Jose Oñorbe

Context. Low-surface-brightness galaxies (LSBGs) are defined as galaxies with central surface brightness levels fainter than the night sky, making them challenging to observe. A key open question is whether their faint appearance arises from intrinsic properties or from stochastic events in their formation histories. Aims. We aim to trace the formation histories of LSBGs to assess whether their evolutionary paths differ from those of high-surface brightness galaxies (HSBGs) and to identify the key physical drivers behind these differences. Methods. We present a fast and efficient method to estimate stellar surface brightness densities in hydrodynamical simulations and a statistically robust exploration of over 150 properties in the reference run REF-L0100N1504 of the EAGLE simulation. To minimise any biases, we carefully matched the stellar and halo mass distributions of the selected LSB and HSB samples. Results. At ɀ = 0, LSBGs are typically extended, rotation-supported systems with lower stellar densities, older stellar populations, reduced star formation activity, and higher specific stellar angular momenta (j_*) than their HSBG counterparts. They also exhibit larger radii of maximum circular velocity R_υmax). We identified key transition redshifts that mark the divergence of LSBG and HSBG properties: j_* diverges at ɀ ~ 5–7 and R_υmax at ɀ ~ 2–3. Star formation activity and large-scale environment appear to play only a minimal role in the development of LSB features. Conclusions. LSBGs follow mass-dependent evolutionary pathways, where early rapid formation and later slowdowns, combined with their distinct structural properties, influence their response to external factors (e.g. mergers and gas accretion). Their LSB nature emerges from intrinsic dynamical and structural factors rather than environmental influences, with angular momentum as a key driver of divergence at high redshifts.

Strong low-ionization transitions like the Mg ii λ2796, 2804Å doublet are believed to produce a detectable ‘metal-line forest’, if metals pollute the neutral IGM. We measure the auto-correlation of the Mg II forest transmission using ten ground based z ≥ 6.80 quasar spectra probing the redshift range |$5.96 < z_{\rm Mg\, {\small II}} < 7.42$| (⁠|$z_{\rm Mg\, {\small II}, median} = 6.47$|⁠). The correlation function exhibits strong small-scale clustering and a pronounced peak at the doublet velocity (Δv = 768 km s⁻¹) arising from discrete absorbers in the CGM of galaxies. After these strong absorbers are identified and masked the signal is consistent with noise. Our measurements are compared to a suite of models generated by combining a large hydrodynamical simulation with a semi-numerical reionization topology, assuming a simple uniform enrichment model. We obtain a 95% credibility upper limit of [Mg/H] < −3.73 at |$z_{\rm Mg\, {\small II},median} = 6.47$|⁠, assuming uninformative priors on [Mg/H] and the IGM neutral fraction |$x_{\rm {H\, {\small I}}}$|⁠. Splitting the data into low-z (⁠|$5.96 < z_{\rm Mg\, {\small II}} < 6.47$|⁠; |$z_{\rm Mg\, {\small II},median} = 6.235$|⁠) and high-z (⁠|$6.47 < z_{\rm Mg\, {\small II}} < 7.42$|⁠; |$z_{\rm Mg\, {\small II},median} = 6.72$|⁠) subsamples again yields null-detections and 95% upper limits of [Mg/H] < −3.75 and [Mg/H] < −3.45, respectively. These first measurements set the stage for making the Mg II forest an emerging tool to precisely constrain the Universe reionization and enrichment history.

The Lyα forest (LAF) at z > 5 probes the thermal and reionization history of the intergalactic medium (IGM) and the nature of dark matter, but its interpretation requires comparison to cosmological hydrodynamical simulations. At high-z, convergence of these simulations is more exacting since transmission is dominated by underdense voids that are challenging to resolve. With evidence mounting for a late end to reionization, small structures down to the sub-kpc level may survive to later times than conventionally thought due to the reduced time for pressure smoothing to impact the gas, further tightening simulation resolution requirements. We perform a suite of simulations using the Eulerian cosmological hydrodynamics code Nyx, spanning domain sizes of 1.25 − 10 h⁻¹ Mpc and 5 − 80 h⁻¹ kpc cells, and explore the interaction of these variables with the timing of reionization on the properties of the matter distribution and the simulated LAF at z = 5.5. In observable Lyα power, convergence within 10% is achieved for k < 0.1 s/km, but larger k shows deviation of up to 20 per cent. While a later reionization retains more small structure in the density field, because of the greater thermal broadening there is little difference in the convergence of LAF power between early (z = 9) and later (z = 6) reionizations. We conclude that at z ∼ 5.5, resolutions of 10 kpc are necessary for convergence of LAF power at k < 0.1 s/km, while higher-k modes require higher resolution, and that the timing of reionization does not significantly impact convergence given realistic photoheating.

The Lyman-Werner (LW) radiation field is a key ingredient in the chemo-thermal evolution of gas in the Early Universe, as it dissociates H₂ molecules, the primary cooling channel in an environment devoid of metals and dust. Despite its important role, it is still not implemented in cosmological simulations on a regular basis, in contrast to the ionising UV background. This is in part due to uncertainty in the source modelling, their spectra and abundance, as well as the detailed physics involved in the propagation of the photons and their interactions with the molecules. The goal of this work is to produce an accurate model of the LW radiation field at z ≥ 6, by post-processing the physics-rich high-resolution FiBY simulation. Our novelties include updated cross sections for H₂, |$\rm {H^-}$| and |$\rm {H^+_2}$| chemical species, IGM absorption by neutral Hydrogen and various spectral models for Population III and Population II stars. With our fiducial set of parameters, we show that the mean LW intensity steadily increases by three orders of magnitude from z ∼ 23 to z ∼ 6, while spatial inhomogeneities originate from massive star-forming galaxies that dominate the photon budget up to a distance of ∼100 proper kpc. Our model can be easily applied to other simulations or semi-analytical models as an external radiation field that regulates the formation of stars and massive black hole seeds in high-z low-mass halos.