arXiv subjects

Gabriele Pezzulli

Publications and source records attributed to Gabriele Pezzulli.

At least 19 recordsLinked to original sources

arXiv2026

A Dusty Quenching-candidate AGN host at z=5.7: massive quiescent galaxies may quench already during the dust-obscured phase

We present a spectro-photometric analysis of RUBIES-EGS-9809, a broad-line AGN host at z=5.7 with a Balmer break and a dominant point source. The source is dust reddened ($A_V>$3 mag), shows hot- and cold-dust emission, and is X-ray Compton thick, with X-ray-to-bolometric ratio consistent with luminous AGN. The large $A_V$ agrees with strong NaI absorption (EW=17$\pm 2 \AA$), possibly from a neutral-gas outflow (3 $\sigma$), while broad [O III] traces an ionized outflow, suggesting AGN feedback may be affecting the host. We estimate a stellar mass log(M*/M$_\odot$)=10.7$\pm$0.3, Lbol$\sim4\times10^{46}$ erg s$^{-1}$ , and black-hole mass log(M$_{BH}/M_\odot)\sim8.1$ (subject to large systematics). The resolved Balmer break (0.1 arcsec, 0.6 kpc) is consistent with a stellar origin, implying the galaxy is already old in stars while still heavily obscured. This shows that massive galaxies can host evolved stellar populations during their compact, dust-obscured phase, consistent with formation in an earlier dusty starburst. SED modelling suggests a declining SFR, but low-resolution spectroscopy alone cannot constrain the recent SFR, so we treat the source as a strong quenching candidate rather than a secure post-starburst system. Because quenching follows the decline in SFR, star-formation-driven outflows are disfavoured, leaving AGN feedback as a plausible driver. We speculate that such rapid feedback ($\sim$100 Myr timescale) may precipitate quenching while the post-quenching phase stays dust-enshrouded, concealing the UV-bright quenched phase that would otherwise be easily detected, if dust free. By the time dust clears, the UV-luminous stars may have already dwindled, leaving a classic dust-free quiescent galaxy. This 'dusty path' to quiescence may explain the lack of intermediate-age quenched progenitors (ages 50-100 Myr) linking dusty starbursts to massive quiescent systems at z=3-7.

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arXiv2026

The impact of hot mode accretion and angular momentum conservation on metallicity gradients of galactic discs

The hot circumgalactic medium (CGM) of a galaxy inevitably rotates more slowly than the cold gas in the disc it surrounds, due to a higher pressure support against gravity. If it accretes vertically onto the disc, angular momentum conservation leads to radial flows, advection of metals inwards and the steepening of a metallicity gradient. The observed gradient hence carries information on the kinematics of the accreting gas. Previous models built on this premise parametrised the angular momentum mismatch as a simple function of radius, constant in time, rather than derived from the properties of the CGM itself. Here, we present a semi-analytic chemical evolution model of a galaxy with a cosmologically motivated evolution, a self-consistent and time-evolving potential and an isothermal rotating CGM. The model includes inside-out formation and the radial flows induced both by the deepening of the potential well (acting on gas and stars) and by accretion, following their combined effect on the gas-phase metallicity gradient over cosmic time. We use a Bayesian framework to fit simultaneously the structural properties and metallicity gradient of a galaxy, inferring both the required inside-out growth and the CGM temperature needed to drive the required radial flows and abundance gradient. Fitting our model to the Milky Way, we find a mild preference for a subvirial corona ($T_\mathrm{CGM}/T_{200}=0.82^{+0.44}_{-0.38}$), in mild tension with observations, and we discuss possible resolutions. The predicted CGM rotation velocity lies between ~100-160 km/s at the present time and is substantially lower at earlier epochs. We further recover an evolution of the gradient over the past ~8Gyr consistent with observations. Our approach is not restricted to the Milky Way and can be directly applied to external galaxies, offering a new handle to constrain hot mode accretion onto galactic discs.

