arXiv subjects

Junehyoung Jeon

Publications and source records attributed to Junehyoung Jeon.

14 recordsLinked to original sources

arXiv2026

Early Supermassive Black Holes and Little Red Dots Require Free-Fall Growth

Supermassive black holes/active galactic nuclei (SMBHs/AGN), forming only a few hundred million years after the Big Bang as observed with the James Webb Space Telescope (JWST), challenge theoretical understanding. How could they grow so massive $(M_{\rm BH} > 10^6 {\rm \,M}_\odot)$ so quickly after initial seeding? Is this rapid growth related to the numerous and enigmatic Little Red Dots (LRDs), compact sources with AGN-like characteristics, discovered by JWST? To address these mysteries, we consider the first-order constraint on SMBH growth: enough baryonic material has to reach the vicinity of the SMBH seed, located near the bottom of the gravitational potential well of the host dark matter halo. We specifically examine cold-mode accretion, where gas from the cosmic environment flows into the virialized halo in cold streams without being shock-heated, efficiently reaching the center on a free-fall timescale. We find that cold mode accretion is necessary to supply material for the SMBHs to reach the observed masses, whereas for shock-heated gas inflow the required amount could only be supplied by implausibly rare halos. Moreover, cold-mode inflow in rare $(\sim1$ Gpc$^{-3}$) halos matches the mass and number of the massive quasars, and halos able to support super-Eddington accretion for massive SMBHs ($\sim10^7$ M$_\odot$) match LRD number densities. The decreasing LRD abundance at lower redshifts may then reflect the termination of cold-mode accretion in the growing host halos. The populations of massive SMBHs and LRDs at early times may thus arise naturally from cosmological structure formation, based on the abundance of halos capable of supplying sufficient material through cold accretion.

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arXiv2026

The Road to Normalcy: Environment-Driven Evolutionary Pathways for Primordial Black Holes

We investigate how cosmological environment regulates the evolution of primordial black hole (PBH) seeds in the early universe using a suite of hydrodynamical simulations. Unlike traditional seeding channels, PBHs provide early BH seeds without the need of special conditions for gas collapse/fragmentation that favor overdense regions, thus covering diverse large-scale environments. We find that PBHs follow distinct evolutionary pathways depending on the gas supply and halo assembly history. In underdense regions, limited inflow suppresses both accretion and star formation, producing faint, metal-poor systems, while in overdense environments, sustained gas inflow drives rapid BH growth and the formation of compact, centrally concentrated stellar components. These differences lead to large variations in BH-to-stellar mass ratio, metallicity, and morphology, collectively bracketing a range of possible evolutionary pathways for PBH-seeded systems. We show that compact, BH-dominated sources resembling recently observed Little Red Dots naturally arise as one phase within these evolutionary pathways before evolving into more extended galaxy--AGN systems. Our results suggest that both the initial seed properties and cosmological environment jointly shape early BH--galaxy co-evolution, while subsequent environmental regulation can erase the memory of the initial seeding channel.

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arXiv2026

A Rapid Evolution in the Observed Mbh/M* Relation at z > 3 Revealed via Spectro-photometric SED-Modeling

Spectroscopic observations from JWST have uncovered a plethora of active galactic nuclei (AGN) at z > 4 with black hole (BH) mass (Mbh) to stellar mass (M*) ratios significantly above the local relation when using standard virial mass scaling relations. However, M* estimates of AGN may be inaccurate due to limitations in spectral energy distribution (SED) fitting codes, exemplified by a lack of physically-motivated AGN line emission models. Here, we fit NIRSpec/PRISM spectra of 39 galaxies at z ~ 3.5-7 selected as broad-line AGN from the CEERS and RUBIES surveys. Applying kinematic decompositions from NIRSpec/G395M spectra, we fit their continuum and narrow-component line fluxes using the BEAGLE-AGN SED fitting tool. While limitations of BEAGLE-AGN make it difficult to model little red dots (LRDs), we find that M* estimates of non-LRDs are, surprisingly, only modestly impacted by the inclusion or not of AGN narrow-line region (NLR) and continuum emission model components. We further find that non-LRD AGN at z < 3.5 are consistent with the local Mbh/M* relation while those at z > 4.5 display elevated ratios. While we cannot rule out observational biases or systematic uncertainties as partial causes, this transition over just ~500 Myr is driven entirely by changes in M* rather than an evolving Mbh distribution. These findings are consistent with models in which rapid BH growth results in elevated Mbh/M* ratios at early times, with a swift late-time assembly of host galaxies returning sources to the local relation at z < 4.

