How much metal did the first stars provide to the ultra-faint dwarfs?

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 140 M ⊙ , 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 140 M ⊙ 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.

Metallicity Distribution Functions of 13 Ultra-faint Dwarf Galaxy Candidates from Hubble Space Telescope Narrowband Imaging

“…In the right panel of Figure 12, we compare the dwarf galaxy data against simulated UFDs from the independent simulations of Jeon et al (2017), Wheeler et al (2019), Agertz et al (2020), andSanati et al (2023). While simulations have begun to reproduce the mean metallicities of UFDs more luminous than 10 4 L e , fainter UFDs still remain a theoretical challenge to simulate.…”

“…In the case of FIRE-2 simulations, which implement strong feedback, many of the lowest-luminosity UFDs never enrich beyond the initialized metallicity floor (Wheeler et al 2019), as shown by the lowerluminosity FIRE UFDs in Figure 12 at [Fe/H]= −4.0. So far, there have been numerical experiments varying parameters such as star formation, radiative transfer, SNe energies (Agertz et al 2020;Sanati et al 2023), metallicity-dependence of Type Ia SNe DTDs (Gandhi et al 2022), and top heaviness and stochastic sampling of the IMF (Applebaum et al 2020;Prgomet et al 2022 respectively), in order to explore the parameter space of physics that elevate enrichment levels in UFDs. Overall, this area of study is still ongoing.…”

Dwarf Galaxy Formation with and without Dark Matter–Baryon Streaming Velocities

“…In the future, we plan on including the formation of Pop III stars in the simulation, as carried out in a recent study by Sanati et al (2023) with the smoothed-particle hydrodynamics(SPH) code GEAR. It will be interesting to see if this result is influenced by pure Pop III star formation since Pop III stars are expected to survive in pristine clumps down to redshift z = 6 (Liu & Bromm 2020).…”

The Impact of Patchy Reionization on Ultrafaint Dwarf Galaxies

“…Ultrafaint dwarf galaxies (UFDs), which are known as the smallest unit of galaxies with the lowest luminosity (L < 10 5 L ☉ ) and stellar mass (10 2 M e < M * < 10 5 M e ) in the Universe, have been considered as the fundamental building blocks of massive galaxies (reviewed in Simon 2019; see also Tolstoy et al 2009;McConnachie 2012). Additionally, due to their shallow potential wells, UFD systems are highly vulnerable to internal and external feedback effects, making them an excellent laboratory for studying how feedback mechanisms change star formation activities in small systems (e.g., Stinson et al 2007;Sawala et al 2010;Simpson et al 2013;Agertz & Kravtsov 2015;Jeon et al 2017;Wheeler et al 2019;Agertz et al 2020;Rey et al 2020;Gutcke et al 2022;Sanati et al 2023).…”