We study how an observationally-motivated, metallicity-dependent initial mass function (IMF) affects the feedback budget and observables of an ultra-faint dwarf galaxy. We model the evolution of a low-mass ($approx 8 , times , 10^{8} , rm M_{odot}$)
dark matter halo with cosmological, zoomed hydrodynamical simulations capable of resolving individual supernovae explosions. We complement the EDGE galaxy formation model from Agertz et al. (2020) with a new prescription for IMF variations according to Geha et al. (2013). At the low metallicities typical of faint dwarf galaxies, the IMF becomes top-heavy, increasing the efficiency of supernova and photo-ionization feedback in regulating star formation. This results in a 100-fold reduction of the final stellar mass of the dwarf compared to a canonical IMF, at fixed dynamical mass. The increase in the feedback budget is nonetheless met by increased metal production from more numerous massive stars, leading to nearly constant iron content at $z=0$. A metallicity-dependent IMF therefore provides a mechanism to produce low-mass ($rm M_{star}sim 10^3 rm M_{odot}$), yet enriched ($rm [Fe/H]approx -2$) field dwarf galaxies, thus opening a self-consistent avenue to populate the plateau in $rm [Fe/H]$ at the faintest end of the mass-metallicity relation.
We introduce and apply a new approach to probe the response of galactic stellar haloes to the interplay between cosmological merger histories and galaxy formation physics. We perform dark-matter-only, zoomed simulations of two Milky Way-mass hosts an
d make targeted, controlled changes to their cosmological histories using the genetic modification technique. Populating each historys stellar halo with a semi-empirical, particle-tagging approach then enables a controlled study, with all instances converging to the same large-scale structure, dynamical and stellar mass at $z=0$ as their reference. These related merger scenarios alone generate an extended spread in stellar halo mass fractions (1.5 dex) comparable to the observed population. Largest scatter is achieved by growing late ($zleq1$) major mergers that spread out existing stars to create massive, in-situ dominated stellar haloes. Increasing a last major merger at $zsim2$ brings more accreted stars into the inner regions, resulting in smaller scatter in the outskirts which are predominantly built by subsequent minor events. Exploiting the flexibility of our semi-empirical approach, we show that the diversity of stellar halo masses across scenarios is reduced by allowing shallower slopes in the stellar mass--halo mass relation for dwarf galaxies, while it remains conserved when central stars are born with hotter kinematics across cosmic time. The merger-dependent diversity of stellar haloes thus responds distinctly to assumptions in modelling the central and dwarf galaxies respectively, opening exciting prospects to constrain star formation and feedback at different galactic mass-scales with the coming generation of deep, photometric observatories.
In the standard Lambda cold dark matter paradigm, pure dark matter simulations predict dwarf galaxies should inhabit dark matter haloes with a centrally diverging density `cusp. This is in conflict with observations that typically favour a constant d
ensity `core. We investigate this `cusp-core problem in `ultra-faint dwarf galaxies simulated as part of the `Engineering Dwarfs at Galaxy formations Edge (EDGE) project. We find, similarly to previous work, that gravitational potential fluctuations within the central region of the simulated dwarfs kinematically heat the dark matter particles, lowering the dwarfs central dark matter density. However, these fluctuations are not exclusively caused by gas inflow/outflow, but also by impulsive heating from minor mergers. We use the genetic modification approach on one of our dwarfs initial conditions to show how a delayed assembly history leads to more late minor mergers and, correspondingly, more dark matter heating. This provides a mechanism by which even ultra-faint dwarfs ($M_* < 10^5,text{M}_{odot}$), in which star formation was fully quenched at high redshift, can have their central dark matter density lowered over time. In contrast, we find that late major mergers can regenerate a central dark matter cusp, if the merging galaxy had sufficiently little star formation. The combination of these effects leads us to predict significant stochasticity in the central dark matter density slopes of the smallest dwarfs, driven by their unique star formation and mass assembly histories.
We study how star formation is regulated in low-mass field dwarf galaxies ($10^5 leq M_{star} leq 10^6 , text{M}_{odot}$), using cosmological high-resolution ($3 , text{pc}$) hydrodynamical simulations. Cosmic reionization quenches star formation in
all our simulated dwarfs, but three galaxies with final dynamical masses of $3 times 10^{9} ,text{M}_{odot}$ are subsequently able to replenish their interstellar medium by slowly accreting gas. Two of these galaxies re-ignite and sustain star formation until the present day at an average rate of $10^{-5} , text{M}_{odot} , text{yr}^{-1}$, highly reminiscent of observed low-mass star-forming dwarf irregulars such as Leo T. The resumption of star formation is delayed by several billion years due to residual feedback from stellar winds and Type Ia supernovae; even at $z=0$, the third galaxy remains in a temporary equilibrium with a large gas content but without any ongoing star formation. Using the genetic modification approach, we create an alternative mass growth history for this gas-rich quiescent dwarf and show how a small $(0.2,mathrm{dex})$ increase in dynamical mass can overcome residual stellar feedback, re-igniting star formation. The interaction between feedback and mass build-up produces a diversity in the stellar ages and gas content of low-mass dwarfs, which will be probed by combining next-generation HI and imaging surveys.
We demonstrate how the least luminous galaxies in the Universe, ultra-faint dwarf galaxies, are sensitive to their dynamical mass at the time of cosmic reionization. We select a low-mass ($sim text{1.5} times 10^{9} , text{M}_{odot}$) dark matter hal
o from a cosmological volume, and perform zoom hydrodynamical simulations with multiple alternative histories using genetically modified initial conditions. Earlier forming ultra-faints have higher stellar mass today, due to a longer period of star formation before their quenching by reionization. Our histories all converge to the same final dynamical mass, demonstrating the existence of extended scatter ($geq$ 1 dex) in stellar masses at fixed halo mass due to the diversity of possible histories. One of our variants builds less than 2 % of its final dynamical mass before reionization, rapidly quenching in-situ star formation. The bulk of its final stellar mass is later grown by dry mergers, depositing stars in the galaxys outskirts and hence expanding its effective radius. This mechanism constitutes a new formation scenario for highly diffuse ($text{r}_{1 /2} sim 820 , text{pc}$, $sim 32 , text{mag arcsec}^2$), metal-poor ($big[ mathrm{Fe}, / mathrm{H} big]= -2.9$), ultra-faint ($mathcal{M}_V= -5.7$) dwarf galaxies within the reach of next-generation low surface brightness surveys.
We apply our recently proposed quadratic genetic modification approach to generating and testing the effects of alternative mass accretion histories for a single $Lambda$CDM halo. The goal of the technique is to construct different formation historie
s, varying the overall contribution of mergers to the fixed final mass. This enables targeted studies of galaxy and dark matter halo formations sensitivity to the smoothness of mass accretion. Here, we focus on two dark matter haloes, each with four different mass accretion histories. We find that the concentration of both haloes systematically decreases as their merger history becomes smoother. This causal trend tracks the known correlation between formation time and concentration parameters in the overall halo population. At fixed formation time, we further establish that halo concentrations are sensitive to the order in which mergers happen. This ability to study an individual halos response to variations in its history is highly complementary to traditional methods based on emergent correlations from an extended halo population.
