Author: Claude-Andre Faucher-Giguere

The past several months have been highly productive for the FIRE project, with a large number of new papers submitted on a variety of science topics.

Among others, the first paper from MassiveFIRE by Feldmann et al. investigates the formation of massive quiescent galaxies at z~2. MassiveFIRE is a major new component of the FIRE project focusing on massive halos relevant for galaxy quenching, sub-millimeter galaxies, and quasars. Simulations from MassiveFIRE have already been incorporated in the following several recent articles:

• van de Voort et al. on “The Impact of Stellar Feedback on Hot Gas in Galaxy Haloes: the Sunyaev-Zel’dovich Effect and Soft X-ray Emission”
• Oklopcic et al. on “Giant Clumps in the FIRE Simulations: a Case Study of a Massive High-Redshift Galaxy”
• Faucher-Giguère et al. on “A Stellar Feedback Origin for Neutral Hydrogen in High-Redshift Quasar-Mass Halos”
• Sravan et al. on “Strongly Time-Variable Ultra-Violet Metal Line Emission from the Circum-Galactic Medium of High-Redshift Galaxies”
• Sparre et al. on “(Star)bursts of FIRE: Observational Signatures of Bursty Star Formation in Galaxies”

In addition, two recent papers have focused on lower mass galaxies, both in the local Universe and during the epoch of reionization:

• El-Badry et al. on “Breathing FIRE: How Stellar Feedback Drives Radial Migration, Rapid Size Fluctuations, and Population Gradients in Low-Mass Galaxies”
• Ma et al. on “Binary Stars Can Provide the ‘Missing Photons’ Needed for Reionization”

In parallel to MassiveFIRE, we are also currently developing FIREBox, a complementary project that will simulate a full cosmological volume at the resolution of the FIRE zoom-in simulations. FIREBox will provide the galaxy statistics necessary to fully interpret high-redshift galaxy surveys and will be ideally suited for studies of the intergalactic medium.

In another recent paper led by Xiangcheng Ma, we use the FIRE simulations to study the origin of the mass-metallicity relation in galaxies. The mass-metallicity relation provides an especially powerful test of galaxy formation models because different state-of-the-art galaxy formation models that all reproduce the observed galaxy stellar mass function diverge strongly on their predictions for the mass-metallicity relation (as shown e.g. in the recent review by R. Somerville and R. Davé). Remarkably, the FIRE simulations reproduce the normalization, slope, and redshift evolution of the observed mass-metallicity relation, over five orders of magnitude in stellar mass, without the need to tune any parameter.

Abstract: We use high-resolution cosmological zoom-in simulations from the Feedback in Realistic Environment (FIRE) project to study the galaxy mass-metallicity relations (MZR) from z=0-6. These simulations include explicit models of the multi-phase ISM, star formation, and stellar feedback. The simulations cover halo masses Mhalo=10^9-10^13 Msun and stellar mass Mstar=10^4-10^11 Msun at z=0 and have been shown to produce many observed galaxy properties from z=0-6. For the first time, our simulations agree reasonably well with the observed mass-metallicity relations at z=0-3 for a broad range of galaxy masses. We predict the evolution of the MZR from z=0-6 as log(Zgas/Zsun)=12+log(O/H)-9.0=0.35[log(Mstar/Msun)-10]+0.93 exp(-0.43 z)-1.05 and log(Zstar/Zsun)=[Fe/H]-0.2=0.40[log(Mstar/Msun)-10]+0.67 exp(-0.50 z)-1.04, for gas-phase and stellar metallicity, respectively. Our simulations suggest that the evolution of MZR is associated with the evolution of stellar/gas mass fractions at different redshifts, indicating the existence of a universal metallicity relation between stellar mass, gas mass, and metallicities. In our simulations, galaxies above Mstar=10^6 Msun are able to retain a large fraction of their metals inside the halo, because metal-rich winds fail to escape completely and are recycled into the galaxy. This resolves a long-standing discrepancy between “sub-grid” wind models (and semi-analytic models) and observations, where common sub-grid models cannot simultaneously reproduce the MZR and the stellar mass functions.

