Preamble

Draft version August 26, 2025 Typeset using L TEX default style in AASTeX631 AGNs in the extremely overdense galaxy region BOSS 1441: A Chandra observation Jiahua Wu Liming Dou Zheng Cai, Yanli Ai, Shiwu Zhang, Zhenya Zheng, Xiaohui Fan, Yuanyuan Su, and Jianfeng Wu 1 Department of Astronomy, Guangzhou University, Guangzhou 510006, China Department of Astronomy, Tsinghua University, Beijing 100084, China College of Engineering Physics, Shenzhen Technology University, Shenzhen 518118, People’s Republic of China Research Center for Astronomical Computing, Zhejiang Laboratory, Hangzhou 311100, China Shanghai Astronomical Observatory, Chinese Academy of Sciences, Shanghai 200030, China Steward Observatory, University of Arizona, 933 North Cherry Avenue, Rm. N204, Tucson, AZ 85721-0065, USA Department of Physics and Astronomy, University of Kentucky, 505 Rose Street, Lexington, KY 40506, USA Department of Astronomy, Xiamen University, Xiamen, Fujian 361005, China

ABSTRACT

We present a Chandra/ACIS-I study of X-ray sources in BOSS 1441, a protocluster at that exhibits a prominent overdensity of Ly emitters (LAEs). Using a 45 ks observation, we identify seven X-ray sources spatially coincident with LAE density peaks. The average X-ray photon index for the seven sources, derived from an absorbed power-law model with Galactic absorption fixed, is 1.49 (ranging from -0.68 to 2.51), corresponding to an average luminosity of 6.85 in the rest-frame 2-33 keV band, with individual luminosities spanning (3.57 – 13.96) . Three sources exhibit relatively flat spectral slopes. Two are associated with the MAMMOTH-1 nebula, while the third, located at the edge of BOSS 1441 with a offset from the LAE density peak, resides in a region with a high submillimeter-band density. We estimate the fraction of X-ray detected AGNs among the LAEs to be 11 %, approximately double that of previously studied LAEs. This elevated fraction suggests BOSS 1441 is in a mature evolutionary stage, with even higher AGN fractions expected in massive LAEs such as PKS 1138-262. In contrast, the submillimeter galaxy population shows a lower AGN fraction (6 %), consistent with their typically obscured nature. These results indicate that the protocluster’s massive galaxies are evolving into the bright red sequence galaxies observed in local clusters, where AGNs likely play a critical role in quenching their star formation.

Keywords

Protoclusters, X-ray active galactic nuclei, Lyman-alpha galaxies

1. INTRODUCTION

Observational studies have shown that in the local universe, galaxy properties exhibit a clear “environmental depen- dence” (e.g., Smith et al. ). Typically, the oldest and most evolved galaxies reside in the central regions of clusters, whereas younger, actively star-forming galaxies tend to inhabit the outer and lower-density areas. In contrast to the star formation–density relation observed in the local universe ( Dressler Goto et al. ), high-redshift galaxies in dense regions are found to experience enhanced star formation, increased interactions, and/or accelerated evolution, along with increased active galactic nuclei (AGNs) activity ( Elbaz et al.

Tran et al. Gr¨utzbauch et al. Koyama et al. Martini et al. ). Studies of X-ray AGN fraction evolution in galaxy clusters ( 3) reveal a significant cosmic-time decline in the prevalence of AGN-hosting cluster galaxies ( Martini et al.

Hashiguchi et al. ). This observed trend underscores the importance of multi-wavelength observations targeting high-redshift overdense environments for elucidating the physical mechanisms driving nuclear activity. Despite their scientific value, comprehensive observational campaigns in these regions remain limited.

Investigating the precursors of local massive clusters, known as protoclusters, may provide essential insights into the earliest and most active phases of cluster formation, particularly on how nuclear activity relates to the local environment. Recent X-ray studies have revealed that AGNs are prevalent in gas-rich protoclusters, where abundant fuel reservoirs likely drive enhanced accretion ( Travascio et al.

Vito et al. ). Notably, these environments also host dusty star-forming galaxies that contribute a growing fraction to the cosmic star-formation rate density Pensabene et al.

