Yucheng Guo

We examined the relation between extended Ly α halos around high-redshift galaxies and the main factors responsible for driving the emission in such halos, in particular at distances around and beyond one virial radius r _vir . To reach the required surface brightness sensitivity we take advantage of the MUSE eXtremely Deep Field (MXDF) survey, allowing us to probe levels as faint as ∼10 ⁻²⁰ erg cm ⁻² s ⁻¹ arcsec ⁻² in individual Ly α halos. Our sample consists of the 21 apparently core- and halo-brightest (yet intrinsically low luminosity of log ₁₀ L _{Ly α} < 42.3 erg s ⁻¹ ) Ly α emitters (LAEs) in the MXDF at 3 < z < 4, with typical virial radii around 20 kpc. We measure their radial surface-brightness profiles out to 50 kpc (more than 2 r _vir ) and investigate the correlations between surface brightness and internal star formation rates of the host galaxies (SFR) or external influences (environmental density, δ + 1). We find a clear break in these correlations at radii around or just below 1 r _vir . Below this break the emission correlates tightly with SFR (as expected) and not at all with δ + 1. Beyond ∼1 r _vir (20 kpc) we observe the opposite trend with no dependence on SFR, but an emerging correlation with δ + 1. We compare our measurements with the expected integrated surface brightness from ultrafaint, individually undetected LAEs and find that the latter is insufficient to drive the observed correlation. We conclude that Ly α emission from the outer halos is regulated by the surrounding environment, but originates mostly from diffuse gas rather than discrete sources.

Small-scale multiphase structure plays a central role in galactic-wind evolution, yet the parsec-scale morphology and excitation of the warm ionized gas remain poorly constrained. We present deep Hubble Space Telescope (HST) narrow-band imaging of the southern wind of Messier 82 (M82) in H α , [O iii ], [S ii ], and [N ii ], designed to resolve the warm ionized phase on parsec scales. The H α emission is detected to ≈2.1 kpc above the disk, while the fainter emission lines are detected over smaller radial extents, with [O iii ] reaching ≈1.5 kpc. We develop a filament-finding pipeline for the H α image and construct a quantitative catalogue of the filamentary structures. The wind forms a highly connected network of strands and knots, dominated by compact filaments with typical projected widths of ≃5.3 pc and lengths of ≃9.5 pc. Both the projected covering fraction and the line-flux contribution of the filamentary component decline with height, showing that the outer wind becomes increasingly dominated by diffuse emission. Optical line-ratio diagnostics indicate that the warm ionized gas occupies an intermediate excitation regime: photoionization by the central starburst can energetically power the observed H α luminosity, while the systematic separation between filamentary and diffuse emission, together with the evolution of the line ratios with vertical distance, suggests an increasing contribution from shocks or other similar heating in the diffuse outer wind. These results show that separating filamentary and diffuse emission in high-resolution imaging provides a powerful way to connect the morphology, excitation, and multiphase structure of galactic winds.

Protoclusters represent sites of accelerated galaxy formation and extreme astrophysical activity characteristic of dense environments. Identifying massive protoclusters and mapping their spatial structures are therefore crucial for understanding how large-scale environment influences galaxy evolution. We combine wide-field Ly α imaging from the One-hundred-deg ² DECam Imaging in Narrowbands survey with extensive Dark Energy Spectroscopic Instrument and ancillary spectroscopy across the extended COSMOS and XMM Large Scale Structure (LSS) fields (≈14 deg ² ) to search for massive protoclusters. We confirm six systems at z ≈ 2.4 and ≈ 3.1, including three newly identified structures and three which overlap with previously known structures and/or systems detected using other tracers. We reconstruct their three-dimensional structures, estimate descendant halo masses, and for one structure at z ≈ 3.12, demonstrate that overlapping narrowband filters (NB497 and N501) provide accurate redshift tomography for emission-line galaxies. One protocluster at z ≈ 2.45 overlaps with one of the LATIS tomographic fields, enabling direct comparison between galaxy and H i overdensities traced by Ly α forest absorption. Another at z ≈ 3.12 hosts a massive quiescent galaxy ( M _* ≈ 1.2 × 10 ¹¹ M _⊙ ), suggesting that overdense environments may play a role in accelerating galaxy assembly and quenching. Comparing Ly α emission properties across environments, we find that protocluster galaxies exhibit higher median line fluxes and a deficit of faint emitters relative to the field. The effect is strongest when combining 2D and 3D density information, indicating that galaxies in the densest protocluster cores are most affected by environmental processes. This effect is stronger at z ≈ 3.1 than at z ≈ 2.4, suggesting possible redshift evolution.

