arXiv subjects

Taysun Kimm

Publications and source records attributed to Taysun Kimm.

At least 37 records · Page 2Linked to original sources

arXiv2025

Synergising semi-analytical models and hydrodynamical simulations to interpret JWST data from the first billion years

The field of high redshift galaxy formation has been revolutionised by JWST, which is yielding unprecedented insights on galaxy assembly at early times. Our key aim is to study the physical mechanisms that can explain the unexpected abundance of bright galaxies at $z \geq 11$, as well as their metal enrichment and spectral properties. We also use recent data to determine the key sources of reionisation. To do so, we implement cold gas fractions and star formation efficiencies derived from the SPHINX20 high-resolution radiation-hydrodynamics simulation into DELPHI, a semi-analytic model that tracks the assembly of dark matter halos and their baryonic components from $z \sim 4.5-40$. In addition, we explore two different methodologies to boost galaxy luminosities at $z \geq 11$: a stellar initial mass function (IMF) that becomes increasingly top-heavy with decreasing metallicity and increasing redshift (eIMF model), and star formation efficiencies that increase with increasing redshift (eSFE model). Our key findings are: (i) both the eIMF and eSFE models can explain the abundance of bright galaxies at $z \geq 11$; (ii) dust attenuation plays an important role for the bright-end of the UV LF at $z \leq 11$; (iii) the mass-metallicity relation is in place as early as $z \sim 17$ in all models although its slope is model-dependent; (iv) within the spread of both models and observations, all of our models are in good agreement with current estimates of $β$ slopes at $z \sim 5-17$ and Balmer break strengths at $z \sim 6-10$; (v) in the eIMF model, galaxies at $z\geq12$ or with $\rm{M_{UV}}\geq-18$ show values of $ξ_{\rm{ion}} \sim 10^{25.55}~{\rm [Hz~erg^{-1}]}$, twice larger than in other models; (vi) star formation in galaxies below $10^{9}\rm{M_{\odot}}$ is the key driver of reionisation, providing the bulk ($\sim 85\%$) of ionising photons down to its midpoint at $z \sim 7$.

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arXiv2024

Impact of star formation models on the growth of simulated galaxies at high redshifts

Star formation is a key process that governs the baryon cycle within galaxies, however, the question of how it controls their growth remains elusive due to modeling uncertainties. To understand the impact of star formation models on galaxy evolution, we performed cosmological zoom-in radiation-hydrodynamic simulations of a dwarf dark matter halo, with a virial mass of $M_{\rm vir} \sim 10^9\, M_\odot$ at $z=6$. We compared two different star formation models: a multi-freefall model combined with a local gravo-thermo-turbulent condition and a more self-consistent model based on a sink particle algorithm, where gas accretion and star formation are directly controlled by the gas kinematics. As the first study in this series, we used cosmological zoom-in simulations with different spatial resolutions and found that star formation is more bursty in the runs with the sink algorithm, generating stronger outflows than in the runs with the gravo-thermo-turbulent model. The main reason for the increased burstiness is that the gas accretion rates on the sinks are high enough to form stars on very short timescales, leading to more clustered star formation. As a result, the star-forming clumps are disrupted more quickly in the sink run due to more coherent radiation and supernova feedback. The difference in burstiness between the two star formation models becomes even more pronounced when the supernova explosion energy is artificially increased. Our results suggest that improving the modeling of star formation on small, sub-molecular cloud scales can significantly impact the global properties of simulated galaxies.

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arXiv2024

On the Origin of the Variety of Velocity Dispersion Profiles of Galaxies

Observed and simulated galaxies exhibit a significant variation in their velocity dispersion profiles. We examine the inner and outer slopes of stellar velocity dispersion profiles using integral field spectroscopy data from two surveys, SAMI (for $z < 0.115$) and CALIFA (for $z < 0.03$), comparing them with results from two cosmological hydrodynamic simulations: Horizon-AGN (for $z = 0.017$) and NewHorizon (for $z\lesssim1$). The simulated galaxies closely reproduce the variety of velocity dispersion slopes and stellar mass dependence of both inner and outer radii ($0.5\,r_{50}$ and $3\,r_{50}$) as observed, where $r_{50}$ stands for half-light radius. The inner slopes are mainly influenced by the relative radial distribution of the young and old stars formed in-situ: a younger center shows a flatter inner profile. The presence of accreted (ex-situ) stars has two effects on the velocity dispersion profiles. First, because they are more dispersed in spatial and velocity distributions compared to in-situ formed stars, it increases the outer slope of the velocity dispersion profile. It also causes the velocity anisotropy to be more radial. More massive galaxies have a higher fraction of stars formed ex-situ and hence show a higher slope in outer velocity dispersion profile and a higher degree of radial anisotropy. The diversity in the outer velocity dispersion profiles reflects the diverse assembly histories among galaxies.

