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The standard cold dark matter plus cosmological constant model predicts that galaxies form within dark-matter haloes, and that low-mass galaxies are more dark-matter dominated than massive ones. The unexpected discovery of two low-mass galaxies lacking dark matter immediately provoked concerns about the standard cosmology and ignited explorations of alternatives, including self-interacting dark matter and modified gravity. Apprehension grew after several cosmological simulations using the conventional model failed to form adequate numerical analogues with comparable internal characteristics (stellar masses, sizes, velocity dispersions and morphologies). Here we show that the standard paradigm naturally produces galaxies lacking dark matter with internal characteristics in agreement with observations. Using a state-of-the-art cosmological simulation and a meticulous galaxy-identification technique, we find that extreme close encounters with massive neighbours can be responsible for this. We predict that ~30% of massive central galaxies (with at least 10 11 solar masses in stars) harbour at least one dark-matter-deficient satellite (with 10 8 –10 9 solar masses in stars). This distinctive class of galaxies provides an additional layer in our understanding of the role of interactions in shaping galactic properties. Future observations surveying galaxies in the aforementioned regime will provide a crucial test of this scenario. A cosmological simulation shows that low-mass galaxies can form with far less dark matter than expected, with results matching some observed characteristics. Roughly one-third of massive central galaxies may host at least one such dark-matter-deficient satellite.

The effects of low molecular weight organic acids (LMWOAs) on the adsorption of quinclorac by sepiolite were investigated using laboratory batch technique. Experiments were conducted with two natural sepiolite samples with different crystal structures and chemical compositions and high-purity sepiolite. The LMWOAs used were acetic, oxalic, and citric acid. And the adsorption mechanism was characterized using X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR). Our analysis revealed that adsorption of quinclorac on α-sepiolite and β-sepiolite was inhibited in the presence of 4 mmol L −1 LMWOAs, whereas LMWOAs stimulated the adsorption of quinclorac in the high-purity sepiolite. Inhibition or stimulation varied across the different types of organic acids. The adsorption isotherms in the presence of 4 mmol L −1 LMWOAs were better explained by Freundlich and linear model. The effect of organic acid concentrations (0–32 mmol L −1 ) on the adsorption of quinclorac by the three sepiolite samples varies greatly depending on the type of organic acid and the property of sepiolite. FTIR, XRD, and XPS analyses showed that LMWOAs bound strongly to the Si–O bond structure, and Si–O-quinclorac-acetic acid (oxalic acid or citric acid) was formed on the surface of β-sepiolite. The adsorption of quinclorac by β-sepiolite was via hydrogen bond, complexation reactions, and charge transfer in the presence of LMWOAs. These results indicate that LMWOAs affect quinclorac adsorption through various interactions involving competition, electrostatic attraction, bridging action, and hydrogen bonding.

