arXiv subjects

Nicolas Laporte

Publications and source records attributed to Nicolas Laporte.

At least 19 recordsLinked to original sources

arXiv2026

HST's Deep Blue: extremely deep UV imaging to reveal the contributors to reionization

Understanding galaxy evolution in the epoch of reionization and the effect these galaxies had on the transformation of the intergalactic medium from neutral to ionized, is a key goal of modern astrophsyics, and is central to both HST's and JWST's missions. The biggest remaining uncertainty is the escape fraction of ionizing photons from galaxies. Quantifying and understanding this at redshifts close to reionization is an objective only achievable through observations with HST, in particular, deep imaging with the WFC3/UVIS instrument to detect ionizing photons from galaxies at 2<z<4. A survey across 20 fields with supporting spectroscopy would both build up a sample similar to the state of the art at low redshift, and also overcome the uncertainty stemming from the unknown transmission of ionizing photons through the intergalactic medium. Such a program would establish the tracers of ionizing photon escape to use within the epoch of reionization and reconcile the growth of the first galaxies with the progression and topology of reionization.

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arXiv2026

Life After the Quasar: Overmassive Black Holes and Remnant Ionised Bubbles in and Around Two z~6.6 Galaxies

Supermassive black holes (SMBH, $M_{\rm{BH}} > 10^8 M_\odot$) powering luminous quasars already exist one billion years after the Big Bang, yet their connection to their star-forming host galaxies, their relation to the general galaxy population and their contribution to Reionisation remains deeply enigmatic. JWST is finding numerous Active Galactic Nuclei (AGN) in high-redshift galaxies with black hole masses that appear to be over-massive compared to their host's stellar mass, but rarely as massive as those found in luminous quasars. Here we report JWST/NIRSpec observations revealing overmassive SMBH in two ultra-luminous Lyman-$\alpha$ emitters at $z\sim6.6$ that exhibit rare double-peaked Lyman-alpha profiles. The broad Balmer lines indicate black hole masses $M_{\rm{BH}}\simeq 2\times10^8 M_\odot$, matching that found in faint $z\sim 6-7$ quasars, and very high BH-to-stellar-mass ratio ($\sim 0.1-0.2$) that exceed the local relation by a factor $\sim$400-800. Stellar population modelling favours young ages ($<50$ Myr), inconsistent with the sustained average Eddington-rate accretion required to reach the observed BH masses by $z=6.6$. The double-peak Lyman-$\alpha$ profiles require a large ionised bubble and high photoionisation rate that is consistent with the ionising output of quasars powered by black holes of similar mass, thus constraining the cessation of the last quasar episode to $<1$ Myr. We interpret both systems as post-quasar galaxies in which AGN feedback has delayed stellar mass assembly, and propose that episodic quasar activity partially explains the unexpected prevalence of large ionised bubbles deep into the Epoch of Reionisation.

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arXiv2026

The search for Population III: Confirmation of a HeII emitter with no metal lines at z=10.6

We report the confirmation of a HeII$\lambda$1640 emitter located at 3 pkpc from the galaxy GN-z11, at z=10.6. The detection, based on JWST NIRSpec-IFU high-resolution spectroscopy, confirms a previous claim based on medium-resolution spectroscopy. The HeII$\lambda$1640 identification is further supported by the independent detection of H$\gamma$ obtained by \"Ubler et al. (2026) at the same location. The HeII emission is spectrally resolved in two components separated by 120 km/s. The Equivalent Width of the HeII emission is extremely high ($>$20 A). No metal lines are detected. We argue that Population III stars are the most plausible explanation for the observed He II emission, with no satisfactory alternative from other classes of sources or mechanisms.