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arXiv2026

Misaligned or chaotic? A strong break of axial symmetry in the local LRD J1025 revealed with VLT/FORS2 spectropolarimetry

Little Red Dots (LRDs) are compact active galactic nuclei (AGN) with unusual spectral energy distributions and broad Balmer emission, candidate signposts of rapid black-hole growth. We present VLT/FORS2 optical linear spectropolarimetry of the closest known LRD, SDSS J102530.29+140207.3, at z=0.1. In total light, we detect spatially extended narrow-H$\alpha$ emission, probably tracing the host galaxy. We measure a nearly grey continuum polarisation $p_{\rm cont}=1.53\pm0.04$(rand.)$\pm$0.20(syst.) per cent, while broad H$\alpha$ is less polarised, $p_{\rm H\alpha}=0.58-0.84$ per cent. We rule out polarisation by Milky Way dust and dilution by an unpolarised line. The polarised continuum with a depolarised line resembles local Seyfert-1 nuclei and favours a single dominant source over multi-source explanations. After Stokes-continuum subtraction, broad H$\alpha$ shows no blue-to-red swing, but there is a significant (48$\pm$4)$^\circ$ continuum-to-line offset in polarisation angle. This implies a break of axial symmetry inside this object, posing interesting geometrical challenges to all existing LRD models and frameworks. We discuss different possible origins of this symmetry break and possible paths to discriminate between them with future observations.

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arXiv2026

The hydrodynamical response of cold circumgalactic clouds to quasar radiation

Recent simulations increasingly resolve the small-scale structure of the circumgalactic medium (CGM), but the dynamical impact of ionising radiation on its cold $10^4$ K component remains poorly understood. We investigate the evolution of cold gas structures exposed to quasars' EUV radiation. We develop an analytical framework to describe the evolution of such clouds, introducing a new threshold that defines when a cloud becomes radiation-shielded. The framework is validated using radiation-hydrodynamic simulations of single static clouds. It predicts three evolutionary paths: (i) an optically thin regime, in which radiation uniformly ionises the cloud; (ii) a radiation-shielded regime, where the cloud remains largely unaffected; and (iii) a rocket-effect regime, in which the propagation of the ionisation front ionises the illuminated side while compressing the opposite side, later accelerating the surviving cold clump. In the latter regime, the cloud's Ly$α$ luminosity can be up to one order of magnitude higher than the optically thin case. Such luminosities are as high as $70\%$ of the values obtained from a fluorescent regime without considering hydrodynamical response. Unless the cloud is shielded, at least $\sim 50$-$60\,\%$ of Ly$α$ emission arises from recombination. Applying this framework to both a ray crossing a population of clouds, and a ray propagating inside a cold stream, we find that the cold CGM around bright quasars ($L_{\mathrm{ν,LL}} \sim 10^{31.6} \, \mathrm{erg\, s^{-1}\, Hz^{-1}}$) is likely fully ionised, whereas the one around faint quasars ($L_{\mathrm{ν,LL}} \sim 10^{28.6} \, \mathrm{erg\, s^{-1}\, Hz^{-1}}$) predominantly experiences a rocket-effect regime. These results imply that the hydrodynamical response of cold CGM structures to quasar radiation must be considered when deriving their physical properties, particularly for faint quasars.

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arXiv2026

The assembly and fate of a giant disc galaxy in a protocluster at $z = 3$

Recent JWST observations revealed two massive ($M_{\star} \gtrsim 10^{11}\,M_{\odot}$), unexpectedly large spiral galaxies at $z \sim 3$, both in overdense environments. We focus on one of these, ADF22.1 at $z = 3.09$, which hosts an active galactic nucleus (AGN), exploiting its extended [CII] emission ($\sim$30 kpc in diameter) with high-resolution observations from the Atacama Large Millimetre Array and JWST. We find a flat outer rotation curve reaching $\sim$530 km s$^{-1}$, and perform, for the first time for a system of this type, a rotation-curve decomposition. We infer a dark-matter halo mass of $\log(M_{200}/M_{\odot})=12.9^{+0.4}_{-0.3}$, a baryon-to-halo mass ratio of $0.4^{+0.6}_{-0.3}$ in units of the cosmological baryon fraction, and a ratio between the baryonic and dark-matter halo specific angular momentum of $1.0^{+0.7}_{-0.5}$. Comparing these quantities with those of local galaxies, we find that ADF22.1 is indistinguishable from $z=0$ giant discs, pointing to the inefficiency of AGN feedback in halting disc growth. Using the Mapping Nearby Galaxies at Apache Point Observatory survey, we identify potential $z=0$ descendants of ADF22.1, suggesting it will evolve into an extreme (in either mass or angular momentum) early-type galaxy. Finally, we argue that cold-stream accretion, invoked to explain disc formation at $z > 1$, cannot simultaneously account for its size, dynamical properties, high specific angular momentum, and baryon-to-halo mass ratio. Instead, sustained accretion from the hot circumgalactic medium, either via spontaneous or fountain-driven condensation, offers a more physically plausible formation pathway.

