arXiv subjects

Michele Fumagalli

Publications and source records attributed to Michele Fumagalli.

At least 19 recordsLinked to original sources

arXiv2026

Cosmic web Ly$α$ emission in a sample of overdense regions

We present a complete and homogeneous analysis of the Ly$α$ emission properties of the cosmic web at $3 \lesssim z \lesssim 5$ as a function of the overdensity of Ly$α$ emitters (LAEs) in the MUSE Ultra Deep Field (MUDF) and the MUSE Extremely Deep Field (MXDF). We identify 41 overdensities, probing environments that are $δ\approx 2 - 10$ times denser than the field. We search for extended emission down to surface brightness (SB) levels of $3 - 5 \times 10^{-20}\ \mathrm{erg\ s^{-1}\ cm^{-2}\ arcsec^{-2}}$, revealing filamentary structures beyond the scale of the circumgalactic medium, confirming that LAEs act as signposts of the cosmic web. A trend with overdensity emerges: non-detections are mainly at $δ< 2$, a $\approx 1$ dex scatter in SB appears for $2 \lesssim δ\lesssim 3.5$, and detections reach a maximum intrinsic SB of $\approx 2 \times 10^{-19}\ \mathrm{erg\ s^{-1}\ cm^{-2}\ arcsec^{-2}}$ for $δ\gtrsim 3.5$. Emitting regions occupy a fraction of projected area $f_s \approx 0.20 - 0.25$ inside filaments, yielding a cosmological incidence for Ly$α$ emission in the cosmic web $\ell(\mathrm{Ly}α)$ of $\approx 1.22$, which is similar to the one of partially-neutral Lyman limit systems. This analysis indicates that the emitting gas is partially ionized at moderate densities ($n_{\mathrm{H}} \approx 10^{-3} - 10^{-1}\ \mathrm{cm}^{-3}$), likely tracing the denser spines of intergalactic filaments and embedded substructures. Finally, we forecast how oriented stacking in larger samples of overdensities from shallower observations could yield an expanded view of the cosmic web before the next-generation wide-field spectroscopic instruments become operational.

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arXiv2026

ALMA-JELLY II. Constraining the radial profiles of the quenching timescales in ram-pressure-stripped galaxies

We investigate the environmental quenching histories and spatial progression of ram-pressure stripping in 11 galaxies in the nearby Coma, Norma, and A1367 clusters. As part of the ALMA-JELLY large program, we combine high-sensitivity VLT/MUSE integral-field spectroscopy with ultraviolet-to-far-infrared photometry in 37 spatially resolved disc apertures. The sample spans $\log(M_*/\mathrm{M}\odot)\sim7.5-10.5$ and includes galaxies undergoing active or recent ram-pressure stripping, identified through prominent H$α$ or UV tails. We use Bayesian spectrophotometric modelling with exponentially declining star-formation histories and show with mock data that combining spectra and photometry breaks parameter degeneracies and improves quenching-age constraints. All galaxies show quenched outer discs and relatively old ($\lesssim1$ Gyr) stellar populations. Massive galaxies ($\log(M_*/\mathrm{M}_\odot)>9.5$) exhibit clear outside-in quenching: inner regions quench about 500 Myr later than their outskirts. The small scatter in quenching age with distance from the truncation radius, despite differences in environment and stripping geometry, points to a common evolutionary sequence. Low-mass and dwarf galaxies instead show weak radial gradients, with quenching occurring more rapidly, typically within 300 Myr.

