arXiv subjects

Jounghun Lee

Publications and source records attributed to Jounghun Lee.

At least 19 recordsLinked to original sources

arXiv2026

A Universal Bilinear Model for the Dependence of Void Asymmetry Distributions on $Ω_{m}$ and $σ_{8}$

$Aims$: We present a new independent diagnostics based on void properties, by which the matter density parameter ($Ω_{m}$) and amplitude of initial density field ($σ_{8}$) can be constrained. This diagnostics utilizes coherent rotation of void galaxies, which can be observed as redshift asymmetry in opposite sides bisected by the projected spin axes of hosting voids. $Methods$: Identifying the voids and their member galaxies in the AbacusSummit of cosmological simulations and searching for their projected spin axes with an iterative method, we numerically determine the void asymmetry distributions both in real and redshift spaces for three different classes of the background cosmologies: the $Λ$CDM, the $w$CDM and the $w_{0}w_{a}$CDM. $Results$: It is discovered that the void asymmetry distributions in real and redshift spaces are well approximated by the generalized Gamma models characterized by the scale and shape parameters for all of the $33$ background cosmologies considered. It turns out that the initial conditions affect only the scale parameter of the void asymmetry distributions that exhibits an almost linear dependence on each of $Ω_{m}$ and $σ_{8}$ with being insensitive to $w$. Developing a bilinear model for the dependence of void asymmetry distributions on $Ω_{m}$ and $σ_{8}$, we show that its coefficients are universal constants over the three cosmological classes. $Conclusions$: Given that the void asymmetry distributions in redshift space are readily measurable properties, our universal model for the scale parameter of the void asymmetry distributions will in principle complement the standard cosmological diagnostics to break the $Ω_{m}$-$σ_{8}$ degeneracy, regardless of $w$.

astro-ph.CO↗

arXiv2026

A Universal Relation Between Primordial Density-Potential Cross-correlation Coefficient and Spin Factor Distribution

Recent studies have revealed that the key properties of visible galaxies like their optical sizes, stellar ages, star formation rates and morphologies are closely linked with the angular momenta of their host dark matter halos. According to the linear tidal torque theory, the halo angular momentum, as a conserved quantity, is directly proportional to the primordial spin factor, $τ$, defined as the degree of misalignment between the principal axes of the initial density and potential Hessian matrices, which were found by numerical experiments to follow a Gamma distribution, fully characterized by its mean and variance. In this study, we heuristically develop an analytic expression for the mean and variance of $τ$ in terms of the initial density-potential cross-correlation coefficient, $q$. Analyzing a dataset from the Multiverse simulations performed for both of the flat $Λ$CDM and $w$CDM cosmologies, we prove that this analytic expression is universally valid in describing how the mean and variance of $τ$ change with $q$, regardless of the smoothing scales for both of the cosmologies. Given the prior finding that the $τ$-distribution can be reconstructed from the observable galaxy size distribution, this universal analytic expression may allow us to determine $q$ from the same observable via the mean and variance of $τ$. We discuss a possibility of constraining the early universe physics from the reconstructed $q$ via our heuristic model, without suffering from cosmological degeneracies.

astro-ph.CO↗

arXiv2026

The effect of dark energy on the void-halo perpendicular alignments

We report a numerical discovery that in a more rapidly accelerating spacetime, the galactic halos on void surfaces develop stronger perpendicular alignments with the directions toward the void centers. We utilize the halo catalogs from the AbacusSummit suite of simulations for $10$ different cosmologies that include one Planck $Λ$CDM, four $w$CDM and five $w_{0}w_{a}$CDM, which share the identical initial conditions except for the dark energy equation of state. For each cosmology, we identify the voids and void-surface galactic halos at $z=0.1$ and determine the probability density functions of the cosines of the angles, $p(\cosθ)$, between the shape axes of void-surface galactic halos and the directions toward the void centers. The numerically obtained $p(\cosθ)$ is fitted to an analytic single-parameter formula derived through an empirical modification of the linear perturbation theory. Eliminating spurious signals caused by the differences in the mass and sphericity distributions of void-surface galactic halos among different cosmologies, we detect a clear net effect of dark energy on the strengths of the perpendicular alignments of void-surface galactic halos, quantified by the single parameter, $d_{t}$. Noting that $d_{t}$ has higher values in the cosmologies where dark energy has more negative pressure and evolves more rapidly, we put forth a bilinear model for the difference in $d_{t}$ between the two cases of $w=-1$ and $w\ne -1$. Demonstrating that this bilinear relation excellently describes the numerical results, we conclude that the perpendicular alignments of void-surface galactic halos should in principle be a powerful independent indicator of the dynamic nature of dark energy.