High-redshift Lyman continuum emitters (LCEs) are crucial for understanding how galaxies ionize the neutral hydrogen in the epoch of reionization. However, detected LCEs at z > 4 are quite rare. Here, we report an LCE candidate at z = 4.444, dubbed LCEz4-M1, which is one of the highest-redshift LCE candidates currently known. The redshift is determined from the Ly α emission line detected in the Very Large Telescope (VLT)/MUSE spectrum. The Lyman continuum (LyC) signal is detected independently in the Hubble Space Telescope F435W image and the VLT/MUSE spectrum at significances of ≃3.7 σ and ≃2.8 σ –3.0 σ , respectively. The LyC centroid is spatially consistent with the James Webb Space Telescope (JWST)/NIRCam continuum within the astrometric uncertainty. Adopting the maximum intergalactic medium transmission, we infer conservative lower-limit escape fractions of f esc ( F435W ) = 0.8 2 − 0.17 + 0.13 and f esc ( MUSE ) = 0.7 5 − 0.28 + 0.18 . Using the combined JWST and MUSE dataset, we characterize the physical properties and morphology of LCEz4-M1. In our fiducial JWST-only spectral energy distribution fit, the galaxy is compact but has a moderate current galaxy-integrated star formation surface density, Σ _SFR = 0.38 M _⊙ yr ⁻¹ kpc ⁻² , suggesting that it is not an extreme compact starburst under this fiducial interpretation. While we find no clear evidence for an ongoing major merger for LCEz4-M1, the presence of a faint companion ( ∼ 0 . ″ 5 ) detected in the F277W band suggests a potential minor interaction. We also find that LCEz4-M1 may lie in a locally overdense region, although the environmental interpretation remains tentative. Finally, the low star formation ratio, SFR _{10 Myr} /SFR _{100 Myr} , low Ly α equivalent width, and relatively weak rest-frame optical emission lines of LCEz4-M1 may indicate a postburst LyC leaking phase.

Galaxies evolve within vast gaseous halos that fuel star formation and carry signatures of feedback-driven outflows. Deep integral field data have enabled the study of Mg II λλ 2796, 2803 halos, which trace galaxy-scale outflows in emission, and while individual detections of Mg II halos have revealed extended circumgalactic structures, their faintness has limited studies to single-object analyses. Here, we present the first statistical study of Mg II -emitting halos using deep MUSE observations of 47 star-forming galaxies at 0.7 < z < 2.0. Building on our previous work, where we developed and applied an outflow modeling framework to a single Mg II halo, we now extend this approach to a larger sample, enabling robust population-level insights into the properties of circumgalactic outflows traced by their extended Mg II emission for the first time. We detected extended Mg II emission out to tens of kiloparsecs and modeled the outflows as an ensemble of radially accelerating shells. Galaxies with Mg II outflows tend to have higher star formation rates (SFRs) and specific star formation rates (sSFRs) and younger stellar populations–consistent with star-formation-driven winds. The observations are consistent with winds that accelerate linearly with radius ( v ∝ r ) from launching velocities of v ∼ 60 km s ⁻¹ up to maximum velocities that correlate with the stellar mass of galaxies, and are of about ∼490 km s ⁻¹ . The inner regions of the outflows are highly opaque (log τ ∼ 2.6), and we also identify a tentative trend between stellar mass and central optical depth. The opening angle of the outflow shows some dependency on the host-galaxy stellar mass, with less massive galaxies showing primarily wide opening angles (i.e., nearly isotropic outflows), and more massive galaxies showing a broader range of values, with both wide and narrow opening angles. The distribution of the spatial extent of Mg II halos exhibits a clear peak at half-light radius (HLR) of ∼5 kpc, with an extended tail of larger HLR values, up to ∼20 kpc. Compact halo sizes (HLR < 8 kpc) correlate with stellar mass, but extended halos do not, which could suggest a difference in the powering mechanism between compact and extended halos. This work provides new insight into the structure and dynamics of the circumgalactic medium and its role in galaxy evolution by constraining, for the first time, the properties of circumgalactic outflows with a statistically significant sample of galaxies with an extended Mg II emission halo.