In a new paper led by Xiangcheng Ma, we use high-resolution simulations of early dwarf galaxies run with the FIRE stellar feedback physics to study the escape fraction of ionizing photons, a crucial quantity to determine whether star forming-galaxies can reionize the intergalactic medium. In this work, we processed the hydrodynamical simulations with a Monte Carlo radiative transfer code to accurately account for ionization effects on the escape of ionizing photons.

We find that clearing of sight lines through the interstellar medium by stellar feedback is key to the escape of ionizing photons from galaxies. Interestingly, standard stellar population synthesis models predict relatively small ~5% time-averaged escape fractions. However, the time-averaged escape fraction can be boosted substantially if stellar populations older than ~3 Myr (i.e., old enough for stellar feedback to have had time to clear sight lines through birth clouds) produce more ionizing photons than in standard models, an effect which could result from the effects of binaries on stellar evolution.

Abstract: We present a series of high-resolution (20-2000 Msun, 0.1-4 pc) cosmological zoom-in simulations at z~6 from the Feedback In Realistic Environment (FIRE) project. These simulations cover halo masses 10^9-10^11 Msun and rest-frame ultraviolet magnitude Muv = -9 to -19. These simulations include explicit models of the multi-phase ISM, star formation, and stellar feedback, which produce reasonable galaxy properties at z = 0-6. We post-process the snapshots with a radiative transfer code to evaluate the escape fraction (fesc) of hydrogen ionizing photons. We find that the instantaneous fesc has large time variability (0.01%-20%), while the time-averaged fesc over long time-scales generally remains ~5%, considerably lower than the estimate in many reionization models. We find no strong dependence of fesc on galaxy mass or redshift. In our simulations, the intrinsic ionizing photon budgets are dominated by stellar populations younger than 3 Myr, which tend to be buried in dense birth clouds. The escaping photons mostly come from populations between 3-10 Myr, whose birth clouds have been largely cleared by stellar feedback. However, these populations only contribute a small fraction of intrinsic ionizing photon budgets according to standard stellar population models. We show that fesc can be boosted to high values, if stellar populations older than 3 Myr produce more ionizing photons than standard stellar population models (as motivated by, e.g., models including binaries). By contrast, runaway stars with velocities suggested by observations can enhance fesc by only a small fraction. We show that “sub-grid” star formation models, which do not explicitly resolve star formation in dense clouds with n >> 1 cm^-3, will dramatically over-predict fesc.

A recent paper by Phil Hopkins introduces GIZMO, a flexible magneto-hydrodynamics+gravity code. GIZMO is derived from Gadget-3, with substantial improvements to the gravity and hydrodynamics solvers, and is now the workhorse of the FIRE project. In addition to P-SPH, GIZMO implements new meshless methods, meshless finite mass (MFM) and meshless finite volume (MFV).

In a series of tests, the new meshless methods are shown to be more accurate than grid-based and smooth particle hydrodynamics solvers, and to be very competitive with moving mesh algorithms. A public version of GIZMO is available to the research community.

We look forward to many productive years of GIZMO!

Several studies have suggested that energy injection by time-variable stellar feedback can significantly affect the inner profiles of dark matter halos, especially in dwarf galaxies. In a recent paper (Onorbe et al.; arXiv:1502.02036), we show that the stellar feedback in the FIRE simulations can indeed produce kiloparsec-scale cores at the center of dwarf galaxy dark matter halos, potentially resolving discrepancies (including the “Too Big to Fail” problem) between observations of Milky Way satellites and dark matter-only simulations in the standard cold dark matter cosmology.