astro-ph.CO↗

arXiv2026

How the cosmic voids contribute to stalling and quenching the giant galaxies on their surfaces

We report a numerical hint that the formations of cosmic voids may be closely linked with the mechanism through which the giant galaxies on void surfaces establish elliptical shapes, redder colors, and lower specific star formation rates (sSFR). Identifying the voids from the TNG300-1 simulations via the Void-Finder algorithm~\cite{HV02} at $z=0$, $0.5$ and $1$, we explore if and how the shapes of the TNG galaxies located on void surfaces are aligned with the directions toward the void centers. Noting that only the giant void-surface galaxies with stellar masses $M_{\star}\ge 10^{10.5}\,h^{-1}\,M_{\odot}$ exhibit significant tendency of perpendicular alignments, we dichotomize them into two $M_{\star}$-controlled samples according to their morphologies (elliptical or spiral), colors (redder or bluer), sSFR (lower or higher) and stellar ages (older or younger). It is found at all of the three redshifts that the perpendicular alignments of void-surface galaxies become stronger for the cases that they have elliptical shapes, redder colors, and lower sSFR, but showing weak dependence on the stellar ages. It is also shown that the numerical results are well described by the analytical one-parameter model developed by Lee~\cite{lee19} under the assumption of the existence of a linear scaling between the covariance matrices of galaxy shape axes and local tidal tensors. We test the robustness of alignment signals against the variation of void-finder algorithms and its feasibility against the redshift-space and projection effects. Our results lead us to speculate that the formation and expansion of voids may have an effect of stalling and quenching the giant void-surface galaxies by compressing adjacent matter and then preventing them from radial infall/accretion.

astro-ph.CO↗

arXiv2025

An Observed Evidence for the Primordial Origin of Galaxy Sizes

We present an observational evidence supporting the scenario that the protogalactic angular momenta play an important role in molding the optical sizes of present galaxies. Analyzing the NASA-Sloan Atlas catalog in the redshift range of $0.02\le z<0.09$, we observationally determine the probability density distributions, $p(r_{50})$ and $p(r_{90})$, where $r_{50}$ and $r_{90}$ denote the galaxy sizes enclosing $50\%$ and $90\%$ of their $r$-band luminosities, respectively. Both of the distributions are found to be well described by a bimodal Gamma mixture model, which is consistent with the recent numerical results. Classifying the local galaxies by their ratios, $r_{50}/r_{90}$, we also show that for the case of late-type galaxies with $r_{50}/r_{90}\ge 0.45$ both of $p(r_{50})$ and $p(r_{90})$ exhibit no bimodal feature, following a unimodal Gamma model. Assuming the existence of a linear causal correlation between $\{r_{50},r_{90}\}$ of the late-type galaxies and the primordial spin factor, $τ$, defined as the degree of misalignments between the initial tidal and protogalaxy inertia tensors, we reconstruct the probability density distributions, $p(τ)$, directly from the observationally determined $p(r_{50})$ and $p(r_{90})$ of the late-type galaxies. It is shown that the reconstructed $p(τ)$ is in an excellent agreement with the real distribution of $τ$ that was determined at the protogalactic stages by numerical experiments. A critical implication of our result on reconstructing the initial conditions from observable galaxy sizes is discussed.

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arXiv2025

Void spin distribution as a powerful probe of $σ_{8}$

We present a numerical proof of the concept that the void spin distributions can provide a tight constraint on the amplitude of matter density fluctuation on the scale of $8\,h^{-1}{\rm Mpc}$ ($σ_{8}$) without being severely deteriorated by the degeneracies of $σ_{8}$ with cold dark matter density parameter multiplied by the dimensionless Hubble parameter square ($Ω_{\rm cdm}h^{2}$), total neutrino mass ($M_ν$) and dark energy equation of state ($w$). Applying the Void-Finder algorithm~\cite{HV02} to a total of $15$ AbacusSummit $N$-body simulations of $15$ different cosmological models~\cite{summit1}, we identify the giant voids and measure the magnitudes of rescaled specific angular momenta of point-like void halos as their spins. The $15$ cosmologies include the Planck $Λ$CDM and $14$ non-Planck models, each of which differs among one another only in one of $\{σ_{8},\ Ω_{\rm cdm}h^{2},\ M_ν,\ w\}$. We determine the probability density distribution of void spins for each model and for the first time find it to be well approximated by the generalized Gamma distribution with two characteristic parameters, $k$ and $θ$. It turns out that the best-fit values of $k$ and $θ$ exhibit very sensitive dependence only on $σ_{8}$, being almost insensitive to $Ω_{\rm cdm}h^{2}$, $M_ν$ and $w$. This exclusive $σ_{8}$-dependence of the void spin distributions is confirmed to be robust against the variation of the mass and number cuts of void halos. We also test an observational feasibility of estimating the void spins from real data on the galaxy redshifts.