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arXiv2024

Exploring lenticular galaxy formation in field environments using NewHorizon: evidence for counter-rotating gas accretion as a formation channel

The formation pathways of lenticular galaxies (S0s) in field environments remain a matter of debate. We utilize the cosmological hydrodynamic simulation, NewHorizon, to investigate the issue. We select two massive star-formation quenched S0s as our main sample. By closely tracing their physical and morphological evolution, we identify two primary formation channels: mergers and counter-rotating gas accretion. The former induces central gas inflow due to gravitational and hydrodynamic torques, triggering active central star formation which quickly depletes the gas of the galaxy. Counter-rotating gas accretion overall has a similar outcome but more exclusively through hydrodynamic collisions between the pre-existing and newly-accreted gas. Both channels lead to S0 morphology, with gas angular momentum cancellation being a crucial mechanism. These formation pathways quench star formation on a short timescale (< Gyr) compared to the timescales of environmental effects. We also discuss how counter-rotating gas accretion may explain the origin of S0s with ongoing star formation and the frequently observed gas-star misaligned kinematics in S0s.

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arXiv2024

The Great Escape: Understanding the Connection Between Ly$α$ Emission and LyC Escape in Simulated JWST Analogues

Constraining the escape fraction of Lyman Continuum (LyC) photons from high-redshift galaxies is crucial to understanding reionization. Recent observations have demonstrated that various characteristics of the Ly$α$ emission line correlate with the inferred LyC escape fraction ($f_{\rm esc}^{\rm LyC}$) of low-redshift galaxies. Using a data-set of 9,600 mock Ly$α$ spectra of star-forming galaxies at $4.64 \leq z \leq 6$ from the SPHINX$^{20}$ cosmological radiation hydrodynamical simulation, we study the escape of Ly$α$ and LyC photons. We find that our mock Ly$α$ observations are representative of high-redshift observations and that typical observational methods tend to over-predict the Ly$α$ escape fraction ($f_{\rm esc}^{\rm Lyα}$) by as much as two dex. We investigate the correlations between $f_{\rm esc}^{\rm LyC}$ and $f_{\rm esc}^{\rm Lyα}$, Ly$α$ equivalent width ($W_λ({\rm Lyα})$), peak separation ($v_{\rm sep}$), central escape fraction ($f_{\rm cen}$), and red peak asymmetry ($A_f^{\rm red}$). We find that $f_{\rm esc}^{\rm Lyα}$ and $f_{\rm cen}$ are good diagnostics for LyC leakage, selecting for galaxies with lower neutral gas densities and less UV attenuation that have recently experienced supernova feedback. In contrast, $W_λ({\rm Lyα})$ and $v_{\rm sep}$ are found to be necessary but insufficient diagnostics, while $A_f^{\rm red}$ carries little information. Finally, we use stacks of Ly$α$, H$α$, and F150W mock surface brightness profiles to find that galaxies with high $f_{\rm esc}^{\rm LyC}$ have less extended Ly$α$ and F150W but larger H$α$ haloes than their non-leaking counterparts. This confirms that Ly$α$ spectral profiles and surface brightness morphology can be used to better understand the escape of LyC photons from galaxies during the Epoch of Reionization.

