publications

2021

1. MNRAS

   Efficient early stellar feedback can suppress galactic outflows by reducing supernova clustering

   Smith, M. C., Bryan, G. L., Somerville, R. S., Hu, C.-Y., Teyssier, R., Burkhart, B., and Hernquist, L.

   2021, MNRAS, 506, 3882

   Abs HTML

   We present a novel set of stellar feedback models, implemented in the moving-mesh code AREPO, designed for galaxy formation simulations with near-parsec (or better) resolution. These include explicit sampling of stars from the IMF, allowing feedback to be linked to individual massive stars, an improved method for the modelling of H II regions, photoelectric heating from a spatially varying FUV field and supernova feedback. We perform a suite of 32 simulations of isolated M_vir = 10¹⁰ M_⊙ galaxies with a baryonic mass resolution of 20 M_⊙ in order to study the non-linear coupling of the different feedback channels. We find that photoionization and supernova feedback are both independently capable of regulating star formation to the same level, while photoelectric heating is inefficient. Photoionization produces a considerably smoother star formation history than supernovae. When all feedback channels are combined, the additional suppression of star formation rates is minor. However, outflow rates are substantially reduced relative to the supernova only simulations. We show that this is directly caused by a suppression of supernova clustering by the photoionization feedback, disrupting star forming clouds prior to the first supernovae. We demonstrate that our results are robust to variations of our star formation prescription, feedback models and the baryon fraction of the galaxy. Our results also imply that the burstiness of star formation and the mass loading of outflows may be overestimated if the adopted star particle mass is considerably larger than the mass of individual stars because this imposes a minimum cluster size.

2. MNRAS

   The sensitivity of stellar feedback to IMF averaging versus IMF sampling in galaxy formation simulations

   Smith, M. C.

   2021, MNRAS, 502, 5417

   Abs HTML

   Galaxy formation simulations frequently use initial mass function (IMF) averaged feedback prescriptions, where star particles are assumed to represent single stellar populations that fully sample the IMF. This approximation breaks down at high mass resolution, where stochastic variations in stellar populations become important. We discuss various schemes to populate star particles with stellar masses explicitly sampled from the IMF. We use Monte Carlo numerical experiments to examine the ability of the schemes to reproduce an input IMF in an unbiased manner while conserving mass. We present our preferred scheme which can easily be added to pre-existing star formation prescriptions. We then carry out a series of high-resolution isolated simulations of dwarf galaxies with supernovae (SNe), photoionization, and photoelectric heating to compare the differences between using IMF averaged feedback and explicitly sampling the IMF. We find that if SNe are the only form of feedback, triggering individual SNe from IMF averaged rates gives identical results to IMF sampling. However, we find that photoionization is more effective at regulating star formation when IMF averaged rates are used, creating more, smaller H II regions than the rare, bright sources produced by IMF sampling. We note that the increased efficiency of the IMF averaged feedback versus IMF sampling is not necessarily a general trend and may be reversed depending on feedback channel, resolution and other details. However, IMF sampling is always the more physically motivated approach. We conservatively suggest that it should be used for star particles less massive than ~500 M_⊙.

2020

1. ApJL

   A Framework for Multiphase Galactic Wind Launching Using TIGRESS

   Kim, C.-G., Ostriker, E. C., Fielding, D. B., Smith, M. C., Bryan, G. L., Somerville, R. S., Forbes, J. C., Genel, S., and Hernquist, L.

   2020, ApJL, 903, L34

   Abs HTML

   Galactic outflows have density, temperature, and velocity variations at least as large as those of the multiphase, turbulent interstellar medium (ISM) from which they originate. We have conducted a suite of parsec-resolution numerical simulations using the TIGRESS framework, in which outflows emerge as a consequence of interaction between supernovae (SNe) and the star-forming ISM. The outflowing gas is characterized by two distinct thermal phases, cool (T ≲ 10⁴ K) and hot (T ≳ 10⁶ K), with most mass carried by the cool phase and most energy and newly injected metals carried by the hot phase. Both components have a broad distribution of outflow velocity, and especially for cool gas this implies a varying fraction of escaping material depending on the halo potential. Informed by the TIGRESS results, we develop straightforward analytic formulae for the joint probability density functions (PDFs) of mass, momentum, energy, and metal loading as distributions in outflow velocity and sound speed. The model PDFs have only two parameters, star formation rate surface density Σ_SFR and the metallicity of the ISM, and fully capture the behavior of the original TIGRESS simulation PDFs over Σ_SFR ∈(10^-4,1) M_⊙ kpc^-2 Myr^-1. Employing PDFs from resolved simulations will enable implementations of subgrid models for galaxy formation with wind velocity and temperature (as well as total loading factors) that are based on theoretical predictions rather than empirical tuning. This is a critical step to incorporate advances from TIGRESS and other high-resolution simulations in future cosmological hydrodynamics and semi-analytic galaxy formation models. We release a Python package to prototype our model and to ease its implementation.