Arrigoni Battaia et al. ), suggesting AGNs play a fundamental role in regulating both galactic growth and SMBH evolution. Protoclusters at 2 have been systematically identified through multiple approaches. Galaxy survey-based methods reveal overdensities via spectroscopic follow-up of LBGs, as demonstrated by the

30 HS 1700+643 (

Steidel et al. ) and

09 SSA22 (

Steidel et al. ) protoclusters. Photometric redshift techniques in wide fields (e.g., COSMOS) enable three-dimensional overdensity mapping ( Chiang et al. ), while deep imaging surveys like CFHTLS identify candidates at 6 through dropout selections Toshikawa et al. ), though all methods face limitations from finite survey volumes and spectroscopic constraints.

Alternatively, tracer-based identification targets rare signposts of massive halos: quasars ( Hu et al. ), radio galaxies Venemans et al. ), submillimeter galaxies (SMGs, e.g., Chapman et al. ), and Ly blobs (LABs, e.g., et al. ). While observational efficient, these tracers introduce selection biases and suffer from brief duty cycles.

A more recent approach to identifying protoclusters capitalizes on the observation that overdense regions in the early universe contain not only an abundance of dark matter and galaxies but also significant reservoirs of cold or warm dense gas.

This gas can be detected via absorption against bright background continuum sources, such as quasars and galaxies.

Cai et al. ) used cosmological simulations to establish a strong correlation between Ly optical depth and mass overdensities on scales of 10 comoving Mpc (cMpc). Their findings revealed that opacity increases with mass overdensities, particularly on scales of approximately 15 cMpc, and that regions traced by strong intergalactic medium (IGM) absorption exhibit higher overdensities than those traced by other rare tracers. These insights led to the development of the MAMMOTH (MApping the Most Massive Overdensity Through Hydrogen) technique, which leverages the extensive quasar spectral library from the SDSS-III/BOSS survey to detect rare, strong HI absorption features in the IGM and identify candidate protoclusters with exceptionally high mass concentrations.

Using the MAMMOTH technique, the massive overdensity of BOSS 1441 at 02 is selected from the early data release of SDSS-III/BOSS ( Cai et al. 2017a ). BOSS 1441 is traced by a strong IGM Ly absorption group with within 15 cMpc. The absorption group is 4 absorption systems within the projected 20 cMpc scale. Follow-up narrowband imaging and spectroscopic observations have constrained the Ly emitters (LAEs) in this field. The LAE overdensity in BOSS 1441 reaches 6 on a 15 cMpc scale. Theoretical modeling indicates that the BOSS 1441 protocluster should collapse into a structure = 0 that resembles a rich local cluster with a total mass M . The number of such massive clusters should be 1 within a 10 volume.

Furthermore, BOSS 1441 is associated with an enormous Ly nebula (ELAN) MAMMOTH-1 with a size of 442 kpc Cai et al. 2017b ), which is used to trace the cool gas reservoir and the cosmic web. High- and the presence of the MAMMOTH-1 nebula imply that this is a gas-rich region where multiple obscured AGNs are expected to be present.

X-ray observations are indispensable for studying AGN in dense environments, as X-ray emission can penetrate gas with high column densities. Chandra’s unparalleled spatial resolution and low background noise have been instru- mental in detecting high-redshift X-ray AGNs. Over the past two decades, deep Chandra surveys have systematically uncovered AGN populations within multiple protoclusters (e.g., Travascio et al.

Vito et al. Tozzi et al. Traina et al. ). Notably, Zhang et al. ) identified a Compton-thick AGN (CT-AGN), designated G-2, at the core of the MAMMOTH-1 nebula through Chandra observations, underscoring the facility’s critical role in probing obscured AGN in overdense regions.

In this study, we conduct a systematic search and analysis of all X-ray-detected sources within the vicinity of the MAMMOTH-1 nebula, utilizing data obtained from Chandra observations. Additionally, we compare the AGN fraction in this region with those observed in other protoclusters to provide further insights into the environmental influence on AGN activity. For consistency, all magnitudes in this work are reported in the AB system. We assume a standard flat ΛCDM cosmology with the following parameters: = 70 km s , Ω= 0 3, and Ω

2.1. Chandra Observation

The MAMMOTH-1 Ly nebula was observed by the Chandra X-ray Observatory on 2019 April 21, using the ACIS-I camera ( Garmire et al. ) in the Very Faint (VFAINT) mode with a single exposure time of ; PI: Z. Cai).

We performed the data reduction using the CIAO software package (v4.16; Fruscione et al. ) with the latest calibration files. We generated level 2 event files using the chandra repro script and applied fine astrometric corrections via the astro script, referencing source positions from the Chandra Source Catalog (CSC 2.1; Evans et al.

