The THESAN project: environmental drivers of Local Group reionization

Abstract

The timing of cosmic reionization across Local Group (LG) analogues provides insights into their early histories and surrounding large-scale structure. Using the radiation-hydrodynamic simulation THESAN-1 and its dark matter-only counterpart THESAN-DARK-1, we track the reionization histories of all haloes, including 224 LG analogues within the proximity of any of the 20 Virgo-like clusters with halo masses above 10^14 Msun at z=0 and their environments. The statistically controlled samples quantify how the reionization redshift (z_reion) correlates with halo mass, local overdensity, and present-day pair properties. Even at fixed mass, haloes in denser regions ionize earlier, and increasing the overdensity smoothing scale systematically suppresses small-scale structure, including local variations and environmental gradients in z_reion. Virgo-like clusters accelerate reionization in their surroundings out to ~5-10 cMpc, beyond which local overdensity again becomes the dominant factor. Within LG pairs, reionization timing offsets reach up to ~150 Myr and correlate with present-day halo separation, reflecting sensitivity to large-scale structure rather than mass ratio in driving asynchronous reionization. The results support an extreme inside-out picture where clustered sources rapidly ionize their immediate neighborhoods, while lower-density regions self-ionize later and voids wait for external homogenization. These links between environment and reionization timing explain the influence of protoclusters and help interpret fossil records in LG dwarfs around the Milky Way. For Milky Way analogues, we find a reionization redshift as early (late) as z_reion = 12.7^{+2.0}_{-1.7} (8.88^{+0.66}_{-0.70}) when considered on 125 ckpc (500 ckpc) scales, with LG analogues following an inside-out reionization picture.

20 pages, 21 figures, comments welcome, please visit www.thesan-project.com for more details

References in corpus (42)

• Introducing the Illustris Project: Simulating the coevolution of dark and visible matter in the Universe
• Too big to fail? The puzzling darkness of massive Milky Way subhaloes
• Properties of galaxies reproduced by a hydrodynamic simulation
• Simulating galaxy formation with black hole driven thermal and kinetic feedback
• UV Luminosity Functions at redshifts z~4 to z~10: 10000 Galaxies from HST Legacy Fields
• Binary Population and Spectral Synthesis Version 2.1: construction, observational verification and new results
• The Quenching of the Ultra-Faint Dwarf Galaxies in the Reionization Era
• The M31 Velocity Vector. II. Radial Orbit Towards the Milky Way and Implied Local Group Mass
• Introducing the THESAN project: radiation-magnetohydrodynamic simulations of the epoch of reionization
• Revealing Galaxy Candidates out to $z\sim 16$ with JWST Observations of the Lensing Cluster SMACS0723
• Galaxy Formation and Reionization: Key Unknowns and Expected Breakthroughs by the James Webb Space Telescope
• Cosmological Radiative Transfer Codes Comparison Project I: The Static Density Field Tests
• First Results from HERA Phase I: Upper Limits on the Epoch of Reionization 21 cm Power Spectrum
• The Star Formation Histories of Local Group Dwarf Galaxies II. Searching For Signatures of Reionization
• Cosmic Reionization On Computers I. Design and Calibration of Simulations
• CEERS Spectroscopic Confirmation of NIRCam-Selected z > 8 Galaxy Candidates with JWST/NIRSpec: Initial Characterization of their Properties
• JWST NIRCam+NIRSpec: Interstellar medium and stellar populations of young galaxies with rising star formation and evolving gas reservoirs
• Imprint of Inhomogeneous Hydrogen Reionization on the Temperature Distribution of the Intergalactic Medium
• The Hestia project: simulations of the Local Group
• The THESAN project: properties of the intergalactic medium and its connection to Reionization-era galaxies
• The Dependence of the Mass Assembly History of Cold Dark Matter Halos on Environment
• z~2-9 Galaxies Magnified by the Hubble Frontier Field Clusters II: Luminosity Functions and Constraints on a Faint-End Turnover
• The THESAN project: Lyman-alpha emission and transmission during the Epoch of Reionization
• The MillenniumTNG Project: The galaxy population at $z\ge 8$
• The THESAN project: ionizing escape fractions of reionization-era galaxies
• The THESAN project: predictions for multi-tracer line intensity mapping in the epoch of reionization
• Constraining the physical properties of the first lensed $z\sim 10-16$ galaxy candidates with JWST
• Connecting Reionization to the Local Universe
• The short ionizing photon mean free path at z=6 in Cosmic Dawn III, a new fully-coupled radiation-hydrodynamical simulation of the Epoch of Reionization
• An Introductory Review on Cosmic Reionization
• Star Formation Histories of Ultra-Faint Dwarf Galaxies: environmental differences between Magellanic and non-Magellanic satellites?
• Large scale simulations of H and He reionization and heating driven by stars and more energetic sources
• The star formation history of the Sextans dwarf spheroidal galaxy: a true fossil of the pre-reionization era
• The THESAN project: connecting ionized bubble sizes to their local environments during the Epoch of Reionization
• The Morphology of Reionization in a Dynamically Clumpy Universe
• The THESAN project: Lyman-alpha emitter luminosity function calibration
• Reionization time of the Local Group and Local-Group-like halo pairs
• JWST ASPIRE: How Did Galaxies Complete Reionization? Evidence for Excess IGM Transmission around $[\mathrm{O}\mathrm{III}]$ Emitters during Reionization
• The galaxy-IGM connection in THESAN: observability and information content of the galaxy-Lyman-$\alpha$ cross-correlation at z$\ge$6
• Emergence of the Temperature-Density Relation in the Low-Density Intergalactic Medium
• The THESAN project: tracking the expansion and merger histories of ionized bubbles during the Epoch of Reionization
• The HST Legacy Archival Uniform Reduction of Local Group Imaging (LAURELIN). I. Photometry and Star Formation Histories for 36 Ultra-faint Dwarf Galaxies