits images, utilizing these Source Extractor-created reference catalogs, each containing ∼1000 sources. The final combined image in F814W is obtained using astrodrizzle, with skymethod = "match" and combine_type = "median." We run Source Extractor again on the final, drizzled F814W image, and use this new catalog (4544 sources) as a reference to match the WFC3/F105W and F125W images to the F814W. In detail, we use tweakreg on the F105W and F125W HST pipeline-reduced flt.fits files, with the F814W catalog as reference, searchrad = 3. and minobj = 6. We drizzled the matched files to obtain the final F125W and F105W images, with the same astrodrizzle parameters used for the F814W filter, and final_scale = 005, in order to match the pixel scale of the WFC3 images to that of ACS.

In order to check the goodness of our match, we compared the coordinates of sources recovered in all three HST final images, considering only the 30 brightest objects. The final mean deviation within the astrometric solutions of the filters is ∼003. If instead we compare their astrometry with the GAIA DR2 catalog (Gaia Collaboration et al. 2018), the mean difference in the coordinates of the recovered sources (10 in F814W, 13 in both F105W and F125W) is ∼005. We note that the final F105W image is affected by an artifact, due to the presence of a satellite trail in one of the flt exposures. We decided not to discard this exposure to obtain the deepest image, but caution is needed when examining sources close to the trail. The final reduced images F814W, F105W, and F125W (hereafter i₈₁₄, Y₁₀₅, and J₁₂₅) are presented in Figure 2 as a red, green, and blue (RGB) color image created with JS9-4L v2.2 (Mandel & Vikhlinin 2018).

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2.1.1. Point-spread Function Matching

We perform an empirical noise calculation of the images to account for the partially correlated noise characteristic of drizzled HST images (Casertano et al. 2000). While Source Extractor calculates the flux uncertainties from individual uncorrelated pixels in the rms map, the procedure described in Papovich et al. (2016) accounts for correlated and uncorrelated noise. We closely follow this empirical noise estimate as described below.

For images with exclusively uncorrelated pixels, the noise is measured in a circular aperture of N pixels, which scale following ${\sigma }_{n}={\sigma }_{1}\times \sqrt{N}$, where σ₁ is the pixel-to-pixel standard deviation of the background. Conversely, the noise from completely correlated pixels is measured as σ_n = σ₁ × N. In our HST images, the noise truly varies among both correlations as N^β where 0.5 < β < 1, and this can be estimated for the whole image with the parameterized equation:

$$\begin{eqnarray}&&{\sigma }_{n}={\sigma }_{1}(\alpha {N}^{\beta }),\end{eqnarray} \tag{ 2 }$$

where α has to be a positive value. Note that we do not include the Poisson correction of the equation from Papovich et al. (2016) as it did not contribute to the calculation of the noise in the HST images. We measure the noise in each of the three HST images by first placing randomly distributed apertures in the sky background with growing sizes from 01–10 in diameter. Then, we use the curve_fit Python function with the Levenberg–Marquardt least-squares method to fit for the noise found in the random apertures with increasing size (see Figure 3). The calculated noise values in each image are applied to estimate the flux errors for the Kron and 04 apertures. For the Kron aperture, N is calculated as the number of pixels in the ellipse defined by the semimajor (A_IMAGE) and semiminor (B_IMAGE) axes measured by Source Extractor.

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The resulting Source Extractor catalogs with the calculated flux errors are then corrected for Galactic dust attenuation following the Cardelli et al. (1989) extinction curve with an R_v = 3.1, as motivated in similar studies of high-redshift galaxies (e.g., Rojas-Ruiz et al. 2020; Finkelstein et al. 2022; Tacchella et al. 2022). We use Schlafly & Finkbeiner (2011) to correct for galactic extinction and find a color excess E(B − V) = 0.025. The zero-points for the final catalog are calculated according to the newest 2020 HST photometric calibrations for ACS and WFC3, which apply to the observed dates of the images. The zero-points in AB magnitude are 25.9360, 26.2646, and 26.2321 for i₈₁₄, Y₁₀₅, and J₁₂₅, respectively. We also apply in all filters an aperture correction from the large (2.5, 3.5) to small (1.2, 1.7) Kron aperture photometry measured in the J₁₂₅, to account for the missing PSF flux in the smaller aperture.

