Connecting the First Galaxies with Ultrafaint Dwarfs in the Local Group: Chemical Signatures of Population III Stars

Abstract

We investigate the star formation history (SFH) and chemical evolution of isolated analogs of Local Group (LG) ultrafaint dwarf galaxies (UFDs; stellar mass range of) and gas-rich, low-mass dwarfs (Leo P analogs; stellar mass range of). We perform a suite of cosmological hydrodynamic zoom-in simulations to follow their evolution from the era of the first generation of stars down to z=0. We confirm that reionization, combined with supernova (SN) feedback, is primarily responsible for the truncated star formation in UFDs. Specifically, halos with a virial mass of ´ M M 2 10 vir 9form 90% of stars prior to reionization. Our work further demonstrates the importance of PopulationIII stars, with their intrinsically high [ ] C Fe yields and the associated external metal enrichment, in producing low-metallicity stars (  -[ ] Fe H 4) and carbon-enhanced metal-poor (CEMP) stars. We find that UFDs are composite systems, assembled from multiple progenitor halos, some of which hosted only PopulationII stars formed in environments externally enriched by SNe in neighboring halos, naturally producing extremely low metallicity Population II stars. We illustrate how the simulated chemical enrichment may be used to constrain the SFHs of true observed UFDs. We find that Leo P analogs can form in halos with~´ M M 4 10 vir 9(z = 0). Such systems are less affected by reionization and continue to form stars until z=0, causing higher-metallicity tails. Finally, we predict the existence of extremely low metallicity stars in LG UFD galaxies that preserve the pure chemical signatures of Population III nucleosynthesis.

The Orbital Histories of Magellanic Satellites Using Gaia DR2 Proper Motions

“…MW2 Mvir [10 10 M ] 100 150 Rvir [kpc] 261 299 cvir 9.86 9.56 M d [10 10 M ] 6.5 5.5 half-light radii, and stellar velocity disperions for UFD galaxies in our sample. Given their low stellar masses, UFDs are also modeled as Plummer spheres but with a total halo mass of 10 9 M (Jeon et al 2017). Applying the Walker et al (2009b) mass estimator to the measured velocity dis- persion and half light radii of Aqu2, Ret2, and Hor1, and searching for the radius at which the mass enclosed within a Plummer profile is equivalent to the dynamical mass yields scale radii of 1.2, 0.8, and 0.6 kpc for these satellites respectively.…”

Efficiency of Metal Mixing in Dwarf Galaxies

“…According to cosmological hydrodynamic simulations of Jeon et al (2017), lowest metallicity stars are formed in external enriched halos accreted later in the main halo. If we consider a fully cosmological structure formation, lower metallicity stars formed by external enrichment would be seen such as in Figure 2a The diffusion coefficient inferred from the Ba abundance is consistent with the theoretical and experimental value.…”

Chemical Abundance Analysis of Three α-poor, Metal-poor Stars in the Ultrafaint Dwarf Galaxy Horologium I*

“…Unfortunately, the moderate-resolution spectroscopy used by Vargas et al (2013) does not permit detailed abundance analysis of many elements. It should be noted, however, that UMa I may not fit the canonical picture of stellar populations (Jeon et al 2017).…”

“…Star formation in these low-mass objects is likely to be highly influenced by only a few nucleosynthetic events (e.g., Ji et al 2015). And since star formation in ultrafaint dwarfs appears to have been quenched early in the history of the universe, perhaps by reionization (Brown et al 2014;Wetzel et al 2015;Jeon et al 2017), a fossil record of the early star formation history of these objects is preserved today.…”

“…This duration is consistent with an early but extended star formation history that would conform to the standard process of chemical evolution in a stellar population. Furthermore, the picture that has emerged to describe star formation in ultrafaint dwarf galaxies is that star formation began quickly, in some cases in a single burst of star formation (Frebel & Bromm 2012), and was soon quenched, possibly by reionization (e.g., Brown et al 2014;Wetzel et al 2015;Jeon et al 2017), leaving the stars in the ultrafaint dwarfs as a fossil record of conditions in the early universe. If we presume a similar chemical evolution timeline for Hor I and use [Fe/H] as an age indicator, our measurements imply that the onset of SNe Ia and the subsequent chemical enrichment of the surrounding gas would have had to occur relatively earlier in Hor I than in other ultrafaint dwarfs.…”