astro-ph.CO↗

arXiv2025

Dependence of Galaxy Stellar Properties on the Primordial Spin Factor

We present a numerical discovery that the observable stellar properties of present galaxies retain significant dependences on the primordial density and tidal fields. Analyzing the galaxy catalogs from the TNG300-1 simulations, we first compute the primordial spin factor, $τ$, defined as the mean degree of misalignments between the principal axes of the initial density and potential hessian tensors at the protogalactic sites. Then, we explore in the framework of Shannon's information theory if and how strongly each of six stellar properties of the present galaxies, namely two stellar sizes ($R_{90\star}$ and $R_{50\star}$), ages, specific star formation rates, optical colors and metallicities, share mutual information with $τ$, measured at $z=127$. Deliberately controlling the TNG galaxy samples to have no differences in the mass, environmental density and shear distributions, we single out net effects of $τ$ on each of the galaxy stellar properties. In the higher stellar mass range of $M_{\star}/(h^{-1}\,M_{\odot})\ge 10^{10}$, significant amounts of mutual information with $τ$ are exhibited by all of the six stellar properties, while in the lower range of $M_{\star}/(h^{-1}\,M_{\odot})< 10^{10}$ only four of the six properties except for the specific star formation rates and colors yield significant signals of $τ$-dependence. Examining how the mean values of the six stellar properties vary with $τ$, we also show that the galaxies originated from the protogalactic sites with higher $τ$ values tend to have larger sizes, later formation epochs, higher specific star formation rates, bluer colors and lower metallicities. It is also discovered that the galaxy stellar sizes, which turn out to be most robustly dependent on $τ$ regardless of $M_{\star}$, follow a bimodal Gamma distribution, the physical implication of which is discussed.

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arXiv2024

The dependence of halo bias on the protohalo shape alignment with the initial tidal field

We present a numerical evidence supporting the primordial origin of secondary halo bias even on the galactic mass scale. Analyzing the data from the IllustrisTNG 300-1 simulations, we investigate the dependence of halo bias on the degree of misalignment between the protohalo inertia and initial tidal tensors, $τ$, measured at redshift, $z_{i}=127$. From the TNG 300-1 galactic halos in logarithmic mass range of $10.5< m\equiv \log[M/(h^{-1}M_{\odot})]\le 13$ identified at $z=0,\ 0.5$ and $1$, a clear signal of $τ$ bias is detected. For the case that $τ$ is measured from the initial tidal field smoothed on the scale of $R_{f}/(h^{-1}\,{\rm Mpc})\lesssim 1$, the halo $τ$ bias is found to be very similar in its tendency and amplitude to the spin bias at all of the three redshifts, if the effects of backsplash halos are properly eliminated. For the case of $R_{f}/(h^{-1}\,{\rm Mpc})=2$, the $τ$ bias at $z=1$ turns out to behave like the age bias, diminishing rapidly in the range of $m> 12$. At $z=0$ and $0.5$, however, the $τ$ and age bias factors show large differences in their overall strengths, which is attributed to the dominant nonlinear effects that undermine the former but enhance the latter. Given these numerical results along with the previous finding that $τ$ shares a large amount of mutual information with the formation epochs and spin parameters of galactic halos, it is concluded that the origins of halo age and spin bias must be closely linked with the primordial factor, $τ$, and that the difference in the tendency between the two bias factors on the galactic mass scale reflects the multi-scale influence of $τ$ on the halo secondary properties.

astro-ph.CO↗

arXiv2024

The impact of constrained interacting dark energy on the bound-zone velocity profile