Gas accretion and feedback are fundamental processes in galaxy evolution models and simulations. In particular, feedback from massive stars is thought to reshape the structure of the interstellar medium (ISM) and self-regulate star formation, leading to the tight observed relationship between star formation rate (SFR) and galaxy mass. However, direct observational constraints on this feedback loop remain limited. Most studies either focus on small samples of galaxies or predominantly probe the warm and hot ionized gas phases, leaving the impact of feedback on the cold neutral gas -- a critical phase of the ISM -- largely unexplored. To address this, we use single-dish HI 21cm observations to statistically detect bulk vertical motions of neutral hydrogen in a sample of nearby, star-forming galaxies. These motions trace gas flows moving perpendicular to the galactic disk. Our analysis reveals that the vertical gas flows exist in a near-equilibrium state: they are driven outwards as galactic winds by momentum injection from star formation, while being simultaneously pulled back by gravitational forces, forming a fountain-like cycle. Such galactic fountains are prevalent in the local Universe. Their mass, momentum, and kinetic energy scale with SFR and can be well explained by a simple dynamical model balancing stellar feedback and gravity. This supports a scenario in which gas recycling, rather than complete ejection, is the dominant outcome of stellar feedback in the local Universe, highlighting the importance of vertical gas circulation in regulating star formation.

Aims. The circumgalactic medium (CGM) is a key component in understanding the physical processes governing the flows of gas around galaxies. Quantifying its evolution and its dependence on galaxy properties is particularly important for our understanding of accretion and feedback mechanisms. Methods. We selected a volume-selected sample of 66 isolated star-forming galaxies at 0.4 < z < 1.5 with log(M_⋆/M_⊙) > 9 from the MusE GAs FLOw and Wind (MEGAFLOW) survey. Using Mg IIλλ2796,2803 absorptions in background quasars, we measured the covering fraction, f_c, and quantified how the cool gas profile depends on galaxy properties (such as star formation rate (SFR), stellar mass (M_⋆) or azimuthal angle relative to the line of sight) and how these dependencies evolve with redshift. Results. The Mg II covering fraction of isolated galaxies is a strong function of impact parameter and is steeper than previously reported. The impact parameter, b₅₀, at which f_c = 50% is b₅₀ = 50 ± 7 kpc for W_r²⁷⁹⁶ > 0.5 Å. It is weakly correlated with SFR (∝SFR^{0.08 ± 0.09}) and decreases with cosmic time (∝(1 + z)^{0.8 ± 0.7}), contrary to the expectation of increasingly larger halos with time. The covering fraction is also higher along the minor axis than along the major axis at the ≈2σ level. Conclusions. The CGM traced by Mg II is similar across the isolated galaxy population. Indeed, among the isolated galaxies with an impact parameter below 55 kpc, all have associated Mg II absorption with W_r²⁷⁹⁶ > 0.3 Å, resulting in a steep covering fraction, f_c(b).