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arXiv2026

What is Powering the Enigmatic He II Emitter Hebe: The First Stars or Black Holes?

Recent high-resolution spectroscopy with the James Webb Space Telescope (JWST) has confirmed the presence of a strong He II $\lambda1640$ emitting clump in the vicinity of GN-z11, with only upper limits on its metallicity. To explain the peculiar properties of this source, now termed Hebe, a cluster of metal-free, Population III (Pop III) stars has been invoked. A less likely source for the hard UV ionizing radiation could be an accreting supermassive black hole (SMBH) embedded inside Hebe. We here provide further constraints on what could power the observed emission lines in Hebe. Comparing with cosmological simulations of Pop III star cluster formation, we assess the maximum Pop III stellar mass that could plausibly form at the location of Hebe, finding stellar masses of a few $10^5\,M_{\odot}$, consistent with those inferred from the observations. Modeling the continuum spectral energy distribution arising from an accreting SMBH, we derive He II and H I ionizing rates and the resulting recombination line luminosities, providing a less natural fit for the combined observations. We thus confirm the interpretation of Hebe as a remarkable, primordial object, with the most plausible power source provided by a massive cluster of Pop III stars, at the limit of what is allowed within the standard model of first star formation.

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arXiv2026

Hunting for the First Explosions at the High-Redshift Frontier

The James Webb Space Telescope (JWST) has spectroscopically confirmed galaxies up to $z\sim14$, 300 Myr after the Big Bang, and several candidates have been discovered at $z\sim15-25$, with one candidate as high as $z\sim30$, only 100 Myr after the Big Bang. Such objects are unexpected, since theoretical studies have not predicted the existence of detectable galaxies at $z\sim30$. While any $z\sim30$ candidates may be contaminants at lower redshifts, we explore whether such extreme redshift sources could be consistent with hyper-energetic transient events linked to the formation of the first, metal-free, stars. Specifically, we consider pair-instability supernovae (PISNe), a predicted class of extreme thermonuclear explosions that leave no remnant behind. Using cosmological simulations, we investigate an overdense cosmic region, where star formation and subsequent PISNe occur at $z\sim30-40$, even within standard cosmology. Assessing the likelihood of such a region, the corresponding number of PISNe at $z\gtrsim20$, and their observed flux, we find that JWST has a non-negligible chance to detect a PISN event at extremely high redshifts. If a transient event were confirmed at $z\sim30$, this would provide a direct glimpse into the epoch of first star formation, dramatically extending the empirical reach of astronomy.

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arXiv2025

A GLIMPSE of Intermediate Mass Black holes in the epoch of reionization: Witnessing the Descendants of Direct Collapse?