Additional Spitzer/IRAC 3.6 and 4.5 μm images covering the same area of the quasar environment explored with HST are available from Cycle 16 archival database (PI: Decarli). Each of the IRAC mosaics has an exposure time of 3.4 hr, and the 3σ limiting depth for point sources is ≈0.8 μJy in both Channels 1 and 2. These additional photometric bands provide crucial information to distinguish true high-redshift galaxies from lower-redshift contaminants. These IRAC bands allow for better building of the spectral energy distribution (SED), and hence to better differentiate between a dusty Balmer-break galaxy at z ∼ 2 and a high-redshift candidate of interest at z ∼ 7.5. The FWHM of the IRAC point response function is ≈18 in Channels 1 and 2; ¹⁸ this is about 2 orders of magnitude larger than in HST. Therefore, in order to match the sources from the two data sets the IRAC PSFs need to be modeled in order to correct for deblending of sources and calculate accurate flux and flux errors. The mosaics and modeling are performed following Kokorev et al. (2022), and are briefly described here.

To obtain photometry from the IRAC imaging, a PSF model method is produced using the tools from the Great Observatories Legacy Fields IR Analysis Tools (GOLFIR; Brammer 2022). This modeling method uses a high-resolution prior, which is built by combining the HST/ACS and WFC3 images. This resulting image is combined with the IRAC PSF using a matching kernel to finally obtain the low-resolution templates. The original IRAC images are divided into homogeneous 4 × 4 patches of 1200, which are allowed to overlap to improve the modeling. The brightest stars and sources with high S/Ns in the IRAC and HST images are manually masked to avoid large residuals from the fit. IRAC model imaging is first generated for the brightest objects in the J₁₂₅ catalog doing a least-squares fit of the low-resolution IRAC patches to the original IRAC data to obtain the modeled fluxes. The flux errors are simply the diagonal of the covariance matrix of the model. Similarly, for the fainter sources in the catalog but the least-squares fit normalizations are then adopted as the IRAC flux densities. The resulting photometry from this PSF modeling method is used for the rest of the analysis.

We use the Easy and Accurate Z_phot from Yale (EAZY; Brammer et al. 2008) version 2015-05-08 to calculate the photometric redshifts of all sources in our catalogs. EAZY calculates the probability distribution function of photometric redshifts P(z) based on a minimized χ² fit of the observed photometry in all given filters to different SED models of known galaxy types. EAZY includes the 12 tweak_fsps_QSF_12_v3 Flexible Stellar Population Synthesis models (Conroy et al. 2009; Conroy & Gunn 2010), the template from Erb et al. (2010) of the young, low-mass, and blue galaxy BX418 at z = 2.3 exhibiting high equivalent width (EW) of nebular lines and Lyα, and a version of this galaxy without the Lyα emission to mimic attenuation from the IGM while preserving strong optical emission lines. All these 14 templates are fed equally into EAZY so that it constructs the best-fitting models from a linear combination of the templates to the flux and flux errors of the source measured in the Kron 1.2, 1.7 elliptical aperture (see Section 3). For each source, EAZY applies IGM absorption following Inoue et al. (2014) for redshift steps of Δz = 0.01. Initially, we consider the redshift probability distribution when giving the templates freedom from z = 0.01 − 12 and assume a flat luminosity prior, as galaxy colors at z ≳ 6 are not yet well understood (Salmon et al. 2018). This wide redshift range is chosen to allow the comparison between the probability of galaxies to be at high (z > 5) and low (z < 5) redshifts.