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arXiv2026

The environmental dependence of the circumgalactic medium in a high-resolution cosmological simulation

There is increasing evidence from observations that the circumgalactic medium (CGM) of galaxies depends on the large-scale structure in which they are embedded. When probing the CGM in absorption using quasar sightlines, studies find an enhanced sky coverage in the CGM of galaxies in overdensities compared to galaxies in isolation. However, the exact reason for this environmental dependence is still unclear. In this work we aim to model for the first time the influence of the large-scale structure on the cool and warm ($T\sim10^{4-5}$ K) gas phases of the CGM. We use a high-resolution ($m_{gas}\approx 4.5\times 10^4$ M$_{\odot}$, $m_{dm}\approx 2.4\times 10^5$ M$_{\odot}$) cosmological simulation based on the EAGLE model of galaxy formation. We select all galaxies at $z=0$ with stellar mass $M_*>10^8$ M$_\odot$ and split them into galaxies in overdensities (group galaxies) and galaxies in isolation using a Friends-of-Friends algorithm. For these two samples, we investigate how the large-scale structure influences the physical properties of the CGM and the measured covering fractions of the cool and warm gas phases. When the two samples of group and isolated galaxies are matched in stellar mass, halo mass, and we use only central galaxies, we do not find any significant difference in the physical properties of the CGM and the measured covering fractions. However, when satellite galaxies are included, we recover the observational trends in the difference of covering fractions with the environment. The difficulty of recovering the observational trends shows the complexity of capturing the multiphase CGM in simulations. However, since our results concerning the admixture of satellites are independent of the employed subgrid physics, this work shows that central galaxies and satellites need to be disentangled in observational studies to clearly discern the role of the environment on the CGM.

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arXiv2026

Forecast for the detectability of patchy hydrogen reionization in WEAVE-QSO measurements of the Lyman-$α$ forest power spectrum at redshift $z \geq 4$

We present the first detailed forecasts for the detectability of patchy hydrogen reionization in the one-dimensional Ly$α$ forest power spectrum to be measured by the WEAVE-QSO survey. Using the Sherwood-relics reionization simulations and a WEAVE-QSO survey configuration, we generate mock spectra in four redshift bins, $z=4.0,4.2,4.4,$ and $4.6$, in which relic ionization and temperature fluctuations from patchy hydrogen reionization enhance the Ly$α$ forest power spectrum on large scales (i.e., at wavenumber $k\sim 10^{-3},\mathrm{s\,km^{-1}}$). Our Ly$α$ forest pipeline forecasts the power spectrum covariance by considering sample size, spectral resolution, noise subtraction, continuum placement, metal contamination, and damping wings from high-column density absorbers. Applying our covariance forecast within a Bayesian parameter inference framework, we find that the signature of patchy hydrogen reionization should be detectable at a significance of $\simeq 4.5σ$. The forthcoming WEAVE-QSO 1D power spectrum measurements should therefore be able to directly detect and characterize the large-scale relic imprint of patchy hydrogen reionization in the Ly$α$ forest power spectrum at $z\geq 4$.

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arXiv2026

DESI as sparse Integral Field Spectrograph I: Spatially resolved chemical enrichment in star-forming galaxies at $z\leq0.1$

We present a spatially resolved chemical abundance analysis of 2291 star-forming galaxies at $z \leq 0.1$, spanning nearly four orders of magnitude in stellar mass ($8 \le \rm log (M_{\star}/M_{\odot}) \le 11.5$), by exploiting the multi-fibre spectra from the Dark Energy Spectroscopic Instrument (DESI) as a sparse integral field spectrograph. In the inner regions ($<2R_e$), the radial gas-phase metallicity profiles show an outward-declining trend for massive galaxies, with the steepest gradient ($\nabla_{log(O/H)}$) $\sim-0.08$ dex/R$_{e}$, whereas low-mass dwarf galaxies exhibit nearly flat profiles ($\nabla_{log(O/H)}\sim-0.02$ dex/R$_{e}$). The large galactocentric radii ($\sim$5 R$_{e}$) probed in this study, reveal flat metallicity profiles near the disk-halo interface. Strikingly, these flat metallicity values are consistent across a wide stellar mass range, likely reflecting the influence of low SFR and metal poor inflows in the outer regions. The metallicity gradient - stellar mass relation exhibits a turnover at $\log(M_\star/M_\odot) \sim 10.5$, beyond which gradients become shallower, possibly driven by the chemical equilibrium in the inner disk of massive galaxies and/or dilution from cosmic gas accretion. At fixed stellar mass, a strong size dependence is observed, where compact galaxies show flatter gradients and higher central enrichment than their extended counterparts. The abundance gradients are further linked with the stellar age distribution within the galactic disk, where galaxies with younger outskirts show steeper gradients than the ones with older outskirts, consistent with ongoing inside-out disc growth sustaining centrally concentrated chemical enrichment. These results underscore the interplay of star formation efficiency, stellar feedback, and metal-poor gas accretion in governing the radial chemical structure in galaxies.