We numerically study the effects of constrained interacting dark energy (CIDER) on the bound-zone velocity profiles around massive dark matter halos. Analyzing the CIDER simulations performed by Baldi (2023) for three different cases of dark sector coupling ($β=0.03$, $0.05$ and $0.08$) as well as for the standard $Λ$CDM cosmology ($β=0$), we determine the mean peculiar velocity profiles in the bound zones around the friends-of-friends halos with masses larger than $M_{\rm cut}=3\times 10^{13}\,h^{-1}M_{\odot}$ at three redshifts, $z=0$, $0.5$ and $1$. It is found that the universal power-law formula proposed by Falco et al. (2024) originally for the $Λ$CDM case still describes well the bound-zone velocity profiles, $V(r)$, even in the CIDER models. The slope of $V(r)$, turns out to be significantly affected by the CIDER, progressively decreasing as $β$ increases. Meanwhile, the amplitude of $V(r)$ exhibits little dependence on $β$, which is ascribed to the identical Hubble parameters shared by the $Λ$CDM and CIDER models in the entire redshift range. Our results imply that the bound-zone velocity slope can break a degeneracy even between the $Λ$CDM and CIDER models with $β\le 0.03$, which the standard cosmological diagnostics fail to distinguish. We devise a simple analytic formula for the bound-zone slope as a function of $β$, and prove its validity at all of the three redshifts. It is concluded that the slope of the mean bound-zone peculiar velocity profile should be in principle a powerful probe of dark sector interaction.

astro-ph.CO↗

arXiv2024

Mutual information between galaxy properties and the initial predisposition

The immense diversity of the galaxy population in the universe is believed to stem from their disparate merging and star formation histories, and multi-scale influences of diverse environments. No single causal factor of the initial state is known to explain how the galaxies formed and evolved to end up possessing such various traits as they have at the present epoch. However, several observational studies have revealed that the key physical properties of the observed galaxies in the local universe appeared to have a much simpler, lower-dimensional correlation structure than expected, the origin of which remains unexplained. Speculating that the emergence of such a simple correlation structure of the galaxy properties must be triggered by nature rather than by nurture, we explore if the present galaxy properties may be correlated with the initial precondition for protogalaxy angular momentum, $τ$, and test it against the data from the IllustrisTNG300-1 hydrodynamic simulation. Employing Shannon's information theory, we discover that $τ$ shares a significantly large amount of mutual information with each of the four basic traits of the TNG galaxies at $z=0$: the spin parameters, formation epochs, stellar-to-total mass ratios, and fraction of kinetic energy in ordered rotation. These basic traits except for the stellar-to-total mass ratios are found to contain even a larger amount of MI about $τ$ than about the total masses and environments for the case of giant galaxies with $11.5\le \log[M_{\rm t}/(h^{-1}\,M_{\odot})]<13$. Our results imply that the initial condition of the universe must be more impactful on the galaxy evolution than conventionally thought.

astro-ph.CO↗

arXiv2024

Galaxy Spin Transition Driven by the Misalignments between the Protogalaxy Inertia and Initial Tidal Tensors

A numerical detection of the $τ$-driven transition of galaxy spins is presented, where $τ$ is the degree of misalignment between the initial tidal field and protogalaxy inertia tensors. Analyzing the data from the IllustrisTNG 300-1 simulations, we first measure the values of $τ$ at the protogalactic sites found by tracing the constituents of the galactic halos in the mass range of $10.5\le \log \left[M_{h}/(h^{-1}M_{\odot})\right] \le 13$ back to the initial stage, $z_{i}=127$. The probability density functions of $τ$ are shown to be well modeled by the $Γ$-distributions, whose shape and scale parameters turn out to have universal values on a certain critical scale. Then, we investigate how the strength and tendency of the galaxy spin alignments with the principal axes of the local tidal fields depend on the initial condition, $τ$. It is found that on the scale lower than the critical one, the galaxy spin transition occurs at two different thresholds from the major to intermediate and from the intermediate to minor principal axes of the local tidal fields, respectively. Noting that the $τ$-dependent spin transition supersedes in strength the previously found mass-dependent, morphology-dependent, and radius-dependent counterparts, we suggest that $τ$ should be the key driver of all types of the galaxy spin transition and that the present galaxy spins are indeed excellent fossil records of the initial condition.

astro-ph.CO↗

arXiv2023

An Observed Transition of Galaxy Spins on the Void Surfaces

In the linear theory, the galaxy angular momentum vectors which originate from the initial tidal interactions with surrounding matter distribution intrinsically develop perpendicular alignments with the directions of maximum matter compression, regardless of galaxy mass. In simulations, however, the galaxy spins exhibit parallel alignments in the mass-range lower than a certain threshold, which depends on redshift, web type, and background cosmology. We show that the observed three dimensional spins of the spiral galaxies located on the void surfaces from the Sloan Digital Sky Survey indeed transit from the perpendicular to the parallel alignments with the directions toward the nearest void centers at the threshold zone, $9.51\le\log [M_{th,\star}/(h^{-1}\,M_{\odot})]\le10.03$. This study presents a first direct observational evidence for the occurrence of the mass-dependent spin transition of the real galaxies with respect to the non-filamentary structures of the cosmic web, opening a way to constrain the initial conditions of the early universe by measuring the spin transition threshold.

