Category: Uncategorized

Robles et al., available on arXiv.

Abstract: We compare a suite of four simulated dwarf galaxies formed in 10^10 Msun 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 dark matter 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 sigma/m = 1 cm^2/g are very similar to those in CDM (spanning Mstar ~ 10^(5.7 – 7.0) Msun) and all runs lie on a similar stellar mass-size relation. The logarithmic dark matter density slope (alpha=d(log rho)/d(log r)) in the central 250-500 pc remains steeper than alpha=-0.8 for the CDM-Hydro simulations with stellar mass Mstar~10^6.6 Msun and core-like in the most massive galaxy. In contrast, every SIDM hydrodynamic simulation yields a flatter profile, with alpha>-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 SIDM will be easier to falsify than CDM using low-mass galaxies. Our FIRE simulations predict that galaxies less massive than Mstar < 3x10^6 Msun provide potentially ideal targets for discriminating models, with SIDM producing substantial cores in such tiny galaxies and CDM producing cusps.

Ma et al., available on arXiv.

Abstract: We present a suite of cosmological zoom-in simulations at z>5 from the Feedback In Realistic Environments project, spanning a halo mass range M_halo~10^8-10^12 M_sun at z=5. We predict the stellar mass-halo mass relation, stellar mass function, and luminosity function in several bands from z=5-12. The median stellar mass-halo mass relation does not evolve strongly at z=5-12. The faint-end slope of the luminosity function steepens with increasing redshift, as inherited from the halo mass function at these redshifts. Below z~6, the stellar mass function and ultraviolet (UV) luminosity function slightly flatten below M_star~10^4.5 M_sun (fainter than M_1500~-14), owing to the fact that star formation in low-mass halos is suppressed by the ionizing background by the end of reionization. Such flattening does not appear at higher redshifts. We provide redshift-dependent fitting functions for the SFR-M_halo, SFR-M_star, and broad-band magnitude-stellar mass relations. We derive the star formation rate density and stellar mass density at z=5-12 and show that the contribution from very faint galaxies becomes more important at z>8. Furthermore, we find that the decline in the z~6 UV luminosity function brighter than M_1500~-20 is largely due to dust attenuation. Approximately 37% (54%) of the UV luminosity from galaxies brighter than M_1500=-13 (-17) is obscured by dust at z~6. Our results broadly agree with current data and can be tested by future observations.

Gonzalez-Samaniego et al., available on arXiv.

Abstract: 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 the Feedback in Realistic Environments (FIRE) project and include twelve systems with stellar masses spanning 10^5-10^7 Msun 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^+0.19_-0.12 and M_Walker/M_true = 1.07^+0.21_-0.15, with errors reflecting the 68% 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 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^-4.2 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.

El Badry et al., available on arXiv.

Abstract: We study the z=0 gas kinematics, morphology, and angular momentum content of isolated galaxies in a suite of cosmological zoom-in simulations from the FIRE project spanning Mstar=10^(6-11) Msun. Gas becomes increasingly rotationally supported with increasing galaxy mass. In the lowest-mass galaxies (Mstar<10^8 Msun), gas fails to form a morphological disk and is primarily dispersion and pressure supported. At intermediate masses (Mstar=10^(8-10) Msun), galaxies display a wide range of gas kinematics and morphologies, from thin, rotating disks, to irregular spheroids with negligible net rotation. All the high-mass (Mstar=10^(10-11) Msun) galaxies form rotationally supported gas disks. Many of the halos whose galaxies fail to form disks harbor reservoirs of high angular momentum gas in their circumgalactic medium. The ratio of the specific angular momentum of gas in the central galaxy to that of the dark-matter halo increases significantly with galaxy mass, from j_gas/j_DM~0.1 at Mstar=10^(6-7) Msun to j_gas/j_DM~2 at Mstar=10^(10-11) Msun. The reduced rotational support in the lowest-mass galaxies owes to (a) stellar feedback and the UV background suppressing the accretion of high-angular momentum gas at late times, and (b) stellar feedback driving large non-circular gas motions. We broadly reproduce the observed scaling relations between galaxy mass, gas rotation velocity, size, and angular momentum, but may somewhat underpredict the incidence of disky, high-angular momentum galaxies at the lowest masses (Mstar=10^6-2x10^7 Msun). In our simulations, stars are uniformly less rotationally supported than gas. The common assumption that stars follow the same rotation curve as gas thus substantially overestimates galaxies' stellar angular momentum, particularly at low masses.

van de Voort et al., available on arXiv.