Hierarchical structure formation in the standard Λ+cold dark matter (CDM) model produces gravitationally bound clumpy halos with abundant substructure. These subhalos are the remnants of dark matter halos that have been accreted by their host halo over cosmic time, and have survived tidal destruction. Understanding halo substructure is extremely important, as subhalos are believed to host satellite galaxies, boost the dark matter annihilation signal, cause tidal heating of fragile structures such as stellar streams and disks, and are potentially responsible for interesting phenomena in gravitational lensing. Most importantly, the demographics of subhalos contain information of the Universe, thus providing a stringent testbed for the cosmological model. This thesis provides a comprehensive study of dark matter subhalos, using a combination of cosmological N-body simulations and semi-analytic modeling. We start with developing a new, semi-analytic model describing halo assembly and subhalo evolution. The model combines Monte-Carlo techniques of generating halo merging histories and simple analytical descriptions for the evolution of subhalos, thus offering extremely fast computation, the agility to experiment with different cosmologies, and the control of specific physical processes. The model accurately predicts the distributions of subhalo mass and structural parameters in cosmological simulations, and outperforms simulations in terms of mass resolution and statistical power. Taking advantage of the speed and agility of the model, we present universal fitting formulae for subhalo mass and maximum circular velocity ( max) functions that are valid for a broad range in host halo mass, redshift and CDM cosmology. The remainder of the dissertation makes use of the model, together with a number of state-of-the-art N-body simulations, to study the statistics of halo substructure. Recent high-resolution CDM simulations reveal ~10 massive Galactic subhalos whose central potential wells are too deep to be consistent with those of the ~10 brightest Milky-Way (MW) satellite galaxies. This inconsistency, dubbed the `too-big-to-fail' problem (TBTF), has become a persistent challenge to the standard ACDM cosmology. However, the number of well resolved Galactic halos in simulations is too small to fully capture the halo-to-halo variance in substructure content, which hinders the interpretation of the inconsistency. Unleashing the power of the semi-analytic model, we generate thousands of MW-size halos with well-resolved subhalo populations, and explicitly demonstrate that a reliable assessment of TBTF requires such large samples. We argue that existing statistics used to address TBTF suffer from the look-elsewhere effect and/or disregard certain aspects of the data on the MW satellite population. We devise a new statistic that is not hampered by these shortcomings, and, using data of the MW satellites with vmax > 15 km s-1, demonstrate that 1.4+3.3-1.1% of MW-size host halos have a subhalo population in statistical agreement with that of the MW. We also discuss how the severity of TBTF depends on halo mass and cosmology. We conclude the thesis with a study of unprecedented statistical power regarding the halo-to-halo variance of substructure. First, we study the mass fraction (fsub) in subhalos as a function of host halo mass, formation redshift, and halo-centric distance. We note that recent measurements of fsub from gravitational lensing are much higher than the average but within the 90th percentile of the fsub distribution. Second, we quantify the deviation of the occupation statistics of subhalos from Poissonian statistics, which is widely assumed in halo occupation distribution (HOD) models. In particular, we clearly reveal the sub-Poissonian statistics at [special characters omitted] ≤ 3, aside from the already-known super-Poissonity at [special characters omitted] » 1, with [special characters omitted] the average number of subhalos. we also quantify the effect of the sub-Poissonity on the galaxy clustering predictions from HOD models. We further show that the extent of nonPoissonity depends on subhalo selection and on halo formation time - selecting subhalos by their mass or vmax at accretion yields weaker super-Poissonity for large [special characters omitted] but stronger sub-Poissonity for small [special characters omitted], compared to selecting by their present-day mass or vmax; earlier-formed halos exhibit less non-Poissonity than later-formed ones. Finally, we use the occupation statistics of the most massive satellites of the MW to put constraint on the mass and formation redshift of the MW halo. In particular, the ` max gap' of MW satellites between ~ 30 km s-1 and 60 km s-1 favors a low-mass, late-formed MW halo, with 0.25 < Mvir/1012 h-1M[special characters omitted] < 1.4 and 0.1 < zf < 1.4 at 90% confidence.

Semi-analytic modeling furnishes an efficient avenue for characterizing the properties of dark matter halos associated with satellites of Milky Way-like systems, as it easily accounts for uncertainties arising from halo-to-halo variance, the orbital disruption of satellites, baryonic feedback, and the stellar-to-halo mass (SMHM) relation. We use the SatGen semi-analytic satellite generator -- which incorporates both empirical models of the galaxy-halo connection in the field as well as analytic prescriptions for the orbital evolution of these satellites after they enter a host galaxy -- to create large samples of Milky Way-like systems and their satellites. By selecting satellites in the sample that match the observed properties of a particular dwarf galaxy, we can then infer arbitrary properties of the satellite galaxy within the Cold Dark Matter paradigm. For the Milky Way's classical dwarfs, we provide inferred values (with associated uncertainties) for the maximum circular velocity \\(v_{max}\\) and the radius \\(r_{max}\\) at which it occurs, varying over two choices of feedback model and two prescriptions for the SMHM relation that populate dark matter halos with physically distinct galaxies. While simple empirical scaling relations can recover the median inferred value for \\(v_{max}\\) and \\(r_{max}\\), this approach provides realistic correlated uncertainties and aids interpretability through variation of the model. For these different models, we also demonstrate how the internal properties of a satellite's dark matter profile correlate with its orbit, and we show that it is difficult to reproduce observations of the Fornax dwarf without strong baryonic feedback. The technique developed in this work is flexible in its application of observational data and can leverage arbitrary information about the satellite galaxies to make inferences about their dark matter halos and population statistics.

