arXiv subjects

Tsang Keung Chan

Publications and source records attributed to Tsang Keung Chan.

10 recordsLinked to original sources

arXiv2026

The Low-Mass Baryon Cycle in QUEST Dwarf Galaxies I: Sample definition and first results

We introduce the QUEST Dwarfs program, which connects low-mass galaxies' stellar populations, interstellar medium, and circumgalactic medium in a large and coherently analyzed sample using a combination of optical spectroscopy, broadband imaging, and FUV absorption spectra of bright background sources .We present initial results from the first-release sample, comprising 14 galaxies with stellar mass $M_\mathrm{star} \leq 10^9 M_\odot$ at $z\approx0.001-0.017$, each with at least one CGM absorption probe at projected distances $d_\mathrm{proj}\lesssim 100$ kpc. This representative sample triples the number of available probes within 1/3 of the halo radius of dwarf galaxies outside of the Local Group. We find that the total silicon column density declines much more rapidly with projected distance than \textsc{Hi}, implying that chemically enriched cool gas is preferentially concentrated in the inner CGM, while the increasing ionization fraction of hydrogen with radius likely enhances this contrast. Accounting for unobserved silicon in higher ionization stages, we infer total metal masses of $\log M_Z/M_\odot\approx4.8$ and $6.5$ in the cool CGM within $0.3 R_\mathrm{vir}$ for dwarfs with median $\log M_\mathrm{star}/M_\odot=7.6$ and 8.6, respectively. These reservoirs correspond to $\approx3$% and $\approx16$% of the total metals produced over the galaxies' lifetimes. More massive galaxies also exhibit systematically stronger metal absorption, suggesting that projected distance governs the radial decline of metal absorption while stellar mass sets the normalization of the CGM metal profile. Individual ions reveal a multiphase structure, with low-ionization species concentrated in the inner halo and higher-ionization species extending farther. The full QUEST Dwarfs survey will provide the statistical power needed to isolate the dominant drivers of CGM enrichment in low-mass halos.

astro-ph.GA↗

arXiv2026

Cosmological Zoom-In Simulations of Milky Way Host Mass Dark Matter Halos with a Blue-Tilted Primordial Power Spectrum

Recent observations from the James Webb Space Telescope revealed a surprisingly large number of galaxies at high redshift, challenging the standard Lambda Cold Dark Matter cosmology with a power-law primordial power spectrum. Previous studies alleviated this tension with a blue tilted primordial power spectrum ($P(k)\propto k^{m_s}$ with $m_s>1$ at small scales $>1~{\rm cMpc}^{-1}$). In this study, we examine whether the blue tilted model can boost dark matter substructures especially at low redshift, thereby addressing other potential challenges to the standard cosmology. First, substructures in the standard cosmological model may not be sufficient to explain the anomalous flux ratio problem observed in strong gravitational lensing. Second, the number of observed nearby satellite galaxies could be higher than the theoretical predictions of the standard cosmology, after completeness correction and tidal stripping by baryonic disks. To study the impact of a blue tilted primordial power spectrum on substructures, we perform high-resolution cosmological zoom-in dark matter-only simulations of Milky Way host mass halos, evolving to redshift $z=0$. At $z=0$, we find that the blue-tilted subhalo mass functions can be enhanced by more than a factor of two for subhalo masses $M_{\rm sub} \lesssim 10^{10} ~M\odot$, whereas the subhalo $V_{\rm max}$ functions can be enhanced by a factor of four for maximum circular velocities $V_{\rm max}\lesssim 30 ~{\rm km/s}$. The blue-tilted scaled cumulative substructure fraction can be an order of magnitude higher at $\sim$10\% of the virial radius. The blue-tilted subhalos also have higher central densities, since the blue-tilted subhalos reach the same $V_{\rm max}$ at a smaller distance $R_{\rm max}$ from the center. We have also verified these findings with higher-resolution simulations.