To identify and exclude periods of high background, we examined the 2.3-7.3 keV band light curve in 100 s time bins, finding no significant flaring events ( ). We produced counts maps, exposure maps, and point-spread function (PSF) maps in three standard energy bands: soft (0.5-2 keV), hard (2-7 keV), and full (0.5-7 keV).

We characterized the X-ray spectral hardness through the hardness ratio (HR), defined as (H (H + S), where H and S represent the background-subtracted counts in the hard and soft bands, respectively. To address low-count statistics, HRs were calculated using the Bayesian Estimation of Hardness Ratios (BEHR) method ( Park et al.

The resulting HR distribution spans a wide range -0.64 to 0.89 (see Table

2.2. X-ray sources detection

We performed X-ray source detection across three energy bands (soft, hard, and full) using the WAVDETECT algorithm Freeman et al. ). The wavelet scales were set in geometric progression (1, 2, 2, 2 2, 4, 4 2, 8, 8 2, and 16 pixels) with a stringent false-positive probability threshold of 10 . Initial source detection across all bands identified 142 candidate sources. Background analysis in source-free regions reveals a mean background level of 2 photons within a 3 aperture in the full band. Poisson statistics give P(X 1%, where X represents photon counts within 3 aperture. This indicates that net counts 5 are statistically significant ( ) against background fluctuations. Applying a conservative selection criterion of 5 net counts in at least one energy band, we obtained 81 statistically significant point sources within the ACIS-I field of view (16 9). In the X-ray sources detection, all CSC sources except three near the field edge were detected. The average positional offset between sources near the aimpoint and their CSC counterparts is . Offsets increase toward the field periphery ( ) due to larger We identified AGNs in the MAMMOTH-1 field through cross-matching of 81 X-ray sources with spectroscopic catalogs from SDSS ( Almeida et al. ) and DESI Early Data Release ( DESI Collaboration et al. ), adopting matching radius. Our analysis yielded three distinct populations: (1) 22 foreground objects at 96; (2) 5 confirmed protocluster members with spectroscopic redshifts 2 342 ( Figure ), where redshifts were determined through Ly emission lines; and (3) 54 unclassified sources requiring additional analysis.

For the unclassified sample, we incorporated optical photometry from DESI Legacy Imaging Surveys ( Dey et al. ) and mid-infrared photometry from AllWISE ( Wright et al. ). Objects with complete spectral coverage across 4 bands were analyzed using Brammer et al. ) for photometric redshift estimation. The full multiwavelength dataset lists in Table . Following the methodology of ( Wright et al. ), we employed WISE color-color diagnostics (their Figure 12 [FIGURE:12]) to effectively separate quasars from stellar and galactic contaminants. All sources in Table include WISE classification according to this scheme.

The MAMMOTH-1 nebula hosts populations of LAEs and SMGs, initially identified by Cai et al. 2017a ) and Arrigoni Battaia et al. ), respectively. Although their spatial distributions exhibit an offset which likely due to the complex gas and dust structure in BOSS 1441, we investigated potential associations with X-ray sources through catalog cross-matching. From the Cai et al. 2017a ) LAE catalog (52 sources within the Chandra field after SDSS/DESI redshift filtering), 15 LAEs have spectroscopic confirmations (13 from MODS observations by Cai et al. 2017a from SDSS/DESI). Six LAEs show X-ray counterparts within a 5 matching radius (LAE10-X1, LAE-X2–X6) . Key associations include LAE10-X1 (G-2 in Zhang et al. ) with CO-confirmed redshift Emonts et al.

Li et al. )) and LAE11-X4, both originally reported by Cai et al. 2017a ). For the SMGs, we combined the 850 m and 450 m catalogs from Arrigoni Battaia et al. ), yielding 29 sources. Only two X-ray counterparts (LAE10-X1 and LAE11-X4) were identified among 59 X-ray sources.

