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We compute the expected X-ray diffuse background and radiative feedback onthe intergalactic medium (IGM) from X-ray binaries prior and during the epochof reionization. The cosmic evolution of compact binaries is followed using apopulation synthesis technique that treats separately neutron stars and blackhole binaries in different spectral states and is calibrated to reproduce theobserved X-ray properties of galaxies at z6. Radiative transfer effects modulate the background spectrum,which shows a characteristic peak between 1 and 2 keV. While the filtering ofX-ray radiation through the IGM slightly increases the mean excess energy perphotoionization, it also weakens the radiation intensity below 1 keV, loweringthe mean photoionization and heating rates. Numerical integration of the rateand energy equations shows that the contribution of X-ray binaries to theionization of the bulk IGM is negligible, with the electron fraction neverexceeding 1%. Direct HeI photoionizations are the main source of IGM heating,and the temperature of the largely neutral medium in between HII cavitiesincreases above the temperature of the cosmic microwave background (CMB) onlyat z0.1.Therefore, in this scenario, it is only at relatively late epochs that the bulkof neutral intergalactic hydrogen may be observable in 21-cm emission againstthe CMB.

Using an isolated Milky Way-mass galaxy simulation, we compare results from nine state-of-the-art gravito-hydrodynamics codes widely used in the numerical community. We utilize the infrastructure we have built for the AGORA High-resolution Galaxy Simulations Comparison Project. This includes the common disk initial conditions, common physics models (e.g., radiative cooling and UV background by the standardized package Grackle) and common analysis toolkit yt, all of which are publicly available. Subgrid physics models such as Jeans pressure floor, star formation, supernova feedback energy, and metal production are carefully constrained across code platforms. With numerical accuracy that resolves the disk scale height, we find that the codes overall agree well with one another in many dimensions including: gas and stellar surface densities, rotation curves, velocity dispersions, density and temperature distribution functions, disk vertical heights, stellar clumps, star formation rates, and Kennicutt-Schmidt relations. Quantities such as velocity dispersions are very robust (agreement within a few tens of percent at all radii) while measures like newly formed stellar clump mass functions show more significant variation (difference by up to a factor of ∼3). Systematic differences exist, for example, between mesh-based and particle-based codes in the low-density region, and between more diffusive and less diffusive schemes in the high-density tail of the density distribution. Yet intrinsic code differences are generally small compared to the variations in numerical implementations of the common subgrid physics such as supernova feedback. Our experiment reassures that, if adequately designed in accordance with our proposed common parameters, results of a modern high-resolution galaxy formation simulation are more sensitive to input physics than to intrinsic differences in numerical schemes.

We use a set of high-resolution N-body simulations of the Galactic disk tostudy its interactions with the population of satellites predictedcosmologically. One simulation illustrates that multiple passages of massivesatellites with different velocities through the disk generate a wobble, havingthe appearance of rings in face-on projections of the stellar disk. They alsoproduce flares in the disk outer parts and gradually heat the disk throughbending waves. A different numerical experiment shows that an individualsatellite as massive as the Sagittarius dwarf galaxy passing through the diskwill drive coupled horizontal and vertical oscillations of stars in underdenseregions, with small significant associated heating. This experiment shows thatvertical excursions of stars in these low-density regions can exceed 1 kpc inthe Solar neighborhood, resembling the coherent vertical oscillations recentlydetected locally. They can also induce non-zero vertical streaming motions aslarge as 10-20 km s$^{-1}$, consistent with recent observations in the Galacticdisk. This phenomenon appears as a local ring, with modest associated diskheating.