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Sebastiano Cantalupo

Publications and source records attributed to Sebastiano Cantalupo.

At least 19 recordsLinked to original sources
arXiv2026

Resolving the physics of Quasar Ly$α$ Nebulae (RePhyNe): II. The dense and clumpy CGM of Quasars at $z \sim$ 3.5

The small scale density distribution of the cold circumgalactic medium (CGM) plays a central role in galaxy evolution, yet it is still poorly understood. We provide new constraints on the cold CGM density distribution by comparing the CGM emission from MUSE observations of quasar (QSO) nebulae at $z \sim 3.8$ with mock observations generated from the new DaLya simulation suite under the assumption of maximal fluorescence. These are further complemented by mock observations generated from the COLIBRE (m5 and m6 resolutions) and HELLO simulations. We first investigate the distribution of the observed and simulated Ly$α$ surface brightness (SB) values, which directly depend on the cold gas density distribution under our assumption, finding stark differences between observations and upper limits provided by simulations with the exception of the COLIBRE higher-resolution run (m5). The observations generally show a much larger covering fraction of regions with relatively large Ly$α$ SB values and suggest that densities reaching at least 10 cm$^{-3}$ must be present in the CGM of QSOs within a radial distance of 50 to 200 ckpc. To understand the origin of this discrepancy, we study the cold CGM density distribution finding in all cases simulated PDFs consistent with skewed lognormal ones with medians $\sim 10^{-2}$ cm$^{-3}$ and clumping factors between 16 and 102. We compare the broadness of these distributions to the one implied by the observed HeII/Ly$α$ CGM ratio, which is independent on the assumption of maximum fluorescence. Assuming lognormal PDFs with median values similar to the simulations, the observed ratio requires a broader PDF with a clumping factor at least 8 times larger than predicted by simulations. These results suggest that higher resolution and/or additional physical mechanisms are needed to reproduce the observed CGM of QSOs at these redshifts.

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arXiv2026

Constraining the density distribution of the cold Circumgalactic Medium of quasars at z>2 through Lya, HeII and Ha emission

Extended Lya emission is routinely detected around high-redshift quasars, revealing large reservoirs of cool circumgalactic medium (CGM). Its resonant nature complicates interpretation of gas density, ionization state, and kinematics, making non-resonant tracers such as HeII and Ha essential. Extending previous analytical models, we use CLOUDY photo-ionization simulations to predict HeII/Ha in quasar-illuminated gas over a broad parameter range, including log-normal density distributions. The ratio constrains density fluctuations, commonly quantified by the clumping factor C. We compare the models with extended Lya, HeII, and Ha observations around quasars at z~2.1-4.5. At z~2.2, we observed 13 quasars with Keck/KCWI and targeted the systems with the most prominent extended Lya and HeII emission with Keck/MOSFIRE, providing the first statistical quasar-CGM sample with combined Lya, HeII, and Ha measurements. Within matched apertures out to ~35 pkpc, stacked spectra give Lya/Ha=9.1, HeII/Ha=0.54, and HeII/Lya=0.06. Single-density models require very high CGM densities (n>10 cm^-3) or implausibly soft AGN spectra to reproduce the low HeII/Ha ratio. Broad log-normal density distributions instead reproduce it at lower median densities but require a substantial emissivity-weighted high-density tail, corresponding formally to C~10^3-10^4 in our parameterization. We also analyze 26 VLT/MUSE quasars at 3.1<z<4.5, where Ha is inaccessible from the ground, finding 14 systems with co-spatial extended Lya and HeII emission. The detected, emission-weighted HeII/Lya component shows no evidence for strong evolution between z~2.2 and z~3.7, though this does not imply an invariant underlying density PDF. These constraints provide a basis for comparison with theoretical models and hydrodynamical simulations of the physical processes shaping quasar CGM at high redshift.