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arXiv2026

MUSE Analysis of Gas around Galaxies (MAGG) -- VII. Emission line galaxies near strong blended Ly$α$ absorption systems at $z\gtrsim3$

We investigate the connection between strong, blended Ly$α$ absorption systems (SBLAs) and $\approx1000$ Ly$α$ emitting galaxies (LAEs) at $z\gtrsim3$ in 28 quasar fields from the MUSE Analysis of Gas around Galaxies (MAGG) survey. Selecting SBLAs as spectral regions with transmitted flux $-0.05 0.25$. The correlation appears to also depend on the width of the spectral window used to identify SBLAs, with a larger window yielding a stronger signal. Our analysis confirms that SBLAs serve as probes of the CGM at the interface between the Ly$α$ forest and the optically-thick Lyman limit systems. The significant dependence of the LAE-SBLA cross-correlation on the spectral binning used to select these absorbers motivates future tests of the current SBLA framework as a tracer of halos.

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arXiv2026

The Qz5 Survey (II): Metallicity Evolution of Damped Lyα Systems Out to z$\sim$5

Damped Ly$α$ absorbers (DLAs) are the highest \HI\ column density (\NHI) absorption line systems detected in the spectra of background quasars. DLAs dominate the neutral gas content of the Universe ($Ω_{\rm HI}$) and are used to measure the metallicity evolution of \HI\ gas. In this work, we introduce a sample of five recently detected DLAs at $z > 4.7$, found in mid to high-resolution spectroscopy from VLT/X-shooter and Keck/HIRES. These DLAs were not pre-selected based on metallicity, enabling an unbiased study of the metallicity of HI gas at $z \sim 5$. We also search for DLAs unbiased in metallicity at $0<z<5.5$ from the literature, we apply a combined correction for dust depletion and $α$-enhancement (assuming no depletion of S, Si, and Zn) to Fe abundances, and we measure the cosmic metallicity evolution of $α$-elements using a linear fit with a slope of$-0.22 \pm 0.05$ dex per unit redshift. For the highest redshift bin, we find an \NHI weighted average of $\langle Z \rangle = -2.00$. This value is $4.4σ$ deviant from the trend recovered at $z < 4.7$ and a K-S test comparison gives a $2.4 σ$ difference. We conclude that the metallicity of HI gas sharply decreases at $z \sim 5$, in agreement with previous tentative evidence. This sharp decrease may be connected with the onset of the enrichment of galaxies' circumgalactic media or with the end of cosmic reionization, though we cannot exclude that it is driven by small sample statistics.

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arXiv2026

The MUSE Ultra Deep Field (MUDF) VIII. The cool gas distribution surrounding galaxies at redshifts z ~ 0.5-2

We use deep MUSE data from the MUDF survey to investigate the cool gas around galaxies at redshifts 0.5 < z < 2. We constructed two samples: one sample for a down-the-barrel analysis, probing outflows via MgII absorption against galaxy continua, and the other sample for projected galaxy pairs to examine the gas around the foreground galaxies in the transverse direction. From down-the-barrel stacked spectra, we detected blueshifted MgII absorption, indicative of outflows, in which the absorption strength increases with stellar mass and star formation rate. Lower-mass galaxies exhibit weaker absorption, but higher outflow velocities, whereas higher-mass systems retain more cool gas with slower outflows. In the transverse direction, the absorption of MgII decreases with the impact parameter, following a shallow profile. Comparing observations with radiative transfer models, we found that extrapolating an expanding halo model constrained with down-the-barrel measurements to halo scales overestimates the observed equivalent widths, likely due to the outflow geometry and the absence of the interstellar medium in the model. Our results highlight that mass, outflow geometry, and gas retention shape the cool circumgalactic medium, and that the combination of absorption and emission diagnostics provides powerful constraints on the properties of the cold halo gas.