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arXiv2024

Formation pathways of the compact stellar systems

The formation pathways of compact stellar systems (CSSs) are still under debate. We utilize the \NH\ simulation to investigate the origins of such objects in the field environment. We identified 55 CSS candidates in the simulation whose properties are similar to those of the observed ultra-compact dwarfs and compact ellipticals. All but two most massive objects (compact elliptical candidates) are a result of a short starburst. Sixteen are formed by tidal stripping, while the other 39 are intrinsically compact from their birth. The stripped objects originate from dwarf-like galaxies with a dark halo, but most of their dark matter is stripped through their orbital motion around a more massive neighbor galaxy. The 39 intrinsically compact systems are further divided into ``associated'' or ``isolated'' groups, depending on whether they were born near a massive dark halo or not. The isolated intrinsic compact objects (7) are born in a dark halo and their stellar properties are older and metal-poor compared to the associated counterparts (32). The stripped compact objects occupy a distinct region in the age-metallicity plane from the intrinsic compact objects. The associated intrinsic compact objects in our sample have never had a dark halo; they are the surviving star clumps of a massive galaxy.

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arXiv2024

The Energy and Dynamics of Trapped Radiative Feedback with Stellar Winds

In this paper, we explore the significant, non-linear impact that stellar winds have on H ii regions. We perform a parameter study using three-dimensional radiative magnetohydrodynamic simulations of wind and ultraviolet radiation feedback from a 35 Msun star formed self-consistently in a turbulent, self-gravitating cloud, similar to the Orion Nebula (M42) and its main ionizing source Theta 1 Ori C. Stellar winds suppress early radiative feedback by trapping ionizing radiation in the shell around the wind bubble. Rapid breakouts of warm photoionized gas ('champagne flows') still occur if the star forms close to the edge of the cloud. The impact of wind bubbles can be enhanced if we detect and remove numerical overcooling caused by shocks crossing grid cells. However, the majority of the energy in the wind bubble is still lost to turbulent mixing between the wind bubble and the gas around it. These results begin to converge if the spatial resolution at the wind bubble interface is increased by refining the grid on pressure gradients. Wind bubbles form a thin chimney close to the star, which then expands outwards as an extended plume once the wind bubble breaks out of the dense core the star formed in, allowing them to expand faster than a spherical wind bubble. We also find wind bubbles mixing completely with the photoionized gas when the H ii region breaks out of the cloud as a champagne flow, a process we term 'hot champagne'.

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arXiv2023

The Sphinx Public Data Release: Forward Modelling High-Redshift JWST Observations with Cosmological Radiation Hydrodynamics Simulations

The recent launch of JWST has ushered in a new era of high-redshift astronomy by providing detailed insights into the gas and stellar populations of galaxies in the epoch of reionization. Interpreting these observations and translating them into constraints on the physics of early galaxy formation is a complex challenge that requires sophisticated models of star formation and the interstellar medium (ISM) in high-redshift galaxies. To this end, we present Version 1 of the Sphinx$^{20}$ public data release. Sphinx$^{20}$ is a full box cosmological radiation hydrodynamics simulation that simultaneously models the large-scale process of cosmic reionization and the detailed physics of a multiphase ISM, providing a statistical sample of galaxies akin to those currently being observed by JWST. The data set contains $\sim14,000$ mock images and spectra of the stellar continuum, nebular continuum, and 52 nebular emission lines, including Ly$α$, for each galaxy in Sphinx$^{20}$ with a star formation rate $\geq0.3\ {\rm M_{\odot}\ yr^{-1}}$. All galaxy emission has been processed with dust radiative transfer and/or resonant line radiative transfer, and data is provided for ten viewing angles for each galaxy. Additionally, we provide a comprehensive set of intrinsic galaxy properties, including halo masses, stellar masses, star formation histories, and ISM characteristics (e.g., metallicity, ISM gas densities, LyC escape fractions). This paper outlines the data generation methods, presents a comparative analysis with JWST ERS and Cycle 1 observations, and addresses data set limitations. The Sphinx$^{20}$ data release can be downloaded at the following URL: https://github.com/HarleyKatz/SPHINX-20-data

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arXiv2023

Emergence and cosmic evolution of the Kennicutt-Schmidt relation driven by interstellar turbulence