astro-ph.CO↗

arXiv2023

Reoriented Memory of Galaxy Spins for the Early Universe

Galaxy spins are believed to retain the initially acquired tendency of being aligned with the intermediate principal axes of the linear tidal field, which disseminates a prospect of using them as a probe of early universe physics. This roseate prospect, however, is contingent upon the key assumption that the observable stellar spins of the present galaxies measured at inner radii have the same alignment tendency toward the initial tidal field as their dark matter counterparts measured at virial limits. We test this assumption directly against a high-resolution hydrodynamical simulation by tracing back the galaxy component particles back to the protogalactic stage. It is discovered that the galaxy stellar spins at $z=0$ have strong but {\it reoriented} memory for the early universe, exhibiting a significant signal of cross-correlation with the {\it major} principal axes of the initial tidal field at $z=127$. An analytic single-parameter model for this reorientation of the present galaxy stellar spins relative to the initial tidal field is devised and shown to be in good accord with the numerical results.

astro-ph.CO↗

arXiv2023

The Density Parity Model for the Evolution of the Subhalo Inner Spin Alignments with the Cosmic Web

We develop a new model within which the radius-dependent transition of the subhalo inner spins with respect to the cosmic web and the variation of the transition threshold radius ($r_{\rm th}$) with subhalo mass ($M_{\rm vir}$), smoothing scale ($r_{f}$), and redshift ($z$) can be coherently explained. The key tenet of this model is that the competition between the pressure effect of the inner mass and the compression effect of the local tidal field determines which principal direction of the tidal field the inner spins are aligned with. If the former predominates, then only the tidal torques turn on, resulting in the alignments of the inner spins with the intermediate principal axes of the tidal field. Otherwise, the subhalo spins acquire a tendency to be aligned with the shortest axes of the subhalo shapes, which is in the major principal directions of the tidal field. Quantifying the two effects in terms of the densities, we make a purely analytical prediction for $r_{\rm th}(M_{\rm vir}, z, r_{f})$. Testing this model against the numerical results from a high-resolution dark matter only N-body simulation in the redshift range of $0\le z\le 3$ on the galactic mass scale of $11.8\le \log[M_{\rm vir}/(h^{-1}M_{\odot})]\le 12.6$ for two different cases of $r_{f}/(h^{-1}{\rm Mpc})=0.5$ and $1$, we find excellent agreements of the model predictions with the numerical results. It is also shown that this model naturally predicts the alignments between the inner spins of the present subhalos with the principal axes of the high-$z$ tidal field at the progenitors' locations.

astro-ph.CO↗

arXiv2022

Radius-Dependent Spin Transition of Dark Matter Halos

A numerical detection of the radius-dependent spin transition of dark matter halos is reported. Analyzing the data from the IllustrisTNG simulations, we measure the halo spin vectors at several inner radii within the virial boundaries and investigate their orientations in the principal frames of the tidal and velocity shear fields, called the Tweb and Vweb, respectively. The halo spin vectors in the high-mass section exhibit a transition from the Tweb intermediate to major principal axes as they are measured at more inner radii, which holds for both of the dark matter and baryonic components. The radius threshold at which the transition occurs depends on the smoothing scale, $R_{f}$, becoming larger as $R_{f}$ decreases. For the case of the Vweb, the occurrence of the radius-dependent spin transition is witnessed only when $R_{f}\ge 1\, h^{-1}$Mpc. Repeating the same analysis but with the vorticity vectors, we reveal a critical difference from the spins. The vorticity vectors are always perpendicular to the Tweb (Vweb) major principal axes, regardless of $R_{f}$, which indicates that the halo inner spins are not strongly affected by the generation of vorticity. It is also shown that the halo spins, as well as the Tweb (Vweb) principal axes, have more directional coherence over a wide range of radial distances in the regions where the vorticity vectors have higher magnitudes. The physical interpretations and implications of our results are discussed.

astro-ph.CO↗

arXiv2022

Disentangling Modified Gravity and Massive Neutrinos with Intrinsic Shape Alignments of Massive Halos