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arXiv2025

Bayesian luminosity function estimation in multi-depth datasets with selection effects: A case study for $3<z<5$ Lyman $α$ emitters

We present a hierarchical Bayesian framework designed to infer the luminosity function of any class of object by jointly modelling data from multiple surveys with varying depth, completeness, and sky coverage. Our method explicitly accounts for selection effects and measurement uncertainties (e.g. in luminosity) and can be generalized to any extensive quantity, such as mass. We validated the model using mock catalogues; from this we determined that deep data reaching $\gtrsim 1.5$ dex below a characteristic luminosity ($\tilde{L}^\star$) are essential to reducing biases at the faint end ($\lesssim 0.1$ dex) and that wide-area data help constrain the bright end. As a proof of concept, we considered a combined sample of 1176 Lyman $α$ emitters at redshift $3 < z < 5$ drawn from several MUSE surveys, ranging from ultra-deep ($\gtrsim 90$ hr) and narrow ($\lesssim 1$ arcmin$^2$) fields to shallow ($\lesssim 5$ hr) and wide ($\gtrsim 20$ arcmin$^2$) fields. With this complete sample, we constrain the luminosity function parameters $\log(Φ^\star/\mathrm{Mpc^{-3}}) = -2.86^{+0.15}_{-0.17}$, $\log(L^\star/\mathrm{erg\,s^{-1}}) = 42.72^{+0.10}_{-0.09}$, and $α= -1.81^{+0.09}_{-0.09}$, where the uncertainties represent the $90\%$ credible intervals. These values are in agreement with the results of studies based on gravitational lensing that reach $\log(L/\mathrm{erg\,s^{-1}}) \approx 41$, although differences in the faint-end slope underscore how systematic errors are starting to dominate. In contrast, wide-area surveys represent the natural extension needed to constrain the brightest Lyman $α$ emitters [$\log(L/\mathrm{erg\,s^{-1}}) \gtrsim 43$], where statistical uncertainties still dominate.

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arXiv2025

Baryonic Ecosystem IN Galaxies (BEINGMgII) -- III. Cool gas reservoirs at $0.3 \le z \le 1.6$ in the Dark Energy Survey

We investigate the origin of intervening cool MgII absorption detected in the spectra of background quasars and the nature of associated galaxies across a broad redshift range of $0.3 \le z \le 1.6$. Using nebular [O II] $λλ$3727,3729 emission lines identified in DESI fiber spectra centered on quasar, we detect 377 galaxies at a typical detection rate of $\sim$0.45% at $z \lesssim 1$, which increases with MgII equivalent width ($W_{2796}$). A significant fraction (74%) of these galaxies are associated with strong absorbers with $W_{2796} \ge$ 2Å. These absorbers trace galaxies spanning stellar masses of $\rm 8.4 \le \log(M_{\star}/M_{\odot}) \le 11.6$ and star formation rates (SFRs) of $\rm -1.2 \le log(SFR~[M_{\odot}yr^{-1}]) \le 2.7$, located at projected galactocentric distances of 4-24 kpc. We find the average MgII absorber strength increases from 2.1Å to 2.9Å between redshifts $z \sim$ 0.4 and 1.2, indicating evolution in the cool gas content of galaxy halos. The relatively constant absorber strength with galactocentric distance implies a clumpy structure of cool gas in the circumgalactic medium (CGM). Further, we find a positive correlation between $W_{2796}$ versus $M_\star$, and SFR, suggesting that the distribution of metal-enriched cool gas in the CGM is closely tied to the properties of the host galaxies. The redshift evolution of gas-phase metallicity suggests that strong MgII absorbers trace the general population of star-forming galaxies. The velocity dispersion of the cool gas increases with halo mass, and the wide range of line of sight velocity offset (-389 to 364 $\rm km\ s^{-1}$) between the galaxy systemic velocity and absorbers highlights the dynamical nature of CGM. However, the majority of this gas remains gravitationally bound to the dark matter halos, consistent with a picture of gas recycling via galactic fountains.