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arXiv2026

Holes in the BH$^\star$? AGN signatures in the FUV spectrum of a black-hole dominated Little Red Dot at $z=7.04$

It has been suggested that "Little Red Dots" (LRDs) might be accreting black holes enshrouded by dense gas in a nearly closed geometry, which completely covers the central black hole, leading to an atmosphere-like structure known as the "black-hole star" ($\rm BH^\star$). We test this scenario by analysing new JWST spectroscopy in the far ultraviolet (FUV, rest-frame) of the prototypical LRD Abell2744-QSO1, at $z=7.04$. We found the presence of broad Ly$α$ emission with an FWHM of $\sim 1000$ km/s, and detections of OI, CIV, and/or FeII emission lines. The NIRCam imaging and NIRSpec slit images indicate that the low-velocity component ($v\lesssim 200$ km/s) of Ly$α$ is likely spatially extended, but the high-velocity component ($v\gtrsim 200$ km/s) of Ly$α$ remains unresolved. Based on the multi-component kinematics and flux of Ly$α$ relative to Balmer lines, we conclude that the observed line profile is unlikely to be broadened by subsequent resonant scattering through the interstellar medium. This suggests that the high-velocity component of Ly$α$ originates in the broad-line region, although resonant scattering in the dense gas likely makes Ly$α$ broader than H$α$ as observed. The nebular features of this LRD indicate that there is at least one relatively optically thin direction where Ly$α$ can escape from the broad-line region (BLR). We also found indications that photons from the BLR are powering fluorescence of FeII and OI on a larger physical scale. The FUV features thus challenge the fully-covered geometry interpretation and suggest that there are "holes" in the $\rm BH^\star$, or the absorbing medium is simply clumpy.

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arXiv2026

The Engine and its Flows: Little Red Dot spectra are shaped by the column densities of their gas envelopes

JWST data have enabled the abundant identification of compact broad Balmer line sources nicknamed the Little Red Dots. While they share broad lines with active galactic nuclei, they are unusually X-ray and infrared weak. We investigate the origin of the Balmer line profiles based on an empirical analysis of 18 broad H$α$-selected sources with high quality spectra at $z\approx3-7$. The H$α$ line profiles vary systematically with Balmer break strength: sources with blue UV to optical colors show a narrow core profile, redder sources with Balmer breaks a blue shifted absorption (P Cygni shape), and the reddest sources display absorption-dominated cores. All H$α$ lines have symmetric exponential wings, which are more dominant and slightly broader in red sources. Balmer absorption is present in $\sim60$ % of the sample, with H$β$ showing relatively stronger absorption. Drawing upon empirical analogies with stellar phenomena, we interpret these trends as being due to radiative processes that depend on variations in the optical depth, ionisation state and column density of a clumpy, partially ionised envelope. We unveil a correlation between the absorber velocity and Balmer break strength, with the densest absorbers inflowing and bluer sources having faster outflows. This indicates viewing angle or evolutionary effects where optically thick gas is inflowing, as suggested in models of super-Eddington accretion, and the engine can more easily drive outflows in directions with lower column densities. This new understanding of Balmer line profiles as tracing gas properties rather than dynamical broadening helps resolve tensions associated with high inferred black hole masses from standard virial calibrations, and reveals the complex gas environment around the hot central engine.

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arXiv2026

The warm outer layer of a Little Red Dot as the source of [Fe II] and collisional Balmer lines with scattering wings