We finally classify six sources (LAE10-X1, LAE-X3, LAE11-X4, LAE-X5, LAE-X6, X7) as confirmed members of the BOSS 1441, supported by spectroscopic redshifts. For LAE-X2, although direct redshift confirmation is unavailable, while lacking direct redshift confirmation, its spatial coincidence with both a LAE and the MAMMOTH-1 nebula 1 All counterparts except LAE10-X1 have matching radii . The larger radius for LAE10-X1 (4.15 ) is justified by its location within the extended MAMMOTH-1 nebula.

z = 2 . 3 0 6 L A E 1 1 - X 4

Rest-frame optical spectra of five BOSS 1441 members from SDSS/DESI observations, normalized to erg s . SDSS observed four sources (LAE-X5, LAE-X6, LAE11-X4, and X7), while LAE-X3 was covered in the DESI Early Data Release. All redshifts are securely confirmed through Ly emission, with supplementary constraints from detected C or Mg absorption features where present.

(Figure ) suggests protocluster membership; we thus adopt 31 for LAE-X2 for subsequent analysis.

Chandra X-ray images of all confirmed members are presented in Figure We generated source and background spectra for the seven BOSS 1441 members using the specextract tool, with response matrix files (RMFs) and ancillary response files (ARFs) produced concurrently. Source extraction regions were centered on each X-ray source, from circular apertures with radii ranging from 2.69 to 7.23 , enclosing of the point spread function encircled energy at 2.3 keV. Background spectra were extracted from concentric annuli with inner radii exceeding the source radii, except for LAE-X5, where a circular 6 aperture avoided contamination from a foreground source (Figure ). The final spectra and response files were utilized for spectral fitting and flux calculations in Section Spatial distribution of X-ray-detected AGN candidates and LAE number density in the BOSS 1441 protocluster field ( 31). The Chandra ACIS-I field of view is outlined by the black dashed square. Green contours show the LAE number density from Cai et al. 2017a ). The Compton-thick AGN G-2 (LAE10-X1; Zhang et al. ) is highlighted with a dotted crosshair. Symbol key: red squares = seven X-ray sources associated with BOSS 1441 (six spectroscopically confirmed; LAE-X2 membership inferred from spatial coincidence); black circles = X-ray sources without optical counterparts; gray squares = 22 spectroscopically confirmed foreground sources ( 2). The inset shows the narrowband image centered on Ly demonstrating the spatial correlation between three X-ray sources (LAE10-X1, LAE-X2, LAE-X5) and the MAMMOTH-1 Ly nebula.

3. X-RAY SPECTRAL ANALYSIS

We performed detailed X-ray spectral analysis of the seven BOSS 1441 members: six LAE counterparts and source X7. Spectra were binned to a minimum of two counts per bin. Spectral analysis was performed in the 0.5 to 7 keV band using the C-statistic ( Kaastra ) in XSPEC( Arnaud , V.12.13). The uncertainties are given at a confidence level 90% for one of the interested parameters, corresponding to ∆

706. All models included

Galactic absorption fixed at N (“phabs” in XSPEC, HI4PI Collaboration et al.

We initially modeled the X-ray spectra using a simple power-law. While all fits were statistically acceptable (Table ), the photon index for LAE10-X1 could not be well constrained (Γ 0), yielding an upper-limit value. The remaining

Chandra/ACIS 25 cutouts of seven sources associated with BOSS 1441, displaying X-ray emission in three energy bands: full (0.5-7 keV), soft (0.5-2 keV), and hard (2-7 keV). All images are binned by a factor of two relative to the native ACIS pixel scale. The black circles indicate the 95% encircled energy radius of the PSF at 2.3 keV, centered on each X-ray source position. sources exhibited Γ = 1.02 – 2.51, with observed 0.5 – 10 keV fluxes in the range (0.85 – 3.80) erg cm corresponding to rest-frame 2 – 33 keV luminosities of (3.57 – 13.96) erg s . The extreme luminosities strongly support the identification of these sources as AGNs.

Given the flat photon indices (Γ = -0.68 – 1.61, 8) measured for LAE10-X1, LAE-X2, and LAE-X3, we applied an intrinsic absorbed power-law model ( zwabs zpowerlaw in XSPEC) with the photon index fixed to Γ = 1 probe their intrinsic absorption N (z). All fits remained statistically acceptable (Table ). The nebula-centered source LAE10-X1 exhibits a Compton-thick (CT) AGN signature with a well-constrained N (z) = 2 . This result is consistent with the CT-AGN candidate G-2 identified by Zhang et al. ), further confirming its heavily obscured nature. For the remaining six sources in our sample (LAE-X2, LAE-X3, LAE11-X4, LAE-X5, LAE-X6, and X7), we were only able to determine upper limits for their N (z) values.