Abstract: We quantify the gas-phase abundance of deuterium in cosmological zoom-in simulations from the Feedback In Realistic Environments project. The cosmic deuterium fraction decreases with time, because mass lost from stars is deuterium-free. At low metallicity, our simulations confirm that the deuterium abundance is very close to the primordial value. The deuterium abundance decreases towards higher metallicity, with very small scatter between the deuterium and oxygen abundance. We compare our simulations to existing high-redshift observations in order to determine a primordial deuterium fraction of (2.549 +/- 0.033) x 10^-5 and stress that future observations at higher metallicity can also be used to constrain this value. At fixed metallicity, the deuterium fraction decreases slightly with decreasing redshift, due to the increased importance of mass loss from intermediate-mass stars. We find that the evolution of the average deuterium fraction in a galaxy correlates with its star formation history. Our simulations are consistent with observations of the Milky Way’s interstellar medium: the deuterium fraction at the solar circle is 83-92% of the primordial deuterium fraction. We use our simulations to make predictions for future observations. In particular, the deuterium abundance is lower at smaller galactocentric radii and in higher mass galaxies, showing that stellar mass loss is more important for fuelling star formation in these regimes (and can even dominate). Gas accreting onto galaxies has a deuterium fraction above that of the galaxies’ interstellar medium, but below the primordial fraction, because it is a mix of gas accreting from the intergalactic medium and gas previously ejected or stripped from galaxies.

Kim et al., available on arXiv.

Abstract: Using a state-of-the-art cosmological simulation of merging proto-galaxies at high redshift from the FIRE project, with explicit treatments of star formation and stellar feedback in the interstellar medium, we investigate the formation of star clusters and examine one of the formation hypothesis of present-day metal-poor globular clusters. We find that frequent mergers in high-redshift proto-galaxies could provide a fertile environment to produce long-lasting bound star clusters. The violent merger event disturbs the gravitational potential and pushes a large gas mass of ~> 1e5-6 Msun collectively to high density, at which point it rapidly turns into stars before stellar feedback can stop star formation. The high dynamic range of the reported simulation is critical in realizing such dense star-forming clouds with a small dynamical timescale, t_ff <~ 3 Myr, shorter than most stellar feedback timescales. Our simulation then allows us to trace how clusters could become virialized and tightly-bound to survive for up to ~420 Myr till the end of the simulation. Because the cluster's tightly-bound core was formed in one short burst, and the nearby older stars originally grouped with the cluster tend to be preferentially removed, at the end of the simulation the cluster has a small age spread.

Hopkins et al., available on arXiv.

Abstract: The Feedback In Realistic Environments (FIRE) project explores the role of feedback in cosmological simulations of galaxy formation. Previous FIRE simulations used an identical source code (FIRE-1) for consistency. Now, motivated by the development of more accurate numerics (hydrodynamic solvers, gravitational softening, supernova coupling) and the 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 show FIRE-2 improvements do not qualitatively change galaxy-scale properties relative to FIRE-1. We then pursue an extensive study of numerics versus physics in galaxy simulations. Details of the star-formation (SF) algorithm, cooling physics, and chemistry have weak effects, provided that we include metal-line cooling and SF occurs at higher-than-mean densities. We present several new resolution criteria for high-resolution galaxy simulations. Most galaxy-scale properties are remarkably robust to the numerics that we test, provided that: (1) Toomre masses (cold disk scale heights) are resolved; (2) feedback coupling ensures conservation and isotropy, and (3) individual supernovae are time-resolved. As resolution increases, stellar masses and profiles converge first, followed by metal abundances and visual morphologies, then properties of winds and the circumgalactic medium. The central (~kpc) mass concentration of massive (L*) galaxies is sensitive to numerics, particularly how winds ejected into hot halos are trapped, mixed, and recycled into the galaxy. Multiple feedback mechanisms are required to reproduce observations: SNe regulate stellar masses; OB/AGB mass loss fuels late-time SF; radiative feedback suppresses instantaneous SFRs and accretion onto dwarfs. We provide tables, initial conditions, and the numerical algorithms required to reproduce our simulations.

