publications

2025

1. The DORCHA Suite: Nature, Nurture, and the Phase Space Distribution of the Milky Way’s High Redshift Progenitors Today

   Sreedhar Balu, Chris Power, Kris Walker, and 1 more author

   Jun 2025

   Abs DOI

   Where in the present-day Milky Way should we search for the remnants of its earliest stars? We address this question using the Dorcha (Gaelic for Dark ; DUR-uh-khuh) suite: a set of 25 high-resolution, dark-matter-only cosmological zoom-in simulations of Milky Way analogue (MWA) haloes evolved to z = 0. Of these, 15 are isolated and the rest are in pairs, similar to the MW and M31. By identifying and tagging the most bound material in high-redshift (z ≥ 5) progenitor haloes – those likely to host early star formation – we track the present-day phase-space distribution of this ancient component. We find that this material is highly centrally concentrated at z = 0, with 90 – 100 per cent residing within r \lessequivlnt 15 h-1 kpc. It exhibits steep density profiles (ρ ∝ r-4), low velocity dispersions (σr/σmax \lessequivlnt 0.6), and radially biased orbits (β \greaterequivlnt 0.5 for r \greaterequivlnt 0.1 R200), consistent with a relaxed, centrally embedded population. These results hold across haloes with diverse formation histories and environments, suggesting that the dynamical signature of early progenitors is robust to later mergers and interactions. Our findings imply that the fossil record of the first generations of stars – including Population III and extremely metal-poor stars – should be sought in the innermost regions of the Milky Way, where they retain distinctive kinematic imprints. While these stellar populations may overlap, we caution that low metallicity does not uniquely identify ancient stars, nor vice versa. The Dorcha suite thus provides a physically motivated baseline for interpreting observations from Galactic Archaeology surveys targeting the bulge and inner halo.

2. On the Use of WGANs for Super Resolution in Dark-Matter Simulations

   John Brennan, Sreedhar Balu, Yuxiang Qin, and 2 more authors

   The Open Journal of Astrophysics, Jun 2025

   Abs DOI

   Super-resolution techniques have the potential to reduce the computational cost of cosmological and astrophysical simulations. This can be achieved by enabling traditional simulation methods to run at lower resolution and then efficiently computing high-resolution data corresponding to the simulated low-resolution data. In this work, we investigate the application of a Wasserstein Generative Adversarial Network (WGAN) model, previously proposed in the literature, to increase the particle resolution of dark-matter-only simulations. We reproduce prior results, showing the WGAN model successfully generates high-resolution data with summary statistics, including the power spectrum and halo mass function, that closely match those of true high-resolution simulations. However, we also identify a limitation of the WGAN model in the form of smeared features in generated high-resolution data, particularly in the shapes of dark-matter halos and filaments. This limitation points to a potential weakness of the proposed WGAN-based super-resolution method in capturing the detailed structure of halos, and underscores the need for further development in applying such models to cosmological data.

3. Semi-Analytical Modelling of Pop. III Star Formation and Metallicity Evolution – II. Impact on 21 Cm Power Spectrum

   Emanuele M Ventura, Yuxiang Qin, Sreedhar Balu, and 1 more author

   Monthly Notices of the Royal Astronomical Society, May 2025

   Abs DOI

   Simulating Population (Pop.) III star formation in mini-haloes in a large cosmological simulation is an extremely challenging task but it is crucial to estimate its impact on the 21 cm power spectrum. In this work, we develop a framework within the semi-analytical code MERAXES to estimate the radiative backgrounds from Pop. III stars needed for the computation of the 21 cm signal. We computed the 21 cm global signal and power spectrum for different Pop. III models varying star formation efficiency, initial mass function and specific X-ray luminosity per unit of star formation (LX/SFR). In all the models considered, we find Pop. III stars have little to no impact on the reionization history but significantly affect the thermal state of the intergalactic medium (IGM) due to the strong injection of X-ray photons from their remnants that heat the neutral IGM at z ≥ 15. This is reflected not only on the 21 cm sky-averaged global signal during the Cosmic Dawn but also on the 21 cm power spectrum at z ≤ 10 where models with strong Pop. III X-ray emission have larger power than models with no or mild Pop. III X-ray emission. We estimate observational uncertainties on the power spectrum using 21CMSENSE and find that models where Pop. III stars have a stronger X-ray emission than Pop. II are distinguishable from models with no or mild Pop. III X-ray emission with 1000 h observations of the upcoming SKA1-low.

