Search

The Feedback In Realistic Environments (FIRE) project explores feedback in cosmological galaxy formation simulations. Previous FIRE simulations used an identical source code ('FIRE-1') for consistency. Motivated by the development of more accurate numerics - including hydrodynamic solvers, gravitational softening, and supernova coupling algorithms - and exploration of new physics (e.g. magnetic fields), we introduce 'FIRE-2', an updated numerical implementation of FIRE physics for the GIZMO code. We run a suite of simulations and compare against FIRE-1: overall, FIRE-2 improvements do not qualitatively change galaxy-scale properties. We pursue an extensive study of numerics versus physics. Details of the star formation algorithm, cooling physics, and chemistry have weak effects provided that we include metal-line cooling and star formation occurs at higher-than-mean densities. We present new resolution criteria for high-resolution galaxy simulations. Most galaxy-scale properties are robust to numerics we test, provided: (1) Toomre masses are resolved; (2) feedback coupling ensures conservation, and (3) individual supernovae are time-resolved. Stellar masses and profiles are most robust to resolution, followed by metal abundances and morphologies, followed by properties of winds and circum-galactic media. Central (~kpc) mass concentrations in massive (> L*) galaxies are sensitive to numerics (via trapping/recycling of winds in hot haloes). Multiple feedback mechanisms play key roles: supernovae regulate stellar masses/winds; stellar mass-loss fuels late star formation; radiative feedback suppresses accretion on to dwarfs and instantaneous star formation in discs. We provide all initial conditions and numerical algorithms used.

We investigate the response of self-interacting dark matter (SIDM) halos to the growth of galaxy potentials using idealized simulations, with each run in tandem with collisionless cold dark matter (CDM). We find that if the stellar potential strongly dominates in the central parts of a galaxy, then SIDM halos can be as dense as CDM halos on observable scales. For extreme cases, core collapse can occur, leading to SIDM halos that are denser and cuspier than their CDM counterparts. If the stellar potential is not dominant, then SIDM halos retain isothermal cores with densities far below CDM predictions. When a disk is present, the inner SIDM halo becomes more flattened in the disk plane than the CDM halo. These results are in excellent quantitative agreement with the predictions of Kaplinghat et al. We also simulated a cluster halo with a central stellar distribution similar to the brightest central galaxy of the cluster A2667. An SIDM halo simulated with the cross-section over mass provides a good match to the measured dark matter (DM) density profile, while an adiabatically contracted CDM halo is denser and cuspier. The profile of the same halo simulated with is not dense enough. Our findings are in agreement with previous results that is disfavored for DM collision velocities above about 1500 km s-1. More generally, the interaction between baryonic potentials and SIDM densities offers new directions for constraining SIDM cross-sections in galaxies where baryons are dynamically important.

We present an empirical approach for interpreting gravitational wave signals of binary black hole mergers under the assumption that the underlying black hole population is sourced by remnants of stellar evolution. Using the observed relationship between galaxy mass and stellar metallicity, we predict the black hole count as a function of galaxy stellar mass. We show, for example, that a galaxy like the Milky Way should host millions of ~30M⊙ black holes and dwarf satellite galaxies like Draco should host ~100 such remnants, with weak dependence on the assumed initial mass function and stellar evolution model. Most low-mass black holes (~10M⊙) typically reside within massive galaxies (M* ≃ 1011M⊙) while massive black holes (~50M⊙) typically reside within dwarf galaxies (M* ≃ 109M⊙) today. If roughly 1 per cent of black holes are involved in a binary black hole merger, then the reported merger rate densities from advanced Laser Interferometer Gravitational-Wave Observatory can be accommodated for a range of merger time-scales, and the detection of mergers with > 50M⊙ black holes should be expected within the next decade. Identifying the host galaxy population of the mergers provides a way to constrain both the binary neutron star or black hole formation efficiencies and the merger time-scale distributions; these events would be primarily localized in dwarf galaxies if the merger time-scale is short compared to the age of the Universe and in massive galaxies otherwise. As more mergers are detected, the prospect of identifying the host galaxy population, either directly through the detection of electromagnetic counterparts of binary neutron star mergers or indirectly through the anisotropy of the events, will become a realistic possibility.