We build the final catalog of galaxy candidates applying the following selection criteria to the results of the photometric-redshift fits from EAZY:

• 1.  
  ${\rm{S}}/{{\rm{N}}}_{{i}_{814}}\lt 2.0$ measured in the 04 circular aperture, implying a nondetection in i₈₁₄.
• 2.  
  ${\rm{S}}/{{\rm{N}}}_{{Y}_{105}}$ or ${\rm{S}}/{{\rm{N}}}_{{J}_{125}}\gt 5.0$ also measured in the 04 circular aperture, to ensure the source is detected at high redshift while also potentially selecting strong Lyα emitters where the flux would only be detected in the Y₁₀₅, or galaxies with strongly absorbed Lyα producing continuum emission only in the J₁₂₅.
• 3.  
  The integrated redshift probability P(z) calculated from EAZY at P(6 < z < 12) > 60%, securing that a high-redshift solution dominates over the total probability distribution.
• 4.  
  The integral of the primary peak of the total integrated distribution P(z_peak) > 50%.
• 5.  
  The redshift probability distribution at z = 7.5 is higher than the neighboring distributions, in Δz = 1 bins: P(6 < z < 7) < P(7 < z < 8) ∧ P(8 < z < 9) < P(7 < z < 8).

We do not place a cut in the half-light radius of the source in order to include in the catalog possible active galactic nuclei (AGN) sources, which would exhibit a more pointlike morphology. However, this parameter is reported in Table 2 and is considered during the visual inspection step. Note that the half-light radius r_0.5 of a star in our survey in the J₁₂₅ band is 2.65 pixels, or 013.

Using the above criteria, we find five LBG candidates where one is the quasar ULAS J1342+0928, and two are identified as diffraction spikes from visual inspection. The succeeding catalog is thus composed of the recovered quasar and two galaxy candidates. We note that decreasing the S/N threshold so that ${\rm{S}}/{{\rm{N}}}_{{Y}_{105}}$ or ${\rm{S}}/{{\rm{N}}}_{{J}_{125}}\gt 3.0$ results in large contamination due to sources with a marginal detection in just one band (25), diffraction spikes (13), bad pixels, or other detector artifacts (25).

An additional test fitting only a lower-redshift solution was performed to better discriminate among possible low-redshift contaminants. For this, we set EAZY to freely fit the 14 SED templates over a redshift span of z = 0.01–5. We then compared the χ² of the best-fit template from this lower-redshift solution ${\chi }_{\mathrm{low}}^{2}{\text{}}z$ to that at higher redshift ${\chi }_{\mathrm{high}}^{2}z$ with the redshift span of z = 0.01–12. If ${\rm{\Delta }}{\chi }_{l-h}^{2}={\chi }_{\mathrm{low}}^{2}z-{\chi }_{\mathrm{high}}^{2}z\lt 4$ the goodness of the fit is lower than the threshold of 95% confidence interval, which means the source can be similarly fit with a high- and a lower-redshift solution (see, e.g., Finkelstein et al. 2022; Bagley et al. 2024). We discarded one candidate that did not pass this test, as it had a ${\rm{\Delta }}{\chi }_{l-h}^{2}=2.4$ with a redshift solution of z_low = 2.5 and z_high = 7.9. The final catalog thus contains the quasar and one LBG candidate at z ∼ 7.5 passing the test with ${\rm{\Delta }}{\chi }_{l-h}^{2}=16.51.$

We recover the quasar with a photometric redshift of z_phot = 7.59, where its systemic redshift measured from [C ii] emission is z = 7.5400 ± 0.0003 (Bañados et al. 2019). We find a new LBG candidate, C-4636, at z_phot = 7.69 (see Figure 4). This photometric redshift is slightly higher than that of the quasar given the very flat redshift probability distribution between z = 7.5 and 8, but the solution dominates among the other redshift distributions with P(7 < z < 8) = 80%, constraining its association with the environment of the quasar. Moreover, C-4636 is at a projected distance of 223 pkpc from the quasar. This distance is similar to that of galaxies found around other high-z quasars in the literature, which showed strong quasar-galaxy clustering (e.g., Morselli et al. 2014; Farina et al. 2017; Mignoli et al. 2020; Meyer et al. 2022). Further exploration of the QSO-LBG clustering for quasar ULAS J1342+0928 and this candidate is described in Section 6.2.