Garrison-Kimmel et al., available on arXiv.

Abstract: Among the most important goals in cosmology is detecting and quantifying small (M_halo ~ 10^6-9 Msun) dark matter (DM) subhalos. Current probes around the Milky Way (MW) are most sensitive to such substructure within ~20 kpc of the halo center, where the galaxy contributes significantly to the potential. We explore the effects of baryons on subhalo populations in Lambda CDM using cosmological zoom-in baryonic simulations of MW-mass halos from the Latte simulation suite, part of the Feedback In Realistic Environments (FIRE) project. Specifically, we compare simulations of the same two halos run using (1) DM-only (DMO), (2) full baryonic physics, and (3) DM with an embedded disk potential grown to match the FIRE simulation. Relative to baryonic simulations, DMO simulations contain ~2x as many subhalos within 100 kpc of halo center; this excess is >~5x within 25 kpc. At z=0, the baryonic simulations are completely devoid of subhalos down to 3×10^6 Msun within 15 kpc of the MW-mass galaxy. Despite the complexities of baryonic physics, the simple addition of an embedded central disk potential to DMO simulations reproduces this subhalo depletion, including trends with radius, remarkably well. Thus, the additional tidal field from the central galaxy is the primary cause of subhalo depletion. Subhalos on radial orbits that pass close to the central galaxy are preferentially destroyed, causing the surviving subhalo population to have tangentially biased orbits compared to DMO predictions. Our method of embedding a disk potential in DMO simulations provides a fast and accurate alternative to full baryonic simulations, thus enabling suites of cosmological simulations that can provide accurate and statistical predictions of substructure populations.

Orr et al., available on arXiv.

Abstract: We present an analysis of the global and spatially-resolved Kennicutt-Schmidt star formation relation in the FIRE (Feedback In Realistic Environments) suite of cosmological simulations, including halos with z=0 masses ranging from 10^10-10^13 Msun. We show that the Kennicutt-Schmidt (KS) relation emerges robustly due to the effects of feedback on local scales, independent of the particular small-scale star formation prescriptions employed. This is true for the KS relation measured using all of the gas and using only the dense (molecular) gas. We demonstrate that the time-averaged KS relation is relatively independent of redshift and spatial averaging scale, and that the star formation rate surface density is weakly dependent on metallicity (~ Z^1/4). Finally, we show that on scales larger than individual giant molecular clouds, the primary condition that determines whether star formation occurs is whether a patch of the galactic disk is thermally Toomre-unstable (not whether it is self-shielding): once a patch can no longer be thermally stabilized against fragmentation, it collapses, becomes self-shielding, cools, and forms stars.

Grudić et al., available on arXiv.

Abstract: We present a suite of 3D multi-physics MHD simulations following star formation in isolated turbulent molecular gas disks ranging from 5 to 500 parsecs in radius. These simulations are designed to survey the range of surface densities between those typical of Milky Way GMCs (~10^2 Msun pc^-2}) and extreme ULIRG environments (~10^4 Msun pc^-2) so as to map out the scaling of star formation efficiency (SFE) between these two regimes. The simulations include prescriptions for supernova, stellar wind, and radiative feedback, which we find to be essential in determining both the instantaneous (eps_ff) and integrated (eps_int) star formation efficiencies. In all simulations, the gas disks form stars until a critical stellar mass has been reached and the remaining gas is blown out by stellar feedback. We find that surface density is a good predictor of eps_int, as suggested by analytic force balance arguments from previous works. Furthermore, SFE eventually saturates to ~1 at high surface density, with very good agreement across different spatial scales. We also find a roughly proportional relationship between eps_ff and eps_int. These results have implications for star formation in galactic disks, the nature and fate of nuclear starbursts, and the formation of bound star clusters. The scaling of eps_ff also contradicts star formation models in which eps_ff~1% universally, including popular subgrid models for galaxy simulations.