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arXiv2025

A warm ultra-luminous infrared galaxy just 600 million years after the Big Bang

We present an Atacama Large Millimeter/submillimeter Array (ALMA) Band 9 continuum detection ($3.3 \sigma$) of MACS0416_Y1 that confirms the suspected warm dust (91$^{+62}_{-35}$ K) of this Lyman-Break Galaxy (LBG) at $z = 8.3$ with $\log_{10} M_{\ast}/$M$_{\odot} = 9.0 \pm 0.1$. A modified black-body fit to the ALMA Bands 3 through 9 data of MACS0416_Y1 finds an intrinsic infrared luminosity of 1.0$^{+1.8}_{-0.6} \times{} 10^{12}\ \mathrm{L_{\odot}}$, placing this UV-selected LBG in the regime of Ultra Luminous Infrared Galaxies (ULIRGs). Its luminous but modest dust reservoir (1.4$^{+1.3}_{-0.5} \times{} 10^{6}\ \mathrm{M_{\odot}}$) is co-spatial to regions with a UV-continuum slope $\beta_{\rm UV} \approx -1.5$ as seen by James Webb Space Telescope (JWST) imaging. Although this implies some dust obscuration, the JWST photometry implies less obscured star formation than seen in the complete characterization by ALMA, implying some spatial separation of dust and stars on scales below 200 pc, i.e., smaller than those probed by JWST and ALMA. This source is an extreme example of dust-obscured star formation contributing strongly to the cosmic build-up of stellar mass, which can only be revealed through direct and comprehensive observations in the (sub)mm regime.

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arXiv2025

JWST COSMOS-3D: Spectroscopic Census and Luminosity Function of [O III] Emitters at 6.75<z<9.05 in COSMOS

We present a catalogue of spectroscopically selected [OIII]+Hb emitters at 6.75<z<9.05 and the resulting [OIII] 5008 \AA Luminosity Function (LF) in the COSMOS field. We leverage the 0.33 deg$^2$ covered by COSMOS-3D using NIRCam/WFSS F444W to perform the largest spectroscopic search for [OIII] emitters at 6.75<z<9.05. We present our catalogue of 249 [OIII] emitters and their associated completeness function. The inferred constraints on the [OIII] LF enable us to characterise the knee of the [OIII] LF, resulting in improved [OIII] LF constraints at z~7,8. Notably, we find evidence for a decline of the [OIII] luminosity density between z~7 and z~8, which could be expected if the metallicity of [OIII] emitters, as well as the cosmic star-formation rate density, is declining at these redshifts. We find that theoretical models that reproduce the z~7,8 [OIII] LF do not reproduce well the [OIII] equivalent width distribution, pointing to potential challenges in the modelling of[OIII] and other nebular lines in the early Universe. Finally, we provide the first constraints on the cosmic variance of [OIII] emitters, estimating at 15% the fractional uncertainty for the z~7,8 [OIII] LF in the 0.33 deg$^2$ field. This estimate is in good agreement with that inferred from clustering, and shows that the [OIII] LF derived from smaller extragalactic legacy fields is strongly affected by cosmic variance. Our results highlight the fundamental role that wide-area JWST slitless surveys play to map the galaxy large-scale structure down into the reionisation era, serving as a springboard for a variety of science cases.

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arXiv2025

A direct black hole mass measurement in a Little Red Dot at the Epoch of Reionization

Recent discoveries of faint active galactic nuclei (AGN) at the redshift frontier have revealed a plethora of broad \Halpha emitters with optically red continua, named Little Red Dots (LRDs), which comprise 15-30\% of the high redshift broad line AGN population. Due to their peculiar spectral properties and X-ray weakness, modeling LRDs with standard AGN templates has proven challenging. In particular, the validity of single-epoch virial mass estimates in determining the black hole (BH) masses of LRDs has been called into question, with some models claiming that masses might be overestimated by up to 2 orders of magnitude, and other models claiming that LRDs may be entirely stellar in nature. We report the direct, dynamical BH mass measurement in a strongly lensed LRD at $z = 7.04$. The combination of lensing with deep spectroscopic data reveals a rotation curve that is inconsistent with a nuclear star cluster, yet can be well explained by Keplerian rotation around a point mass of 50 million Solar masses, consistent with virial BH mass estimates from the Balmer lines. The Keplerian rotation leaves little room for any stellar component in a host galaxy, as we conservatively infer $M_{\rm BH}/M_{*}>2$. Such a ''naked'' black hole, together with its near-pristine environment, indicates that this LRD is a massive black hole seed caught in its earliest accretion phase.