We examine the candidate's i₈₁₄ − Y₁₀₅ and Y₁₀₅ − J₁₂₅ colors to evaluate possible stellar contamination. We take MLT-dwarf stars from the IRTF SpeX Library developed by Burgasser (2014) and compare their colors to those of LBGs and quasars at z = 6.5 − 8.5. The color of candidate C-4636 in Y₁₀₅ − J₁₂₅ = 0.7 and its compact morphology with a half-light radius r_0.5 = 010 is comparable with the average radius of stars in this field of r_0.5 = 013 ± 001, make it a possible MLT-dwarf contaminant (see Figure 6). However, the ratio of the flux in the total Kron to the 04 aperture of 1.45 ± 0.07, and the Source Extractor stellarity parameter of CLASS_STAR =0.08 do not classify this source as a star. Additionally, galaxies at this redshift would show a distinct SED from MLT contaminants at λ > 2 μm, which can be determined even with shallow IRAC imaging (e.g., Finkelstein et al. 2022; Bagley et al. 2024). The nondetection in our IRAC/3.6 and 4.5 μm imaging supports the high-redshift nature of this candidate. Thus, we still consider this source as a good LBG candidate in the physical environment of ULAS J1342+0928. We recognize that additional filter information redder than the J₁₂₅ band would provide further insights into the characterization of this source.

The compact morphology could suggest that C-4636 is an AGN. This is consistent with theoretical simulations, which show that AGNs tend to cluster near quasars with an SMBH of 10⁸–10⁹ M_⊙ (Costa et al. 2014), and some observational cases already seen at z > 5 (e.g., McGreer et al. 2016; Connor et al. 2019; Yue et al. 2021; Maiolino et al. 2023; Scholtz et al. 2023). Existing 45 ks Chandra observations of this field do not show any X-ray signal at this location. (Bañados et al. 2018b). Future JWST NIRSpec spectrum targeting strong nebular emission lines could be used for a Baldwin–Philips–Terlevich diagnostic diagram (Baldwin et al. 1981) to distinguish whether this source is an AGN.

A galaxy candidate at 27 projected pkpc from the quasar ULAS J1342+0928 had been previously identified in Venemans et al. (2020). This candidate was recovered with Atacama Large Millimeter/submillimeter Array (ALMA) observations targeting the rest-frame [C ii]-158 μm emission of the quasar and found to be at z_{[C II]} = 7.5341 ± 0.0009. In order to study the properties of this source in other wavelengths, we looked for any counterpart emission in the HST and Spitzer/IRAC data. However, we did not detect this galaxy in any of the five images (see Figure 5). Since no flux is recovered up to S/N ∼2, the possibility of this galaxy being a low-redshift interloper is strongly disfavored. Furthermore, many studies have concluded that there is a significant population of dust-obscured galaxies that have no rest-frame optical counterpart detected at the current observational limits (e.g., Wang et al. 2019b; Mazzucchelli et al. 2019; Meyer et al. 2022). Therefore, this [C ii] emitter could be one of these dusty star-forming galaxies (DSFG) in the environment of ULAS J1342+0928. Currently, the only detection available of this galaxy is from ALMA 223 GHz observations of its [C ii] emission at 0.10 ± 0.03 Jy km s⁻¹, and no dust continuum was recovered (f_cont < 0.06 mJy; Venemans et al. 2020). Given the information at hand, we are not able to place meaningful constraints or predictions on the SED of this source. Further ALMA or JWST observations are necessary to confirm the nature of this source and study its properties. The current ALMA observations of this field cover only a field of view of ∼39'', i.e., ∼194 kpc, around the quasar. Hence, we are not able to investigate any counterpart for the LBG candidates found in this work.