astro-ph.CO↗

arXiv2026

The Star Formation History and Evolution of the Ultra-Diffuse M81 Satellite F8D1

We present deep HST imaging of one of the nearest ultra-diffuse galaxies (UDGs) outside of the Local Group: F8D1, a satellite of M81 known to be tidally disrupting. UDGs are an enigmatic and diverse population, with evolutionary pathways ranging from tidal processing to bursty feedback and high initial angular momentum. To determine F8D1's evolutionary drivers, we resolve stars in F8D1's central $\sim$1 kpc and in a parallel field $\sim$6 kpc along its major axis to deep photometric limits, reaching below the Red Clump. We also image eight shallower fields along F8D1's major and minor axes. We calculate the star formation history (SFH) in the two deep fields, finding that while currently quiescent, both regions experienced a substantial burst $\sim$2 Gyr ago and a smaller burst $\sim$500 Myr ago, which likely formed F8D1's nuclear star cluster. In the shallow fields, using the ratio of evolved Asymptotic Giant Branch and Red Giant Branch stars out to $\sim$13 kpc along F8D1's known stellar stream, we confirm that F8D1 was globally star-forming until at least $\sim$2 Gyr ago. We estimate a total progenitor stellar mass, including the stream, of $\sim$1.3$\times$10$^8 M_{\odot}$, with an average [M/H] $\sim$ $-$0.8. We compare F8D1's properties to those of Local Group galaxies with similar initial stellar mass. We find that F8D1 is consistent with a progenitor star-forming galaxy similar to NGC 6822 that is in the midst of a transition to a Sagittarius-like system. Notably, this evolutionary sequence can be accomplished through tidal processing alone in galaxies that have experienced sufficiently bursty feedback to have created cored profiles.

astro-ph.GA↗

arXiv2026

TDEs on FIRE: Illuminating the Cosmic Evolution of Tidal Disruption Rates

Tidal disruption events have been extensively studied in the local universe, but their prevalence at high redshifts remains largely unexplored. Using the FIRE-2 cosmological zoom-in simulations, we compute the per-galaxy tidal disruption rate (TDR) over $z=1-10$, covering black holes from IMBHs to SMBHs. The averaged TDR rises from the early universe, peaks at $\sim 4 \times 10^{-4} \, \text{yr}^{-1}$ near $z \sim 2.5$, and declines to $\sim 10^{-5} \, \text{yr}^{-1}$ at $z=1$. The TDR correlates strongly with host galaxy star formation rate and central stellar density at all redshifts. Qualitatively, the TDR trends with the $M_{\rm BH}$ and $M_{\rm gal}$ persist from high redshift to the local universe, suggesting similar BH-galaxy scaling across cosmic time. Satellite galaxies exhibit comparably high TDRs, with their fractional contribution increasing significantly at high redshifts, highlighting their potential for probing IMBHs and early galaxy assembly. This work demonstrates that cosmological simulations offer a promising avenue for constraining the cosmic evolution of the TDR, paving the way for future comparisons with next-generation observations.

astro-ph.HE↗

arXiv2026

SPARCS -- combining radiation hydrodynamics with non-equilibrium metal chemistry in the SWIFT astrophysical code

We present SPARCS, which combines the moment-based radiative transfer method SPH-M1RT with the non-equilibrium metal chemistry solver CHIMES in the modern highly-parallel astrophysical code SWIFT. SPARCS enables on-the-fly radiation hydrodynamics simulations, with multi-frequency ultraviolet radiative transfer coupled with all ionisation states of 11 major elements, in the presence of dust, cosmic ray ionization and heating, and self-gravity. Direct radiation pressure on gas and dust is also accounted for. We validate SPARCS against analytic solutions and standard photo-ionization codes such as CLOUDY in idealized tests. As an example application, we simulate an ionization front propagating through an inhomogeneous interstellar medium with solar metallicity. We produce mock optical emission line observations with the level population calculation code HyLight and the diagnostic radiative transfer code RADMC3D. We find that non-equilibrium effects and inhomogeneities can boost the low ion fractions by up to an order of magnitude. Possible applications of SPARCS include studying the dynamical impact of radiation on gas in star-forming regions, and in the interstellar and circumgalactic medium, as well as interpreting line diagnostics in such environments, and galactic or AGN outflows.

