arXiv subjects

Jun Toshikawa

Publications and source records attributed to Jun Toshikawa.

At least 19 recordsLinked to original sources

arXiv2026

H I Absorbers as Beacons of Hidden Structure at $z \sim 3$: Multi-Component, Metal-Rich Absorption System near a Protocluster

H I gas traces the large-scale structure and provides the primary fuel for star formation. High-$z$ protoclusters are ideal laboratories to study how H I gas is accreted and consumed during the formation of the most massive structures in the Universe. However, much remains unknown about the distribution and physical state of their H I gas. We examine a rare configuration in which a protocluster candidate is located in front of a quasar at $z=3.09$. Our spectroscopic campaign confirms a protocluster at $z=3.079$: however, no corresponding strong H I absorption is found in the background quasar spectrum. Instead, we serendipitously discover a prominent H I absorption feature at $z\sim 3.01$, offset by $\sim 60$ cMpc from the centre of the protocluster. Spanning an exceptionally broad velocity range of $\sim 2000$ km s$^{-1}$ ($\sim 40$ cMpc), this absorption is decoupled from the confirmed member galaxies. Detailed kinematic modelling reveals this absorption comprises five distinct components rather than a single cloud. Moreover, one of these components exhibits a super-solar metallicity ($[\mathrm{O/H}] = +1.19^{+0.91}_{-0.78}$). We propose two physical scenarios for this unique system: (1) an additional, hidden massive protocluster along the line of sight, and/or (2) metal-rich outflows and metal-poor inflows driven by a single massive galaxy. The discovery highlights that while protoclusters are not universally associated with strong H I absorption, targeting the strong H I absorbers may serve as a beacon for uncovering massive, metal-rich protoclusters or complex gas kinematics in the early Universe.

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arXiv2026

EPISODE II: Variability in the CO and H$_2$O rovibrational absorption lines in a periodically variable protostar EC 53

We present two-epoch JWST NIRSpec and MIRI observations of the young protostar EC 53 (V371 Ser), a periodically variable source with well-characterized quiescent and burst phases. The spectra in both epochs show absorption in the CO overtone ($\sim$2.3 $\mu$m) and fundamental ($\sim$4.6 $\mu$m) bands and the H$_2$O stretching ($\sim$2.7 $\mu$m) and bending ($\sim$6.0 $\mu$m) modes. We also obtained high-resolution ($R\approx45{,}000$) IGRINS spectra during the burst to constrain the CO overtone line profiles. LTE slab modeling yields gas temperatures of $\sim$1800 K (CO overtone) and $\sim$1200 K (CO fundamental), consistent with the overtone tracing hotter gas at smaller radii. The H$_2$O stretching-mode absorption shows no compelling evidence for variability, and the current JWST CO overtone data do not provide a robust constraint on overtone variability. In contrast, the CO fundamental and H$_2$O bending-mode features weaken by a factor of $\sim$2 during the burst, which is most naturally explained by continuum changes rather than large variations in absorbing gas. To quantify continuum dilution, we introduce a ``relative veiling'' that treats the quiescent spectrum as an internal reference and measures the change in the continuum excess between the two epochs. This formalism yields burst-to-quiescent hot-continuum ratios of $2.9\pm0.2$ for the CO overtone and $1.71\pm0.11$ for the CO fundamental. Using a viscous-disk prescription, these imply representative accretion-rate ratios of $\sim$3.6 and $\sim$2.0, respectively. The differing ratios suggest that inner-disk regions traced at different temperatures, and thus radii, respond differently across the burst cycle, consistent with episodic mass buildup in the inner disk during quiescence followed by more efficient transport through the innermost disk onto the protostar during the burst.

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arXiv2026

The Evolution of the SFR-M_* relation at 0.1<z<4: Environmental and Morphological Dependencies

We present a comprehensive study of the relationship between star formation rate (SFR) and stellar mass (M_*) from z = 0.1 to z = 4 using a mass-complete sample of approximately 290,000 galaxies from the COSMOS2020 catalog. We find that the SFR-M_* relation exhibits a pronounced high-mass decline that becomes increasingly evident at lower redshifts. Examining environmental and morphological dependencies, we find strikingly different patterns. For all galaxies, we find galaxies in high-density environments exhibit suppressed star formation rates at z < 1 especially at high-mass end, while for star-forming galaxies no apparent environmental effect is found at all redshifts. In contrast, galaxy morphology exerts strong influence on the SFR-M_* relation at z < 2, in a sense that early-type galaxies exhibit systematically lower star formation rates at fixed mass compared to spirals and irregulars, with this trend persisting even within the star-forming population. These results suggest that internal structural properties (bulge components in particular) continuously regulate star formation efficiency independently of whether galaxies are classified as active or quiescent, whereas external environmental processes primarily serve as rapid quenching mechanisms that increase the fraction of quiescent galaxies at low redshifts. We attribute the observed high-mass decline of the SFR-M_* relation to COSMOS2020's superior capability for detecting massive star-forming galaxies undergoing "morphological quenching" processes.

