Two key elements of the FIRE simulations are enabling their improved predictive power:
1) They directly resolve the formation of giant molecular clouds (GMCs), the rate limiting for star formation in galactic disks, and
2) They model the energy and momentum return from the main stellar feedback processes directly following the predictions of stellar population synthesis models.
By resolving the main units for star formation and the stellar feedback processes that regulate their formation, the FIRE simulations are allowing us to free ourselves of sub-resolution models for star formation and stellar feedback that have been standard in cosmological simulations to date, and which have limited their predictive power.
Because the FIRE simulations directly resolve the main structures in the interstellar medium (ISM) of galaxies, they are also allowing us to connect cosmological studies of galaxy formation with studies of star formation on galactic scales, two fields which have traditionally been the focus of distinct communities. This new connection is enabling rapid progress in our understanding of many of the key processes that govern galaxy evolution but which could not be resolved in the cosmological context previously.
The FIRE simulations build on earlier work simulating star formation and stellar feedback in isolated galaxies. These calculations have been used to study the origin of the Kennicutt-Schmidt relation, the structure of the ISM and the properties of GMCs, galactic winds driven by stellar feedback, gas inflow in gas rich disks and during galaxy mergers, and a range of other problems. As part of the FIRE project, we have extended the methods introduced in this earlier work to the case of zoom-in cosmological simulations. Our calculations explicitly follow stellar feedback from radiation pressure, photo-ionization and photo-electric heating, stellar winds (both O-star and AGB), and supernovae (Types I & II):
Radiation pressure: Light (mostly from the youngest stars) scatters off gas and dust in the galaxy. Each time a photon scatters or is absorbed, it imparts some of its momentum to that gas, “pushing” away the gas and dust. This does not “heat up” the gas, but can impart an enormous amount of momentum.
Stellar winds: Young stars blow winds off their surface that can have velocities as large as ~1000 km/s. This shocks and provides a large amount of thermal energy to heat the gas. Older stars blow “slow” winds at just ~10 km/s, but the total mass recycled into the ISM can be very large, ~30% of the original mass in stars.
Photo-Ionization: The light from the stars also ionizes gas, heating it up to ~10^4 K. These ionized ‘bubbles’ can push on the gas significantly in very low-mass galaxies (where the corresponding velocities of the gas are comparable to the disk orbital velocities). It can also destroy molecules, a critical ingredient for the next generation of star formation.
Supernovae: After a few million years, massive stars begin to explode as supernovae. Each such event imparts a large energy to the nearby ISM. Many “overlapping” events can build up huge hot bubbles of gas that generate pressures sufficient to “blow out” of the disk and vent material into the intergalactic medium.
We are currently working to expand FIRE to include the growth of massive black holes and feedback from them.
The first-generation FIRE simulations were run with the GIZMO code in P-SPH mode. P-SPH is a pressure-entropy implementation of smooth particle hydrodynamics (SPH) that resolves the main historical discrepancies between grid-based and SPH methods, particularly for fluid mixing instabilities. We are developing FIRE-2, a second generation of FIRE simulations based on the Meshless Finite Mass (MFM) hydrodynamic solver implemented in GIZMO. MFM has been demonstrated to provide superior accuracy on a wide variety of test problems.
The FIRE collaboration has grown significantly over the past few years, and includes: Norm Murray (CITA), Jose Onorbe (MPIA), Freeke van de Voort (UC Berkeley), Alexander Muratov (UC San Diego), Daniel Anglés-Alcázar (Northwestern), Robert Feldmann (UC Berkeley), Chris Hayward (Caltech), Andrew Wetzel (Caltech), Xiangcheng Ma (Caltech), T. K. Chan (UC San Diego), and Zach Hafen (Northwestern). In addition, we collaborate closely with the UC Irvine group led by James Bullock and the UT Austin group led by Mike Boylan-Kolchin on dwarf galaxy projects.
We gratefully acknowledge grant support from the NSF and NASA, as well as computing resources from NSF’s Extreme Science and Engineering Discovery Environment (XSEDE) and NASA’s High-End Computing facilities.