Mahsa Sanati

Abstract:

Recent observations by James Webb Space Telescope reveal an unexpectedly abundant population of rapidly growing supermassive black holes (SMBHs) in the early Universe, underscoring the need for improved models for their origin and growth. Employing new full radiative transfer hydrodynamical simulations of galaxy formation, we investigate the local and intergalactic feedback of SMBH progenitors for the Population III.1 (Pop III.1) scenario, i.e. efficient formation of supermassive stars from pristine, undisturbed dark matter minihaloes. Our cosmological simulations capture the R-type expansion phase of these Pop III.1 stars, with their H-ionizing photon luminosities of generating H ii regions that extend deep into the intergalactic medium, reaching comoving radii of . We vary both the Pop III.1 ionization flux and cosmological formation environments, finding the former regulates their final , whereas the latter is more important in setting their formation redshift. We use the results from our radiation-hydrodynamics simulations to estimate the cosmic number density of SMBHs, , expected from Pop III.1 progenitors. We find , consistent with the results inferred from recent observations of the local and high-redshift universe. Overall, this establishes Pop III.1 progenitors as viable candidates for the formation of the first SMBH, and emphasizes the importance of exploring heavy mass seed scenarios.

Abstract:

Aims: The true nature of primordial magnetic fields (PMFs) and their role in the formation of galaxies still remains elusive. To shed light on these unknowns, we investigate their impact by varying two sets of properties: (i) accounting for the effect of PMFs on the initial matter power spectrum, and (ii) accounting for their magneto-hydrodynamical effects on the formation of galaxies. By comparing both we can determine the dominant agent in shaping galaxy evolution.

Methods: We use the magneto-hydrodynamics code RAMSES, to generate multiple new zoom-in simulations for eight different host halos of dwarf galaxies across a wide luminosity range of 103 − 106 L⊙. These halos are selected from a ΛCDM cosmological box, tracking their evolution down to redshift z = 0. We explore a variety of primordial magnetic field (comoving) strengths Bλ ranging from 0.05 to 0.50 nG.

Results: We find magnetic fields in the interstellar medium not only modify star formation in dwarf spheroidal galaxies but also completely prevent the formation of stars in less compact ultra-faints with halo mass and stellar mass below ∼ 2.5 · 109 and 3 · 106 M⊙, respectively. At high redshifts, the impact of PMFs on host halos of dwarf galaxies through the modification of the matter power spectrum is more dominant than the influence of magneto-hydrodynamics in shaping their gaseous structure. Through the amplification of small perturbations ranging in mass from 107 to 109 M⊙ in the ΛCDM+PMFs matter power spectrum, primordial fields expedite the formation of the first dark matter halos, leading to an earlier onset and a higher star formation rate at redshifts z > 12. We investigate the evolution of various energy components and demonstrate that magnetic fields with an initial strength of Bλ ≥ 0.05 nG exhibit a strong growth of magnetic energy, accompanied by a saturation phase, that starts quickly after the growth phase. These trends persist consistently, regardless of the initial conditions, whether it is the classical ΛCDM or modified by PMFs. Lastly, we investigate the impact of PMFs on the present-time observable properties of dwarf galaxies, namely, the half light radius, V-band luminosity, mean metallicity and velocity dispersion profile. We find that PMFs with moderate strengths of Bλ ≤ 0.10 nG show great agreement with the scaling relations of the observed Local group dwarfs. However, stronger fields lead to large sizes and high velocity dispersion.

Abstract:

Numerical simulations of dwarf galaxies have so far failed to reproduce the observed metallicity-luminosity relation, down to the regime of ultra-faint dwarfs (UFDs). We address this issue by exploring how the first generations of metal-free stars (Pop III) could help increase the mean metallicity ([Fe/H]) of those small and faint galaxies. We ran zoom-in chemo-dynamical simulations of 19 halos extracted from a λ Cold Dark Matter (CDM) cosmological box and followed their evolution down to redshift z = 0. Models were validated not only on the basis of galaxy global properties, but also on the detailed investigation of the stellar abundance ratios ([α/Fe]). We identified the necessary conditions for the formation of the first stars in mini-halos and derived constraints on the metal ejection schemes. The impact of Pop III stars on the final metallicity of UFDs was evaluated by considering different stellar mass ranges for their initial mass function (IMF), the influence of pair-instability supernovae (PISNe), and their energetic feedback, as well as the metallicity threshold that marks the transition from the first massive stars to the formation of low-mass long-lived stars. The inclusion of Pop III stars with masses below 140M⊙, and a standard IMF slope of -1.3 does increase the global metallicity of UFDs, although these are insufficient to resolve the tension with observations. The PISNe with progenitor masses above 140M⊙ do allow the metal content of UFDs to further increase. However, as PISNe are very rare and sometimes absent in the faintest UFDs, they have a limited impact on the global faint end of the metallicity-luminosity relation. Despite a limited number of spectroscopically confirmed members in UFDs, which make the stellar metallicity distribution of some UFDs uncertain, our analysis reveals that this is essentially the metal-rich tail that is missing in the models. The remaining challenges are thus both observational and numerical: (i) to extend high-resolution spectroscopy data samples and confirm the mean metallicity of the faintest UFDs; and (ii) to explain the presence of chemically enriched stars in galaxies with very short star formation histories.