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arXiv2025

Narrowband Imaging of a z=3.24 Protocluster: Insights from [O III] Emitting Galaxies

We present a narrowband imaging on a spectroscopically confirmed protocluster ``D4UD01'' at z=3.24 using CFHT/WIRCam. We identify a sample of 24 [O III] emission line galaxies in the field, which forms a large overdensity in the protocluster region. The protocluster is expected to evolve into a Virgo-like cluster by z=0. Utilizing multiwavelength data, we derive the physical properties of these [O III] emitters and find they are medium mass normal star-forming galaxies ($\sim10^{10}$M$_\odot$) roughly following the star-forming main sequence. The [O III] emitters trace an overdensity spatially offset from that of photometric-redshift and quiescent galaxies, suggesting these distinct galaxy populations may inhabit dark matter halos that formed at different epochs. A comparative analysis of [O III] emitter properties shows similar characteristics in both protocluster and field environments. This protocluster likely represents an evolved structure that has progressed beyond its peak star-formation phase, although our limited sample size may prevent detection of subtle environmental effects.

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arXiv2024

Mining for Protoclusters at $z\sim4$ from Photometric Datasets with Deep Learning

Protoclusters are high-$z$ overdense regions that will evolve into clusters of galaxies by $z=0$, making them ideal for studying galaxy evolution expected to be accelerated by environmental effects. However, it has been challenging to identify protoclusters beyond $z=3$ only by photometry due to large redshift uncertainties, hindering statistical study. To tackle the issue, we develop a new deep-learning-based protocluster detection model, PCFNet, which considers a protocluster as a point cloud. To detect protoclusters at $z\sim4$ using only optical broad-band photometry, we train and evaluate PCFNet with mock $g$-dropout galaxies based on the N-body simulation with the semi-analytic model. We use the sky distribution, $i$-band magnitude, $(g-i)$ color, and the redshift probability density function surrounding a target galaxy on the sky. PCFNet achieves to detect five times more protocluster member candidates while maintaining high purity (recall $=7.5\pm0.2$%, precision $=44\pm1$%) than conventional methods. Moreover, PCFNet is able to detect more progenitors ($M_\mathrm{halo}^{z=0}=10^{14-14.5}\,M_\odot$) that are less massive than supermassive clusters like the Coma cluster. We apply PCFNet to the observational photometric dataset of the HSC-SSP Deep/UltraDeep layer ($\sim17\mathrm{\,deg^2}$) and detect $121$ protocluster candidates at $z\sim4$. We find the rest-UV luminosities of our protocluster member candidates are brighter than those of field galaxies, which is consistent with previous studies. Additionally, the quenching of satellite galaxies depends on both the core galaxy's halo mass at $z\sim4$ and accumulated mass until $z=0$ in the simulation. PCFNet is very flexible and can find protoclusters at other redshifts or in future extensive surveys by Euclid, LSST, and Roman.

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arXiv2024

Environments around quasars at $z\sim$3 revealed by wide-field imaging with Subaru HSC and CFHT

We examine the local density environments around 67 quasars at $z\sim3$, by combining the imaging data of Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP) and Canada-France-Hawaii Telescope Large Area U-band Survey (CLAUDS) over about 20 deg$^{2}$. Our measurements exploit $U$-dropout galaxies in the vicinities of quasars taken from Sloan Digital Sky Survey (SDSS). We find that the quasars have the indistinguishable surrounding density distribution from the $U$-dropout galaxies, and that three quasars are associated with protocluster candidates within a projected separation of 3 arcmin. According to a halo evolutionary model, our results suggest that quasars at this epoch occupy haloes with a typical mass of $1.3^{+1.4}_{-0.9} \times 10^{13} h^{-1} \mathrm{M_{\odot}}$. We also investigate the dependence of the local galaxy overdensity on ultraviolet (UV) luminosities, black hole (BH) masses, and proximity zone sizes of the quasars, but no statistically-significant correlation was found. Finally, we find that the local density of faint $U$-dropout galaxies are lower than that of bright $U$-dropout galaxies within a projected distance of $0.51\pm0.05$ physical Mpc, where the quasar UV radiation is 30 times intenser than backgroung UV radiation. We argue that photoevaporation may suppress galaxy formation at short distances where the quasar UV intensity is strong, even in massive haloes.