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arXiv2025

Before its time: a remarkably evolved protocluster core at z=7.88

Protoclusters represent the most extreme environments in the very early Universe. They form from large-scale dark matter overdensities, harbouring an overabundance of galaxies fed by large gas reservoirs. Their early and accelerated evolution results in a distinct difference in the properties of galaxies resident in protoclusters versus the field, which is known to be in place by $z\sim 5-6$. We utilise JWST NIRCam observations of the A2744-z7p9OD protocluster at $z=7.88$ to constrain the properties of resident galaxies. We identify seven new protocluster members, bringing the total number to 23 and the total stellar mass of the protocluster to in excess of $10^{10}\ \rm{M_{\odot}}$. These galaxies are remarkably evolved just 650 Myr after the Big Bang, preferentially showing redder UV-slopes and stronger Balmer breaks than is typical of field galaxies. We use the PROSPECTOR spectral energy distribution fitting code to derive key galaxy properties, finding distinct populations in the core versus the outskirts of the protocluster. The core is largely composed of dusty, massive galaxies which can be characterised as undergoing a synchronised lulling phase, while galaxies in the protocluster outskirts are undergoing recent bursts of star formation. Finally, a strong suppression of the continuum around the Ly$\alpha$-break evidences extreme neutral hydrogen column densities in many resident galaxies ($N_{\rm HI}\gtrsim10^{22.5}\ {\rm cm^{-2}}$). The A2744-z7p9OD system is the most extreme, evolved overdensity yet observed at $z>7$, with higher stellar masses, gas densities, and dust attenuation, revealing the intersection of local environment and high-redshift galaxy formation at their extremes.

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arXiv2025

A black hole in a near-pristine galaxy 700 million years after the Big Bang

The recent discovery of a large number of massive black holes within the first two billion years after the Big Bang, as well as their peculiar properties, have been largely unexpected based on the extrapolation of the properties of luminous quasars. These findings have prompted the development of several theoretical models for the early formation and growth of black holes, which are, however, difficult to differentiate. We report the metallicity measurement around a gravitationally lensed massive black hole at redshift 7.04 (classified as a Little Red Dot), hosted in a galaxy with very low dynamical mass. The weakness of the [OIII]5007 emission line relative to the narrow H$\beta$ emission indicates extremely low metallicity, about $4\times 10^{-3}$ solar, and even more metal poor in the surrounding few 100 pc. We argue that such properties cannot be uncommon among accreting black holes around this early cosmic epoch. Explaining such a low chemical enrichment in a system that has developed a massive black hole is challenging for most theories. Models assuming heavy black hole seeds (such as Direct Collapse Black Holes) or super-Eddington accretion scenarios struggle to explain the observations, although they can potentially reproduce the observed properties in some cases. Models invoking "primordial black holes" (i.e. putative black holes formed shortly after the Big Bang) may potentially explain the low chemical enrichment associated with this black hole, although this class of models also requires further developments for proper testing.

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arXiv2025

JWST & ALMA Joint Analysis with [OII]$\lambda\lambda$3726,3729, [OIII]$\lambda$4363, [OIII]88$\mu$m, and [OIII]52$\mu$m: Multi-Zone Evolution of Electron Densities at $\mathbf{z\sim0-14}$ and Its Impact on Metallicity Measurements