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arXiv2025

CLUES III: Do User Choices Impact The Results of SED Fitting? Tests of 'Off-The-Shelf' Stellar Population and Dust Extinction Models in the CLUES Sample

The simple stellar population models produced by stellar population and spectral synthesis (SPS) codes are used as spectral templates in a variety of astrophysical contexts. In this paper, we test the predictions of four commonly used stellar population synthesis codes (YGGDRASIL, BPASS, FSPS, and a modified form of GALAXEV which we call GALAXEVneb) by using them as spectral templates for photometric SED fitting with a sample of 18 young stellar clusters. All clusters have existing HST COS FUV spectroscopy that provide constraints on their ages as well as broadband photometry from HST ACS and WFC3. We use model spectra that account for both nebular and stellar emission, and additionally test four extinction curves at different values of $R_V$. We find that for individual clusters, choice of extinction curve and SPS model can introduce significant scatter into the results of SED fitting. Model choice can introduce scatter of 34.8 Myr in age, a factor of 9.5 in mass, and 0.40mag in extinction. Extinction curve choice can introduce scatter of up to a factor of 32.3 Myr in age, a factor of 10.4 in mass, and 0.41mag in extinction. We caution that because of this scatter, one-to-one comparisons between the properties of individual objects derived using different SED fitting setups may not be meaningful. However, our results also suggest that SPS model and extinction curve choice do not introduce major systematic differences into SED fitting results when the entire cluster population is considered. The distribution of cluster properties for a large enough sample is relatively robust to user choice of SPS code and extinction curve.

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arXiv2025

Automated quasar continuum estimation using neural networks: a comparative study of deep-learning architectures

Context. Ongoing and upcoming large spectroscopic surveys are drastically increasing the number of observed quasar spectra, requiring the development of fast and accurate automated methods to estimate spectral continua. Aims. This study evaluates the performance of three neural networks (NN) - an autoencoder, a convolutional NN (CNN), and a U-Net - in predicting quasar continua within the rest-frame wavelength range of $1020~\textÅ$ to $2000~\textÅ$. The ability to generalize and predict galaxy continua within the range of $3500~\textÅ$ to $5500~\textÅ$ is also tested. Methods. The performance of these architectures is evaluated using the absolute fractional flux error (AFFE) on a library of mock quasar spectra for the WEAVE survey, and on real data from the Early Data Release observations of the Dark Energy Spectroscopic Instrument (DESI) and the VIMOS Public Extragalactic Redshift Survey (VIPERS). Results. The autoencoder outperforms the U-Net, achieving a median AFFE of 0.009 for quasars. The best model also effectively recovers the Ly$α$ optical depth evolution in DESI quasar spectra. With minimal optimization, the same architectures can be generalized to the galaxy case, with the autoencoder reaching a median AFFE of 0.014 and reproducing the D4000n break in DESI and VIPERS galaxies.