For sources with identical degrees of freedom ( ), the -statistic yields no statistically significant preference between the obscured and unobscured spectral models, indicating that both provide acceptable fits to the data. To further constrain the spectral properties of low-count sources (LAE10-X1, LAE-X2, and LAE-X5), we derived the “effective” photon index (Γ) and intrinsic column density (N (z)) from HRs, following Traina et al. ). We obtain Γ for LAE10-X1 and LAE-X2; and Γ 3 or N for LAE-X5, which are consistent with the -statistic spectral fitting.

4. X-RAY NUMBER COUNTS

Protoclusters host enhanced AGN incidence rates relative to field environments, likely facilitated by rich gas reser- voirs. To quantify the X-ray AGN overdensity, we compute cumulative number counts (log ) as:

N ( > S ) = �

S i >S 1 / Ω , (1)

where represents the flux of the i-th source and Ωis the corresponding sky coverage.

All fluxes were derived using the srcflux tool, assuming a power-law spectrum with photon index (Γ = 1 7) and Galactic absorption (N HI4PI Collaboration et al.

We first examine the line-of-sight distribution of all 81 X-ray sources. Figure shows the soft (0.5 – 2 keV) and hard-band (2 – 10 keV) number counts for sources at 342, with shaded regions indicating 1 (68.27%) Poisson confidence intervals ( Gehrels ). For comparison, we overlay predictions from: (1) the AGN population synthesis model of Gilli et al. ), which includes an exponential decline at 7, and (2) galaxy counts from the 7 Ms Chandra Deep Field-South survey ( Luo et al. ). Our measurements show good agreement with this composite model across both energy bands.

Figure further displays the log distribution for sources in the protocluster redshift range of 2 < z < 342. We find enhanced normalization and a flatter slope relative to field populations, primarily driven by sources with fluxes erg cm , which are predominantly identified as BOSS 1441 members. For comparison, we include PKS 1138-262, a similar protocluster at a redshift 2, using deep 700 ks Chandra data alongside our shallower BOSS 1441 observations. Both systems exhibit comparable soft-band overdensities, but PKS 1138-262 shows significantly stronger hard-band enhancement. This difference may partly arise from incomplete source detection in our shallower BOSS 1441 data.

N ( > S ) d e g 2

Cumulative number counts for X-ray sources at

342. Left: Soft-band log

distribution within the Chandra field, compared to the composite model combining the Gilli et al. ) AGN population synthesis (with exponential cutoff), and Luo et al. ) galaxy counts.

Right: Hard-band distribution with identical model comparison.

Shaded regions indicate 1 uncertainty.

5.1. X-ray properties of BOSS 1441 members

Among the seven X-ray-detected AGNs in BOSS 1441, three sources (LAE10-X1, LAE-X2, and LAE-X3) show spectral characteristics indicative of obscuration, with flat photon indices ( 8) and elevated hardness ratios. LAE10- X1, the most extreme case, is confirmed as a Compton-thick (CT) AGN through direct column density measurement.

Applying the standard gas-to-dust ratio ( Bohlin et al. )), we derive extreme extinction values: 1119 mag for LAE10-X1, with upper limits of 840 mag for LAE-X2 and mag for LAE-X3. These obscuration estimates are consistent with DESI photometric observations, where LAE10- X1 (g = 23.26 mag) and LAE-X3 (g = 23.09 mag) appear as faint optical counterparts, while LAE-X2 remains undetected optically. The high fraction of obscured AGNs in this sample supports theoretical models of gas-rich, dense environments ( Gilli et al.

Vito et al. ). Particularly noteworthy is the spatial coincidence of two heavily

N ( > S ) d e g 2

Cumulative number counts within the protocluster redshift range (2 < z < 342).

The plotting format follows Figure . The observed excess at fluxes erg cm is attributed to AGN members of the BOSS 1441 system.

The dashed line shows the cumulative number counts for the PKS 1138-262 protocluster from Tozzi et al. ), included for comparison with BOSS 1441. obscured AGNs (LAE10-X1 and LAE-X2, both with HR 8) with the MAMMOTH-1 nebula, a known reservoir of massive gas content.

LAE-X3 displays a flat spectral slope (Γ = 1 ), despite its offset from the LAE density peak. Intriguingly, SCUBA-2/JCMT observations place this source within a region of elevated submillimeter source density (see Fig. 10 [FIGURE:10] Arrigoni Battaia et al. ), indicating significant obscuration by both gas and dust components. This spatial configuration reveals the complex and inhomogeneous matter distribution within the BOSS 1441 system.