We present a JWST and ALMA detailed study of the ISM properties of high-redshift galaxies. Our JWST/NIRSpec IFU spectroscopy targeting three galaxies at $z=6-7$ detects key rest-frame optical emission lines, allowing us to derive [OII]$\lambda\lambda$3726,3729-based electron densities of $n_\mathrm{e,optical}\sim1000$ cm$^{-3}$ on average and [OIII]$\lambda$4363-based metallicities of $\mathrm{12+log(O/H)}=8.0-8.2$ in two galaxies. New ALMA Band 9/10 observations detect the [OIII]52$\mu$m line in one galaxy but do not in the others, resulting in FIR-based densities of $n_\mathrm{e,FIR}\lesssim500$ cm$^{-3}$ from the [OIII]52$\mu$m/[OIII]88$\mu$m ratios, systematically lower than the optical [OII]-based measurements. These low FIR-based densities are comparable to those at both $z\sim0$ and $z>6$ in the literature, including JADES-GS-z14-0 at $z=14.18$, suggesting little evolution up to $z\sim14$, in contrast to the increasing trend of optical-based densities with redshift. By conducting a JWST and ALMA joint analysis using emission lines detected with both telescopes, we find that the observed FIR [OIII]52,88$\mu$m luminosities are too high to be explained by the optical-based densities at which they would be significantly collisionally de-excited. Instead, a 2-zone model with distinct high- and low-density regions is required to reproduce all observed lines, indicating that FIR [OIII] emission arises predominantly from low-density gas, while optical [OIII] and [OII] lines trace both regions. We further demonstrate that the direct-$T_\mathrm{e}$ method can sometimes significantly underestimate metallicities up to 0.8 dex due to the presence of the low-density gas not fully traced by optical lines alone, highlighting the importance of combining optical and FIR lines to accurately determine gas-phase metallicities in the early universe.

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arXiv2025

Glimmers in the Cosmic Dawn. II. A variability census of supermassive black holes across the Universe

Understanding the origin and evolution of supermassive black holes (SMBH) stands as one of the most important challenges in astrophysics and cosmology, with little current theoretical consensus. Improved observational constraints on the cosmological evolution of SMBH demographics are needed. Here we report results of a search via photometric variability for SMBHs appearing as active galactic nuclei (AGN) in the cosmological volume defined by the Hubble Ultra Deep Field (HUDF). This work includes particular focus on a new observation carried out in 2023 with the \textit{Hubble Space Telescope (HST)} using the WFC3/IR/F140W, which is compared directly to equivalent data taken 11 years earlier in 2012. Two earlier pairs of observations from 2009 to 2012 with WFC3/IR/F105W and WFC3/IR/F160W are also analysed. We identify 521, 188, and 109 AGN candidates as nuclear sources that exhibit photometric variability at a level of 2, 2.5 and 3~$\sigma$ in at least one filter. This sample includes 13, 3, and 2 AGN candidates at redshifts $z>6$, when the Universe was $\lesssim900$~Myr old. After variability and luminosity function (down to $M_{\rm UV}=-17\:$mag) completeness corrections, we estimate the co-moving number density of SMBHs, $n_{\rm SMBH}(z)$. At $z \gtrsim 6$, $n_{\rm SMBH}\gtrsim 6\times10^{-3}\:{\rm cMpc^{-3}}$. At low-$z$ our observations are sensitive to AGN fainter than $M_{\rm UV}=-17 \:$mag, and we estimate $n_{\rm SMBH}\gtrsim 10^{-2}\:{\rm cMpc^{-3}}$. We discuss how these results place strong constraints on a variety of SMBH seeding theories.

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arXiv2024

ALMA Lensing Cluster Survey: Dust mass measurements as a function of redshift, stellar-mass and star formation rate, from z=1 to z=5

Understanding the dust content of galaxies, its evolution with redshift and its relationship to stars and star formation is fundamental for our understanding of galaxy evolution. Using the ALMA Lensing Cluster Survey (ALCS) wide-area band-6 continuum dataset ($\sim\,$110 arcmin$^2$ across 33 lensing clusters), we aimed at constraining the dust mass evolution with redshift, stellar mass and star formation rate (SFR). After binning sources according to redshift, SFR and stellar mass -- extracted from an HST-IRAC catalog -- we performed a set of continuum stacking analyses in the image domain using \textsc{LineStacker} on sources between $z=1$ and $z=5$, further improving the depth of our data. The large field of view provided by the ALCS allows us to reach a final sample of $\sim4000$ galaxies with known coordinates and SED-derived physical parameters. We stack sources with SFR between $10^{-3}$ and $10^{3}$ M$_\odot$ per year, and stellar mass between $10^{8}$ and $10^{12}$ M$_\odot$, splitting them in different stellar mass and SFR bins. Through stacking we retrieve the continuum 1.2\,mm flux, a known dust mass tracer, allowing us to derive the dust mass evolution with redshift and its relation with SFR and stellar mass. We observe clear continuum detections in the majority of the subsamples. From the non detections we derive 3-$\sigma$ upper limits. We observe a steady decline in the average dust mass with redshift. Moreover, sources with higher stellar mass or SFR have higher dust mass on average, allowing us to derive scaling relations. Our results are mostly in good agreement with models at $z\sim1$-3, but indicate typically lower dust-mass than predicted at higher redshift.