The population of the Little Red Dots (LRDs) may represent a key phase of supermassive black hole (SMBH) growth. A cocoon of dense excited gas is emerging as key component to explain the most striking properties of LRDs, such as strong Balmer breaks and Balmer absorption, as well as the weak IR emission. To dissect the structure of LRDs, we analyze new deep JWST/NIRSpec PRISM and G395H spectra of FRESCO-GN-9771, one of the most luminous known LRDs at $z=5.5$. These reveal a strong Balmer break, broad Balmer lines and very narrow [O III] emission. We unveil a forest of optical [Fe II] lines, which we argue is emerging from a dense ($n_{\rm H}=10^{9-10}$ cm$^{-3}$) warm layer with electron temperature $T_{\rm e}\approx7000$ K. The broad wings of H$α$ and H$β$ have an exponential profile due to electron scattering in this same layer. The high $\rm Hα:Hβ:Hγ$ flux ratio of $\approx10.4:1:0.14$ is an indicator of collisional excitation and resonant scattering dominating the Balmer line emission. A narrow H$γ$ component, unseen in the other two Balmer lines due to outshining by the broad components, could trace the ISM of a normal host galaxy with a star formation rate $\sim5$ M$_{\odot}$ yr$^{-1}$. The warm layer is mostly opaque to Balmer transitions, producing a characteristic P-Cygni profile in the line centers suggesting outflowing motions. This same layer is responsible for shaping the Balmer break. The broad-band spectrum can be reasonably matched by a simple photoionized slab model that dominates the $λ>1500$ Å continuum and a low mass ($\sim10^8$ M$_{\odot}$) galaxy that could explain the narrow [O III], with only subdominant contribution to the UV continuum. Our findings indicate that Balmer lines are not directly tracing gas kinematics near the SMBH and that the BH mass scale is likely much lower than virial indicators suggest.

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arXiv2026

A quiescent galaxy in a gas-rich cosmic web node at z~3

Recent JWST observations have unveiled a large number of quiescent galaxies at $z\gtrsim3$, bringing potential challenges to current galaxy formation models. Since star formation is expected to be fed by external gas accretion, the knowledge about the circumgalactic media (CGM) of these galaxies is essential to understanding how they quench. In this work, we present the discovery of a massive and passive galaxy ($M_\star\simeq10^{11}\,M_\odot$) within the MQN01 structure at z~3.25, containing one of the largest overdensities of galaxies and active galactic nuclei (AGN) found so far at $z\gtrsim3$. The passive galaxy has a star-formation rate of $4^{+6}_{-2}~M_\odot$/yr, placing it more than 1 dex below the star-forming main sequence, and has no detectable molecular gas ($M_\mathrm{H2}<7\times10^{9}\,M_\odot$). Surprisingly, it is located at the center of a large cool gas reservoir, as traced by bright Ly$\alpha$ and H$\alpha$ emission. By taking advantage of deep multi-wavelength information unique to this field, including deep Chandra X-ray data, we argue that the inefficient gas accretion from the CGM onto this galaxy over the last few hundreds of Myr, as suggested by the observations, could be caused by an AGN jet of a nearby star-forming galaxy located at a projected distance of 48 kpc. In particular, we argue that the jet feedback may have maintained a high level of CGM turbulence around the passive galaxy and thus caused a reduced gas accretion over the required time-scales. In addition, the elevated ionizing field provided by the AGN overdensity, including the nearby AGN, can illuminate the passive galaxy's cool CGM and make it visible through fluorescent emission. Our study demonstrates that the star formation rates of high-redshift galaxies could be substantially reduced and maintained at a low level even within gas-rich and overdense environments in particular situations.

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arXiv2025

A weak Ly$α$ halo for an extremely bright Little Red Dot: Indications of enshrouded SMBH growth

The abundant population of "Little Red Dots" (LRDs)-compact objects with red UV to optical colors and broad Balmer lines at high redshift-is unveiling new insights into the properties of early active galactic nuclei (AGN). Perhaps the most surprising features of this population are the presence of Balmer absorption and ubiquitous strong Balmer breaks. Recent models link these features to an active supermassive black hole (SMBH) cocooned in very dense gas ($N_{\rm H}\sim10^{24}\,\rm cm^{-2}$). We present a stringent test of such models using VLT/MUSE observations of A2744-45924, the most luminous LRD known to date ($L_{\rm Hα}\approx10^{44}~\rm erg\,s^{-1}$), located behind the Abell-2744 lensing cluster at $z=4.464$ ($μ=1.8$). We detect a moderately extended Ly$α$ nebula ($h\approx5.7$ pkpc), spatially offset from the point-like H$α$ seen by JWST. The Ly$α$ emission is narrow ($\rm FWHM=270\pm 15~km\,s^{-1}$), spatially offset to H$α$, and faint ($\rm Lyα=0.07Hα$) compared to Ly$α$ nebulae typically observed around quasars of similar luminosity. We detect compact N$\,$IV]$λ$1486 emission, spatially aligned with H$α$, and a spatial shift in the far-UV continuum matching the Ly$α$ offset. We discuss that H$α$ and Ly$α$ have distinct physical origins: H$α$ originates from the AGN, while Ly$α$ is powered by star formation. In the environment of A2744-45924, we identify four extended Ly$α$ halos ($Δz<0.02$, $Δr<100$ pkpc). Their Ly$α$ luminosities match expectations based on H$α$ emission, indicating no evidence for radiation from A2744-45924 affecting its surroundings. The lack of strong, compact, and broad Ly$α$ and the absence of a luminous extended halo, suggest that the UV AGN light is obscured by dense gas cloaking the SMBH with covering factor close to unity.