The spatial distribution of AGNs in BOSS 1441 reveals an interesting pattern: two unobscured AGNs (LAE11-X4 and LAE-X5) reside near the LAE density peak, with LAE11-X4 additionally exhibiting an SMG counterpart. Their redshift measurements (z=2 0005 and z=2 0011), combined with their projected positions, suggests these sources lie in the protocluster’s periphery. Their lower obscuration is naturally explained by the reduced gas densities characteristic of such outer environments.

In addition, the presence of multiple AGNs is a well-documented feature of enormous Ly nebulae (ELANe) at 2, as demonstrated by Cantalupo et al. ) and Hennawi et al. ). In MAMMOTH-1, we observe three X-ray sources spatially coinciding with Ly surface brightness peaks (Figure inset; Cai et al. 2017b ), indicating that AGN activity significantly impacts Ly morphology. This interaction may operate through two primary mechanisms: (1) enhanced UV radiation from AGNs potentially amplifying Ly emission, and (2) a denser circumgalactic medium facilitating intensified fluorescence via scattering processes ( Cai et al.

5.2. X-ray AGN Fraction

The environmental dependence of AGN fraction provides critical insight into SMBH fueling mechanisms. X-ray studies have quantified this relationship across diverse environments (e.g., Lehmer et al.

Lehmer et al. Digby-North et al. Macuga et al. Polletta et al. ). For LAEs and SMGs in BOSS 1441, we define the X-ray AGN fraction as:

f = N AGN N LAE / SMG , (2)

with Poissonian uncertainties computed as double-sided 68.27% (1 ) confidence intervals following Gehrels Our measurements yield: % for all 52 LAEs; % for the 15 spectroscopically confirmed LAEs; and % for 29 SMGs.

Figure compares these values with other protoclusters covered by sensitive X-ray observations ( Vito et al. ). Notably, represents one of the highest AGN fractions observed in protoclusters, while is significantly lower.

This dichotomy reflects intrinsic differences in AGN incidence

between galaxy populations. Within SMGs, typical obscuration and low AGN luminosities limit detectability to only the brightest sources ( Vito et al. , Fig. 4 [FIGURE:4]).

This AGN fraction for LAEs ( %) represents a factor of 2 increase over the SSA22 protoclus- Lehmer et al. ), and is elevated by 17.9 relative to the field. Fisher’s Exact Test confirms this enhance- ment is statistically significant ( 99% confidence). These results contrast with measurements from the protocluster HS 1700+64 ( Digby-North et al. ), where a similar LAE selection yielded an AGN fraction comparable to SSA22, but with only 87% confidence. The extremely high AGN activity in BOSS 1441 suggests distinct environmental con- ditions may be driving enhanced SMBH growth. This is further supported by (Figure ), demonstrating that the high X-ray source density cannot be solely attributed to elevated galaxy number density.

X-ray AGN fractions across protoclusters at 1

5. Red stars denote AGN fractions for LAEs in BOSS 1441

(this work), while the red diamond represents SMGs in the same structure. Circles indicate AGN fractions from published protocluster studies ( Vito et al. ), computed using heterogeneous galaxy tracers. Filled symbols designate measurements using spectroscopically confirmed members when available; open symbols correspond to results based on photometric member candidates.

The MAMMOTH technique has identified three massive overdensities: BOSS 1441, BOSS 1244, and BOSS 1542. As characterized by Shi et al. ), these structures exhibit distinct evolutionary states: BOSS 1441 shows signatures of advanced virialization, BOSS 1244 comprises one or two merging protoclusters, and BOSS 1542 forms a filamentary network. These morphologies align with the three-phase cluster formation framework of Shimakawa et al. ): (1) Growing phase ( 3): large-scale structure collapse; (2) Maturing phase ( 3): transition from dusty starbursts to quenched populations; (3) Declining phase ( 2): emergence of red sequences.

Our Chandra observations reveal an elevated X-ray AGN fraction in BOSS 1441 ( %). This enhanced AGN activity provides direct observational evidence for ongoing SMBH growth, strongly supporting Shi et al. ’s virial- ization hypothesis. The measurement confirms the predicted correlation between structural maturity and AGN activity during the critical maturation phase at The dense protocluster PKS 1138-262, associated with a radio galaxy galaxy at 156 ( Carilli et al. ), exhibits striking evidence for AGN-driven quenching.