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arXiv2024

Charting the Lyman-alpha escape fraction in the range 2.9<z<6.7 and consequences for the LAE reionisation contribution

The escape of Lyman-alpha photons at redshifts greater than two is an ongoing subject of study and an important quantity to further understanding of Lyman-alpha emitters (LAEs), the transmission of Lyman-alpha photons through the interstellar medium and intergalactic medium, and the impact these LAEs have on cosmic reionisation. This study aims to assess the Lyman-alpha escape fraction over the redshift range 2.9<z<6.7, focusing on VLT/MUSE-selected, gravitationally lensed, intrinsically faint LAEs. These galaxies are of particular interest as the potential drivers of cosmic reionisation. We assessed the Lyman-alpha escape fraction in two ways: through an individual study of 96 LAEs behind the A2744 lensing cluster, with JWST/NIRCam and HST data, and through a study of the global evolution of the escape fraction using the state-of-the-art luminosity functions for LAEs and the UV-selected `parent' population (dust-corrected). We compared these studies to those in the literature based on brighter samples. We find a negligible redshift evolution of the escape fraction for our individual galaxies; it is likely that it was washed out by significant intrinsic scatter. We observed a more significant evolution towards higher escape fractions with decreasing UV magnitude and fit this relation. When comparing the two luminosity functions to derive the escape fraction in a global sense, we saw agreement with previous literature when integrating the luminosity functions to a bright limit. However, when integrating using a faint limit equivalent to the observational limits of our samples, we observed enhanced escape fraction values, particularly around z~6, where the escape fraction becomes consistent with 100%. This indicates for the faint regimes we sampled that galaxies towards reionisation tend to allow very large fractions of Lyman-alpha photons to escape. (shortened)

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arXiv2024

Rising from the ashes: evidence of old stellar populations and rejuvenation events in the very early Universe

While JWST has observed galaxies assembling as early as $z\sim14$, evidence of galaxies with significant old stellar populations in the Epoch of Reionisation (EoR) -- the descendants of these earliest galaxies -- are few and far between. Bursty star-formation histories (SFHs) have been invoked to explain the detectability of the earliest UV-bright galaxies, but also to interpret galaxies showing Balmer breaks without nebular emission lines. We present the first spectroscopic evidence of a $z\sim7.9$ galaxy, A2744-YD4, which shows a Balmer break and emission lines, indicating the presence of both a mature and young stellar population. The spectrum of A2744-YD4 shows peculiar emission line ratios suggesting a relatively low ionisation parameter and high gas-phase metallicity. A median stack of galaxies with similar emission line ratios reveals a clear Balmer break in their stacked spectrum. This suggests that a mature stellar population ($\sim 80$ Myr old) has produced a chemically enriched, disrupted interstellar medium. Based on SED-fitting and comparison to simulations, we conclude that the observed young stellar population is in fact the result of a rejuvenation event following a lull in star formation lasting $\sim 20$ Myr, making A2744-YD4 and our stack the first spectroscopic confirmation of galaxies that have rejuvenated following a mini-quenched phase. These rejuvenating galaxies appear to be in an exceptional evolutionary moment where they can be identified. Our analysis shows that a young stellar population of just $\sim 30 \%$ of the total stellar mass would erase the Balmer break. Hence, 'outshining' through bursty SFHs of galaxies in the early Universe is likely plaguing attempts to measure their stellar ages and masses accurately.