2024

1. Semi-Analytic Modelling of Pop. III Star Formation and Metallicity Evolution – I. Impact on the UV Luminosity Functions at z = 9–16

   Emanuele M Ventura, Yuxiang Qin, Sreedhar Balu, and 1 more author

   Monthly Notices of the Royal Astronomical Society, Feb 2024

   Abs DOI

   ABSTRACT We implemented Population III (Pop. III) star formation in mini-haloes within the Meraxes semi-analytic galaxy formation and reionization model, run on top of a N-body simulation with L = 10 h-1 cMpc with 20483 particles resolving all dark matter haloes down to the mini-haloes (∼105 M⊙). Our modelling includes the chemical evolution of the IGM, with metals released through supernova-driven bubbles that expand according to the Sedov–Taylor model. We found that SN-driven metal bubbles are generally small, with radii typically of 150 ckpc at z = 6. Hence, the majority of the first galaxies are likely enriched by their own star formation. However, as reionization progresses, the feedback effects from the UV background become more pronounced, leading to a halt in star formation in low-mass galaxies, after which external chemical enrichment becomes more relevant. We explore the sensitivity of the star formation rate density and stellar mass functions to the unknown values of free parameters. We also discuss the observability of Pop. III dominated systems with JWST, finding that the inclusion of Pop. III galaxies can have a significant effect on the total UV luminosity function at z = 12–16. Our results support the idea that the excess of bright galaxies detected with JWST might be explained by the presence of bright top-heavy Pop. III dominated galaxies without requiring an increased star formation efficiency.

2. The Impact of Ionization Morphology and X-ray Heating on the Cosmological 21-Cm Skew Spectrum

   Jadien H Cook, Sreedhar Balu, Bradley Greig, and 4 more authors

   Monthly Notices of the Royal Astronomical Society, Mar 2024

   Abs DOI

   ABSTRACT The cosmological 21-cm signal offers a potential probe of the early Universe and the first ionizing sources. Current experiments probe the spatially dependent variance (Gaussianity) of the signal through the power spectrum (PS). The signal, however, is expected to be highly non-Gaussian due to the complex topology of reionization and X-ray heating. We investigate the non-Gaussianities of X-ray heating and reionization, by calculating the skew spectrum (SS) of the 21-cm signal using meraxes, which couples a semi-analytic galaxy population with seminumerical reionization simulations. The SS is the cross-spectrum of the quadratic temperature brightness field with itself. We generate a set of seven simulations from z = 30 to 5, varying the halo mass threshold for hosting star formation, the X-ray luminosity per star formation rate, and the minimum X-ray energy escaping host galaxies. We find the SS is predominantly negative as a function of redshift, transitioning to positive towards the start of reionization, and peaking during the mid-point of reionization. We do not see a negative dip in the SS during reionization, likely due to the specifics of modelling ionization sources. We normalize the SS by the PS during reionization isolating the non-Gaussianities. We find a trough (\k}sim }, 0.1}, }textrm {Mpc}^{-1}\) and peak (\k}sim }, 0.4{-}1}, }textrm {Mpc}^{-1}\) in the normalized SS during the mid-to-late periods of reionization. These correlate to the ionization topology, and neutral islands in the IGM. We calculate the cosmic variance of the normalized SS, and find these features are detectable in the absence of foregrounds with the SKA_LOW.