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arXiv2026

MUSEQuBES: Probing Anisotropies in Gas and Metal Distributions in the Circumgalactic Medium

We investigate the azimuthal dependence of HI and OVI-bearing gas in the circumgalactic medium (CGM) of 113 isolated galaxies in the redshift range $0.12<z<0.75$, including 91 new measurements from the MUSE Quasar-fields Blind Emitters Survey (MUSEQuBES). Of these, measurements for 46 galaxies lie within the virial radius ($R_{\rm vir}$), including 36 non-face-on systems for which azimuthal angle ($ϕ$) measurements are robust. The HI covering fraction (k_HI) within $R_{\rm vir}$ of low-mass ($7<\log(M/M_{\odot})\leq 9$) galaxies, for a threshold column density of ${\rm log}_{10}(N(HI)/{\rm cm}^{-2})=14.5$, exhibits an enhancement along both the disk plane ($ϕ\lesssim20^{\circ}$) and in the polar direction ($ϕ\gtrsim70^{\circ}$). In contrast, such a bimodal distribution is not observed for higher mass galaxies ($9<\log(M/M_{\odot})\leq 11.3$). Similarly, the OVI covering fraction (k_OVI), for a threshold of ${\rm log}_{10}(N(OVI)/{\rm cm}^{-2})=14.0$, shows a tentative enhancement along both the projected major and minor axes for low-mass galaxies. In contrast, \OVI-bearing gas around higher-mass galaxies appears more uniformly distributed, with no significant azimuthal dependence. Finally, using the halo circular-velocity-normalized pixel-velocity two-point correlation function (TPCF), we find that OVI absorbers are kinematically narrower along the disk plane compared to the polar directions of the host galaxies with similar stellar mass distributions. The observed isotropic distribution of OVI in high-mass halos suggests that its spatial distribution is governed by global halo properties; however, the OVI kinematics retain memory of the site of origin.

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arXiv2026

A Partial Lyman Limit Absorber in the Halo of a Galaxy Pair: A Possible Signature of Gas Inflow

We present an analysis of a partial Lyman limit system at z = 0.87641 detected in the HST/COS spectrum of the background quasar LBQS 0107-0235. The absorber exhibits a simple kinematic structure, with the metal-lines and the H I Lyman-series absorption well described by a single component. Photoionization modeling yields a gas metallicity of one-tenth solar and a hydrogen number density of n_H \approx 8.5 \times 10^{-4} cm^{-3} (log_{10}(n_H/cm^{-3}) \approx -3.1). At the absorber redshift, the VLT/MUSE data show two galaxies (G1 and G2) at normalized impact parameters of ρ/R_{vir} \approx 0.9 and velocity separations of |Δv| = 18 and 99 km s^{-1}, respectively, from the absorber. Both galaxies have rotating disks with stellar masses of M_* \approx 6 \times 10^9 and \approx 2.2 \times 10^{10} M_{\odot}. Their 100-Myr-averaged star formation rates are \approx 2.5 and \approx 2.2 M_{\odot} yr^{-1}, though their instantaneous rates place G2 on the star-forming main sequence and G1 above it, which is actively star-forming at this redshift. The absorber is positioned very close to the projected major axis of both galaxies. The absorber's orientation, kinematics, and sub-solar metallicity (log_{10}(Z/Z_{\odot}) = -1.05) are consistent with the absorption tracing a sub-solar metallicity inflowing stream, though a galaxy-galaxy interaction origin cannot be excluded. We discuss these scenarios in the context of cosmological simulations of cold-mode accretion and CGM gas flows around galaxies with halos of mass M_h \lesssim 10^{12} M_{\odot}.

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arXiv2026

A Comprehensive Study of Morphology and Kinematics in Extended Nebulae Around UV Luminous Quasars at $z\approx1$

Gas flows between galaxies and the circumgalactic medium (CGM) play a central role in galaxy evolution and can become observable as giant nebulae when illuminated by the quasars. We present an ensemble study of nebulae around 30 UV-luminous quasars at z=0.4-1.4 from the CUBS and MUSEQuBES surveys, 27 of which are detected in extended [O II] and/or [O III] emission. Based on a joint analysis of nebular morphology and surrounding galaxy environments, we introduce three morpho-kinematic classifications. We identify eleven irregular, large-scale (>50 kpc) systems, many of which are likely interaction-related; twelve compact host-galaxy-scale nebula, likely tracing CGM/ISM gas; and four systems with complex morphologies of uncertain origin. We introduce a quantitative measure of the spatial and kinematic association between nebulae and quasar-host group galaxies, finding a statistically significant association for ten nebulae, most of which are irregular, large-scale nebulae, consistent with qualitative analysis. Radio jets are detected in six systems, with no strong correlation found between radio activity and nebular emission. The [O II] nebulae are more asymmetric than their Ly$α$ counterparts at $z>2$, but bear more similarity to H I gas observed in 21 cm around local elliptical galaxies. Blueshifted-redshifted patterns, likely tracing gas rotation, are observed in roughly 30% of the systems, though disturbed kinematics suggest that feedback may also be important. These results show that giant quasar nebulae are not a uniform class of objects, but instead arise through multiple pathways shaped by host-galaxy gas, galaxy interactions, group environments, and quasar activity, with the most striking cases associated with galaxy interactions.