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arXiv2024

JWST, ALMA, and Keck Spectroscopic Constraints on the UV Luminosity Functions at z~7-14: Clumpiness and Compactness of the Brightest Galaxies in the Early Universe

We present the number densities and physical properties of the bright galaxies spectroscopically confirmed at $z\sim7-14$. Our sample is composed of 60 galaxies at $z_\mathrm{spec}\sim7-14$, including recently-confirmed galaxies at $z_\mathrm{spec}=12.34-14.32$ with JWST, as well as new confirmations at $z_\mathrm{spec}=6.583-7.643$ with $-24< M_\mathrm{UV}< -21$ mag using ALMA and Keck. Our JWST/NIRSpec observations have also revealed that very bright galaxy candidates at $z\sim10-13$ identified from ground-based telescope images before JWST are passive galaxies at $z\sim3-4$, emphasizing the necessity of strict screening and spectroscopy in the selection of the brightest galaxies at $z>10$. The UV luminosity functions derived from these spectroscopic results are consistent with a double power-law function, showing tensions with theoretical models at the bright end. To understand the origin of the overabundance of bright galaxies, we investigate their morphologies using JWST/NIRCam high-resolution images obtained in various surveys including PRIMER and COSMOS-Web. We find that $\sim70\%$ of the bright galaxies at $z\sim7$ exhibit clumpy morphologies with multiple sub-components, suggesting merger-induced starburst activity, which is consistent with SED fitting results showing bursty star formation histories. At $z\gtrsim10$, bright galaxies are classified into two types of galaxies; extended ones with weak high-ionization emission lines, and compact ones with strong high-ionization lines including NIV]$\lambda$1486, indicating that at least two different processes (e.g., merger-induced starburst and compact star formation/AGN) are shaping the physical properties of the brightest galaxies at $z\gtrsim10$ and are responsible for their overabundance.

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arXiv2024

A complex node of the cosmic web associated with the massive galaxy cluster MACS J0600.1-2008

MACS J0600.1-2008 (MACS0600) is an X-ray luminous, massive galaxy cluster at $z_{\mathrm{d}}=0.43$, studied previously by the REionization LensIng Cluster Survey (RELICS) and ALMA Lensing Cluster Survey (ALCS) projects which revealed a complex, bimodal mass distribution and an intriguing high-redshift object behind it. Here, we report on the results of a combined analysis of the extended strong lensing (SL), X-ray, Sunyaev-Zeldovich (SZ), and galaxy luminosity-density properties of this system. Using new JWST and ground-based Gemini-N and Keck data, we obtain 13 new spectroscopic redshifts of multiply imaged galaxies and identify 12 new photometric multiple-image systems and candidates, including two multiply imaged $z\sim7$ objects. Taking advantage of the larger areal coverage, our analysis reveals an additional bimodal, massive SL structure which we measure spectroscopically to lie adjacent to the cluster and whose existence was implied by previous SL-modeling analyses. While based in part on photometric systems identified in ground-based imaging requiring further verification, our extended SL model suggests that the cluster may have the second-largest critical area and effective Einstein radius observed to date, $A_{\mathrm{crit}}\simeq2.16 \mathrm{arcmin}^2$ and $\theta_{\mathrm{E}}=49.7''\pm5.0''$ for a source at $z_{\mathrm{s}}=2$, enclosing a total mass of $M(<\theta_{\mathrm{E}})=(4.7\pm0.7)\times10^{14} \mathrm{M}_{\odot}$. These results are also supported by the galaxy luminosity distribution, the SZ and X-ray data. Yet another, probably related massive cluster structure, discovered in X-rays $5'$ (1.7 Mpc) further north, suggests that MACS0600 is part of an even larger filamentary structure. This discovery adds to several recent detections of massive structures around SL galaxy clusters and establishes MACS0600 as a prime target for future high-redshift surveys with JWST.