The scaling relations between the gas content and star formation rate of galaxies provide useful insights into processes governing their formation and evolution. We investigate the emergence and the physical drivers of the global Kennicutt-Schmidt (KS) relation at $0.25 \leq z \leq 4$ in the cosmological hydrodynamic simulation NewHorizon capturing the evolution of a few hundred galaxies with a resolution of $\sim$ 40 pc. The details of this relation vary strongly with the stellar mass of galaxies and the redshift. A power-law relation $Σ_{\rm SFR} \propto Σ_{\rm gas}^{a}$ with $a \approx 1.4$, like that found empirically, emerges at $z \approx 2 - 3$ for the most massive half of the galaxy population. However, no such convergence is found in the lower-mass galaxies, for which the relation gets shallower with decreasing redshift. At the galactic scale, the star formation activity correlates with the level of turbulence of the interstellar medium, quantified by the Mach number, rather than with the gas fraction (neutral or molecular), confirming previous works. With decreasing redshift, the number of outliers with short depletion times diminishes, reducing the scatter of the KS relation, while the overall population of galaxies shifts toward low densities. Using pc-scale star formation models calibrated with local Universe physics, our results demonstrate that the cosmological evolution of the environmental and intrinsic conditions conspire to converge towards a significant and detectable imprint in galactic-scale observables, in their scaling relations, and in their reduced scatter.

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arXiv2023

On the Significance of the Thick Disks of Disk Galaxies

Thick disks are a prevalent feature observed in numerous disk galaxies including our own Milky Way. Their significance has been reported to vary widely, ranging from a few to 100% of the disk mass, depending on the galaxy and the measurement method. We use the NewHorizon simulation which has high spatial and stellar mass resolutions to investigate the issue of thick disk mass fraction. We also use the NewHorizon2 simulation that was run on the same initial conditions but additionally traced nine chemical elements. Based on a sample of 27 massive disk galaxies with M* > 10^10 M_{\odot} in NewHorizon, the contribution of the thick disk was found to be 34 \pm 15% in r-band luminosity or 48 \pm 13% in mass to the overall galactic disk, which seems in agreement with observational data. The vertical profiles of 0, 22, and 5 galaxies are best fitted by 1, 2, or 3 sech2 components, respectively. The NewHorizon2 data show that the selection of thick disk stars based on a single [α/Fe] cut is severely contaminated by stars of different kinematic properties while missing a bulk of kinematically thick disk stars. Vertical luminosity profile fits recover the key properties of thick disks reasonably well. The majority of stars are born near the galactic mid-plane with high circularity and get heated with time via fluctuation in the force field. Depending on the star formation and merger histories, galaxies may naturally develop thick disks with significantly different properties.

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arXiv2023

Simulating the diversity of shapes of the Lyman-$α$ line

The Ly$α$ line is a powerful probe of distant galaxies, which contains information about inflowing/outflowing gas through which Ly$α$ photons scatter. To develop our understanding of this probe, we post-process a zoom-in radiation-hydrodynamics simulation of a low-mass ($M_* \sim 10^9 M_\odot$) galaxy to construct 22500 mock spectra in 300 directions from $z = 3$ to 4. Remarkably, we show that one galaxy can reproduce the variety of a large sample of spectroscopically observed Ly$α$ line profiles. While most mock spectra exhibit double-peak profiles with a dominant red peak, their shapes cover a large parameter space in terms of peak velocities, peak separation and flux ratio. This diversity originates from radiative transfer effects at ISM and CGM scales, and depends on galaxy inclination and evolutionary phase. Red-dominated lines preferentially arise in face-on directions during post-starburst outflows and are bright. Conversely, accretion phases usually yield symmetric double peaks in the edge-on direction and are fainter. While resonant scattering effects at $< 0.2\times R_{\rm vir}$ are responsible for the broadening and velocity shift of the red peak, the extended CGM acts as a screen and impacts the observed peak separation. The ability of simulations to reproduce observed Ly$α$ profiles and link their properties with galaxy physical parameters offers new perspectives to use Ly$α$ to constrain the mechanisms that regulate galaxy formation and evolution. Notably, our study implies that deeper Ly$α$ surveys may unveil a new population of blue-dominated lines tracing inflowing gas.