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arXiv2025

The Qz5 Survey (I): How the HI Mass Density of the Universe Evolves With Cosmic Time

We report that the neutral hydrogen (HI) mass density of the Universe ($ρ_{HI}$) increases with cosmic time since $z \sim 5$, peaks at $z \sim 3$, and then decreases toward $z \sim 0$. This is the first result of Qz5, our spectroscopic survey of 63 quasars at $z \gtrsim 5$ with VLT/X-SHOOTER and Keck/ESI aimed at characterizing intervening HI gas absorbers at $z \sim 5$. The main feature of Qz5 is the high resolution ($R \sim 7000 - 9000$) of the spectra, which allows us to (1) accurately detect high column density HI gas absorbers in an increasingly neutral intergalactic medium at $z \sim 5$ and (2) determine the reliability of previous $ρ_{HI}$ measurements derived with lower resolution spectroscopy. We find 5 intervening Damped Ly$α$ absorbers (DLAs) at $z > 4.5$, which corresponds to the lowest DLA incidence rate ($0.034^{0.05}_{0.02}$) at $z \gtrsim 2$. We also measure the lowest $ρ_{HI}$ at $z \gtrsim 2$ from our sample of DLAs and subDLAs, corresponding to $ρ_{HI} = 0.56^{0.82}_{0.31} \times 10^8~$M$_{\odot}~$Mpc$^{-3}$ at $z \sim 5$. Taking into account our measurements at $z \sim 5$ and systematic biases in the DLA detection rate at lower spectral resolutions, we conclude that $ρ_{HI}$ doubles from $z \sim 5$ to $z \sim 3$. From these results emerges a qualitative agreement between how the cosmic densities of HI gas mass, molecular gas mass, and star-formation rate build up with cosmic time.

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arXiv2025

The MUSE Ultra Deep Field: A 5 Mpc stretch of the z $\approx$ 4 cosmic web revealed in emission

We detect Ly$α$ emission from a ~5 Mpc-long (comoving) portion of the cosmic web hosting an overdensity ($δ\approx 25$) of 19 Ly$α$ emitters (LAEs) at $z\approx 4$ within the MUSE Ultra Deep Field (MUDF), reaching an average surface brightness (SB) of $5\times 10^{-20}~\rm{erg~s^{-1}~ cm^{-2}~arcsec^{-2}}$. This large-scale structure has an average SB similar to the filament across the two MUDF quasars at $z\approx 3.22$. However, deep multiwavelength data do not show a clear presence of active galactic nuclei, suggesting that the emission is mainly regulated by the underlying gas density. We find $\approx 0.2$ dex higher star formation compared to control samples and a remarkable predominance (5/7) of blue-peaked emission lines in the spectra of the embedded LAEs, indicative of favorable conditions for gas accretion. Lastly, we quantify the contribution of intragalactic gas to the Ly$α$ SB profile at large distances from LAEs. By studying samples of filaments detected in emission within diverse environments, we are finally gaining new insight into the physics of gas accretion within the cosmic web.

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arXiv2025

High-definition imaging of a filamentary connection between a close quasar pair at z=3

Filaments connecting halos are a long-standing prediction of cold dark matter theories. We present a detection of the cosmic web emission connecting two quasar-host galaxies at redshift z ~3.22 in the MUSE Ultra Deep Field (MUDF). The very deep observations unlock a high-definition view of the filament morphology, a measure of the transition radius between the intergalactic and circumgalactic medium, and the characterization of the surface brightness profiles along the filament and in the transverse direction. Through systematic comparisons with simulations, we validate the filaments' typical density predicted in the current cold dark-matter model. Our analysis of the MUDF field, an excellent laboratory for quantitatively studying filaments in emission, opens a new avenue to constrain the physical properties of the cosmic web and to trace the distribution of dark matter on large scales.

astro-ph.CO↗

arXiv2025

Galaxy Formation and Symbiotic Evolution with the Inter-Galactic Medium in the Age of ELT-ANDES

High-resolution absorption spectroscopy toward bright background sources has had a paramount role in understanding early galaxy formation, the evolution of the intergalactic medium and the reionisation of the Universe. However, these studies are now approaching the boundaries of what can be achieved at ground-based 8-10m class telescopes. The identification of primeval systems at the highest redshifts, within the reionisation epoch and even into the dark ages, and of the products of the first generation of stars and the chemical enrichment of the early Universe, requires observing very faint targets with a signal-to-noise ratio high enough to detect very faint spectral signatures. In this paper, we describe the giant leap forward that will be enabled by ANDES, the high-resolution spectrograph for the ELT, in these key science fields, together with a brief, non-exhaustive overview of other extragalactic research topics that will be pursued by this instrument, and its synergistic use with other facilities that will become available in the early 2030s.