Shimakawa et al. ) report a stellar mass-dependent AGN fraction among H emitters (HAEs), with 60% (4 6) of massive systems (M ) hosting X-ray AGNs. These HAEs occupy the post-starburst region in rest-frame color space, displaying intermediate properties between star-forming galaxies and passive populations, suggesting AGN feedback may drive their transition from star-forming to quiescent states.

For BOSS 1441, a protocluster in the maturing phase with ongoing galaxy quenching, we similarly predict elevated AGN fractions among massive LAEs. This aligns with findings in SSA22, where Lehmer et al. ) demonstrated that LAEs/LBGs with stellar masses 1.2–1.8 higher than field galaxies host proportionally enhanced AGN incidence.

Together, these results underscore SMBH-galaxy co-evolution signatures during cluster assembly.

For five SDSS/DESI AGNs (LAE-X3, LAE11-X4, LAE-X5, LAE-X6, and X7), we estimate the virial SMBH masses via C emission: + 2 log

� = a + b log � λL λ 10 44 erg s − 1

where 66 and 53 are empirical coefficients for C Wu & Shen ). The calculated values = (2.24 – 8.32) align with the DR16Q sample average ( Wu & Shen ), suggesting that environmental mechanisms, rather than individual SMBH growth processes, dominate the elevated AGN fraction in BOSS 1441.

This apparent contradiction may arise from obscured AGNs in gas-rich environments: our X-ray spectral analysis (Section ) reveals several Compton-thick candidates with intrinsic erg s , where dense circumnuclear material could simultaneously obscure AGNs and fuel rapid SMBH growth.

6. CONCLUSION

Chandra X-ray study of the BOSS 1441 protocluster, host to the MAMMOTH-1 nebula, reveals a distinct population of AGNs embedded in its gas-rich, overdense environment. The main findings are summarized as follow: 1. The MAMMOTH-1 neblua is associated with three X-ray AGNs (LAE10-X1, LAE-X2, LAE-X5), indicating that its Ly luminosity is powered by multiple AGNs rather than a single ionizing source. 2. X-ray hardness ratios and spectral modeling identify heavily obscured AGNs, including a Compton-thick AGN with N (z) = 2 . This obscuration aligns with the protocluster’s gas-rich conditions, where dense material both shrouds AGNs and fuels rapid supermassive black hole growth. 3. An obscured object (LAE-X3) is spatially offset from the LAE density peak yet coincides with a submillimeter- band overdensity. This configuration reveals concurrent gas/dust obscuration and traces inhomogeneous matter distribution within the protocluster.

4. The X-ray AGN fraction in BOSS 1441 is 11

% for the full LAE sample (52 sources), representing a statistically significant enhancement ( 99% confidence). For the subset of 15 spectroscopically confirmed LAEs, this fraction rises to 33 %, ranking among the highest values observed in protoclusters. Both fractions rank among the highest observed in protoclusters. These elevated fractions support BOSS 1441’s classification as a system in a maturing phase of virialization, contrasting with younger protoclusters (e.g., SSA22) and mirroring evolved counterparts (e.g., PKS 1138-262) where AGN feedback likely drives galaxy quenching. These signatures indicate BOSS 1441 is transitioning to a phase of declining star formation, with massive members evolving into red-sequence galaxies.

Collectively, these findings establish gas-rich protoclusters as pivotal laboratories for SMBH-galaxy co-evolution, where environmental fueling mechanisms govern the emergence of massive quiescent galaxies in the early universe.

LAE11-X4 ULIRG/LINER; Obscured AGN ULIRG; LINER; Starburst LAE-X6 ULIRG; LINER; Starburst LAE-X3 ULIRG; LINER; Starburst Seyfert galaxy Seyfert galaxy

XID α X β X R 95 zsp zph Netcts S 0 . 5 − 2 keV S 2 − 10 keV HR g r z W 1 W 2 W 3 W 4 Type

41+13

LAE10-X1 220.3520 40.0520 2.71 ′′ 2.312 2 . 210 5 . 96+3 . 00 − 2 . 40 < 0 . 03 0 . 42+0 . 20 − 0 . 16 0 . 86+0 . 14 − 0 . 01 23.26 23.03 23.11 19.45 19.07 17.68 15.40 QSO