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arXiv2023

Translators of galaxy morphology indicators between observation and simulation

Based on the recent advancements in the numerical simulations of galaxy formation, we anticipate the achievement of realistic models of galaxies in the near future. Morphology is the most basic and fundamental property of galaxies, yet observations and simulations still use different methods to determine galaxy morphology, making it difficult to compare them. We hereby perform a test on the recent NewHorizon simulation which has spatial and mass resolutions that are remarkably high for a large-volume simulation, to resolve the situation. We generate mock images for the simulated galaxies using SKIRT that calculates complex radiative transfer processes in each galaxy. We measure morphological indicators using photometric and spectroscopic methods following observer's techniques. We also measure the kinematic disk-to-total ratios using the Gaussian mixture model and assume that they represent the true structural composition of galaxies. We found that spectroscopic indicators such as $V/σ$ and $λ_{R}$ closely trace the kinematic disk-to-total ratios. In contrast, photometric disk-to-total ratios based on the radial profile fitting method often fail to recover the true kinematic structure of galaxies, especially for small galaxies. We provide translating equations between various morphological indicators.

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arXiv2022

PRISM: A Non-Equilibrium, Multiphase Interstellar Medium Model for Radiation Hydrodynamics Simulations of Galaxies

We introduce the PRISM interstellar medium (ISM) model for thermochemistry and its implementation in the RAMSES-RTZ code. The model includes a non-equilibrium primordial, metal, and molecular chemistry network for 115 species coupled to on-the-fly multifrequency radiation transport. PRISM accurately accounts for the dominant ISM cooling and heating processes in the low-density regime (i.e. $ρ<10^5\ {\rm cm^{-3}}$), including photoheating, photoelectric heating, H$_2$ heating/cooling, cosmic-ray heating, H/He cooling, metal-line cooling, CO cooling, and dust cooling (recombination and gas-grain collisions). We validate the model by comparing 1D equilibrium simulations across six dex in metallicity to existing 1D ISM models in the literature. We apply PRISM to high-resolution (4.5 pc) isolated dwarf galaxy simulations that include state-of-the-art models for star formation and stellar feedback to take an inventory of which cooling and heating processes dominate each different gas phase of a galaxy and to understand the importance of non-equilibrium effects. We show that most of the ISM gas is either close to thermal equilibrium or exhibits a slight cooling instability, while from a chemical perspective, the non-equilibrium electron fraction is often more than three times higher or lower than the equilibrium value, which impacts cooling, heating, and observable emission lines. Electron enhancements are attributed to recombination lags while deficits are shown to be due to rapid cosmic-ray heating. The PRISM model and its coupling to RAMSES-RTZ is applicable to a wide variety of astrophysical scenarios, from cosmological simulations to isolated giant molecular clouds, and is particularly useful for understanding how changes to ISM physics impact observable quantities such as metallic emission lines.

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arXiv2022

Two Modes of LyC Escape From Bursty Star Formation: Implications for [C II] Deficits and the Sources of Reionization

We use the SPHINX$^{20}$ cosmological radiation hydrodynamics simulation to study how Lyman Continuum (LyC) photons escape from galaxies and the observational signatures of this escape. We define two classes of LyC leaker: Bursty Leakers and Remnant Leakers, based on their star formation rates (SFRs) that are averaged over 10 Myr (SFR$_{10}$) or 100 Myr (SFR$_{100}$). Both have $f_{\rm esc}>20\%$ and experienced an extreme burst of star formation, but Bursty Leakers have ${\rm SFR_{10}>SFR_{100}}$, while Remnant Leakers have ${\rm SFR_{10} 20\%$ dominate the ionizing photon budget at $z\gtrsim7.5$ but the contribution from galaxies with $f_{\rm esc}<5\%$ becomes significant at the tail-end of reionization.

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arXiv2022

Star formation history and transition epoch of cluster galaxies based on the Horizon-AGN simulation

Cluster galaxies exhibit substantially lower star formation rates than field galaxies today, but it is conceivable that clusters were sites of more active star formation in the early universe. Herein, we present an interpretation of the star formation history (SFH) of group/cluster galaxies based on the large-scale cosmological hydrodynamic simulation, Horizon-AGN. We find that massive galaxies in general have small values of e-folding timescales of star formation decay (i.e., ``mass quenching'') regardless of their environment, whilst low-mass galaxies exhibit prominent environmental dependence. In massive host halos (i.e., clusters), the e-folding timescales of low-mass galaxies are further decreased if they reside in such halos for a longer period of time. This ``environmental quenching'' trend is consistent with the theoretical expectation from ram pressure stripping. Furthermore, we define a ``transition epoch'' as where cluster galaxies become less star-forming than field galaxies. The transition epoch of group/cluster galaxies varies according to their stellar and host cluster halo masses. Low-mass galaxies in massive clusters show the earliest transition epoch of $\sim 7.6$ Gyr ago in lookback time. However, it decreases to $\sim 5.2$ Gyr for massive galaxies in low-mass clusters. Based on our findings, we can describe cluster galaxy's SFH with regard to the cluster halo-to-stellar mass ratio.