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Simcoe et al. (2020) inspected the optical/NIR spectrum of ULAS J1342+0928, and identified a strong metal absorber at z = 6.84 spanning ∼150 km s⁻¹ on its line of sight. This galaxy has not been directly observed in emission yet. Hence, we also explore candidates at z ∼ 7 to search for any counterparts (e.g., Neeleman et al. 2019). We begin by selecting all sources with P(6 < z < 12) > 60%, and a P(6 < z < 7) higher than neighboring distributions in Δz = 1 bins. After visually inspecting the candidates and evaluating the best fits for the high- and lower-redshift solutions ${\rm{\Delta }}{\chi }_{l-h}^{2}$, similar to the process used for the z ∼ 7.5 galaxy candidates search, we identify two galaxy candidates. C-4966 has a photometric redshift of z_phot = 6.91 and a probability distribution of P(6 < z < 7) = 55% and P(6.5 < z < 7.5) = 71%. C-5764 is found at z_phot = 6.89 and has a P(6 < z < 7) = 68% and P(6.5 < z < 7.5) = 79% (see Figures 2, 4, and Table 1). Both candidates favor a redshift solution closer to z = 6.5 − 7.5. Evaluating different Lyα properties for these candidates using our color models for LBGs at z = 6.5 − 8.5 in Figure 6, we find that similar colors would be reproduced with either (FWHM =1000 Å; EW =15Å) or (FWHM = 2000 Å; EW =15 Å), rather than a more typical narrow Lyα line for LAEs (FWHM = 200 Åand EW = 100 Å or lower). Spectroscopically confirming these galaxies could point to galaxy clustering in the field at z ∼ 6.8, near the absorber.

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The Lyman break of LBG candidates at z ∼ 7.5 falls at the observed wavelength of λ ∼ 1μm, which is positioned right in the middle of the Y₁₀₅ band used in this work (see Figure 1). This implies that even if the source is detected in Y₁₀₅, its Y₁₀₅ − J₁₂₅ color would be red. No contiguous filter (e.g., HST JH₁₄₀ or H₁₆₀) is available to us to robustly measure the rest-frame UV color redward of the Lyα break. Although our photometry in the Spitzer/IRAC 3.6 and 4.5 μm bands complement the galaxy SED and help determine its dust components to rule out low-redshift contaminants, this is only possible for galaxies already robustly selected at high redshift with HST imaging up to λ ∼ 2μm. Our data currently has a wider gap in the wavelength coverage (1.4–3.6 μm, or J₁₂₅ − IRAC/3.6 μm), causing a considerable number of galaxies to never enter the selection catalog. Taking into account all these limitations, we compare the found number density (one new LBG) to what we would recover with a consistent selection of galaxy candidates in the same redshift span of z ∼ 7–8, using comparable data from blank fields.

The Cosmic Assembly Near-Infrared Deep Extragalactic Legacy Survey GOODS North and South Deep survey presented in Finkelstein et al. (2015) provides a similar filter coverage (I₈₁₄, Y₁₀₅, J₁₂₅, H₁₆₀) at comparative depths to our HST survey around ULAS J1342+0928. This filter coverage helps to assess the completeness of our data since we can attempt to reproduce the number density of LBGs in a blank field, but using just the I₈₁₄, Y₁₀₅, and J₁₂₅ bands and our selection criteria for high-redshift galaxies. We build the photometry catalog for EAZY using the fluxes from the GOODS catalog, and the flux errors from our limiting magnitudes in the HST filters to match the noise to that of our images. We run EAZY in the same setup and select LBG candidates with the criteria employed for the analysis of the quasar field (see Section 4.2). Using these three HST filters alone, we recover 31 sources. This is much lower than the 125 sources with a photometric redshift between z = 7 and 8 in the GOODS catalog that are selected when including the additional photometry in H₁₆₀. Hence, we recover in total only 31/125, i.e., ∼25% of the sources.

This test shows that the selection of z ∼ 7.5 galaxies based on our HST filter set is strongly incomplete. The recovered fraction of galaxies and their magnitudes in J₁₂₅ is shown in bins of Δ0.5 mag in Figure 7. Note that the faintest galaxy candidate recovered from this catalog has J₁₂₅ = 27.53, whereas the GOODS catalog from Finkelstein et al. (2015) has sources as dim as J₁₂₅ = 28.76. For galaxies in GOODS at J₁₂₅ < 26.5, i.e., in the range of C-4636 (J₁₂₅ = 26.41; see Table 2), we obtain the highest recovery rate of 43%. The completeness drops to ∼10% between J₁₂₅ = 27.5 and 28.0. However, we note that our 5σ limiting magnitude in this band is deeper reaching up to J_125,5σ = 27.46 (see the dashed line in Figure 7).

