The Environment Of Galaxies And Groups Of Galaxies

In this thesis, we employ two numerical tools - semi- analytical models and N-body plus hydrodynamical simulations of large scale structure and individual galaxies - to explore the underlying physics governing the formation and evolution of groups of galaxies, and the role of environment in generating polar structures around disk galaxies. Using phenomenological models of baryonic physics imposed upon large-scale dissipationless simulations of the Universe, semi-analytic models (SAMs) are one of the principal methods used to model large samples of model galaxies. We sought to examine the properties of groups of galaxies with a range of densities using SAMs applied specifically to the industry-standard Millennium Run; for this work, we make use of the well-known Munich and Durham models, and their descendants. We are especially interested in how group properties change as we change the linking length of our Friends- of-Friends group finder. We compare the group populations and richness in these models and compare them both with observations and high-resolution N- body simulations. This leads us to the conclusion that the Durham models produce a much larger population of compact objects than the Munich models. We also explore the group dynamics and morphology as a function of density. We compare the luminosity distributions of galaxy groups using publicly available SAMs in order to explor

The environment of galaxies and groups of galaxies

In this thesis, we employ two numerical tools - semi- analytical models and N-body plus hydrodynamical simulations of large scale structure and individual galaxies - to explore the underlying physics governing the formation and evolution of groups of galaxies, and the role of environment in generating polar structures around disk galaxies. Using phenomenological models of baryonic physics imposed upon large-scale dissipationless simulations of the Universe, semi-analytic models (SAMs) are one of the principal methods used to model large samples of model galaxies. We sought to examine the properties of groups of galaxies with a range of densities using SAMs applied specifically to the industry-standard Millennium Run; for this work, we make use of the well-known Munich and Durham models, and their descendants. We are especially interested in how group properties change as we change the linking length of our Friends- of-Friends group finder. We compare the group populations and richness in these models and compare them both with observations and high-resolution N- body simulations. This leads us to the conclusion that the Durham models produce a much larger population of compact objects than the Munich models. We also explore the group dynamics and morphology as a function of density. We compare the luminosity distributions of galaxy groups using publicly available SAMs in order to exploreEThOS - Electronic Theses Online ServiceGBUnited Kingdo

The Milky Way as a High Redshift Galaxy: The Importance of Thick Disk Formation in Galaxies

We compare the star-formation history and dynamics of the Milky Way (MW) with the properties of distant disk galaxies. During the first ~4 Gyr of its evolution, the MW formed stars with a high star-formation intensity (SFI), Sigma_SFR~0.6 Msun/yr/kpc2 and as a result, generated outflows and high turbulence in its interstellar medium. This intense phase of star formation corresponds to the formation of the thick disk. The formation of the thick disk is a crucial phase which enables the MW to have formed approximately half of its total stellar mass by z~1 which is similar to "MW progenitor galaxies" selected by abundance matching. This agreement suggests that the formation of the thick disk may be a generic evolutionary phase in disk galaxies. Using a simple energy injection-kinetic energy relationship between the 1-D velocity dispersion and SFI, we can reproduce the average perpendicular dispersion in stellar velocities of the MW with age. This relationship, its inferred evolution, and required efficiency are consistent with observations of galaxies from z~0-3. The high turbulence generated by intense star formation naturally resulted in a thick disk, a chemically well-mixed ISM, and is the mechanism that links the evolution of MW to the observed characteristics of distant disk galaxies.Comment: 5 pages, 4 figures; accepted to ApJ Letter

On the Effects of Local Environment on Active Galactic Nucleus (AGN) in the Horizon Run 5 Simulation

We use the Horizon Run 5 cosmological simulation to study the effect of galaxy intrinsic properties and the local environment on active galactic nuclei (AGNs) characterized by their threshold of the accretion rate. We select galaxies in the stellar mass range 10 9.5 ≤ M * / M ⊙ ≤ 10 10.5 in the snapshot at redshift z = 0.625. Among various intrinsic properties, we find that the star formation rate of the host galaxy is most correlated to the AGN activity. To quantify the environment, we use background galaxy number density (large-scale environment) and distance and morphological type of the nearest neighbors (small-scale environment), and study their relative effects on the AGN properties. We find that, compared to the background density, the nearest neighbor environment is the dominant quantity determining the bolometric luminosity, star formation rate, and kinematic properties of AGNs and better dictates the gas mass of the host galaxy. We show that the cold gas content in the host galaxies is crucial in triggering AGN activity. However, when the nearest neighbor environment effects start to act at the neighbor distance of less than about half the virial radius of the neighbor, the neighbor environmental effects are the most dominant factor for quasar activity