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arXiv2026

Cosmic ray heating of cold streams: Implications for the gas supply and growth of massive galaxies

Recent observations have demonstrated the presence of cosmic rays (CRs) in cosmic-web filaments. Cold streams supply gas inflows from these filaments into massive galaxies during the cosmic noon. As these streams are expected to be magnetised, external cosmic-web CRs may become entrained with this inflowing gas. We aim to determine whether this externally-supplied CR population can deposit energy to alter or disrupt the supply of cold gas to galaxies. We couple a spectrally-resolved CR transport calculation to a redshift-dependent analytical model of magnetised cold streams in galaxy haloes and investigate whether externally-supplied CRs can modify gas supply through this channel. We find CR energy deposition can alter the thermal state of cold streams. Dense stream cores remain largely resilient and only experience weak heating. Their temperature is raised by less than a factor of 10, which is insufficient to overcome radiative cooling at the stream-CGM interface. In more diffuse streams, and in partially mixed interface gas of the most massive haloes near the virial radius, CR heating becomes strong enough that radiative cooling can no longer balance it, and the gas is heated toward or above the mixing-layer temperature. This weakens the stability of the stream, making it more susceptible to disruption. Complete evaporation is possible only in extreme cases. Cold streams are therefore more vulnerable to CR heating at larger galactocentric radii, higher halo masses, and in more diffuse or partially-mixed stream material. By preferentially heating diffuse gas, externally supplied CRs may introduce additional selectivity into cold-gas accretion that modifies the gas supply and growth of massive galaxies. These CRs weaken fragile streams and erode their cold envelope, and may cause surviving cold flow components to appear thinner and more sharply confined far into galaxy haloes.

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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$α$ and H$α$ 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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arXiv2026

Strong environmental AGN enhancement among DSFGs in z > 2 protoclusters

Galaxy protoclusters (PCs) at z > 2 are dense regions in which cold gas availability and elevated galaxy interaction rates trigger intense, often dust-obscured, star formation. These mechanisms are also expected to promote super-massive black hole (SMBH) growth, but this effect remains unclear, largely due to heterogeneous galaxy selections and active galactic nuclei (AGN) identification methods in previous studies. We quantitatively assess the impact of PC environment on SMBH growth by measuring the incidence of X-ray AGN among dusty star-forming galaxies (DSFGs) in PCs and in a homogeneously selected control field sample, and investigate the physical mechanisms driving any difference. We consider ALMA-detected DSFGs in sub-mm/mm continuum of seven PCs at 2 < z < 4.5, and construct a selection-matched control sample from the COSMOS survey. We statistically compare X-ray AGN incidence and host galaxy physical properties obtained through uniform spectral energy distribution fitting. We find a significant enhancement of X-ray AGN fraction in PCs by ~2.7x (Poisson significance p = 3e-4). Similar values are found in two redshift bins: ~2.7x at z = 2-3 (p = 0.003) and ~2.6x at z = 3-4.5 (p = 0.03). PC and field DSFG samples are well matched in stellar mass, star-formation rate, and dust mass, ruling out selection effects or systematically higher host masses as the driver. Our results provide quantitative evidence that the dense PC environment enhances AGN incidence and SMBH growth in DSFGs beyond what host galaxy properties alone predict, likely through increased gas availability and interaction-driven fueling. This work is a first step toward a homogeneous assessment of environmental effects on SMBH growth across cosmic time.