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arXiv2024

Galaxy properties from the outskirts to the core of a protocluster at z=3.699

We present follow-up spectroscopy on a protocluster candidate selected from the wide-field imaging of the Hyper SuprimeCam Subaru Strategic Programme. The target protocluster candidate was identified as a $4.5\sigma$ overdense region of $g$-dropout galaxies, and the redshifts of $g$-dropout galaxies are determined by detecting their Ly$\alpha$ emission. Thirteen galaxies, at least, are found to be clustering in the narrow redshift range of $\Delta z<0.05$ at $z=3.699$. This is clear evidence of the presence of a protocluster in the target region. Following the discovery of the protocluster at $z=3.699$, the physical properties and three-dimensional distribution of its member galaxies are investigated. Based on spectroscopically-confirmed $g$-dropout galaxies, we find an overabundance of rest-frame ultraviolet (UV) bright galaxies in the protocluster. The UV brightest protocluster member turns out to be an active galactic nucleus, and the other UV brighter members tend to show smaller Ly$\alpha$ equivalent widths than field counterparts. The member galaxies tend to densely populate near the centre of the protocluster, but the separation from the nearest neighbour rather than the distance from the centre of the protocluster is more tightly correlated to galaxy properties, implying that the protocluster is still in an early phase of cluster formation and only close neighbours have a significant impact on the physical properties of protocluster members. The number density of massive galaxies, selected from an archival photometric-redshift catalogue, is higher near the centre of the protocluster, while dusty starburst galaxies are distributed on the outskirts. These observational results suggest that the protocluster consists of multiple galaxy populations, whose spatial distributions may hint at the developmental phase of the galaxy cluster.

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arXiv2023

Nature vs. Nurture: Revisiting the environmental impact on star formation activities of galaxies

We present a systematic study of the environmental impact on star formation activities of galaxies using a mass-complete sample of $\sim$170k galaxies at $z<4$ from the latest COSMOS2020 catalog. At $z<1$, we find that the mean star-formation rate (SFR) of all galaxies decreases with increasing density of the environment. However when we consider only star-forming galaxies, the mean SFR becomes independent of the environment at $z<1$. At $z>2$ we observe a clear positive correlation between the SFR and density of the environment for all the galaxies. On the other hand, stellar mass of the galaxies increases significantly with the environments at all redshifts except for star-forming galaxies at $z<1$. The fraction of quiescent galaxies increases with increasing density of environment at $z<2$, and the ``morphology-density'' relation is confirmed to be present up to $z\sim1$. We also find that environmental quenching is negligible at $z>1$, whereas mass quenching is the dominant quenching mechanism for massive galaxies at all redshifts. Based on these results, we argue that stellar mass regulated physical processes might be the major driving force for star formation activities of galaxies. At low redshift ($z<1$) massive galaxies are quenched primarily due to their high mass, resulting in a normal ``SFR-density'' relation. At high redshift ($z>2$) most of the galaxies are star-forming ones tightly following the star-forming main sequence, and the difference in their stellar mass at different environments naturally leads to a reversal of ``SFR-density'' relation.

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arXiv2023

An enhanced abundance of bright galaxies in protocluster candidates at z~3-5

We present a protocluster search covering $z\sim3$ to $z\sim5$ based on the combination of the Hyper SuprimeCam Subaru Strategic Programme and the CFHT Large Area $U$-band Deep Survey. We identify about 30 protocluster candidates per unit redshift over the $\sim25\,\mathrm{deg^2}$ area of the Deep/Ultra-Deep layer. Protocluster candidates are selected as regions with a significantly enhanced surface density of dropout galaxies. With this large sample, we characterise the properties of their individual member galaxies. We compare the number counts of dropout galaxies in protocluster candidates with that of coeval field galaxies. Rest-frame UV bright galaxies are over-abundant in protocluster candidates, a trend seen across the full redshift range studied. We do not find evidence for their spatial distribution within protocluster candidates to be distinct from their fainter counterparts, nor for their UV colour to be different from that of field galaxies with the same brightness. Cosmological simulations predict this bright-end excess, with the main cause being a richer population of massive galaxies, with only a minor contribution from an enhancement in star formation activity (and therefore UV emission) at fixed mass. $U$-to-$K$ SED modelling of our observed samples supports this interpretation. This environmental differentiation in number counts is already in place at $z\sim5$, with no significant redshift dependence over the range in lookback times probed. These observational results and model predictions suggest that the cosmic clock is ahead in high-density environments.