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arXiv2022

The Obelisk simulation: galaxies contribute more than AGN to HI reionization of protoclusters

We present the Obelisk project, a cosmological radiation-hydrodynamics simulation following the assembly and reionization of a protocluster progenitor during the first two billions of years from the big bang, down to $z = 3.5$. The simulation resolves haloes down to the atomic cooling limit, and tracks the contribution of different sources of ionization: stars, active galactic nuclei, and collisions. The Obelisk project is designed specifically to study the coevolution of high redshift galaxies and quasars in an environment favouring black hole growth. In this paper, we establish the relative contribution of these two sources of radiation to reionization and their respective role in establishing and maintaining the high redshift ionizing background. Our volume is typical of an overdense region of the Universe and displays star formation rate and black hole accretion rate densities similar to high redshift protoclusters. We find that hydrogen reionization happens inside-out and is completed by $z \sim 6$ in our overdensity, and is predominantly driven by galaxies, while accreting black holes only play a role at $z \sim 4$.

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arXiv2022

Impact of Radiation Feedback on the Formation of Globular Cluster Candidates during Cloud-Cloud Collisions

To understand the impact of radiation feedback during the formation of a globular cluster (GC), we simulate a head-on collision of two turbulent giant molecular clouds (GMCs). A series of idealized radiation-hydrodynamic simulations is performed, with and without stellar radiation or Type II supernovae. We find that a gravitationally bound, compact star cluster of mass $M_{\rm GC} \sim 10^5\,M_\odot$ forms within $\approx 3\,{\rm Myr}$ when two GMCs with mass $M_{\rm GMC}=3.6\times10^5\,M_\odot$ collide. The GC candidate does not form during a single collapsing event but emerges due to the mergers of local dense gas clumps and gas accretion. The momentum transfer due to the absorption of the ionizing radiation is the dominant feedback process that suppresses the gas collapse and photoionization becomes efficient once a sufficient number of stars form. The cluster mass is larger by a factor of $\sim 2$ when the radiation feedback is neglected, and the difference is slightly more pronounced (16\%) when extreme $\rm Lyα$ feedback is considered in the fiducial run. In the simulations with radiation feedback, supernovae explode after the star-forming clouds are dispersed, and their metal ejecta are not instantaneously recycled to form stars.

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arXiv2022

The Challenges of Identifying Population III Stars in the Early Universe

The recent launch of JWST has enabled the exciting prospect of detecting the first generation of metal-free, Population III (Pop. III) stars. Determining the emission line signatures that robustly signify the presence of Pop. III stars against other possible contaminants represents a key challenge for interpreting JWST data. To this end, we run high-resolution (sub-pc) cosmological radiation hydrodynamics simulations of the region around a dwarf galaxy at $z\geq10$ to predict the emission line signatures of the Pop. III/Pop. II transition. We show that the absence of metal emission lines is a poor diagnostic of Pop. III stars because metal-enriched galaxies in our simulation can maintain low [OIII] 5007${\rm Å}$ emission that may be undetectable due to sensitivity limits. Combining spectral hardness probes (e.g. HeII 1640${\rm Å}$/H$α$) with metallicity diagnostics is more likely to probe the presence of metal-free stars, although contamination from Wolf-Rayet stars, X-ray binaries, or black holes may be important. The hard emission from Pop. III galaxies fades fast due to the short stellar lifetimes of massive Pop. III stars, which could further inhibit detection. Similarly, Pop. III stars may be detectable after they evolve off the main-sequence due to the cooling radiation from nebular gas or a supernova remnant; however, these signatures are also short-lived (i.e. few Myr), and contaminants such as flickering black holes might confuse this diagnostic. While JWST will provide a unique opportunity to spectroscopically probe the nature of the earliest galaxies, both the short timescales associated with pristine systems and ambiguities in interpreting key diagnostic emission lines may hinder progress. Special care will be needed before claiming the discovery of systems with pure Pop. III stars.

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