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arXiv2026

Constraining the nature of active galactic nuclei through circumgalactic Lya emission at z=2-3

We present a comprehensive analysis of circumgalactic Lya nebulae around 59 unobscured and 26 obscured quasars at z=2-3, observed with the Keck Cosmic Web Imager (KCWI), to constrain the nature of active galactic nuclei (AGN) at cosmic noon. We find that Lya nebulae around unobscured quasars are significantly less symmetric having a symmetry parameter of a_w=0.2-0.6 and more spatially extended having a scale length of r_h=10.7+/-0.5 kpc than those around obscured quasars (a_w=0.6-0.8; r_h=6.6-7.7 kpc).Unobscured quasars also exhibit steeply declining velocity dispersion profiles with the slope of -4.3+/-0.4 km s^-1 kpc^-1, indicative of large-scale outflows, whereas obscured quasars display flat profiles (-0.2+/-0.7 and -0.6+/-0.4 km s^-1 kpc^-1). The degree of quasar obscuration appears to be intrinsically linked to nebular asymmetry and extent, a relationship that could be in tension with the standard orientation-based AGN unified model, as it expects unobscured-quasar nebulae to be more symmetric and compact. These results naturally fit the evolutionary scenario, where AGN feedback drives a transition from an obscured to an unobscured phase-progressively redistributing gas to larger radii, introducing anisotropy, and driving turbulence. Taken together, our findings favor the evolutionary scenario over the purely orientation-based unified model for quasars at cosmic noon.

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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

MUSEQuBES: Investigating the Physical and Chemical Properties of Circumgalactic Gas Around Ly$α$ Emitters at $z \approx 3.3$

We present a detailed study of the physical properties of absorbers associated with $z\approx3.3$ \lya\ emitters (LAEs) from the MUSEQuBES survey. Using \HI\ and ionic column densities derived from Voigt profile fitting, we determine the density and metallicity of 75 absorbers associated with 59 LAEs through a custom-built Bayesian framework. The overall sample shows a median density $\log\,(n_{\rm H}/{\rm cm^{-3}})=-2.7\pm0.7$ and metallicity \met$=-1.6^{+1.2}_{-0.9}$ with $\approx15\%$ ($\approx 40\%$ including upper limits) of systems showing metallicities consistent with the IGM at these redshifts (\met$\lesssim -2.8$). Intriguingly, absorbers with $\log\, N(\rm HI)/{\rm cm^{-2}} \gtrsim 16.5$, corresponding to an overdensity of $\sim 100$ at $z=3$, exhibit a bimodal metallicity distribution with peaks at \met$=-3.8\pm 0.2$ and $-1.8\pm 0.6$. The latter are observed at large impact parameters ($\gtrsim150$~pkpc) and often exhibit low-ionization species (e.g., \SiII, \AlII). We interpret the former as pristine inflowing gas from the cosmic web, while the latter likely traces metal-enriched CGM associated with outflows from faint galaxies not detected in Ly$α$ emission. We find no significant correlations between absorber metallicity or density and host galaxy properties, including redshift, impact parameter, SFR, Ly$α$ luminosity, or environment. Absorbers separated by $\lesssim500$~\kms\ show $\sim$1~dex metallicity and $\sim$0.5~dex density variations, indicating physical and chemical inhomogeneity of the medium around these LAEs.

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arXiv2026

MUSEQuBES: Physical conditions, origins, and multi-element abundances of the circumgalactic medium of an isolated, star-forming dwarf galaxy at z=0.57

In dwarf galaxy models, outflows expel metal-enriched interstellar medium (ISM) into the circumgalactic medium (CGM) to reproduce their observed low metallicities, but measurements of dwarf CGM properties are scarce. We present a study of the CGM of an isolated dwarf at $z=0.5723$ with a stellar mass of $\approx5\times10^7\rm\,M_{\odot}$ and star-formation rate ($\approx0.05\,\rm M_\odot\,yr^{-1}$) and ISM metallicity ($\rm [O/H]\approx-0.9$) consistent with the star-forming main sequence and mass-metallicity relation. A background quasar sightline with archival UV spectra probes the dwarf's CGM at a projected distance of 28 kpc, corresponding to approximately half of the estimated virial radius. The dwarf's CGM is detected in \ion{H}{1}, intermediate metal ions of \ion{C}{3}, \ion{O}{3}, \ion{O}{4}, and \textcolor{black}{\ion{S}{5}}, and kinematically broader, highly-ionized \ion{O}{6}, but is undetected in \ion{N}{4} and \ion{Ne}{8}. Photoionization modeling of the intermediate ions indicates a modest volume-filling factor ($\sim 6\%$ along the sightline or $\sim 2\%$ globally), and a mass of $\sim2\times10^8 {\rm\,M_\odot}$, $\sim4\times$ higher than the dwarf's stellar mass, but $\sim10\times$ less than the highly ionized CGM. The \ion{O}{6} kinematics are comparable to the dwarf's estimated virial velocity, suggesting it is likely associated with cool, photoionized, and volume-filling CGM, with bulk motion or turbulence dominating over thermal pressure. The metallicity inferred for the intermediate ions is $\rm [O/H]=-0.6$, but with low relative abundances of $\rm [C/O]=-0.6$ and \textcolor{black}{$\rm [N/O]<-1.0$}. The [N/O] is below levels expected of the dwarf's ISM, but consistent with core-collapse supernova ejecta, suggesting that supernova-enriched gas escaped the dwarf without mixing significantly with ISM enriched in nitrogen from evolved, low-mass stars.