astro-ph.GA↗

arXiv2026

The HyLight model for hydrogen emission lines in simulated nebulae

Hydrogen recombination lines provide key diagnostics of ionized gas in galaxies, yet most hydrodynamical simulations estimate hydrogen level populations using interpolated emissivity tables rather than computing them directly from local physical conditions. We present HyLight, a Python-based atomic model that calculates hydrogen level populations and line emissivities from the gas density, temperature, and ionization state, enabling accurate predictions in both equilibrium and non-equilibrium environments. Benchmark comparisons show that HyLight reproduces Cloudy predictions for Balmer, Paschen, and Brackett emissivities to within 1 per cent under typical photoionized nebular conditions, while discrepancies of several tens of per cent arise relative to other published calculations. As an illustrative application, we use HyLight to compute photoionization-to-line intensity ratios in an HII nebula and generate synthetic hydrogen emission maps from a radiation-hydrodynamical simulation that includes non-equilibrium thermochemistry. Combining physical consistency with flexibility, HyLight provides a robust framework for connecting hydrodynamical simulations with observational diagnostics of photoionized regions, and enhances our ability to interpret hydrogen emission in complex, non-equilibrium astrophysical environments.

astro-ph.GA↗

arXiv2025

Emission line tracers of galactic outflows driven by stellar feedback in simulations of isolated disk galaxies

Hydrodynamic simulations can connect outflow observables to the physical conditions of outflowing gas. Here, we use simulations of isolated disk galaxies ranging from dwarf mass ($M_{200} = 10^{10}\mathrm{M}_{\odot}$) to Milky Way mass ($M_{200} = 10^{12}\mathrm{M}_{\odot}$), based on the FIRE-2 subgrid models to investigate multiphase galactic outflows. We use the CHIMES non-equilibrium chemistry module to create synthetic spectra of common outflow tracers ([CII]$_{158\rm{μm}}$, $\mathrm{CO}_{J(1-0)}$, H$α$ and $[\mathrm{OIII}]_{5007\text{A}}$). Using our synthetic spectra we measure the mass outflow rate, kinetic power and momentum flux using observational techniques. In [CII]$_{158\rm{μm}}$ we measure outflow rates of $10^{-4}$ to $1$ $\mathrm{M_{\odot}yr^{-1}}$ across an SFR range of $10^{-3}$ to $1$ $\text{M}_{\odot}\text{yr}^{-1}$, which is in reasonable agreement with observations. The significant discrepancy is in $\mathrm{CO}_{J(1-0)}$, with the simulations lying $\approx1$ dex below the observational sample. We test observational assumptions used to derive outflow properties from synthetic spectra. We find the greatest uncertainty lies in measurements of electron density, as estimates using the SII doublet can overestimate the actual electron density by up to 2 dex, which changes mass outflow rates by up to 4 dex. We also find that molecular outflows are especially sensitive to the conversion factor between CO luminosity and H2 mass, with outflow rates changing by up to 4 dex in our least massive galaxy. Comparing the outflow properties derived from the synthetic spectra to those derived directly from the simulation, we find that [CII]$_{158\rm{μm}}$ probes outflows at greater distances from the disk, whilst we find that molecular gas does not survive at large distances within outflows within our modestly star-forming disk galaxies simulated in this work.

astro-ph.GA↗

arXiv2024

SWIFT: A modern highly-parallel gravity and smoothed particle hydrodynamics solver for astrophysical and cosmological applications