We present a suite of 15 cosmological zoom-in simulations of isolated dark matter haloes, all with masses of Mhalo ≈ 1010M⊙ at z = 0, in order to understand the relationship among halo assembly, galaxy formation and feedback's effects on the central density structure in dwarf galaxies. These simulations are part of the Feedback in Realistic Environments (FIRE) project and are performed at extremely high resolution (mbaryon = 500M⊙, mdm = 2500M⊙). The resultant galaxies have stellar masses that are consistent with rough abundance matching estimates, coinciding with the faintest galaxies that can be seen beyond the virial radius of the Milky Way (M*/M⊙ ≈ 105 - 107). This non-negligible spread in stellar mass at z = 0 in haloes within a narrow range of virial masses is strongly correlated with central halo density or maximum circular velocity Vmax, both of which are tightly linked to halo formation time. Much of this dependence of M* on a second parameter (beyond Mhalo) is a direct consequence of the Mhalo ~ 1010M⊙ mass scale coinciding with the threshold for strong reionization suppression: the densest, earliest-forming haloes remain above the UV-suppression scale throughout their histories while late-forming systems fall below the UV-suppression scale over longer periods and form fewer stars as a result. In fact, the latest-forming, lowest-concentration halo in our suite fails to form any stars. Haloes that form galaxies with M ≳ 2 × 106 M⊙ have reduced central densities relative to dark-matter-only simulations, and the radial extent of the density modifications is well-approximated by the galaxy half-mass radius r1/2. Lower-mass galaxies do not modify their host dark matter haloes at the mass scale studied here. This apparent stellar mass threshold of M ≈ 2 × 106-2 × 10-4 Mhalo is broadly consistent with previous work and provides a testable prediction of FIRE feedback models in Λcold dark matter.

We present Feedback in Realistic Environment (FIRE)/GIZMO hydrodynamic zoom-in simulations of isolated dark matter haloes, two each at the mass of classical dwarf galaxies (Mvir ≃ 1010 M o˙) and ultra-faint galaxies (Mvir ≃ 109 M o˙), and with two feedback implementations. The resulting central galaxies lie on an extrapolated abundance matching relation from M* ≃ 106 to 104 M o˙ without a break. Every host is filled with subhaloes, many of which form stars. Each of our dwarfs with M* ≃ 106 M o˙ has 1-2 well-resolved satellites with M* = 3-200 × 103 M o˙. Even our isolated ultra-faint galaxies have star-forming subhaloes. If this is representative, dwarf galaxies throughout the Universe should commonly host tiny satellite galaxies of their own. We combine our results with the Exploring the Local Volume in Simulations (ELVIS) simulations to show that targeting ~50 kpc regions around nearby isolated dwarfs could increase the chances of discovering ultra-faint galaxies by ~35 per cent compared to random pointings, and specifically identify the region around the Phoenix dwarf galaxy as a good potential target. The well-resolved ultra-faint galaxies in our simulations (M* ≃ 3-30 × 103 M o˙) form within Mpeak ≃ 0.5-3 × 109 M o˙ haloes. Each has a uniformly ancient stellar population (>10 Gyr) owing to reionization-related quenching. More massive systems, in contrast, all have late-time star formation. Our results suggest that Mhalo ≃ 5 × 109 M o˙ is a probable dividing line between haloes hosting reionization 'fossils' and those hosting dwarfs that can continue to form stars in isolation after reionization.