JWST has revealed an abundance of supermassive black holes (BHs) in the early Universe, and yet the lowest mass seed black holes that gave rise to these populations remain elusive. Here we present a systematic search for broad-line Active Galactic Nuclei (AGNs) in some of the faintest high-$z$ galaxies surveyed yet by combining ultra-deep JWST/NIRSpec G395M spectroscopy with the strong lensing aid in Abell S1063. By employing the profile of the [OIII]$\lambda 5007$ emission lines as a template for narrow-line components and carefully cross-validating with mock observations, we identify a sample of ten broad-line AGNs at $4.5<z<7.0$ (eight secure, two tentative). The inferred BH masses from the broad H$\alpha$ line explore the intermediate BH mass regime down to $\sim 10^{5.5}\,M_\odot$. The stellar mass ($M_*$) is estimated with a galaxy+AGN composite model, and we find the BH to stellar mass ratio spans down to $M_{\rm BH}/M_*\lesssim 0.1\%$, unveiling populations on the empirical $M_{\rm BH}-M*$ relation observed in the local universe. We also derive the black hole mass function and investigate its low-mass end at this epoch. While we confirm the agreement of our results with previous studies at $M_{\rm BH}\gtrsim10^{6.5}M_{\odot}$, we find the mass range of $\sim 10^{5.5}\,M_\odot$ features an enhanced abundance with respect to the extrapolated best-fit Schechter function. Comparison with theoretical models suggests that a possible origin for this enhanced abundance is the direct-collapse BH formation, supporting the scenario that the direct collapse of massive gas clouds is a significant pathway for the earliest supermassive BHs.

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arXiv2025

Little Red Dots and their Progenitors from Direct Collapse Black Holes

The James Webb Space Telescope (JWST) has discovered a new population of objects, the Little Red Dots (LRDs), characterized by V-shaped spectra indicative of strong breaks around the Balmer limit and compact morphology that gave them their name. A popular explanation is that they are a sub-population of active galactic nuclei/supermassive black holes (AGN/SMBHs) predominantly found in the high-redshift Universe ($z\gtrsim3$). Similarly, direct collapse black holes (DCBHs), theorized to form from collapsing massive, extremely metal-poor gas clouds, have been invoked to explain high-redshift quasars, the most massive AGN sub-population. Here, we employ the semi-analytical code A-SLOTH to produce a population of DCBHs and compare them against observed LRD demographics and properties. Specifically, we compare the DCBH-seeded SMBH population against the standard stellar-remnant seeds and find that DCBH models agree better with observed LRD population statistics and host halo properties. Furthermore, for the most extreme and earliest LRD detections, interpreted to be systems with an AGN but little stellar component, DCBHs are able to reproduce the observed spectral shape and properties under multiple scenarios - high dust attenuation or AGN surrounded by dense gas - that have been proposed to explain the unique shape of LRD spectra. Even when super-Eddington accretion, invoked previously to explain the nature of LRDs, is enforced on stellar remnant seeds, the spectral characteristics of extreme LRDs cannot be reproduced. We emphasize the importance of gas-metallicity observations as an additional dimension besides the widely used SMBH-stellar mass ratios to further constrain the progenitors of LRDs.

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arXiv2025

How do Massive Primordial Black Holes Impact the Formation of the First Stars and Galaxies?

We investigate the impact of massive primordial black holes (PBHs; $m_{\rm BH}\sim 10^6~M_{\odot}$) on the star formation and first galaxy assembly process using high-resolution hydrodynamical simulations from $z = 1100$ to $z \sim 9$. We find that PBH accretion is self-regulated by feedback, suppressing mass growth unless feedback is weak. PBHs accelerate structure formation by seeding dark matter halos and gravitationally attracting gas, but strong feedback can delay cooling and suppress star formation. In addition, the presence of baryon-dark matter streaming creates an offset between the PBH location and the peaks induced in gas density, promoting earlier and more efficient star formation compared to standard $\Lambda$CDM. By $z \sim 10$, PBH-seeded galaxies form dense star clusters, with PBH-to-stellar mass ratios comparable to observed high-$z$ AGN like UHZ-1. Our results support PBHs as viable SMBH seeds but do not exclude alternative scenarios. We emphasize that PBH-seeding provides a natural explanation for some of the newly-discovered overmassive SMBHs at high redshift, in particular those with extreme ratios of BH-to-dynamical (virial) mass that challenge standard formation channels. Future studies with ultra-deep JWST surveys, the Roman Space Telescope, and radio surveys with facilities such as SKA and HERA will be critical in distinguishing PBH-driven SMBH growth from other pathways.