We present two new diagnostics based on the intrinsic shape alignments of group/cluster size dark matter halos to disentangle the effect of $f(R)$ gravity from that of massive neutrinos. Using the snapshot data from a series of the {\small DUSTGRAIN}-{pathfinder} $N$-body simulations for the Planck $Λ$CDM cosmology and three $f(R)$ gravity models with massive neutrinos ($ν$), we first determine the probability density functions of the alignment angles between the shape orientations of massive halos and the minor principal axes of the local tidal fields. The numerically obtained results turn out to agree very well with the analytic formula derived under the assumption that the anisotropic merging along the cosmic web induces the halo shape alignments. The four cosmologies, which several standard diagnostics failed to discriminate, are found to yield significantly different best-fit values of the single parameter that characterizes the analytic formula. We also numerically determine the spatial cross-correlations between the shape orientations of neighbor group/cluster halos, and find them to be in good agreements with a fitting formula characterized by two parameters, whose best-fit values are found to substantially differ among the four models. We also discuss the limitations and caveats of these new diagnostics that must be overcome for the application to real observational data.

astro-ph.CO↗

arXiv2022

Combined Effects of $f(R)$ Gravity and Massive Neutrinos on the Turn-Around Radii of Dark Matter Halos

We present a new statistics based on the turn-around radii of cluster halos to break the dark sector degeneracy between the $Λ$CDM model and the alternative ones with $f(R)$ gravity and massive neutrinos ($ν$) characterized by the strength of the fifth force, $\vert f_{R0}\vert$, and the total neutrino mass, $M_ν$. Analyzing the rockstar halo catalogs at the present epoch from the {\small DUSTGRAIN}-{pathfinder} $N$-body simulations performed for four different cosmologies, namely, $Λ$CDM ($\vert f_{R0}\vert=0$, $M_ν=0.0$eV), fR6 ($\vert f_{R0}\vert=10^{-6}$, $M_ν=0.0$eV), fR6+$0.06$eV ($\vert f_{R0}\vert=10^{-6}$, $M_ν=0.06$eV) and fR5+$0.15$eV ($\vert f_{R0}\vert=10^{-5}$, $M_ν=0.15$eV), which are known to yield very similar conventional statistics to one another. For each model, we select those cluster halos which do not neighbor any other larger halos in their bound zones and construct their bound-zone peculiar velocity profiles at $z=0$. Then, we determine the radial distance of each selected halo at which the bound-zone velocity becomes equal to the recession speed of the Hubble flow as its turn around radius, and evaluate the cumulative probability distribution of the ratios of the turn-around radii to the virial counterparts, $P(r_{t}/r_{v}\ge α)$. The degeneracy between the fR6 and fR5+$0.15$eV models is found to be readily broken by the $10σ_{ΔP}$ difference in the value of $P(α=4)$, while the $3.2σ_{ΔP}$ difference between the $Λ$CDM and fR6+$0.06$eV models is detected in the value of $P(α=8.5)$. It is also found that the four models yield smaller differences in $P(α)$ at higher redshifts.

astro-ph.CO↗

arXiv2022

Merger Effects on the Spin and Shape Alignments of Galaxy Stellar, Cold and Hot Gas, and Dark Matter Components

We present a numerical evidence supporting the scenario that the peculiar alignments of the galaxy stellar spins with the major principal axes of the local tidal tensors are produced during the quiescent evolution period when the galaxies experience no recent merger events. Analyzing the merger tree from the TNG300-1 simulation of the IllustrisTNG project, we find the latest merger epochs, $a(z_{m})$, of the galaxies, and create four $a(z_{m})$-selected samples that are controlled to share the identical mass and density distributions. For each sample, we determine the spin and shape vectors of the galaxy stellar, cold and hot gas, and dark matter components separately, and compute the average strengths of their alignments with the principal directions of the local tidal fields as well as their mutual alignment tendencies. It is found that the stellar (cold gas) spin axes of the galaxies whose latest merger events occur at earlier epochs are more strongly aligned (weakly anti-aligned) with the major principal axes of the tidal fields. It is also shown that although the mass-dependent transition of the galaxy DM spins have little connection with the merger events, the morphologies, spin-shape and shape-shear alignment strengths of the galaxy four components sensitively depend on $a(z_{m})$. Noting that the stellar components of the galaxies which undergo long quiescent evolution have distinctively oblate shapes and very strong spin-shape alignments, we suggest that the local tidal field might be traced by using the stellar shapes of galaxies without signatures of mergers as a proxy of their stellar spins.

astro-ph.GA↗