Numerical simulations have become one of the key tools used by theorists in all the fields of astrophysics and cosmology. The development of modern tools that target the largest existing computing systems and exploit state-of-the-art numerical methods and algorithms is thus crucial. In this paper, we introduce the fully open-source highly-parallel, versatile, and modular coupled hydrodynamics, gravity, cosmology, and galaxy-formation code SWIFT. The software package exploits hybrid shared- and distributed-memory task-based parallelism, asynchronous communications, and domain-decomposition algorithms based on balancing the workload, rather than the data, to efficiently exploit modern high-performance computing cluster architectures. Gravity is solved for using a fast-multipole-method, optionally coupled to a particle mesh solver in Fourier space to handle periodic volumes. For gas evolution, multiple modern flavours of Smoothed Particle Hydrodynamics are implemented. SWIFT also evolves neutrinos using a state-of-the-art particle-based method. Two complementary networks of sub-grid models for galaxy formation as well as extensions to simulate planetary physics are also released as part of the code. An extensive set of output options, including snapshots, light-cones, power spectra, and a coupling to structure finders are also included. We describe the overall code architecture, summarise the consistency and accuracy tests that were performed, and demonstrate the excellent weak-scaling performance of the code using a representative cosmological hydrodynamical problem with $\approx$$300$ billion particles. The code is released to the community alongside extensive documentation for both users and developers, a large selection of example test problems, and a suite of tools to aid in the analysis of large simulations run with SWIFT.

astro-ph.IM↗

arXiv2024

The impact and response of minihalos and the inter-halo medium on cosmic reionization

An ionization front (I-front) that propagates through an inhomogeneous medium is slowed down by self-shielding and recombinations. We perform cosmological radiation hydrodynamics simulations of the I-front propagation during the epoch of cosmic reionization. The simulations resolve gas in minihalos (halo mass $10^4\lesssim M_h[{\rm M}_\odot]\lesssim 10^8)$ that could dominate recombinations, in a computational volume that is large enough to sample the abundance of such halos. The numerical resolution is sufficient (gas particle mass $\sim 20{\rm M}_\odot$, spatial resolution $< 0.1\;{\rm ckpc}$) to allow accurate modelling of the hydrodynamic response of gas to photo-heating. We quantify the photo-evaporation time of minihalos as a function of $M_h$ and its dependence on the photo-ionization rate, $Γ_{-12}$, and the redshift of reionization, $z_i$. The recombination rate can be enhanced over that of a uniform medium by a factor $\sim 10-20$ early on. The peak value increases with $Γ_{-12}$ and decreases with $z_i$, due to the enhanced contribution from minihalos. The clumping factor, $c_r$, decreases to a factor of a few at $\sim 100\;{\rm Myr}$ after the passage of the I-front when the minihalos have been photo-evaporated; this asymptotic value depends only weakly on $Γ_{-12}$. Recombinations increase the required number of photons per baryon to reionize the Universe by 20-100 per cent, with the higher value occurring when $Γ_{-12}$ is high and $z_i$ is low. We complement the numerical simulations with simple analytical models for the evaporation rate and the inverse Strömgren layer. The study also demonstrates the proficiency and potential of SPHM1RT to address astrophysical problems in high-resolution cosmological simulations.

astro-ph.CO↗

arXiv2019

Reconciling observed and simulated stellar halo masses

We use cosmological hydrodynamical simulations of Milky-Way-mass galaxies from the FIRE project to evaluate various strategies for estimating the mass of a galaxy's stellar halo from deep, integrated-light images. We find good agreement with integrated-light observations if we mimic observational methods to measure the mass of the stellar halo by selecting regions of an image via projected radius relative to the disk scale length or by their surface density in stellar mass . However, these observational methods systematically underestimate the accreted stellar component, defined in our (and most) simulations as the mass of stars formed outside of the host galaxy, by up to a factor of ten, since the accreted component is centrally concentrated and therefore substantially obscured by the galactic disk. Furthermore, these observational methods introduce spurious dependencies of the estimated accreted stellar component on the stellar mass and size of galaxies that can obscure the trends in accreted stellar mass predicted by cosmological simulations, since we find that in our simulations the size and shape of the central galaxy is not strongly correlated with the assembly history of the accreted stellar halo. This effect persists whether galaxies are viewed edge-on or face-on. We show that metallicity or color information may provide a way to more cleanly delineate in observations the regions dominated by accreted stars. Absent additional data, we caution that estimates of the mass of the accreted stellar component from single-band images alone should be taken as lower limits.

astro-ph.GA↗