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arXiv2022

A Wide and Deep Exploration of Radio Galaxies with Subaru HSC (WERGS). IX. The Most Overdense Region at z~5 Inhabited by a Massive Radio Galaxy

We report on the galaxy density environment around a high-z radio galaxy (HzRG) at z=4.72, HSC J083913.17+011308.1 (HSC J0839+0113), probed using an r-dropout Lyman break galaxy (LBG) sample from the Hyper Suprime-Cam Subaru Strategic Program data. We find that HSC J0839+0113 resides in the outskirt of an overdense region identified by the r-dropout galaxies at a 4.7 sigma significance level. The projected distance between HSC J0839+0113 and the peak position of the overdense region is 0.4 physical Mpc which is shorter than the typical protocluster radius in this epoch. According to the extended Press Schechter and the light cone models, the HSC J0839+0113-hosted overdense region is expected to evolve into a halo > 10^14 Msun at z=0 with a high probability of >80 %. These findings suggest that HSC J0839+0113 is associated with a protocluster. The HSC J0839+0113 rich-system is the most overdense region of LBGs among the known protoclusters with LBGs in the same cosmic epoch.

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arXiv2022

3D intrinsic shapes of quiescent galaxies in observations and simulations

We study the intrinsic 3D shapes of quiescent galaxies over the last half of cosmic history based on their axial ratio distribution. To this end, we construct a sample of unprecedented size, exploiting multi-wavelength $u$-to-$K_s$ photometry from the deep wide area surveys KiDS+VIKING paired with high-quality $i$-band imaging from HSC-SSP. Dependencies of the shapes on mass, redshift, photometric bulge prominence and environment are considered. For comparison, the intrinsic shapes of quenched galaxies in the IllustrisTNG simulations are analyzed and contrasted to their formation history. We find that over the full $0 10^{11}\ M_{\odot}$, with the effect being most pronounced at lower redshifts. In TNG, the most massive galaxies feature the highest ex-situ stellar mass fractions, pointing to violent relaxation via mergers as the mechanism responsible for their 3D shape transformation. Larger differences between observed and simulated shapes are found at low to intermediate masses. At any mass, the most spheroidal quiescent galaxies in TNG feature the highest bulge mass fractions, and conversely observed quiescent galaxies with the highest bulge-to-total ratios are found to be intrinsically the roundest. Finally, we detect an environmental influence on galaxy shape, at least at the highest masses, such that at fixed mass and redshift quiescent galaxies tend to be rounder in denser environments.

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arXiv2022

Cool outflows in MaNGA: a systematic study and comparison to the warm phase

This paper investigates the neutral gas phase of galactic winds via the Na I D$λλ5890,5895$Å feature within $z \sim 0.04$ MaNGA galaxies, and directly compares their incidence and strength to the ionized winds detected within the same parent sample. We find evidence for neutral outflows in 127 galaxies ($\sim 5$ per cent of the analysed line-emitting sample). Na I D winds are preferentially seen in galaxies with dustier central regions and both wind phases are more often found in systems with elevated SFR surface densities, especially when there has been a recent upturn in the star formation activity according to the SFR$_{5Myr}$/SFR$_{800Myr}$ parameter. We find the ionized outflow kinematics to be in line with what we measure in the neutral phase. This demonstrates that, despite their small contributions to the total outflow mass budget, there is value to collecting empirical measurements of the ionized wind phase to provide information on the bulk motion in the outflow. Depending on dust corrections applied to the ionized gas diagnostics, the neutral phase has $\sim 1.2 - 1.8$ dex higher mass outflow rates ($\dot{M}_{out}$), on average, compared to the ionized phase. We quantify scaling relations between $\dot{M}_{out}$ and the strengths of the physical wind drivers (SFR, $L_{AGN}$). Using a radial-azimuthal stacking method, and by considering inclination dependencies, we find results consistent with biconical outflows orthogonal to the disk plane. Our work complements other multi-phase outflow studies in the literature which consider smaller samples, more extreme objects, or proceed via stacking of larger samples.