2019

1. MNRAS

   Cosmological simulations of dwarfs: the need for ISM physics beyond SN feedback alone

   Smith, M. C., Sijacki, D., and Shen, S.

   2019, MNRAS, 485, 3317

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   The dominant feedback mechanism in low-mass haloes is usually assumed to take the form of massive stars exploding as supernovae (SNe). We perform very high resolution cosmological zoom-in simulations of five dwarf galaxies to z = 4 with our mechanical SN feedback model. This delivers the correct amount of momentum corresponding to the stage of the SN remnant evolution resolved, and has been shown to lead to realistic dwarf properties in isolated simulations. We find that in four out of our five simulated cosmological dwarfs, SN feedback has insufficient impact resulting in excessive stellar masses, extremely compact sizes and central supersolar stellar metallicities. The failure of the SN feedback in our dwarfs is physical in nature within our model and is the result of the build-up of very dense gas in the early universe due to mergers and cosmic inflows prior to the first SN occurring. We demonstrate that our results are insensitive to resolution (provided that it is high enough), details of the (spatially uniform) UV background and reasonable alterations within our star formation prescription. We therefore conclude that the ability of SNe to regulate dwarf galaxy properties is dependent on other physical processes, such as turbulent pressure support, clustering, and runaway of SN progenitors and other sources of stellar feedback.

2. MNRAS

   Fast and energetic AGN-driven outflows in simulated dwarf galaxies

   Koudmani, S., Sijacki, D., Bourne, M. A., and Smith, M. C.

   2019, MNRAS, 484, 2047

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   The systematic analysis of optical large-scale surveys has revealed a population of dwarf galaxies hosting active galactic nuclei (AGN), which have been confirmed by X-ray follow-up observations. Recently, the MaNGA survey identified six dwarf galaxies that appear to have an AGN that is preventing on-going star formation. It is therefore timely to study the physical properties of dwarf galaxies, in particular whether the presence of an AGN can affect their evolution. Using the moving mesh code AREPO, we have investigated different models of AGN activity, ranging from simple energy-driven spherical winds to collimated, mass-loaded, bipolar outflows in high-resolution simulations of isolated dwarf galaxies hosting an active black hole. Our simulations also include a novel implementation of star formation and mechanical supernova (SN) feedback. We find that AGN outflows have a small but systematic effect on the central star formation rates (SFRs) for all set-ups explored, while substantial effects on the global SFR are only obtained with strong SNe and a sustained high-luminosity AGN with an isotropic wind. This suggests that AGN feedback in dwarf galaxies is unlikely to directly regulate their global SFRs. There is, however, a significant effect on outflow properties, which are notably enhanced by the AGN to much higher outflow temperatures and velocities, in agreement with kinematic signatures from the MaNGA survey. This indicates that AGN may play an indirect role in regulating the baryon cycle in dwarf galaxies by hindering cosmic gas inflows.

2018

1. MNRAS

   Supernova feedback in numerical simulations of galaxy formation: separating physics from numerics

   Smith, M. C., Sijacki, D., and Shen, S.

   2018, MNRAS, 478, 302

   Abs HTML

   While feedback from massive stars exploding as supernovae (SNe) is thought to be one of the key ingredients regulating galaxy formation, theoretically it is still unclear how the available energy couples to the interstellar medium and how galactic scale outflows are launched. We present a novel implementation of six sub-grid SN feedback schemes in the moving-mesh code AREPO, including injections of thermal and/or kinetic energy, two parametrizations of delayed cooling feedback and a ‘mechanical’ feedback scheme that injects the correct amount of momentum depending on the relevant scale of the SN remnant resolved. All schemes make use of individually time-resolved SN events. Adopting isolated disc galaxy set-ups at different resolutions, with the highest resolution runs reasonably resolving the Sedov-Taylor phase of the SN, we aim to find a physically motivated scheme with as few tunable parameters as possible. As expected, simple injections of energy overcool at all but the highest resolution. Our delayed cooling schemes result in overstrong feedback, destroying the disc. The mechanical feedback scheme is efficient at suppressing star formation, agrees well with the Kennicutt-Schmidt relation, and leads to converged star formation rates and galaxy morphologies with increasing resolution without fine-tuning any parameters. However, we find it difficult to produce outflows with high enough mass loading factors at all but the highest resolution, indicating either that we have oversimplified the evolution of unresolved SN remnants, require other stellar feedback processes to be included, and require a better star formation prescription or most likely some combination of these issues.