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Abstract:

We present results of Population III (Pop III) formation in the megatron suite of simulations, which self-consistently follows radiation and non-equilibrium chemistry, and resolves gas at near-pc resolution in a Milky Way-mass progenitor at Cosmic Dawn. While the very first Pop III stars form in haloes with masses well below the atomic cooling limit, the majority of Pop III stars form in more massive systems above the K atomic cooling threshold as a Lyman–Werner (LW) background of is rapidly established. We find that the global Pop III star formation rate stabilizes to a value of at . Among the three processes that quench Pop III star formation in minihaloes, the LW background, gas starvation, and external chemical enrichment, the LW background is most important. A small fraction of haloes undergo multiple episodes of Pop III star formation when the earlier forming stars all directly collapse to black holes. If the haloes become massive enough, they can form up to Pop III stars in a single burst, which may be observable by James Webb Space Telescope with moderate gravitational lensing. Pop III stars form at a wide range of distances from UV-bright galaxies, with only per cent of Pop III stars forming within the virial radius of galaxies with . Finally, by tracking Pop III star remnants down to , we find that per cent reside in the stellar halo of our simulated Milky Way analogue, while the remainder are gravitationally bound to lower mass systems, including satellite haloes.

Abstract:

Abstract We explore how the fraction of red (quenched) galaxies varies in the dwarf galaxy regime (107 M⊙ < M⋆ < 109.5 M⊙), using a mass-complete sample of ∼5900 dwarfs at z < 0.15, constructed using deep multi-wavelength data in the COSMOS field. The red fraction decreases steadily until M⋆ ∼ 108.5 M⊙ and then increases again towards lower stellar masses. This ‘U’ shape demonstrates that the traditional notion of ‘downsizing’ (i.e. that progressively lower mass galaxies maintain star formation until later epochs) is incorrect – downsizing does not continue uninterrupted into the dwarf regime. The U shape persists regardless of environment, indicating that it is driven by internal processes rather than external environment-driven mechanisms. Our results suggest that, at M⋆ ≲ 108 M⊙, the quenching of star formation is dominated by supernova (SN) feedback and becomes more effective with decreasing stellar mass, as the potential well becomes shallower. At M⋆ ≳ 109 M⊙, the quenching is driven by a mix of SN feedback and AGN feedback (which becomes more effective with increasing stellar mass, as central black holes become more massive). The processes that quench star formation are least effective in the range 108 M⊙ < M⋆ < 109 M⊙, likely because the potential well is deep enough to weaken the impact of SN feedback, while the effect of AGN feedback is still insignificant. The cosmological simulations tested here do not match the details of how the red fraction varies as a function of stellar mass – we propose that the red fraction vs stellar mass relation (particularly in the dwarf regime) is a powerful calibrator for the processes that regulate star formation in galaxy formation models.

Abstract:

Jellyfish galaxies provide direct evidence of ram pressure stripping in cluster environments. We investigate the role of magnetic fields in the formation of jellyfish galaxies with a multiphase interstellar medium (ISM) using radiation magnetohydrodynamic simulations. We impose magnetized (magnetohydrodynamic; MHD) and nonmagnetized (hydrodynamic; HD) winds on the gas-rich dwarf galaxies containing the magnetized or nonmagnetized ISM. The MHD winds strip the disk gas more effectively than the HD winds because of the magnetic force acting against the local density gradient, which results in remarkably different ram pressure stripped features. The magnetic fields induced by the MHD winds generate a strong magnetic pressure, which forms smoothed disks and tail gas features. Since the stripped ISM in MHD wind cases travels while being nearly isolated from the intracluster medium (ICM), the stripped ISM mostly forms stars within 20 kpc of the galactic disks. In contrast, nonmagnetized winds facilitate the efficient mixing of the stripped ISM with the ICM, resulting in the formation of abundant warm clouds that cool and collapse in the distant (∼50–100 kpc) tails at times of a few hundred Myr. Consequently, distant tail star formation occurs only in the HD wind runs. Finally, despite the different tail features, the star formation rates in the disk remain similar owing to the interplay between the increased gas stripping and the gas density increase in the disks of the MHD wind runs. These results suggest that the magnetized ICM may have a significant influence on jellyfish galaxies, whereas the magnetized ISM play a minor role.