My Research

Cosmic Dawn & Epoch of Reionization

Reionization is the last global phase transition of the intergalactic medium (IGM), and the question of which galaxies drove it -- abundant faint sources or rarer bright ones -- remains open. I build semi-analytic and semi-numerical frameworks in which the ionizing emissivity of galaxies is calibrated against radiative-transfer hydrodynamic simulations, then coupled to MCMC samplers to jointly fit the full set of available constraints: JWST UV luminosity functions and luminosity densities, the ionizing emissivity at z∼5, the neutral hydrogen fraction at z∼6–8, and the CMB Thomson optical depth.

This joint approach exposes tensions that single-probe analyses hide. Models with weak feedback and a large faint-galaxy contribution reproduce the reionization observables but undershoot the elevated JWST UV luminosity functions at z>9; models with strong feedback and a larger bright-galaxy contribution match JWST at z≥10 but overshoot the bright end at lower redshift. The strong-feedback solution predicts a more gradual, extended reionization history whose optical depth is consistent with the CMB and which eases the photon-budget problem. Earlier work in this program showed that requiring consistency between the cosmic dawn 21-cm signal and post-reionization observables favors faint-galaxy-dominated models. Current work extends this to semi-numerical reionization simulations with the SimFast21 code, in which the ionizing photon escape fraction is allowed to vary with halo mass and redshift rather than held fixed, and to the contribution of AGN to the ionizing budget in light of the overmassive black hole population revealed by JWST.

Selected publications

  • Bera, A., Hassan, S., Feldmann, R., Davé, R. & Finlator, K. (2026), Towards Reconciling Reionization with JWST: The Role of Bright Galaxies and Strong Feedback, ApJ (accepted), arXiv:2511.19600

  • Bera, A., Hassan, S., Smith, A., Cen, R., Garaldi, E., Kannan, R. & Vogelsberger, M. (2023), Bridging the Gap between Cosmic Dawn and Reionization Favors Models Dominated by Faint Galaxies, ApJ 959, 2, arXiv:2209.14312

  • Sheoran, P., Bera, A., Hassan, S. (2026), Does the Escape Fraction Shape Reionization Morphology? , in prep.

  • Oneil, Q., Bera, A., Hassan, S., Madarasinghe, R., (2026), AGN contribution to reionization in light of recent JWST observations, in prep.

Large Scale Structure & Early Galaxies

The formation histories of dark matter halos set the baseline rate at which galaxies accrete gas, and therefore the pace at which they form stars. I study how halo mass accretion rates (MARs) and their formation times can be described universally -- across cosmologies, redshifts, and simulation suites -- and how those histories propagate into the observable galaxy population at high redshift.

Using halo catalogs from dark matter-only and hydrodynamic simulations (Erebos, IllustrisTNG, Thesan) spanning z=0–14 and cosmologies as different as ΛCDM and Einstein–de Sitter, I have shown that median MARs and half-mass formation times are captured by a compact, universal fitting function of physically motivated variables rather than of halo mass itself. Such fitting functions provide the accretion backbone for semi-analytic models of galaxy formation, and let one map halo growth directly onto the star formation rates and UV luminosities that JWST now measures at z≳8. A parallel strand of this work asks the inverse question: what star formation efficiency, as a function of halo mass and redshift, is required for the observed UV luminosity function and the halo mass accretion rate to be mutually consistent?

Selected publications

  • Bera, A. & Diemer, B. (2026), A universal model for the accretion rates and formation times of dark matter halos, arXiv:2606.09997

  • Bera, A. & Diemer, B. (2026), Star formation efficiencies of high-redshift galaxies informed by JWST, in prep.

Credit: 21cmSpace

21-cm Cosmology

The redshifted 21-cm line of neutral hydrogen is the only probe that tracks the IGM continuously from the dark ages through cosmic dawn and reionization. Its amplitude is set by the different competing mechanisms including Lyman-α coupling and the thermal history of the gas, which makes it a sensitive probe of the first sources and of any non-standard physics that heats or cools the IGM.

I model the global 21-cm signal semi-analytically, tracking the processes that set the spin and kinetic temperatures and asking what a detection would actually constrain. This includes the heating of the IGM by cosmic ray protons from Pop III and Pop II supernovae, which links the absorption depth directly to the early star formation rate density; the evolution of primordial magnetic fields, whose dissipation via ambipolar diffusion and decaying turbulence proceeds differently in a "colder IGM" background than in the standard scenario, tightening the bounds one can place on the field amplitude; and the effect of a modified recombination history on the predicted signal. Together these set the interpretive framework needed for upcoming measurements with SKA, HERA, and global-signal experiments, such as EDGES, SARAS, and connect them back to the galaxy population that JWST is now characterizing.

Selected publications

  • Barkana, R., Basquette, O., Bera, A. et al. (2026), High-Redshift Signatures from the Cosmic Dawn and the Epoch of Reionization, AASKAII, arXiv:2606.29903

  • Bera, A., Samui, S. & Datta, K. K. (2023), Impact of cosmic rays on the global 21-cm signal during cosmic dawn, MNRAS 519, 4, arXiv:2202.12308

  • Bera, A., Ghara, R., Chatterjee, A., Datta, K. K. & Samui, S. (2023), Studying cosmic dawn using redshifted HI 21-cm signal: A brief review, JApA 44, 10, arXiv:2210.12164

  • Bera, A., Datta, K. K. & Samui, S. (2020), Primordial magnetic fields during the cosmic dawn in light of EDGES 21-cm signal, MNRAS 498, 918, arXiv:2005.14206

  • Datta, K. K., Kundu, A., Paul, A. & Bera, A. (2020), Cosmic recombination history in light of EDGES measurements of the cosmic dawn 21-cm signal, PRD 102, 083502,
    arXiv:2001.06497

Bera A.

― Contacts


ankitabm@umd.edu

Department of Astronomy, University of Maryland PSC 1105, 4296 Stadium Drive College Park, Maryland 20742-2421