We investigate the effect of self-interacting dark matter (SIDM) on the density profiles of Vmax ≃ 40km s-1 isolated dwarf dark matter haloes-the scale of relevance for the too big to fail problem (TBTF)-using very high resolution cosmological zoom simulations. Each halo has millions of particles within its virial radius. We find that SIDM models with cross-sections per unitmass spanning the range s/m=0.5-50 cm2 g-1 alleviate TBTF and produce constantdensity cores of size 300-1000 pc, comparable to the half-light radii of M* ~ 105-7M⊙ dwarfs. The largest, lowest density cores develop for cross-sections in the middle of this range, σ/m ~ 5-10 cm2 g-1. Our largest SIDM cross-section run (σ/m = 50 cm2 g-1) develops a slightly denser core owing to mild core-collapse behaviour, but it remains less dense than the cold dark matter case and retains a constant-density core profile. Our work suggests that SIDM cross-sections as large or larger than 50 cm2 g-1 remain viable on velocity scales of dwarf galaxies (vrms ~ 40 km s-1). The range of SIDM cross-sections that alleviate TBTF and the cusp/core problem spans at least two orders of magnitude and therefore need not be particularly fine-tuned.

We compare a suite of four simulated dwarf galaxies formed in 10 10 M ☉ haloes of collisionless cold dark matter (CDM) with galaxies simulated in the same haloes with an identical galaxy formation model but a non-zero cross-section for DM self-interactions. These cosmological zoom-in simulations are part of the Feedback In Realistic Environments (fire) project and utilize the fire-2 model for hydrodynamics and galaxy formation physics. We find the stellar masses of the galaxies formed in self-interacting dark matter (SIDM) with σ/m = 1 cm 2 g -1 are very similar to those in CDM (spanning M ★ ≈ 10 5.7-7.0 M ☉ ) and all runs lie on a similar stellar mass-size relation. The logarithmic DM density slope ( α = d log ρ/d log r ) in the central 250-500 pc remains steeper than α = -0.8 for the CDM-Hydro simulations with stellar mass M ★ ~ 10 6.6 M ☉ and core-like in the most massive galaxy. In contrast, every SIDM hydrodynamic simulation yields a flatter profile, with α > -0.4. Moreover, the central density profiles predicted in SIDM runs without baryons are similar to the SIDM runs that include fire-2 baryonic physics. Thus, SIDM appears to be much more robust to the inclusion of (potentially uncertain) baryonic physics than CDM on this mass scale, suggesting that SIDM will be easier to falsify than CDM using low-mass galaxies. Our fire simulations predict that galaxies less massive than M ★ ≲ 3 × 10 6 M ☉ provide potentially ideal targets for discriminating models, with SIDM producing substantial cores in such tiny galaxies and CDM producing cusps.

We use a suite of high-resolution cosmological dwarf galaxy simulations to test the accuracy of commonly used mass estimators from Walker et al. (2009) and Wolf et al. (2010), both of which depend on the observed line-of-sight velocity dispersion and the 2D half-light radius of the galaxy, Re . The simulations are part of the Feedback in Realistic Environments (fire) project and include 12 systems with stellar masses spanning 10 5 –10 7 M⊙ that have structural and kinematic properties similar to those of observed dispersion-supported dwarfs. Both estimators are found to be quite accurate: M Wolf ∕ M true = 0.98 − 0.12 + 0.19 and M Walker ∕ M true = 1.07 − 0.15 + 0.21 , with errors reflecting the 68 per cent range over all simulations. The excellent performance of these estimators is remarkable given that they each assume spherical symmetry, a supposition that is broken in our simulated galaxies. Though our dwarfs have negligible rotation support, their 3D stellar distributions are flattened, with short-to-long axis ratios c / a ≃ 0.4–0.7. The median accuracy of the estimators shows no trend with asphericity. Our simulated galaxies have sphericalized stellar profiles in 3D that follow a nearly universal form, one that transitions from a core at small radius to a steep fall-off ∝ r −42 at large r ; they are well fit by Sérsic profiles in projection. We find that the most important empirical quantity affecting mass estimator accuracy is Re . Determining Re by an analytic fit to the surface density profile produces a better estimated mass than if the half-light radius is determined via direct summation.