Context. The hydrogen Lyman-alpha (Lyα) line, the brightest rest-frame ultraviolet line of high-redshift galaxies, exhibits a large variety of shapes, which is due to factors at different scales, from the interstellar medium to the intergalactic medium (IGM). Aims. The aim of this work is to provide a systematic inventory and classification of the spectral shapes of Lyα emission lines to better understand the general population of high-redshift Lyα emitting galaxies (LAEs). Methods. Using the unprecedentedly deep data from the MUSE eXtremely Deep Field (MXDF; up to 140 hour exposure time), we selected 477 galaxies observed in the ∼2.8−6.6 redshift range, 15 of which have a systemic redshift from nebular lines. We developed a method to classify Lyα emission lines in four spectral and three spatial categories by combining a pure spectral analysis with a narrow-band image analysis. We measured spectral properties, such as the peak separation and the blue-to-total flux ratio for the double-peaked galaxies. Results. To ensure a robust sample for statistical analysis, we define two unbiased subsets, inclusive and restrictive, by applying thresholds for signal-to-noise ratio, peak separation, and Lyα luminosity, yielding a final unbiased sample of 206 galaxies. Our analysis reveals that between 32% and 51% of the galaxies exhibit double-peaked profiles, with peak separations ranging from 150 km s⁻¹ to nearly 1600 km s⁻¹. The fraction of double-peaked galaxies seems to evolve dependently with the Lyα luminosity, while we do not see a severe decrease in this fraction with redshift, which is expected given the IGM attenuation at high redshift. An artificial increase in the number of double-peaked galaxies at the highest redshifts may cause the observation of a plateau instead of a decrease. A notable number of these double-peaked profiles show blue-dominated spectra, suggesting unique gas dynamics and inflow characteristics in some high-redshift galaxies. The consequent fraction of blue-dominated spectra needs to be confirmed by obtaining new systemic redshift measurements. Among the double-peaked galaxies, 4% are spurious detections, that is, the blue and red peaks do not come from the same spatial location. Around 20% out of the 477 sources of the parent sample lie in a complex environment, meaning there are other clumps or galaxies at the same redshift within a distance of 30 kpc. Conclusions. Our results suggest that the double-peaked LAE fraction may trace the evolution of IGM attenuation, but the faintest galaxies must be observed at high redshift. We also need more data to confirm the trend seen at low redshift. In addition, it is crucial to obtain secure systemic redshifts for LAEs to better constrain the nature of the Lyα double-peaked lines. Statistical samples of double-peaked and triple-peaked galaxies are a promising probe of the evolution of the physical properties of galaxies across cosmic time.

Context. The circumgalactic medium (CGM) serves as a baryon reservoir that connects galaxies to the intergalactic medium and fuels star formation. The spatial distribution of the metal-enriched cool CGM has not yet been directly revealed at cosmic noon (z ≃ 2–4), as bright emission lines at these redshifts are not covered by optical integral field units. Aims. To remedy this situation, we performed the first-ever detections and exploration of extended Si II* emission in the low-ionization state (LIS), referred to as Si II* halos, at redshifts ranging from z = 2 to 4 as a way to trace the metal-enriched cool CGM. Methods. We used a sample of 39 galaxies with systemic redshifts of z = 2.1–3.9 measured with the [C III] doublet in the MUSE Hubble Ultra Deep Field catalog, whose integration times span from ≃30 to 140 hours. We searched for extended Si II* λ1265, 1309, 1533 emission (fluorescent lines) around individual galaxies. We also stacked a subsample of 14 UV-bright galaxies. Results. We report five individual detections of Si II* λ1533 halos. We also confirm the presence of Si II* λ1533 halos in stacks for the subsample containing UV-bright sources. The other lines do not show secure detections of extended emission in individual or in stacking analyses. These detections may imply that the presence of metal-enriched CGM is a common characteristic for UV-bright galaxies. To investigate whether the origin of Si II* is continuum pumping, as suggested in previous studies, we checked the consistency of the equivalent width (EW) of Si II* emission and the EW of Si II absorption for the individual halo object with the most reliable detection. We confirm the equivalence, suggesting that photon conservation works for this object and points toward continuum pumping as the source of Si II*. We also investigated Si II* lines in a RAMSES-RT zoom-in simulation including continuum pumping, and find the ubiquitous presence of extended halos.

The extended Lyα haloes (LAHs) have been found to be prevalent around high-redshift star-forming galaxies. However, the origin of the LAHs is still a subject of debate. The spatially resolved analysis of Lyα profiles provides an important diagnostic. We analyse the average spatial extent and spectral variation of the circumgalactic LAHs by stacking a sample of 155 Lyα emitters (LAEs) at redshifts of 3 < z < 4 in the MUSE Extremely Deep Field. Our analysis reveals that, with respect to the Lyα line of the target LAE, the peak of the Lyα line at large distances becomes increasingly more blueshifted up to a projected distance of 60 kpc (≈3× virial radius), with a velocity offset of ≈250 km/s. This trend is evident in both the mean and median stacks, suggesting that it is a general property of our LAE sample, which typically has a Lyα luminosity of ≈10^41.1 erg s⁻¹. However, due to the absence of systemic redshift data, it remains unclear whether the Lyα line peak at large projected distances is less redshifted compared to the inner regions or truly blueshifted with respect to the systemic velocity. We explore various scenarios to explain the large-scale kinematics of the Lyα line.