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arXiv2024

Glimmers in the Cosmic Dawn: A Census of the Youngest Supermassive Black Holes by Photometric Variability

We report first results from a deep near infrared campaign with the Hubble Space Telescope to obtain late-epoch images of the Hubble Ultra-Deep Field (HUDF), 10-15 years after the first epoch data were obtained. The main objectives are to search for faint active galactic nuclei (AGN) at high redshifts by virtue of their photometric variability, and measure (or constrain) the comoving number density of supermassive black holes (SMBHs), n_{SMBH}, at early times. In this Letter we present an overview of the program and preliminary results concerning eight objects. Three variables are supernovae, two of which are apparently hostless with indeterminable redshifts, although one has previously been recorded at a z\approx 6 object precisely because of its transient nature. Two further objects are clear AGN at z= 2.0 and 3.2, based on morphology and/or infrared spectroscopy from JWST. Three variable targets are identified at z = 6-7, which are also likely AGN candidates. These sources provide a first measure of n_{SMBH} in the reionization epoch by photometric variability, which places a firm lower limit of 3 \times 10^{-4} cMpc^{-3}. After accounting for variability and luminosity incompleteness, we estimate n_{SMBH} \gtrsim 8 \times 10{-3} cMpc{-3}, which is the largest value so far reported at these redshifts. This SMBH abundance is also strikingly similar to estimates of n_{SMBH} in the local Universe. We discuss how these results test various theories for SMBH formation.

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arXiv2023

GA-NIFS: JWST discovers an offset AGN 740 million years after the Big Bang

A surprising finding of recent studies is the large number of Active Galactic Nuclei (AGN) associated with moderately massive black holes ($\rm \log(M_\bullet/M_\odot)\sim 6-8$), in the first billion years after the Big Bang ($z>5$). In this context, a relevant finding has been the large fraction of candidate dual AGN, both at large separations (several kpc) and in close pairs (less than a kpc), likely in the process of merging. Frequent black hole merging may be a route for black hole growth in the early Universe; however, previous findings are still tentative and indirect. We present JWST/NIRSpec-IFU observations of a galaxy at $z=7.15$ in which we find evidence for a $\rm \log(M_\bullet/M_\odot)\sim7.7$ accreting black hole, as traced by a broad component of H$\beta$ emission, associated with the Broad Line Region (BLR) around the black hole. This BLR is offset by 620 pc in projection from the centroid of strong rest-frame optical emission, with a velocity offset of $\sim$40 km/s. The latter region is also characterized by (narrow) nebular emission features typical of AGN, hence also likely hosting another accreting black hole, although obscured (type 2, narrow-line AGN). We exclude that the offset BLR is associated with Supernovae or massive stars, and we interpret these results as two black holes in the process of merging. This finding may be relevant for estimates of the rate and properties of gravitational wave signals from the early Universe that will be detected by future observatories like LISA.

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arXiv2023

The growth of the gargantuan black holes powering high-redshift quasars and their impact on the formation of early galaxies and protoclusters

High-redshift quasars ($z\gtrsim6$), powered by black holes (BHs) with large inferred masses, imply rapid BH growth in the early Universe. The most extreme examples have inferred masses of $\sim \! 10^9\,$M$_\odot$ at $z = 7.5$ and $\sim \! 10^{10}\,$M$_\odot$ at $z = 6.3$. Such dramatic growth via gas accretion likely leads to significant energy input into the quasar host galaxy and its surroundings, however few theoretical predictions of the impact of such objects currently exist. We present zoom-in simulations of a massive high-redshift protocluster, with our fiducial FABLE model incapable of reproducing the brightest quasars. With modifications to this model to promote early BH growth, such as earlier seeding and mildly super-Eddington accretion, such `gargantuan' BHs can be formed. With this new model, simulated host dust masses and star formation rates are in good agreement with existing JWST and ALMA data from ultraluminous quasars. We find the quasar is often obscured as it grows, and that strong, ejective feedback is required to have a high probability of detecting the quasar in the rest-frame UV. Fast and energetic quasar-driven winds expel metal-enriched gas, leading to significant metal pollution of the circumgalactic medium (CGM) out to twice the virial radius. As central gas densities and pressures are reduced, we find weaker signals from the CGM in mock X-ray and Sunyaev-Zeldovich maps, whose detection - with proposed instruments such as Lynx, and even potentially presently with ALMA - can constrain quasar feedback.

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