Bimodality of [alpha Fe]-[Fe/H] distributions is a natural outcome of dissipative collapse and disc growth in Milky Way-type galaxies

By using chemo-dynamical simulations of the Milky Way-type galaxies formation, we find that the two alpha-sequences are formed in quite different physical environments. The high-alpha sequence is formed early from a burst of star formation in a turbulent, compact gaseous disc that forms a thick disc. The low-alpha stellar population is the result of quiescent star formation supported by the slow accretion of enriched gas on to a radially extended thin disc. Feedback-driven outflows during the formation of the thick disc are responsible for the enrichment of the surrounding gaseous halo, which subsequently feeds the disc on a longer time-scale. During the thin disc phase, chemical evolution reaches an equilibrium metallicity and abundance, where the stars pile-up. This equilibrium metallicity decreases towards the outer disc, generating the ridge line that forms the low-alpha sequence. We identify a second mechanism capable of creating a low-alpha sequence in one of our simulations. A rapid shutdown of the star formation due to feedback at the end of the thick disc phase, suppresses the chemical enrichment of the halo gas, which, once accreted on to the star-forming disc, dilutes the interstellar medium at the beginning of the thin disc formation. Therefore, the bimodality is independent of any particular merger history, suggesting that it could be much more widespread than has been claimed. We also find that radial migration has a negligible effect on the [alpha/Fe]-[Fe/H] distribution over time, suggesting that alpha-bimodality results purely from the presence of different star formation regimes over the galaxy's formation

Low-Surface-Brightness Galaxies are missing in the observed Stellar Mass Function

We investigate the impact of the surface brightness (SB) limit on the galaxy stellar mass functions (GSMFs) using mock surveys generated from the Horizon Run 5 (HR5) simulation. We compare the stellar-to-halo-mass relation, GSMF, and size-stellar mass relation of the HR5 galaxies with empirical data and other cosmological simulations. The mean SB of simulated galaxies are computed using their effective radii, luminosities, and colors. To examine the cosmic SB dimming effect, we compute $k$-corrections from the spectral energy distributions of individual simulated galaxy at each redshift, apply the $k$-corrections to the galaxies, and conduct mock surveys based on the various SB limits. We find that the GSMFs are significantly affected by the SB limits at a low-mass end. This approach can ease the discrepancy between the GSMFs obtained from simulations and observations at $0.625\le z\le 2$. We also find that a redshift survey with a SB selection limit of \left^e = 28 mag arcsec${}^{-2}$ will miss 20% of galaxies with $M_\star^g=10^{9}~{\rm M_\odot}$ at $z=0.625$. The missing fraction of low-surface-brightness galaxies increases to 50%, 70%, and 98% at $z=0.9$, 1.1, and 1.9, respectively, at the SB limit.Comment: 27 pages, 30 figures, accepted for publication in Ap

Identification of Galaxy Protoclusters Based on the Spherical Top-hat Collapse Theory

We propose a new method for finding galaxy protoclusters that is motivated by structure formation theory and also directly applicable to observations. We adopt the conventional definition that a protocluster is a galaxy group whose virial mass $M_{\rm vir} < M_{\rm cl}$ at its epoch, where $M_{\rm cl}=10^{14}\,M_{\odot}$, but would exceed that limit when it evolves to $z=0$. We use the critical overdensity for complete collapse at $z = 0$ predicted by the spherical top-hat collapse model to find the radius and total mass of the regions that would collapse at $z=0$. If the mass of a region centered at a massive galaxy exceeds $M_{\rm cl}$, the galaxy is at the center of a protocluster. We define the outer boundary of protocluster as the zero-velocity surface at the turnaround radius so that the member galaxies are those sharing the same protocluster environment and showing some conformity in physical properties. We use the cosmological hydrodynamical simulation Horizon Run 5 (HR5) to calibrate this prescription and demonstrate its performance. We find that the protocluster identification method suggested in this study is quite successful. Its application to the high-redshift HR5 galaxies shows a tight correlation between the mass within the protocluster regions identified according to the spherical collapse model and the final mass to be found within the clusters at $z=0$, meaning that the regions can be regarded as the bona fide protoclusters with high reliability. We also confirm that the redshift-space distortion does not significantly affect the performance of the protocluster identification scheme.Comment: 23 pages, 21 figures. Accepted for publication in Ap