LAE-X5

LAE-X2 220.3525 40.0473 2.69 ′′ — — 7 . 80+3 . 24 − 2 . 90 < 0 . 03 0 . 65+0 . 25 − 0 . 21 0 . 89+0 . 11 − 0 . 01 — — — — — — — —

. X-ray Source Catalog in the Chandra Field of View ULIRG; LINER; Starburst ULIRG; LINER; Starburst Seyfert galaxy Seyfert galaxy Seyfert galaxy

X36 220.3579 40.0682 2.91 ′′ — — 5 . 77+2 . 64 − 2 . 38 < 0 . 04 0 . 39+0 . 22 − 0 . 17 0 . 85+0 . 15 − 0 . 01 — — — — — — — —

XID α X β X R 95 zsp zph Netcts S 0 . 5 − 2 keV S 2 − 10 keV HR g r z W 1 W 2 W 3 W 4 Type

X55 220.5038 40.1324 10.82 ′′ — — 5 . 41+4 . 58 − 4 . 22 < 0 . 11 0 . 76+0 . 53 − 0 . 48 0 . 60+0 . 40 − 0 . 04 24.68 24.31 22.92 — — — — —

X54 220.2622 40.0582 4.02 ′′ — — 3 . 80+2 . 70 − 2 . 33 < 0 . 07 0 . 55+0 . 28 − 0 . 23 0 . 65+0 . 35 − 0 . 06 24.21 24.10 22.69 — — — — —

X58 220.3714 40.1209 4.01 ′′ — — 4 . 80+2 . 92 − 2 . 56 < 0 . 06 0 . 42+0 . 25 − 0 . 20 0 . 75+0 . 25 − 0 . 04 — — — — — — — —

net counts; (8) 2keV: Soft-band flux [erg cm 1]; (9) 10keV: Hard-band flux [erg cm 1]; (10) HR: Hardness ratio ( ); (11)-(13) DESI ] photometry [AB mag]; (14)-(17) WISE [ 4] photometry [AB mag]; (18) Class: WISE classification ( Wright et al. ). Catalog contains 59 X-ray sources after foreground exclusion. Fluxes computed with srcflux — Column (1) XID: Source designation; (2) R.A.: Right ascension (J2000) [deg]; (3) Dec.: Declination (J2000) [deg]; (4) 95: 95% encircled-energy PSF radius [arcsec]; spec: Spectroscopic redshift (SDSS/DESI/CO); (6) phot: Photometric redshift computed by EAZY ( Brammer et al. ); (7) CountsFB: Full-band (0.5-7 keV)

Best-fit parameters for the simple power-law spectral model LAE10-X1 LAE-X2 LAE-X3 LAE11-X4 LAE-X5 LAE-X6 —Column (1) XID: Source designation; (2) : Power-law normalization at 1 keV (10 photons keV ); (3) Γ: Photon index; (4) log 10keV : Observed 0.5-10 keV flux (erg ); (5) log : Rest-frame 2-7 keV luminosity (erg s ); (6) log 33keV : Rest- frame 7-33 keV luminosity (erg s ); (7) C/ : Cash statistic and degrees of freedom. All flux and luminosity values are corrected for Galactic absorption. For LAE-X2, we assume Best-fit parameters for the intrinsic absorbed power-law model (Γ fixed to 1.8) LAE10-X1 LAE-X2 LAE-X3 LAE11-X4 LAE-X5 LAE-X6 —Column (1) XID: Source designation; (2) : Power-law normalization at 1 keV [photons ]; (3) : Intrinsic column density [10 ]; (4) log 10keV : Observed 0.5- 10 keV flux [erg cm ]; (5) log : Rest-frame 2-7 keV luminosity [erg s ]; (6) log 33keV : Rest-frame 7-33 keV luminosity [erg s ]; (7) C-stat/dof: Cash statistic and degrees of freedom. All flux and luminosity values are corrected for both Galactic and intrinsic absorption.

Luminosities assume 31 for LAE-X2.

This work was supported by the Joint Research Foundation in Astronomy under the cooperative agreement between the National Science Foundation of China and the CAS (U1731104, U1731109), National Science Foundation of China (NSFC-11833007). L.M.D. also acknowledges the support from the Key Laboratory for Astronomical Observation and Technology of Guangzhou, the Astronomy Science and Technology Research Laboratory of the education department of Guangdong Province.

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