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arXiv2021

A Wide and Deep Exploration of Radio Galaxies with Subaru HSC (WERGS). VI. Distant Filamentary Structures Pointed by High-z Radio Galaxies at z~4

We present the environmental properties around high-$z$ radio galaxies (HzRGs) at $z\sim4$, which have been poorly investigated because of their rarity. We use the largest samples of HzRGs and $g$-dropout galaxy overdense regions at $z\sim4$, which were constructed from Hyper Suprime-Cam Subaru Strategic Program, to characterize the HzRG environments statistically. We measure the $g$-dropout galaxy overdensities around 21 HzRGs whose rest-frame 1.4 GHz radio luminosities ($L_{1.4\mathrm{GHz}}$) are $10^{26-27}$ W Hz$^{-1}$. We find that the overdensities around the faint HzRGs with $L_{1.4\mathrm{GHz}}\sim10^{26.0-26.5}$ W Hz$^{-1}$ tend to be higher than that of the $g$-dropout galaxies. On the other hand, no significant difference of density environments is found between the luminous HzRGs with $ L_{1.4\mathrm{GHz}}\sim10^{26.5-27.0} $ W Hz$^{-1}$ and the $g$-dropout galaxies. The HzRGs are found to occupy more massive halos than $g$-dropout galaxies through a cross-correlation between the HzRGs and $g$-dropout galaxies. This trend is more pronounced in the faint HzRGs. These results are consistent with a scenario where HzRGs get older and more massive as the radio-luminosity decreases. The HzRGs are expected to trace the progenitors of local cluster halos from their calculated halo mass. In addition, we find that surrounding galaxies tend to distribute along the radio-jet major axis of the HzRGs at angular distances less than $\lesssim500$ physical kpc. Our findings imply the onset of the filamentary structures around the HzRGs at $z\sim4$.

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arXiv2021

Interrelation of the environment of Ly$α$ emitters and massive galaxies at $2<z<4.5$

We present a comparison of the spatial distributions of Ly$α$ emitters (LAEs) and massive star-forming and quiescent galaxies (SFGs and QGs) at $2 -20$). From the overdensity distributions, LAEs are located in less dense regions than SFGs and QGs, whereas SFGs and QGs tend to be in the same overdensity distributions. The different spatial distributions of LAEs compared to those of massive galaxies may be attributed to assembly bias or large amounts of neutral hydrogen gas associated with massive halos. These results reinforce the importance of exploring multiple galaxy populations in quantifying the intrinsic galaxy environment of the high-$z$ universe.

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arXiv2021

Incidence, scaling relations and physical conditions of ionised gas outflows in MaNGA

In this work, we investigate the strength and impact of ionised gas outflows within $z \sim 0.04$ MaNGA galaxies. We find evidence for outflows in 322 galaxies ($12\%$ of the analysed line-emitting sample), 185 of which show evidence for AGN activity. Most outflows are centrally concentrated with a spatial extent that scales sublinearly with $R_{\rm e}$. The incidence of outflows is enhanced at higher masses, central surface densities and deeper gravitational potentials, as well as at higher SFR and AGN luminosity. We quantify strong correlations between mass outflow rates and the mechanical drivers of the outflow of the form $\dot{M}_{\rm out} \propto \rm SFR^{0.97}$ and $\dot{M}_{\rm out} \propto L_{\rm AGN}^{0.55}$. We derive a master scaling relation describing the mass outflow rate of ionised gas as a function of $M_{\star}$, SFR, $R_{\rm e}$ and $L_{\rm AGN}$. Most of the observed winds are anticipated to act as galactic fountains, with the fraction of galaxies with escaping winds increasing with decreasing potential well depth. We further investigate the physical properties of the outflowing gas finding evidence for enhanced attenuation in the outflow, possibly due to metal-enriched winds, and higher excitation compared to the gas in the galactic disk. Given that the majority of previous studies have focused on more extreme systems with higher SFRs and/or more luminous AGN, our study provides a unique view of the non-gravitational gaseous motions within `typical' galaxies in the low-redshift Universe, where low-luminosity AGN and star formation contribute jointly to the observed outflow phenomenology.