Spatial Distribution of Intracluster Light versus Dark Matter in Horizon Run 5

One intriguing approach for studying the dynamical evolution of galaxy clusters is to compare the spatial distributions among various components, such as dark matter, member galaxies, gas, and intracluster light (ICL). Utilizing the recently introduced Weighted Overlap Coefficient (WOC) \citep{2022ApJS..261...28Y}, we analyze the spatial distributions of components within 174 galaxy clusters ($M_{\rm tot}> 5 \times 10^{13} M_{\odot}$, $z=0.625$) at varying dynamical states in the cosmological hydrodynamical simulation Horizon Run 5. We observe that the distributions of gas and the combination of ICL with the brightest cluster galaxy (BCG) closely resembles the dark matter distribution, particularly in more relaxed clusters, characterized by the half-mass epoch. The similarity in spatial distribution between dark matter and BCG+ICL mimics the changes in the dynamical state of clusters during a major merger. Notably, at redshifts $>$ 1, BCG+ICL traced dark matter more accurately than the gas. Additionally, we examined the one-dimensional radial profiles of each component, which show that the BCG+ICL is a sensitive component revealing the dynamical state of clusters. We propose a new method that can approximately recover the dark matter profile by scaling the BCG+ICL radial profile. Furthermore, we find a recipe for tracing dark matter in unrelaxed clusters by including the most massive satellite galaxies together with BCG+ICL distribution. Combining the BCG+ICL and the gas distribution enhances the dark matter tracing ability. Our results imply that the BCG+ICL distribution is an effective tracer for the dark matter distribution, and the similarity of spatial distribution may be a useful probe of the dynamical state of a cluster.Comment: 23 pages, 12 figures, accepted for publication in Ap

The Horizon Run 5 Cosmological Hydrodynamic Simulation: Probing Galaxy Formation from Kilo- to Giga-parsec Scales

Horizon Run 5 (HR5) is a cosmological hydrodynamical simulation which captures the properties of the Universe on a Gpc scale while achieving a resolution of 1kpc. Inside the simulation box we zoom-in on a high-resolution cuboid region with a volume of 1049×114×114cMpc3.The sub-grid physics chosen to model galaxy formation includes radiative heating/cooling, UV background, star formation, supernova feedback, chemical evolution tracking the enrichment of oxygen and iron, the growth of supermassive black holes and feedback from active galactic nuclei (AGN) in the form of a dual jet-heating mode. For this simulation we implemented a hybrid MPI-OMP version of RAMSES, specifically targeted for modern many-core many thread parallel architectures. In addition to the traditional simulation snapshots, light-cone data was generated on the fly. For the post-processing, we extended the Friends-of-Friend (FoF) algorithm and developed a new galaxy finder PGalF to analyse the outputs of HR5. The simulation successfully reproduces observations, such as the cosmic star formation history and connectivity of galaxy distribution, We identify cosmological structures at a wide range of scales, from filaments with a length of several cMpc, to voids with a radius of ~100 cMpc. The simulation also indicates that hydrodynamical effects on small scales impact galaxy clustering up to very large scales near and beyond the baryonic acoustic oscillation (BAO) scale. Hence, caution should be taken when using that scale as a cosmic standard ruler: one needs to carefully understand the corresponding biases. The simulation is expected to be an invaluable asset for the interpretation of upcoming deep surveys of the Universe

Spatial Distribution of Intracluster Light versus Dark Matter in Horizon Run 5

One intriguing approach for studying the dynamical evolution of galaxy clusters is to compare the spatial distributions among various components such as dark matter, member galaxies, gas, and intracluster light (ICL). Utilizing the recently introduced weighted overlap coefficient (WOC), we analyze the spatial distributions of components within 174 galaxy clusters (M tot > 5 × 1013 M ⊙, z = 0.625) at varying dynamical states in the cosmological hydrodynamical simulation Horizon Run 5. We observe that the distributions of gas and the combination of ICL with the brightest cluster galaxy (BCG) closely resembles the dark matter distribution, particularly in more relaxed clusters, characterized by the half-mass epoch. The similarity in spatial distribution between dark matter and BCG+ICL mimics the changes in the dynamical state of clusters during a major merger. Notably, at redshifts >1, BCG+ICL traced dark matter more accurately than the gas. Additionally, we examined the one-dimensional radial profiles of each component, which show that the BCG+ICL is a sensitive component revealing the dynamical state of clusters. We propose a new method that can approximately recover the dark matter profile by scaling the BCG+ICL radial profile. Furthermore, we find a recipe for tracing dark matter in unrelaxed clusters by including the most massive satellite galaxies together with the BCG+ICL distribution. Combining the BCG+ICL and the gas distribution enhances the dark matter tracing ability. Our results imply that the BCG+ICL distribution is an effective tracer for the dark matter distribution, and the similarity of the spatial distribution may be a useful probe of the dynamical state of a cluster