Abstract: We present ultra-high resolution cosmological hydrodynamic simulations of M_*~10^(4-6) Msun dwarf galaxies that form within M_v=10^(9.5-10) Msun dark matter halos. Our simulations rely on the FIRE implementation of star formation feedback and were run with high enough force and mass resolution to directly resolve stellar and dark matter structure on the ~200 pc scales of interest for classical and ultra-faint dwarfs in the Local Group. The resultant galaxies sit on the M_* vs. M_v relation required to match the Local Group stellar mass function. They have bursty star formation histories and also form with half-light radii and metallicities that broadly match those observed for local dwarfs at the same stellar mass. For the first time we demonstrate that it is possible to create a large (~1 kpc) dark matter core in a cosmological simulation of an M_*~10^6 Msun dwarf galaxy that resides within an M_v=10^10 Msun halo — precisely the scale of interest for resolving the Too Big to Fail problem. However, these large cores are not ubiquitous and appear to correlate closely with the star formation histories of the dwarfs: dark matter cores are largest in systems that form their stars late (z<~2), after the early epoch of cusp building mergers has ended. Our M_*~10^4 Msun dwarf retains a cuspy dark matter halo density profile that matches almost identically that of a dark-matter only run of the same system. Despite forming in a field environment, this very low mass dwarf has observable properties that match closely to those of ultra-faint satellite galaxies of the Milky Way, including a uniformly old stellar population (>10 Gyr). Though ancient, most of the stars in our ultra-faint form after reionization; the UV field acts mainly to suppress fresh gas accretion, not to boil away gas that is already present in the proto-dwarf.

One of the key aspects of the FIRE simulations is that the explicit implementation of stellar feedback processes (photoionization, radiation pressure, stellar winds, and supernovae) naturally generates galactic winds, without adjusted parameters. Cosmological simulations previously demonstrated that powerful galactic outflows are critical to explain the observationally-inferred suppression galaxy stellar masses and the enrichment of the intergalactic medium, but most simulations to date have relied on adjusted parameters to produce galactic winds with adequate mass outflow rates and velocities.

A new paper by Muratov et al. (arXiv:1501.03155) analyzes the basic properties of galactic winds in the FIRE simulations and provides convenient analytic approximations for use in future sub-resolution models. The paper also highlights the inflow-star formation-outflow cycles that galaxies undergo in our simulations, another distinguishing relative to lower-resolution simulations with coarser interstellar medium models that predict star formation histories that are smoother in time.

Abstract: We present an analysis of the galaxy-scale gaseous outflows from the FIRE (Feedback in Realistic Environments) simulations. This suite of hydrodynamic cosmological zoom simulations provides a sample of halos where star-forming giant molecular clouds are resolved to z=0, and features an explicit stellar feedback model on small scales. In this work, we focus on quantifying the gas mass ejected out of galaxies in winds and how this material travels through the halo. We correlate these quantities to star formation in galaxies throughout cosmic history. Our simulations reveal that a significant portion of every galaxy’s evolution, particularly at high redshift, is dominated by bursts of star formation, which are followed by powerful gusts of galactic outflow that sweep up a large fraction of gas in the interstellar medium and send it through the circumgalactic medium. The dynamical effect of these outflows can significantly limit the amount of star formation within the affected galaxy. At low redshift, however, sufficiently massive galaxies corresponding to L*-progenitors develop stable disks and switch into a continuous and quiescent mode of star formation that does not drive outflows into the halo. We find inflow to be more continuous than outflow, although filamentary accretion onto the galaxy can be temporarily disrupted by recently ejected outflows. Using a variety of techniques, we measure outflow rates and use them to derive mass-loading factors, and their dependence on circular velocity, halo mass, and stellar mass for a large sample of galaxies in the FIRE simulation suite, spanning four decades in halo mass, six decades in stellar mass, and a redshift range of 4.0 > z > 0. Mass-loading factors for L*-progenitors are eta ~= 10 at high redshift, but decrease to eta << 1 at low redshift.