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arXiv2025

Irony at z=6.68: a bright AGN with forbidden Fe emission and multi-component Balmer absorption

We present the deepest medium-resolution JWST/NIRSpec spectroscopy to date of a bright Little Red Dot (LRD) AGN, Irony at z=6.68. The data reveal broad Balmer emission from H$α$-H$δ$ and Balmer absorption in H$α$-H$ε$. The absorption lines are kinematically split: H$α$ is blueshifted while higher-order lines are redshifted suggesting complex gas kinematics; their relative ratios are inconsistent with a single, passive absorbing screen. The line depths require absorption of both the BLR and the continuum, ruling out a stellar origin, consistent with the smooth Balmer break. We fit the broad H$γ$-H$α$ lines and find the data favor a double-Gaussian effective profile, although exponential wings are evident. Depending on the adopted profile, single-epoch virial estimates give log(M$_\bullet$/M$_\odot$)=7.86-8.39 and $λ_{\rm Edd}$=1.7-0.4. The dynamical mass implied by the narrow lines is low log(Mdyn/M$_\odot$)=9.1, suggesting an overmassive black hole. The narrow lines display little attenuation, A$_V<0.5$ mag; while broad H$α$/H$β\sim9$ and the broad Balmer decrements are inconsistent with standard dust attenuation curves, suggesting collisional processes. The forbidden-line spectrum includes auroral [S II] and [N II], and a forest of [Fe II] lines. Line ratios and kinematics indicate a stratified narrow-line region with both low (n$_{\rm e}$=420 cm$^{-3}$) and high densities (n$_{\rm e}\gtrsim 6.3\times10^5$ cm$^{-3}$). We detect metal absorption lines in both the optical (Ca II and Na I) and UV range (Fe II UV1-UV3). Our results support a picture of a compact AGN embedded in a dense, high covering-factor and stratified cocoon, with complex neutral-gas kinematics. While the choice of broad-line profile affects the virial estimates of M$_\bullet$, we find the effect to be of order 0.6 dex between the different approaches.

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arXiv2025

X-ray view of a massive node of the Cosmic Web at z=3 II. Discovery of extended X-ray emission around a hyperluminous QSO

While the warm, ionized gas in the CGM at z>3 is now routinely observed around bright QSOs in Lya emission, little is known about the CGM hot phase due to its expected faintness in the X-ray band, often referred to as the ICM. Here, we report the analysis of 634 ks of Chandra X-ray observations in the MQN01 Cosmic Node, a region containing one of the brightest Lya nebulae and the largest galaxy overdensity discovered so far at z>3. We detect 66 net counts of X-ray emission in the 0.5-2 keV band extending to at least 30 kpc from the brightest QSO in MQN01. The morphology and spectrum are consistent with thermal emission from hot plasma in CIE. Photoionization is negligible, and IC is disfavored. A joint spatial and spectral MCMC analysis provides consistency with a beta-model with a steep density profile and a gas temperature kT~1.8 keV and virial halo mass Mvir~3e13 Mo. The inferred hot gas mass is Mhot(<Rvir)~2.6e12 Mo, which is ~8.3% of Mvir, or ~56% of the theoretical cosmological baryon budget of the halo. The hot gas also emits an exceptionally high Lx, with a measured L2-10~2.3e45 erg/s within the central 30 kpc. This system is a clear outlier in the Lx-Tx plane, indicating a thermodynamic state distinct from that of evolved lower-redshift hot halos. The cooling time in the inner 15-30 kpc is comparable to the local dynamical time, suggesting that the gas could become locally unstable in the absence of heating or feedback. Moreover, the thermal pressure associated with the detected CGM hot phase is large enough to confine the cold and dense clumps, which are required to reproduce the high Lya emission associated with the inner regions of the MQN01 structure. Although limited to a single system, our results provide unique information on the multi-phase properties of the CGM and a view of the nascent thermal hot gas phase observed in local galaxy clusters.

