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ABSTRACT We explore the origin of stellar metallicity gradients in simulated and observed dwarf galaxies. We use FIRE-2 cosmological baryonic zoom-in simulations of 26 isolated galaxies as well as existing observational data for 10 Local Group dwarf galaxies. Our simulated galaxies have stellar masses between 105.5 and 108.6 M⊙. Whilst gas-phase metallicty gradients are generally weak in our simulated galaxies, we find that stellar metallicity gradients are common, with central regions tending to be more metal-rich than the outer parts. The strength of the gradient is correlated with galaxy-wide median stellar age, such that galaxies with younger stellar populations have flatter gradients. Stellar metallicty gradients are set by two competing processes: (1) the steady ‘puffing’ of old, metal-poor stars by feedback-driven potential fluctuations and (2) the accretion of extended, metal-rich gas at late times, which fuels late-time metal-rich star formation. If recent star formation dominates, then extended, metal-rich star formation washes out pre-existing gradients from the ‘puffing’ process. We use published results from ten Local Group dwarf galaxies to show that a similar relationship between age and stellar metallicity-gradient strength exists among real dwarfs. This suggests that observed stellar metallicity gradients may be driven largely by the baryon/feedback cycle rather than by external environmental effects. 

ABSTRACT A standard prediction of galaxy formation theory is that the ionizing background suppresses galaxy formation in haloes with peak circular velocities smaller than $$V_{\rm peak}\simeq 20 \, \rm km \, s^{-1}$$, rendering the majority of haloes below this scale completely dark. We use a suite of cosmological zoom simulations of Milky Way-like haloes that include central Milky Way disc galaxy potentials to investigate the relationship between subhaloes and ultrafaint galaxies. We find that there are far too few subhaloes within 50 kpc of the Milky Way that had $$V_{\rm peak}\gtrsim 20\, \rm km \, s^{-1}$$ to account for the number of ultrafaint galaxies already known within that volume today. In order to match the observed count, we must populate subhaloes down to $$V_{\rm peak}\simeq 6\, \rm km \, s^{-1}$$ with ultrafaint dwarfs. The required haloes have peak virial temperatures as low as 1500 K, well below the atomic hydrogen cooling limit of 104 K. Allowing for the possibility that the Large Magellanic Cloud contributes several of the satellites within 50 kpc could potentially raise this threshold to $$10\, \rm km \, s^{-1}$$ (4000 K), still below the atomic cooling limit and far below the nominal reionization threshold. 

ABSTRACT We study stellar-halo formation using six Milky-Way-mass galaxies in FIRE-2 cosmological zoom simulations. We find that $$5{-}40{{\ \rm per\ cent}}$$ of the outer (50–300 kpc) stellar halo in each system consists of in-situ stars that were born in outflows from the main galaxy. Outflow stars originate from gas accelerated by superbubble winds, which can be compressed, cool, and form co-moving stars. The majority of these stars remain bound to the halo and fall back with orbital properties similar to the rest of the stellar halo at z = 0. In the outer halo, outflow stars are more spatially homogeneous, metal-rich, and alpha-element-enhanced than the accreted stellar halo. At the solar location, up to $$\sim \!10 {{\ \rm per\ cent}}$$ of our kinematically identified halo stars were born in outflows; the fraction rises to as high as $$\sim \!40{{\ \rm per\ cent}}$$ for the most metal-rich local halo stars ([Fe/H] >−0.5). Such stars can be retrograde and create features similar to the recently discovered Milky Way ‘Splash’ in phase space. We conclude that the Milky Way stellar halo could contain local counterparts to stars that are observed to form in molecular outflows in distant galaxies. Searches for such a population may provide a new, near-field approach to constraining feedback and outflow physics. A stellar halo contribution from outflows is a phase-reversal of the classic halo formation scenario of Eggen, Lynden-Bell & Sandange, who suggested that halo stars formed in rapidly infalling gas clouds. Stellar outflows may be observable in direct imaging of external galaxies and could provide a source for metal-rich, extreme-velocity stars in the Milky Way. 

ABSTRACT We explore the radial variation of star formation histories (SFHs) in dwarf galaxies simulated with Feedback In Realistic Environments (FIRE) physics. The sample contains 26 field dwarf galaxies with Mstar = 105–109 M⊙. We find age gradients are common in our dwarfs, with older stars dominant at large radii. The strength of the gradient correlates with overall galaxy age such that earlier star formation produces a more pronounced gradient. The relation between formation time and strength of the gradient is driven by both mergers and star formation feedback. Mergers can both steepen and flatten the age gradient depending on the timing of the merger and SFHs of the merging galaxy. In galaxies without significant mergers, feedback pushes stars to the outskirts. The strength of the age gradient is determined by the subsequent evolution of the galaxy. Galaxies with weak age gradients constantly grow to z  = 0, meaning that young star formation occurs at a similar radius to which older stars are heated to. In contrast, galaxies with strong age gradients tend to maintain a constant half-mass radius over time. If real galaxies have age gradients as we predict, stellar population studies that rely on sampling a limited fraction of a galaxy can give a biased view of its global SFH. Central fields can be biased young by Gyrs while outer fields are biased old. Fields positioned near the 2D half-light radius will provide the least biased measure of a dwarf galaxy’s global SFH.