The FIRE simulations are uniquely well suited for studies of the circum-galactic medium (CGM), where the cosmological inflows and galactic winds that regulate star formation in galaxies are mediated. In particular, our simulations self-consistently generate galactic winds with a realistic multiphase structure, which is important for comparison with spectroscopic measurements of halo gas.

We have just completed the first paper from the FIRE project (Faucher-Giguère et al., available on the arXiv). This first paper focuses on HI in galaxy halos at z=2-4, which includes the peak of the cosmic star formation history. In future papers, we will use the FIRE simulations to study other observational diagnostics of the CGM, including metal absorption and emission lines, and gas kinematics.

Abstract: Gas inflows and outflows regulate star formation in galaxies. Probing these processes is one of the central motivations for spectroscopic measurements of the circum-galactic medium. We use high-resolution cosmological zoom-in simulations from the FIRE project to make predictions for the covering fractions of neutral hydrogen around galaxies at z=2-4. These simulations resolve the interstellar medium of galaxies and explicitly implement a comprehensive set of stellar feedback mechanisms. Our simulation sample consists of 16 main halos covering the mass range M_h~2×10^9-8×10^12 Msun at z=2, including 12 halos in the mass range M_h~10^11-10^12 Msun corresponding to Lyman break galaxies (LBGs). We process our simulations with a ray tracing method to compute the ionization state of the gas. Galactic winds increase the HI covering fractions in galaxy halos by direct ejection of cool gas from galaxies and through interactions with gas inflowing from the intergalactic medium. Our simulations predict HI covering fractions for Lyman limit systems (LLSs) consistent with measurements around z~2-2.5 LBGs; these covering fractions are a factor ~2 higher than our previous calculations without galactic winds. The fractions of HI absorbers arising in inflows and in outflows are on average ~50% but exhibit significant time variability. For our most massive halos, we find a factor ~3 deficit in the LLS covering fraction relative to what is measured around quasars at z~2, suggesting that the presence of a quasar may affect the properties of halo gas on ~100 kpc scales. The predicted covering fractions peak at M_h~10^11-10^12 Msun, near the peak of the star formation efficiency in dark matter halos. In our simulations, star formation and galactic outflows are highly time dependent; HI covering fractions are also time variable but less so because they represent averages over large areas.

In a new paper (van de Voort et al.), available on the arXiv, we use simulations of a Milky Way-mass galaxy from the FIRE project to study the origin of r-process elements. These are the first cosmological simulations in which r-process elements are self-consistently advected by the fluid. Contrary to previous findings from more approximate semi-analytic models, we find that neutron star mergers may produce the majority of the r-process nuclei in the Universe.

Abstract: We quantify the stellar abundances of neutron-rich r-process nuclei in cosmological zoom-in simulations of a Milky Way-mass galaxy from the Feedback In Realistic Environments project. The galaxy is enriched with r-process elements by binary neutron star (NS) mergers and with iron and other metals by supernovae. These calculations include key hydrodynamic mixing processes not present in standard semi-analytic chemical evolution models, such as galactic winds and hydrodynamic flows associated with structure formation. We explore a range of models for the rate and delay time of NS mergers, intended to roughly bracket the wide range of models consistent with current observational constraints. We show that NS mergers can produce [r-process/Fe] abundance ratios and scatter that appear reasonably consistent with observational constraints. At low metallicity, [Fe/H]<-2, we predict there is a wide range of stellar r-process abundance ratios, with both supersolar and subsolar abundances. Low-metallicity stars or stars that are outliers in their r-process abundance ratios are, on average, formed at high redshift and located at large galactocentric radius. Because NS mergers are rare, our results are not fully converged with respect to resolution, particularly at low metallicity. However, the uncertain rate and delay time distribution of NS mergers introduces an uncertainty in the r-process abundances comparable to that due to finite numerical resolution. Overall, our results are consistent with NS mergers being the source of most of the r-process nuclei in the Universe.