When dark matter has a large cross section for self scattering, halos can undergo a process known as gravothermal core collapse, where the inner core rapidly increases in density and temperature. To date, several methods have been used to implement Self-Interacting Dark Matter~(SIDM) in N-body codes, but there has been no systematic study of these different methods or their accuracy in the core-collapse phase. In this paper, we compare three different numerical implementations of SIDM, including the standard methods from the GIZMO and Arepo codes, by simulating idealized dwarf halos undergoing significant dark matter self interactions (\\(\\sigma/m = 50\\)~cm\\(^2\\)/g). When simulating these halos, we also vary the mass resolution, time-stepping criteria, and gravitational force-softening scheme. The various SIDM methods lead to distinct differences in a halo's evolution during the core-collapse phase, as each results in spurious scattering rate differences and energy gains/losses. The use of adaptive force softening for gravity can lead to numerical heating that artificially accelerates core collapse, while an insufficiently small simulation time step can cause core evolution to stall or completely reverse. Additionally, particle numbers must be large enough to ensure that the simulated halos are not sensitive to noise in the initial conditions. Even for the highest-resolution simulations tested in this study (\\(10^6\\) particles per halo), we find that variations of order \\(10\\%\\) in collapse time are still present. The results of this work underscore the sensitivity of SIDM modeling on the choice of numerical implementation and motivate a careful study of how these results generalize to halos in a cosmological context.

The properties of galaxies are intricately linked to the characteristics of their host dark-matter haloes. We use a suite of controlled simulations of isolated galaxies to quantify how halo spin, concentration, inner density profile, and baryon fraction regulate galaxy sizes, at fixed halo mass of \\(M_{\\rm{vir}}=10^{11} M_\\odot\\). We generate initial conditions of haloes and inhabitant spherical gas distributions in equilibrium, on a parameter grid spanned by these four halo parameters, and evolve the systems with the \\(\\texttt{GIZMO}\\) code and the \\(\\texttt{FIRE-3}\\) physics. The resulting half-mass radii of stars and cold baryons depend systematically on halo structure and baryon content: galaxy size increases with halo spin, decreases with halo concentration, is weakly sensitive to the inner density slope except in highly cuspy haloes, and is strongly suppressed at high baryon fractions. We evaluate the relative importance of the halo parameters on galaxy size using different metrics including the quadratic response-surface method and random-forest regression, and consistently find halo concentration to be the most informative predictor of size. The baryon fraction shows a subtle, non-monotonic impact on size, by modulating how galaxy size depends on halo spin. Our results clarify which secondary parameters of host dark-matter haloes dominate the scatter in galaxy sizes at the massive-dwarf mass scale.

We compare the internal stellar structures of central galaxies in the TNG50 and TNG100 simulations and field galaxies in the CALIFA survey. The luminosity fractions of the dynamically cold, warm, and hot components in both TNG50 and TNG100 galaxies exhibit general consistency with those observed in CALIFA galaxies. For example, they all exhibit a minimum luminosity fraction of the dynamically hot component in galaxies with intermediate stellar masses, and the morphology of each orbital component in the TNG50 and TNG100 galaxies closely resembles that found in the CALIFA galaxies. We therefore use the simulations to quantify the physical origins of the different components, focusing on the dynamically hot component in TNG50. We identify three primary regimes and thus physical processes: (1) in low mass galaxies that have not experienced major mergers, stars are born with a wide range of circularity distributions and have remained relatively unchanged until the present day. Consequently, hot stars in such galaxies at redshift 0 are predominantly born hot. (2) In higher mass galaxies lacking major mergers, most stars are initially born cold but are subsequently heated through secular evolution. (3) In galaxies across the entire mass range, mergers, if they occurred, significantly increased the hot orbital fraction. As a result, the dynamically hot bulge within \\(R_e\\) of present-day galaxies does not indicate their past merger histories; instead, the hot stars in the outer regions are mostly heated or accreted by mergers, thus indicating galaxy merger history. The massive galaxies are initially born with cold, rotationally supported structures, consistent with recent observations from the James Webb Space Telescope (JWST) regarding high-redshift galaxies.