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arXiv2025

The galaxy-halo connection of disc galaxies over six orders of magnitude in stellar mass

(Abridged) The relations between stellar ($M_\ast$), gas ($M_{\rm gas}$), baryonic ($M_{\rm bar} = M_\ast + M_{\rm gas}$), and dark matter halo mass ($M_{200}$) provide unique constraints on galaxy formation and cosmology. The shape of the relations constrains how galaxies regulate their growth through gas accretion, star formation, and feedback; their scatter probes the stochasticity of galaxy assembly. Here, we assemble a sample of 49 nearby gas-rich dwarf and massive disc galaxies with unmatched ancillary data. We obtain their gas kinematics and derive their dark matter properties through rotation curve decomposition. Our sample allows us to study the galaxy-halo connection across nearly six orders of magnitude in $M_\ast$. We find that the $M_{\rm gas}-M_{200}$ relation rises monotonically, with galaxies having around 4 per cent of the average cosmological baryon fraction in cold gas. Contrastingly, the $M_\ast-M_{200}$ relation shows a more complex behaviour. A particularly interesting finding is that of a population of baryon-deficient' dwarfs (BDDs) with stellar masses $\sim 1-1.5$ orders of magnitude lower than expected from current models. Yet, baryon-rich galaxies also exist, and we find a large spread in the baryon retention fraction across our galaxies. We compare our findings with semi-analytic and hydrodynamical galaxy formation simulations. While the simulations broadly reproduce most observed features, they struggle to match the BDDs and do not capture the diversity in baryon fractions. Understanding these differences will shed new light on how feedback regulates galaxy formation. Finally, we study the dark matter halo concentration-mass relation. We find that below $M_{200} \sim 10^{11}\,M_\odot$, the concentrations are systematically lower than expected. We discuss whether these results stem from the influence of baryonic physics or the environment.

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arXiv2024

A Giant Disk Galaxy Two Billion Years After The Big Bang

Observational studies showed that galaxy disks are already in place in the first few billion years of the universe. The early disks detected so far, with typical half-light radii of 3 kiloparsecs at stellar masses around 10^11 M_sun for redshift z~3, are significantly smaller than today's disks with similar masses, in agreement with expectations from current galaxy models. Here, we report observations of a giant disk at z=3.25, when the universe was only 2 billion years old, with a half-light radius of 9.6 kiloparsecs and stellar mass of 3.7^+2.6_-2.2x10^11 M_sun. This galaxy is larger than any other kinematically-confirmed disks at similar epochs and surprisingly similar to today's largest disks regarding size and mass. JWST imaging and spectroscopy reveal its spiral morphology and a rotational velocity consistent with local Tully-Fisher relation. Multi-wavelength observations show that it lies in an exceptionally dense environment, where the galaxy number density is over ten times higher than the cosmic average and mergers are frequent. The discovery of such a giant disk suggests the presence of favorable physical conditions for large-disk formation in dense environments in the early universe, which may include efficient accretion of gas carrying coherent angular momentum and non-destructive mergers between exceptionally gas-rich progenitor galaxies.

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arXiv2024

Anatomy of an ionized bubble: NIRCam grism spectroscopy of the $z=6.6$ double-peaked Lyman-$α$ emitter COLA1 and its environment

The increasingly neutral intergalactic gas at $z>6$ impacts the Lyman-$α$ flux observed from galaxies. One luminous galaxy, COLA1, stands out because of its unique double-peaked Ly$α$ line at $z=6.6$, unseen in any simulation of reionization. Here we present JWST/NIRCam wide-field slitless spectroscopy in a 21 arcmin$^2$ field centered on COLA1. We find 141 galaxies spectroscopically-selected through the [OIII]($\lambda4969,5008$) doublet at $5.35 M_{\rm UV}>-19.5$).