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Table 2. Photometry of HST and Spitzer Selected Galaxy Candidates

ID	S/Ni	S/NY	S/NJ	S/N3.6μm	S/N4.5μm	i814	Y105	J125	3.6 μm	4.5 μm	r0.5
						(AB mag)	(AB mag)	(AB mag)	(AB mag)	(AB mag)	(arcsec)
QSO-6381	−0.01	3461.1	3772	100.87	118.14	>28.62	21.18	20.61	20.16	20.15	014
C-4636	−1.20	8.63	12.03	−1.0	0.09	>28.71	27.11	26.41	>24.14	>24.22	010
C-4966	1.88	9.45	6.62	0.26	0.98	>28.77	26.34	26.43	>23.96	>24.04	014
C-5764	0.24	10.64	6.16	0.90	0.19	>28.80	26.93	27.08	>23.68	>23.73	015

Note. This table presents the photometry of the high-redshift galaxy candidates. Column (1) is the candidate ID. Columns (2)–(6) are the calculated S/N values from the 04 diameter circular aperture in the HST bands, and from the Spitzer photometry. Columns (7)–(11) are the calculated AB magnitudes, where the limiting magnitudes correspond to 3σ estimates. Column (12) is the half-light radius of the object in arcseconds.

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In the context of clustering, the probability of finding an excess of LBGs around a quasar is determined by the two-point correlation function, represented as 1 + ξ_QG(r). Here, the quasar and LBG (QSO-LBG) cross-correlation is expressed in a power-law form ${\xi }_{\mathrm{QG}}(r)={\left(r/{r}_{0}^{{QG}}\right)}^{-\gamma }$, where r^QG ₀ signifies the cross-correlation length and γ denotes the slope of the function. The highest redshift at which the QSO-LBG clustering has been studied is z ∼ 4 (García-Vergara et al. 2017), resulting in ${r}_{0}^{{\rm{QG}}}=8.83{h}^{-1}$ comoving Mpc (cMpc) with a fixed slope of γ = 2.0. We adopt these measurements and assume no evolution of the QSO-LBG cross-correlation between z = 4 and z = 7.5 (830 Myr). The resulting LBG excess as a function of the comoving radius around ULAS J1342+0928 is presented with the magenta curve in Figure 8. A quasar field presenting a QSO-LBG excess consistent or above this curve would be considered to reside in a high-density region, suggestive of an overdensity. We calculate the excess based on our observation of one QSO-LBG pair relative to the expected number of LBGs in the field. This expected number is calculated from the number density of LBGs at z = 7.5 as interpolated from the z = 7 and 8 rest-UV luminosity functions from Finkelstein et al. (2015); our completeness fraction from Section 6.1, and the comoving cylindrical volume with a radius equivalent to the comoving distance from the quasar to candidate C-4636, and a comoving line of sight from the quasar's redshift Δz = ± 0.3, corresponding to the redshift uncertainties on the C-4636 z_phot estimated with EAZY (see Section 5.1 and Table 1). Note that in this estimation we do not account for candidate C-[C ii] identified with ALMA in the environment of the quasar, given that this galaxy is not UV bright. Figure 8 shows that at the comoving radius to C-4636 corresponding to 1.9 cMpc, we observe an LBG excess of ${0.46}_{-0.08}^{+1.52}$ (green circle with error bars), indicating that this quasar field is consistent with cosmic density (black-dashed line where LBG excess is = 1), or slightly underdense. This field is incompatible with an overdensity of UV-bright galaxies as our result is at least 1.4 times below the clustering expectations (magenta curve) around z = 4 quasars from García-Vergara et al. (2017). Note that this measurement is limited by low number statistics. Therefore, a larger area coverage or optimal set of filters to improve the completeness of LBGs, and spectroscopic follow-ups, would be necessary to fully characterize the environment of this quasar.