We study the connection between galaxy morphology and host dark matter (DM) halo structure using cosmological simulations. Introducing a new kinematic decomposition scheme, we robustly separate thin and thick discs and measure halo properties, including cosmic web locations, internal structures, and assembly histories. In the TNG50 simulation, we find that the orbital-circularity threshold for disc differentiation varies systematically with galaxy mass and redshift. Similarly, the energy threshold between stellar halos and inner galaxies depends on mass and redshift, minimizing at sub-Galactic halo mass where the circularity threshold approaches its peak. Revisiting galaxy size predictors, we show that disc sizes in TNG50 correlate with three structural parameters beyond virial mass and redshift: 1) a positive correlation with halo spin \\(\\lambda\\) across redshifts -- stronger than previously reported for zoom-in simulations but still weaker than the simple \\(r_{1/2}/R_{\\rm vir} \\propto \\lambda\\) scaling; 2) an anti-correlation with DM concentration \\(c\\); 3) larger discs in more actively accreting haloes. Disc mass fraction is higher in rounder haloes and in cosmic knots and filaments, implying that disc development needs both stable halo conditions and continuous material supply. Our methodology is public and adaptable to other simulations.

We present a statistical study of black hole (BH) formation and growth seeded by gravothermal core collapse of self-interacting dark matter (SIDM) halos at high redshift, using a cosmological semi-analytical framework based on Monte Carlo merger trees. We demonstrate that gravothermal collapse naturally leads to BH formation in high-concentration halos at a characteristic mass scale set by the SIDM cross section, and occurs predominantly in the early Universe. This mechanism is particularly promising for explaining the abundance of the little red dots (LRDs) -- a population of early, apparently galaxy-less active galactic nuclei hosting supermassive BHs. By incorporating this seeding process with simple models of BH growth and assuming a 100% duty cycle, we reproduce the observed LRD mass function for velocity-dependent cross sections of \\(\\sigma_{0m} \\sim 30\\,\\mathrm{cm}^2\\,\\mathrm{g}^{-1}\\) and \\(\\omega \\sim 80\\,\\mathrm{km}\\,\\mathrm{s}^{-1}\\), which are consistent with independent constraints from local galaxies. While higher values of \\(\\sigma_{0m}\\) (or \\(\\omega\\)) would overpredict the low-mass (or high-mass) end of the BH mass function, such deviations could be reconciled by invoking a reduced duty cycle or lower Eddington ratio. Our results suggest that the demographics of high-redshift BHs can serve as a novel and complementary probe of SIDM physics.

Recent observations of supermassive black holes (SMBHs) at high redshifts pose challenges to standard seeding mechanisms. Among competing models, the collapse of self-interacting dark matter (SIDM) halos provide a plausible explanation for early SMBH formation. While previous studies on modeling the gravothermal collapse of SIDM halos have primarily focused on non-relativistic evolution under the assumption of hydrostatic equilibrium, We advance this framework by relaxing the equilibrium assumption and additionally incorporating general-relativistic effects. To this end, we introduce the Misner-Sharp formalism to the SIDM context for the first time. Our model reproduces the standard hydrostatic models in the early long-mean-free-path (LMFP) regime, but displays interesting distinct behavior in the late short-mean-free-path (SMFP) regime, where intense outward heat flux drives a rapid expansion of the outer envelope, removing mass from the core and significantly decelerating the collapse. Our general relativistic treatment enables us to follow halo evolution to the final stage when the apparent horizon forms. Our simulation yields a seed black hole mass of approximately \\(3\\times10^{-8}\\) of the halo mass at horizon formation, suggesting that additional mechanisms such as baryonic effects are critical for seeding black holes that are sufficiently massive to account for SMBHs in the early Universe.