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arXiv2024

Large-scale excess HI absorption around $z\approx4$ galaxies detected in a background galaxy spectrum in the MUSE eXtremely Deep Field

Observationally mapping the relation between galaxies and the intergalactic medium (IGM) is of key interest for studies of cosmic reionization. Diffuse hydrogen gas has typically been observed in HI Lyman-$α$ (Ly$α$) absorption in the spectra of bright background quasars. However, it is important to extend these measurements to background galaxies as quasars become increasingly rare at high redshift and rarely probe closely separated sight-lines. Here we use deep integral field spectroscopy in the MUSE eXtremely Deep Field to demonstrate the measurement of the Ly$α$ transmission at $z\approx4$ in absorption to a background galaxy at $z=4.77$. The HI transmission is consistent with independent quasar sight-lines at similar redshifts. Exploiting the high number of spectroscopic redshifts of faint galaxies (500 between $z=4.0-4.7$ within a radius of 8 arcmin) that are tracers of the density field, we show that Ly$α$ transmission is inversely correlated with galaxy density, i.e. transparent regions in the Ly$α$ forest mark under-dense regions at $z\approx4$. Due to large-scale clustering, galaxies are surrounded by excess HI absorption over the cosmic mean out to 4 cMpc/h. We also find that redshifts from the peak of the Ly$α$ line are typically offset from the systemic redshift by +170 km/s. This work extends results from $z\approx 2 - 3$ to higher redshifts and demonstrates the power of deep integral field spectroscopy to simultaneously measure the ionization structure of the IGM and the large-scale density field in the early Universe.

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arXiv2024

The density of the Milky Way's corona at $z\approx 1.6$ through ram pressure stripping of the Draco dSph galaxy

Satellite galaxies within the Milky Way's (MW) virial radius $R_{\mathrm{vir}}$ are typically devoid of cold gas due to ram pressure stripping by the MW's corona. The density of this corona is poorly constrained today and essentially unconstrained in the past, but can be estimated using ram pressure stripping. In this paper, we probe the MW corona at $z\approx 1.6$ using the Draco dwarf spheroidal galaxy. We assume that i) Draco's orbit is determined by its interaction with the MW, whose dark matter halo we evolve in time following cosmologically-motivated prescriptions, ii) Draco's star formation was quenched by ram pressure stripping and iii) the MW's corona is approximately smooth, spherical and in hydrostatic equilibrium. We used GAIA proper motions to set the initial conditions and Draco's star formation history to estimate its past gas content. We found indications that Draco was stripped of its gas during the first pericentric passage. Using 3D hydrodynamical simulations at a resolution that enables us to resolve individual supernovae and assuming no tidal stripping, which we estimate to be a minor effect, we find a density of the MW corona $\geq 8\times 10^{-4}$ cm$^{-3}$ at a radius $\approx 0.72R_{\mathrm{vir}}$. This provides evidence that the MW's corona was already in place at $z\approx 1.6$ and with a higher density than today. If isothermal, this corona would have contained all the baryons expected by the cosmological baryon fraction. Extrapolating to today shows good agreement with literature constraints if feedback has removed $\lesssim 30$% of baryons accreted onto the halo.

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arXiv2023

Resolving the physics of Quasar Ly$α$ Nebulae (RePhyNe): I. Constraining Quasar host halo masses through Circumgalactic Medium kinematics

Ly$α$ nebulae ubiquitously found around z>2 quasars can supply unique constraints on the properties of the Circumgalactic Medium, such as its density distribution, provided the quasar halo mass is known. We present a new method to constrain quasar halo masses based on the line-of-sight velocity dispersion maps of Ly$α$ nebulae. By using MUSE-like mock observations obtained from cosmological hydrodynamic simulations under the assumption of maximal quasar fluorescence, we show that the velocity dispersion radial profiles of Ly$α$-emitting gas are strongly determined by gravity and that they are thus self-similar with respect to halo mass when rescaled by the virial radius. Through simple analytical arguments and by exploiting the kinematics of HeII1640Ȧ emission for a set of observed nebulae, we show that Ly$α$ radiative transfer effects plausibly do not change the shape of the velocity dispersion profiles but only their normalisation without breaking their self-similarity. Taking advantage of these results, we define the variable $η^{140-200}_{40-100}$ as the ratio of the median velocity dispersion in two specifically selected annuli and derive an analytical relation between $η^{140-200}_{40-100}$ and the halo mass which can be directly applied to observations. We apply our method to 37 observed quasar Ly$α$ nebulae at 3<z<4.7 and find that their associated quasars are typically hosted by ~$10^{12.16 \pm 0.14}$ M$_{\odot}$ haloes independent of redshift within the explored range. This measurement, which is completely independent of clustering methods, is consistent with the lowest mass estimates based on quasar auto-correlation clustering at z~3 and with quasar-galaxies cross-correlation results.

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