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arXiv2025

Little red dot variability over a century reveals black hole envelope via a giant Einstein cross

"Little red dots" (LRDs) represent a new population of astronomical objects uncovered by JWST whose nature remains debated. Although many LRDs are suspected as active galactic nuclei (AGN), they show little variability on days-years timescales. We report the discovery of two gravitationally lensed LRDs at redshift $\sim$4.3 behind the cluster RXCJ2211-0350, one of which (RX1) is quadruply imaged with time delays spanning $\sim$130 years. RX1 exhibits intrinsic color and brightness variations of up to 0.7 magnitude among its images. These changes are consistent with blackbody-temperature variations of a photosphere, indicating long-term variability analogous to Cepheid-like pulsations but in a far more extended ($R \sim 2000$ AU) and massive ($M \gtrsim 10^6 \, M_{\odot}$) systems. These results suggest LRDs as a distinct class of AGN with stellar-like envelopes.

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arXiv2025

MUSEQuBES: The Column Density, Covering Fraction, Mass, and Environmental Dependence of Cool HI Gas Around Low-Redshift Galaxies

We investigate cool HI gas traced by Lyman series absorption around 256 galaxies at z ~ 0.48 (median stellar mass, log10(M*/Msun) = 8.7) using 15 background quasars (median impact parameter, D = 140 pkpc), as part of the MUSE Quasar-fields Blind Emitters Survey (MUSEQuBES). We find that the HI column density (N(HI)) profile around isolated star-forming galaxies spanning ~3 dex in stellar mass is well described by a power law with slope ~ -3 when expressed as a function of normalized impact parameter D/Rvir. The HI covering fraction (k) within the virial radius for log10(N(HI)/cm^{-2}) = 14 is significantly lower in high-mass passive galaxies than in isolated star-forming galaxies. The k-profile of isolated star-forming galaxies suggests a characteristic size of the HI-rich CGM of ~ 1.5 Rvir across the stellar mass range. The mean HI mass in the outer CGM (0.3-1 Rvir ) increases with stellar mass, ranging from ~ 10^5 to 10^6.6 Msun. The b-parameters of the strongest HI components correlate and anti-correlate with specific star-formation rate (sSFR) and mass, respectively, with >2 sigma significance. Broad Lya absorbers (BLAs) with b > 60 km/s are predominantly associated with high-mass galaxies, likely tracing the warm-hot phase of the CGM. The velocity centroids of H i components indicate that absorbers at D < Rvir are largely consistent with being gravitationally bound to their galaxies, independent of stellar mass. Finally, leveraging ~ 3000 galaxies from the wide-field Magellan follow-up of six MUSEQuBES fields, we find that non-isolated galaxies exhibit an HI-rich environment extending roughly three times farther than in isolated counterparts.

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arXiv2025

Kinematic analysis of an Ultra-Strong MgII absorber at z~1.13 linking to Circumgalactic Gas Structures