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arXiv2021

Accelerated galaxy growth and environmental quenching in a protocluster at z=3.24

We present a multiwavelength study of galaxies around D4UD01, a spectroscopically confirmed protocluster at z = 3.24 to investigate environmental trends. 450 galaxies are selected based on Ks band detection with photometric redshifts (photo-z) at 3.0 < z < 3.4, among which ~ 12% are classified as quiescent galaxies. The quiescent galaxies are among the most massive and reddest ones in the entire sample. We identify a large photo-z galaxy overdensity in the field, which lies close to the previously spectroscopically confirmed sources of the protocluster. We find that the quiescent galaxies are largely concentrated in the overdense protocluster region with a higher quiescent fraction, showing a sign of environmental quenching. Galaxies in the protocluster are forming faster than the field counterparts as seen in the stellar mass function, suggesting early and accelerated mass assembly in the overdense regions. Although weak evidence of suppressed star-formation is found in the protocluster, the statistics are not significant enough to draw a definite conclusion. Our work shed light on how the formation of massive galaxies is affected in the dense region of a protocluster when the Universe was only 2 Gyr old.

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arXiv2020

Faint Quasars Live in the Same Number Density Environments as Lyman Break Galaxies at z ~ 4

Characterizing high-z quasar environments is key to understanding the co-evolution of quasars and the surrounding galaxies. To restrict their global picture, we statistically examine the g-dropout galaxy overdensity distribution around 570 faint quasar candidates at z ~ 4, based on the Hyper Suprime-Cam Subaru Strategic Program survey. We compare the overdensity significances of g-dropout galaxies around the quasars with those around g-dropout galaxies, and find no significant difference between their distributions. A total of 4 (22) out of the 570 faint quasars, 0.7_{-0.4}^{+0.4} (3.9_{-0.8}^{+0.8}) %, are found to be associated with the > 4 sigma overdense regions within an angular separation of 1.8 (3.0) arcmin, which is the typical size of protoclusters at this epoch. This is similar to the fraction of g-dropout galaxies associated with the > 4 sigma overdense regions. This result is consistent with our previous work that 1.3_{-0.9}^{+0.9} % and 2.0_{-1.1}^{+1.1} % of luminous quasars detected in the Sloan Digital Sky Survey exist in the > 4 sigma overdense regions within 1.8 and 3.0 arcmin separations, respectively. Therefore, we suggest that the galaxy number densities around quasars are independent of their luminosity, and most quasars do not preferentially appear in the richest protocluster regions at z ~ 4. The lack of an apparent positive correlation between the quasars and the protoclusters implies that: i) the gas-rich major merger rate is relatively low in the protocluster regions, ii) most high-z quasars may appear through secular processes, or iii) some dust-obscured quasars exist in the protocluster regions.

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arXiv2020

Statistical correlation between the distribution of Ly$α$ emitters and IGM HI at $z\sim2.2$ mapped by Subaru/Hyper Suprime-Cam

The correlation between neutral Hydrogen (HI) in the intergalactic medium (IGM) and galaxies now attracts great interests. We select four fields which include several coherently strong Ly$α$ absorption systems at $z\sim2.2$ detected by using background quasars from the whole SDSS/(e)BOSS database. Deep narrow-band and $g$-band imaging are performed using the Hyper Suprime-Cam on the Subaru Telescope. We select out 2,642 Ly$α$ emitter (LAE) candidates at $z=2.177\pm0.023$ down to the Ly$α$ luminosity of $L_{\text{Ly}α}\approx 2 \times 10^{42} {\rm erg~s}^{-1}$ to construct the galaxy overdensity maps, covering an effective area of 5.39 deg$^2$. Combining the sample with the Ly$α$ absorption estimated from 64 (e)BOSS quasar spectra, we find a moderate to strong correlation between the LAE overdensity $δ_{\rm LAE}$ and the effective optical depth $τ_{\rm LoS}$ in line-of-sights, with $P$-value$=0.09\%$ ($<0.01\%$) when the field that contains a significant quasar overdensity is in(ex)cluded. The cross-correlation analysis also clearly suggests that up to $4\pm1$ pMpc, LAEs tend to cluster in the regions rich in HI gas, indicated by the high $τ_{\rm LoS}$, and avoid the low $τ_{\rm LoS}$ region where the HI gas is deficient. By averaging the $τ_{\rm LoS}$ as a function of the projected distance ($d$) to LAEs, we find a $30\%$ excess signal at $2σ$ level at $d<200$ pkpc, indicating the dense HI in circumgalactic medium, and a tentative excess at $400<d<600$ pkpc in IGM regime, corroborating the cross-correlation signal detected at about $0.5$ pMpc. These statistical analyses indicate that galaxy$-$IGM HI correlations exist on scales ranging from several hundred pkpc to several pMpc at $z\sim2.2$.

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