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JWST observations from program GTO 1219 (PI: Luetzgendorf) aim at confirming the LBG candidate C-4636 at z ∼ 7.5 using NIRSpec MSA spectroscopy. The observations cover 0.7–3.1 μm with the G140H/F070LP and G235H/F170LP grating and filter combination. This setup offers a high-resolution power of ∼1000 and ∼2700, respectively, enabling sensitivity to UV metal emission lines such as C iv λ1549, C iii] λ λ 1907,1909, and Mg ii λ2798. These lines would characterize the ionization and chemical enrichment of the galaxy (e.g., Hutchison et al. 2019). The Lyman break at z ∼ 7.5 from the galaxy would potentially be observed providing additional confirmation of the candidate. Recent studies using JWST NIRCam/WFSS spectra have demonstrated overdensities around quasars at slightly lower redshifts. Wang et al. (2023) found 10 [O iii] emitting galaxies in the environment of the quasar J0305-3150 at z = 6.6, probing an overdensity of galaxies in this field. Furthermore, Kashino et al. (2023) compiled a comprehensive catalog of [O iii] emitting galaxies at 5.3 < z < 7.0 in the field of the quasar J0100+2802 at z = 6.327. Among the 117 [O iii] emitters, 24 were associated with the quasar environment, revealing a clear overdensity of galaxies. These findings therefore demonstrate the efficacy of investigating quasar environments by observing strong UV-rest emission lines of galaxies. Applying a similar strategy to the z = 7.54 quasar ULAS J1342+0928, the use of NIRCam/WFSS with the F430M filter would be suitable for identifying such [O iii]-emitting galaxies.

Even though simulations show that massive quasars such as ULAS J1342+0928 are good indicators of galaxy overdensities, the opposite has also been observed (e.g., Bañados et al. 2013; Simpson et al. 2014; Mazzucchelli et al. 2017a). There is no evidence for an evolutionary trend of overdensities with redshift, as, for example, Mignoli et al. (2020) found both LBGs and LAEs in the environment of a quasar at z = 6.31. On the contrary, Goto et al. (2017) did not find any LAEs around a z = 6.4 quasar and pointed to the possibility that the quasar formation drains out the available matter within ∼1 pMpc. There are indeed many physical processes at play in the formation of a quasar and its environment. A possible method to suppress or delay star/galaxy formation within a few proper megaparsecs from the quasar is its UV radiation (e.g., Ota et al. 2018; Costa et al. 2019; Lambert et al. 2024).

In a different scenario, supernova-driven galactic winds could simply sparse the galaxies further away from the quasar and thus reduce the number density observed (e.g., by a factor of up to 3.7 in the HST/ACS area; Costa et al. 2014). Finally, there is also the possibility of the environment being fully dominated by dust-obscured galaxies, and no LBGs or LAEs can be found with traditional photometric techniques. In order to probe this scenario for the ULAS J1342+0928 field, further ALMA observations covering a larger area could unveil this population of galaxy candidates. One can further explore their chemical properties by, e.g., rest-frame optical observations with JWST (e.g., Decarli et al. 2017; García-Vergara et al. 2022).

Analysis of the z = 6.84 absorber detected in the spectrum of ULAS J1342+0928 by Simcoe et al. (2020) suggests that this system may be classified as a damped Lyα system, with a fiducial column density of N_HI = 10^20.6 cm⁻² (Simcoe et al. 2020). Galaxies originating such absorbers at z ∼ 4 are typically located at impact parameters of ≲50 pkpc (e.g., Neeleman et al. 2017, 2019). However, recent studies based on z < 2 Mg ii (λ λ2796, 2803 Å) absorbers, showed that group environment may give rise to stronger and more widespread absorption systems within a projected distance of ≲480 pkpc (e.g., Nielsen et al. 2018; Fossati et al. 2019; Dutta et al. 2020). If this behavior holds at high redshift (see Doughty & Finlator 2023, for the implication of the end of the reionization on absorption systems), this can explain the relatively large impact parameters observed for our two candidates C-4966 and C-5764 (290 pkpc, 476 pkpc, assuming z = 6.9, respectively). The two z ∼ 6.9 galaxy candidates need to be spectroscopically confirmed to establish the physical link with the metal absorption system detected at z ∼ 6.8 by Simcoe et al. (2020).