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arXiv2025

The Emerging Black Hole Mass Function in the High-Redshift Universe

Observations with the James Webb Space Telescope (JWST) have identified an abundant population of supermassive black holes (SMBHs) already in place during the first few hundred million years of cosmic history. Most of them appear overmassive relative to the stellar mass in their host systems, challenging models of early black hole seeding and growth. Multiple pathways exist to explain their formation, including heavy seeds formed from direct collapse/supermassive stars or sustained super-Eddington accretion onto light stellar remnant seeds. We use the semi-analytical code A-SLOTH to predict the emerging SMBH mass function under physically motivated models for both light and heavy seed formation, to be compared with upcoming ultra-deep JWST surveys. We find that both pathways can reproduce observations at $z\sim5-6$, but have distinct features at higher redshifts of $z\sim10$. Specifically, JWST observations have the potential to constrain the fraction of efficiently accreting (super-Eddington) SMBHs, as well as the existence and prevalence of heavy seeds, in particular through ultra-deep observations of blank fields and/or gravitational lensing surveys. Such observations will provide key insights to understand the process of SMBH formation and evolution during the emergence of the first galaxies. We further emphasize the great promise of possible SMBH detections at $z\gtrsim 15$ with future JWST observations to break the degeneracy between light- and heavy-seed models.

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arXiv2024

Broad-Line AGN at 3.5<z<6: The Black Hole Mass Function and a Connection with Little Red Dots

We present a sample of 50 H-alpha detected broad-line active galactic nuclei (BLAGN) at redshifts 3.5 0), independent of the contributions of emission lines to the broadband photometry. We construct the black hole (BH) mass function at 3.5<z<6 after computing robust observational and line detection completeness corrections. This BH mass function shows broad agreement with both recent JWST/NIRSpec and JWST/NIRCam WFSS based BH mass functions, though we extend these earlier results to log(M(BH)/M(sun)) < 7. The derived BH mass function is consistent with a variety of theoretical models, indicating that the observed abundance of black holes in the early universe is not discrepant with physically-motivated predictions. The BH mass function shape resembles a largely featureless power-law, suggesting that any signature from black-hole seeding has been lost by redshift z~5-6. Finally, we compute the BLAGN UV luminosity function and find good agreement with JWST-detected BLAGN samples from recent works, finding that BLAGN hosts constitute <10% of the total observed UV luminosity at all but the brightest luminosities.

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arXiv2024

Physical Pathways for JWST-Observed Supermassive Black Holes in the Early Universe

Observations with the James Webb Space Telescope (JWST) have revealed active galactic nuclei (AGN) powered by supermassive black holes (SMBHs) with estimated masses of $10^7-10^8$ M$_\odot$ at redshifts $z\sim7-9$. Some reside in overmassive systems with higher AGN to stellar mass ratios than locally. Understanding how massive black holes could form so early in cosmic history and affect their environment to establish the observed relations today are some of the major open questions in astrophysics and cosmology. One model to create these massive objects is through direct collapse black holes (DCBHs) that provide massive seeds ($\sim10^5-10^6$ M$_\odot$), able to reach high masses in the limited time available. We use the cosmological simulation code GIZMO to study the formation and growth of DCBH seeds in the early Universe. To grow the DCBHs, we implement a gas swallowing model set to match the Eddington accretion rate as long as the nearby gaseous environment, affected by stellar and accretion disk feedback, provides sufficient fuel. We find that to create massive AGN in overmassive systems at high redshifts, massive seeds accreting more efficiently than the fiducial Bondi-Hoyle model are needed. We assess whether the conditions for such enhanced accretion rates are realistic by considering limits on plausible transport mechanisms. We also examine various DCBH growth histories and find that mass growth is more sustained in overdense cosmological environments, where high gas densities are achieved locally. We discuss the exciting prospect to directly probe the assembly history of the first SMBHs with upcoming, ultra-deep JWST surveys.