2023

1. Thermal and Reionization History within a Large-Volume Semi-Analytic Galaxy Formation Simulation

   Sreedhar Balu, Bradley Greig, Yisheng Qiu, and 4 more authors

   Monthly Notices of the Royal Astronomical Society, Apr 2023

   Abs DOI

   We predict the 21-cm global signal and power spectra during the Epoch of Reionization using the meraxes semi-analytic galaxy formation and reionization model, updated to include X-ray heating and thermal evolution of the intergalactic medium. Studying the formation and evolution of galaxies together with the reionization of cosmic hydrogen using semi-analytic models (such as M eraxes) requires N-body simulations within large volumes and high-mass resolutions. For this, we use a simulation of side-length 210 h-1 Mpc with 43203 particles resolving dark matter haloes to masses of \5}times 10^8 }rm{ }h^{-1}}, }mathrm{M_}odot }\. To reach the mass resolution of atomically cooled galaxies, thought to be the dominant population contributing to reionization, at z = 20 of {}sim 2}times 10^7 }text{ }h^{-1}}, }mathrm{M_}odot } we augment this simulation using the darkforest Monte Carlo merger tree algorithm (achieving an effective particle count of ∼1012). Using this augmented simulation, we explore the impact of mass resolution on the predicted reionization history as well as the impact of X-ray heating on the 21-cm global signal and the 21-cm power spectra. We also explore the cosmic variance of 21-cm statistics within 703 h-3 Mpc3 sub-volumes. We find that the midpoint of reionization varies by ∆z ∼ 0.8 and that the cosmic variance on the power spectrum is underestimated by a factor of 2–4 at k ∼ 0.1–0.4 Mpc-1 due to the non-Gaussian nature of the 21-cm signal. To our knowledge, this work represents the first model of both reionization and galaxy formation which resolves low-mass atomically cooled galaxies while simultaneously sampling sufficiently large scales necessary for exploring the effects of X-rays in the early Universe.

2. Fisher Matrix Forecasts on the Astrophysics of Galaxies during the Epoch of Reionization from the 21-Cm Power Spectra

   Sreedhar Balu, Bradley Greig, and J Stuart B Wyithe

   Monthly Notices of the Royal Astronomical Society, Aug 2023

   Abs DOI

   ABSTRACT The hyperfine 21-cm transition of neutral hydrogen from the early Universe (z > 5) is a sensitive probe of the formation and evolution of the first luminous sources. Using the Fisher matrix formalism we explore the complex and degenerate high-dimensional parameter space associated with the high-z sources of this era and forecast quantitative constraints from a future 21-cm power spectrum (21-cm PS) detection. This is achieved using {}rm {}small ERAXES} a coupled semi-analytic galaxy formation model and reionization simulation, applied to an N-body halo merger tree with a statistically complete population of all atomically cooled galaxies out to z ∼ 20. Our mock observation assumes a 21-cm detection spanning z ∈ [5, 24] from a 1000 h mock observation with the forthcoming Square Kilometre Array, and is calibrated with respect to ultraviolet luminosity functions (UV LFs) at z ∈ [5, 10], the optical depth of CMB photons to Thompson scattering from Planck, and various constraints on the IGM neutral fraction at z > 5. In this work, we focus on the X-ray luminosity, ionizing UV photon escape fraction, star formation, and supernova feedback of the first galaxies. We demonstrate that it is possible to recover five of the eight parameters describing these properties with better than 50 per cent precision using just the 21-cm PS. By combining with UV LFs, we are able to improve our forecast, with five of the eight parameters constrained to better than 10 per cent (and all below 50 per cent).

3. Implications of z \greaterequivlnt 12 JWST Galaxies for Galaxy Formation at High Redshift

   Yuxiang Qin, Sreedhar Balu, and J Stuart B Wyithe

   Monthly Notices of the Royal Astronomical Society, Sep 2023

   Abs DOI

   ABSTRACT Using a semi-analytic galaxy formation model, we study analogues of eight z \greaterequivlnt 12 galaxies recently discovered by James Webb Space Telescope (JWST). We select analogues from a cosmological simulation with a (311 cMpc)3 volume and an effective particle number of 1012 enabling the resolution of every atomic-cooling galaxy at z ≤ 20. We vary model parameters to reproduce the observed ultraviolet (UV) luminosity function at 5 < z < 13, aiming for a statistically representative high-redshift galaxy mock catalogue. Using the forward-modelled JWST photometry, we identify analogues from this catalogue and study their properties as well as possible evolutionary paths and local environment. We find faint JWST galaxies (MUV \greaterequivlnt - 19.5) to remain consistent with the standard galaxy formation model and that our fiducial catalogue includes large samples of their analogues. The properties of these analogues broadly agree with conventional spectral energy distribution-fitting results, except for having systematically lower redshifts due to the evolving ultraviolet luminosity function, and for having higher specific star formation rates as a result of burstier histories in our model. On the other hand, only a handful of bright galaxy analogues can be identified for the observed z ∼ 12 galaxies. Moreover, in order to reproduce the z \greaterequivlnt 16 JWST galaxy candidates, boosting star-forming efficiencies through reduced feedback regulation and increased gas depletion rate is necessary relative to models of lower redshift populations. This suggests star formation in the first galaxies could differ significantly from their lower redshift counterparts. We also find that these candidates are subject to low-redshift contamination, which is present in our fiducial results as both the dusty or quiescent galaxies at z ∼ 5.