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arXiv2024

Modeling Mg II resonance doublet spectra from galaxy haloes at z $\sim$ 1

We investigate the properties of cold gas at $10^4~\rm K$ around star-forming galaxies at $z~\sim~1$ using Mg II spectra through radiative transfer modeling. We utilize a comprehensive dataset of 624 galaxies from the MAGG and MUDF programs. We focus on Mg II emission from galaxies and their outskirts to explore the cold gas within galaxies and the circumgalactic medium (CGM). We model Mg II spectra for 167 individual galaxies and stacked data for different stellar mass bins. The Mg II spectrum and surface brightness vary significantly with stellar mass. In low-mass galaxies ($M_*/M_\odot<10^9$), Mg II emission is observed in both core ($R_{\rm p}<$ 10 kpc) and halo regions (10 kpc $ 10^{10}$), strong core absorption and more extended halo emission are prominent. This indicates that more massive galaxies have more cold gas. Radiative transfer modeling allows us to investigate key parameters such as the Mg II column density $N_{\rm MgII}$ and the outflow velocity $v_{\rm exp}$. We identify a negative correlation between $N_{\rm MgII}$ and $v_{\rm exp}$. Since higher stellar mass galaxies exhibit a higher $N_{\rm MgII}$ and lower $v_{\rm exp}$, this suggests an abundance of slowly moving cold gas in massive galaxies. In addition, the fitting results of halo spectra indicate the presence of intrinsic Mg II absorption and strong anisotropy of the cold gas distribution around massive galaxies. This study is not only a proof-of-concept of modeling spatially varying Mg II spectra but also enhances our understanding of the CGM and provides insights into the mass-dependent properties of cold gas in and around galaxies.

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arXiv2024

Baryonic Ecosystem in Galaxies (BEINGMgII). Host Galaxies of Ultra-strong MgII Absorbers in Subaru Hyper Suprime-Cam Survey

We study the galaxies hosting ultra-strong MgII (USMgII) absorbers at small impact parameters of $\sim$2" (5 - 20 kpc), spanning a redshift range of $0.4 \le z \le 1.7$, using deep, high-resolution images from Hyper Suprime-Cam Subaru Strategic Survey and spectra from SDSS survey. From a total of 418 USMgII absorbers with $W_{2796}\ \ge 3 \mathring{A}$, along 412 quasar sightlines, we detect 50 galaxies based on [O II] $λλ$3727,3729 nebular emission detected at $\ge 2σ$ level. Utilizing the [O II] emission from the stacked spectrum and employing the best-fit galaxy SED template, we further identify 86 galaxies, leading to a total of 136 bona fide USMgII galaxies. With a prerequisite of having a minimum of four HSC passbands available, we find a detection rate of $\sim$38% at an average impact parameter of 11.4 kpc. We find that galaxies hosting USMgII systems are typically star-forming main sequence galaxies, with 21% exhibiting a starburst nature. The non-zero [O II] emission along the `clear' sightlines, with no stellar counterpart, hints that the USMgII absorbers may likely emanate from the unseen faint galaxies near the quasar. The USMgII absorbers preferentially align along the major and minor axes of the galaxy, which suggests that they originate in the disk or large-scale wind. We show that the distribution of $W_{2796}$ as a function of impact parameter indicates a discernible radial dependence for the `disk' and `wind' subsets, with the observed large scatter in $W_{2796}$ potentially attributed to large-scale outflows. The quasar sightline hosting USMgII systems show a factor three higher galaxy surface density at impact parameters of $\lesssim 50$kpc, highlights the multiple pathways giving rise to USMgII absorption.

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