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arXiv2023

Observability of Low-Luminosity AGN in the Early Universe with JWST

Active galactic nuclei (AGN) in the early Universe are thought to be prominent sources of energy and ionizing photons that affected the growth of their host galaxy and their environment. However, it is still unclear how the supermassive black holes (SMBHs) that fuel these AGN grew to the observed high masses already at high redshifts. Observations of high-redshift SMBH progenitors or lower-luminosity AGN will thus help characterize the evolution of SMBHs and their impact on the surroundings. With the launch of the JWST, fainter objects at high redshifts can now be detected, including lower-luminosity AGN. We assess the observability of such low luminosity AGN, using the cosmological simulation code GIZMO to provide a realistic environment for black hole growth in the early Universe. Soon after the first stars are born in the simulation run, we insert stellar-remnant black hole seeds of various initial masses, between $300$ and $10^4 {\rm \ M}_{\odot}$, at the center of a dark matter halo and follow their growth until $z\sim6$. Such stellar black hole seeds placed in a typical high-$z$ environment do not significantly accrete and grow to reach masses that can be observed with the JWST under conditions of standard Bondi-Hoyle accretion, as energy input from stellar feedback and chaotic dynamics prevent efficient gas accretion onto the black holes. To be observed with the JWST, rarer but still physically feasible growth regimes, involving Eddington or super-Eddington accretion, would be required. Alternatively, AGN observability may be boosted under even rarer conditions of extreme gravitational lensing.

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arXiv2022

Maximal X-ray feedback in the pre-reionization universe

X-ray feedback in the pre-reionization Universe provided one of the major energy sources for reionization and the thermal evolution of the early intergalactic medium. However, X-ray sources at high redshift have remained largely inaccessible to observations. One alternative approach to study the overall effect of X-ray feedback in the early Universe is a full cosmological simulation. Toward this goal, in this paper we create an analytic model of X-ray feedback from accretion onto supermassive black holes (SMBHs), to be used as a sub-grid model in future cosmological simulations. Our analytic model provides a relation between the mass of a dark matter halo and the SMBH it hosts, where the efficiency is governed by an energy balance argument between thermal feedback and the confining gravitational potential of the halo. To calibrate the model, we couple the halo-level recipe with the Press-Schechter halo mass function and derive global mass and energy densities. We then compare our model to various observational constraints, such as the resulting soft X-ray and IR cosmic radiation backgrounds, to test our choice of model parameters. We in particular derive model parameters that do not violate any constraints, while providing maximal X-ray feedback prior to reionization. In addition, we consider the contribution of SMBH X-ray sources to reionization and the global 21 cm absorption signal.

astro-ph.CO↗

arXiv2022

SED Analysis of 13 Spectroscopically Confirmed Galaxies at z$\simeq$6 to Constrain UV-Slope, Model Dust Attenuation and Escape Fractions

The reionization of the hydrogen in the Universe is thought to have completed by redshift $z\simeq5.5-6$. To probe this era, galaxy observations in the Subaru Deep Field (SDF) have identified more than 100 galaxies at $z\simeq6$, many spectroscopically confirmed through follow-up observations. We model the spectral energy distributions (SEDs) of 13 SDF galaxies with the CIGALE and Dense Basis codes using available optical/IR data. Modeling deep IR photometry has the potential to constrain the galaxy's Lyman continuum (LyC) escape fraction (\fesc). We use the modeled nebular emission lines and find that the implied escape fractions ranges from \textbf{0 to 0.8 with a median of $\sim$0.35 for Dense Basis and $\sim$0.55 for CIGALE.} Significant uncertainties in the data exist, so that fitting results in a large range of \fesc\ for individual objects. The implied median \fesc-values may be high enough for galaxies to finish reionization by $z\sim6$. Furthermore, we find no strong trends between the UV-slope $β$ or \EBminV with model \fesc. If true, the lack of trends suggest that other factors besides nebular emission or dust extinction could have led to LyC escaping, such as the presence of holes in the ISM with sufficiently wide opening angles from outflows of supernovae and/or weak AGN, resulting in a range of implied \fesc-values depending on the viewing angle of each galaxy. The current \textit{HST, Spitzer} and ground-based photometric and model errors for the galaxies remain large, so IR spectroscopic observations with the \textit{James Webb Space Telescope} are needed to constrain this possibility.

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