We present a spectroscopic and imaging analysis of the $z_{gal} \approx 1.1334$ ultra-strong MgII absorption system identified in the $VLT$/UVES spectrum of a background quasar located at $ρ\approx 18$ kpc from a star-forming galaxy. Low ionization metal lines like MgI, FeII, and CaII are also detected for this absorber. The HI lines are outside of the wavelength coverage. The MgII has a rest-frame equivalent width of $W_r(2796) =3.185 +/- 0.032 A^{\circ} $, with the absorption spread across $Δv \approx 460$ km~s$^{-1}$ in several components. A component-by-component ionization modeling shows several of these components having solar and higher metallicities. The models also predict a total HI column density of $log[N(HI)/cm^{-2}] \approx 22.5$, consistent with ultra-strong MgII absorbers being sub-Damped Lyman Alpha and Damped Lyman Alpha systems. The absorber is well within the virial radius of the nearest galaxy which has a stellar mass $M_* = 4.7 \times 10^{10}$~M$_\odot$, and a star formation rate of $\approx 8.3$~M$_\odot$~yr$^{-1}$. The absorption is along the projected major axis of the galaxy with a velocity spread that is wider than the galaxy's disk rotation. From the kinematic analysis of the absorber and the galaxy, the origin of the absorption can be attributed to a combination of circumgalactic gas structures, some corotating with the disk and the rest at line-of-sight velocities outside of the disk rotation.

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arXiv2025

H I Properties of Field Galaxies at $\boldsymbol{z\approx 0.2}$-0.6: Insights into Declining Cosmic Star Formation

We report statistically significant detection of H I 21-cm emission from intermediate-redshift ($z\approx0.2$-0.6) galaxies. By leveraging multi-sightline galaxy survey data from the Cosmic Ultraviolet Baryon Survey (CUBS) and deep radio observations from the MeerKAT Absorption Line Survey (MALS), we have established a sample of $\approx6000$ spectroscopically identified galaxies in 11 distinct fields to constrain the neutral gas content at intermediate redshifts. The galaxies sample a broad range in stellar mass -- $8\lesssim\log{M_\rm{star}/\rm{M}_\odot}\lesssim11$ with a median of $\langle\log{M_\rm{star}/\rm{M}_\odot}\rangle_\rm{med}\approx10$ -- and a wide range in redshift -- $0.24\lesssim z\lesssim0.63$ with a median of $\langle z\rangle_\rm{med}=0.44$. Our detected emission-line signal exceeds $4\,σ$ significance in the stacked spectra of all subsamples, and the observed total H I 21-cm line flux translates to a H I mass $M_\rm{H\;I}\approx10^{10}\rm{M}_\odot$. We find a high H I-to-stellar mass ratio of $M_\mathrm{H\;I}/M_\rm{star}\approx6$ for low-mass galaxies with $\langle\log{M_\rm{star}/\rm{M}_\odot}\rangle \approx9.3$ ($>3.7\,σ$). For galaxies with $\langle\log{M_\rm{star}/\rm{M}_\odot}\rangle\approx10.6$, we find $M_\mathrm{H\;I}/M_\rm{star}\approx0.3$ ($>4.7\,σ$). Additionally, the redshift evolution of H I mass in both low- and high-mass field galaxies, inferred from the stacked emission-line signal, aligns well with the expectation from the cosmic star formation history. This suggests that the overall decline in the cosmic star formation activity across the general galaxy population may be connected to a decreasing supply of neutral hydrogen. Finally, our analysis has revealed significant 21-cm signals at distances greater than 75 kpc from these intermediate-redshift galaxies, indicating a substantial reservoir of H I gas in their extended surroundings.

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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

Formation and Environmental Context of Giant Bulgeless Disk Galaxies in the Early Universe: Insights from Cosmological Simulations

Giant bulgeless disk galaxies, theoretically expected to be rare in the early Universe, have been confirmed by the James Webb Space Telescope (JWST) to exist as early as 2 billion years after the Big Bang. These morphologically extreme systems offer valuable insights into the physics of disk formation and the interplay between galaxies and their dark-matter halos. Using cosmological simulations, we identify analogs of such galaxies with stellar masses around $10^{11} M_\odot$ and half-light radii up to 6 kpc at $z \sim 3$ and characterize the factors that contribute to their formation. These galaxies form in young cosmic knots, populating host halos of high spin, low concentration, and spherical shapes. They feature dynamically coherent circum-galactic medium, as well as gas-rich, coherent mergers, which preserve their disk morphology and drive their large sizes. Interestingly, all the simulated giant disks harbor a compact, aligned inner disk, marginally resolvable in JWST images with a Sérsic index near unity. These findings highlight the environmental and structural conditions necessary for forming and sustaining giant bulgeless disks and provide a theoretical framework for interpreting JWST observations of extreme disk morphologies in the early Universe.

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