Outflows and Chemical Enrichment in the First Galaxies Jeremy Ritter Chalence Safranek-Shrader, Miloš Milosavljević, Volker Bromm 1

The First Galaxies Ancient stellar systems containing dark matter Primitive chemical evolution history: contributing nucleosynthetic sources not homogenized Not directly observable before James Webb Space Telescope Surviving relics in the local universe: Milky Way’s satellites (“near-field cosmology”) 2

Alan W. McConnachie 2012 The Astronomical Journal

Segue 1: M. Geha (DEIMOS) 4

Ultra Faint Dwarfs Could they be examples of the first galaxies? – The lowest luminosity dark matter dominated stellar systems known Why are the mass to light ratios so large? – Dark matter-stellar mass ratio ~ 1000 – Compare to the cosmological ratio Ω dm /Ω b ~ 5 How did they lose their baryons? – Ram pressure stripping? – Supernova explosions can unbind gas 5

Evan N. Kirby et al ApJ

UFD Chemical Evolution History Low metallicities −2.5 < [Fe/H] < −1.0 Significant metallicity spread σ [Fe/H] ~ 0.5 – Merging of chemically heterogeneous progenitors? – Inhomogeneous local enrichment? Metallicity decreases with luminosity – Is metallicity an accurate cosmic clock, as normally assumed? 7

Luis C. Vargas et al ApJ

UFD Chemical Evolution History Enhanced α-element abundances [α/Fe] > 0 α-enhancement declines above a critical [Fe/H] – Signature of SN Ia enrichment at very low metallicities Segue 1 – [α/Fe] ~ 0.5, average [Fe/H] ~ −2.5 – pure core collapse chemistry: no pair-instability, SN Ia, or AGB (s-process) signatures. 9

Frebel and Bromm 2012 ApJ Frebel et al ApJ

Marla Geha et al ApJ

Safranek-Shrader et al. MNRAS (2014) 440 (1): L76-L80. 12

Ritter et al. 2014, in prep. 13

Research Collaborators Volker Bromm Miloš Milosavljević Alan SluderChalence Safranek-Shrader 14

Open Questions Can moderate-mass Pop III stars, which are expected to explode as core-collapse supernovae, explain UFD abundance patterns? How far from a minihalo is supernova ejecta dispersed? How long does it take for the ejecta to fall back and enrich gas that can cool, collapse, and form new stars? Does the hydrodynamics of the supernova remnant expansion and the fallback homogenize the abundances? 15

Methodology Cosmological simulations with dark matter and baryons – Simulate a Pop III star forming halo at high redshift – FLASH AMR hydro code embedded with N-body dark matter particle integration and multi-grid gravity solver – Non-equilibrium atomic and molecular chemistry leading to Pop III stars, and photoionization and cooling tables tabulated from CLOUDY after Pop III stars have been inserted Track aspherical H II region in 3072 angular pixels Insert supernovae in the kinetic-energy-dominated, free-expansion phase, and follow the gas and ejecta hydrodynamics for several tens of Myr 16

Test of H II Region Photoheating 200 M  star in a cosmological simulation Integrate for 2.2 Myr 2.6 × ionizing photons per second Flat inner profile approaches 0.1 cm −3 A dense shell expanding to >100 pc Spherically averaged radial density profile 17

Test of Supernova Remnant Hydrodynamics M ejecta = 40 M  E kinetic = 6×10 51 ergs M Z / M ejecta = 0.3 (α-enhanced composition) 18

8.5 Myr after a erg supernova with 40 M  of ejecta Single Core Collapse Supernova Circle encloses R vir = 200 pc of a 10 6 M  minihalo 19

Ejecta Fallback Less than 50% of metal ejecta has enough momentum to leave the virial radius after 40 Myr 20

Before SN, M halo = 10 6 M  40 Myr after SN, M halo = 4×10 6 M  Gas thermodynamic profile recovers after only a few tens of Myr 21

Sustained inflow of mass after ~ 1 Myr – Metals return after ~ 5 Myr – Total mass inflow rate ~ 2×10 −3 M  yr −1 – Average metallicity ~ 10 −2 Z  – Metal highly inhomogeneous in returning gas Baryons Metals 22

baryon density slice + ejecta particles in projection 360 pc1100 pc Inhomogeneous Enrichment 23

Red = Z from Eulerian tracer, Blue = Z from Lagrangian tracer 24

Challenges for Resolving Metallicity Inhomogeneity in Metal Enriched Star Formation Artificial numerical diffusion of passive mass scalars is inherent in Eulerian finite volume methods – Introduce Lagrangian passive tracer particles to show where metals can travel Small scale instabilities in blastwaves are difficult to resolve – AMR grid is dynamically refined to ensure the Jeans length is resolved – New supernova remnants are inserted by zooming in to extremely high resolution while maintaining previous remnants at moderately high resolution – The smallest scale structure is invariably not resolved 25

Multiple Supernovae in a Minihalo Protostellar disk fragmentation produces Pop III stars in small, compact clusters We sample a Pop III initial mass function to compute delay times of consecutive supernovae Pop III stars of different masses should have distinct nucleosynthetic signatures (though the detailed yields are not currently known) Time separation of supernova events opens the possibility of complexity in metal dispersal 26

Multiple Supernovae: Methodology Pop III star cluster sampled from a flat IMF – Masses: 80, 63, 50, 40, 32, 25, and 20 M  – Lifetimes: 3.1, 3.25, 3.6, 4.1, 4.9, 6.1, and 7.7 Myr – Combined ionizing luminosity: 2.2 × photon/s Kinetic blastwaves all inserted with ergs – Initial radius at high density 0.6 pc (cell size 0.02 pc) and at low density 2.9 pc (cell size 0.09 pc) much less than the radius of equivalent mass Track the ejecta from each supernova separately using individually tagged passive tracer particles 27

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Source Heterogeneity: Tracing Individual Ejecta Exterior viewCross sectional view (kpc) 30

Some Limitations of the Methodology Ionization front kinematics are not accurately reproduced – But large scale thermodynamic response to photoheating and the failure to photoevaporate cold, dense filaments are consistent with other simulations and analytical models Not incorporating thermal conduction at contact discontinuities (e.g., at the interface of the cold shell and the hot interior of the supernova remnant) Artificial mass diffusion at hot-cold discontinuities can lead to overcooling Sub-grid turbulence, turbulent dissipation, turbulent metal diffusivity are not treated Magnetic field is not treated 31

Summary of Preliminary Findings Some ejecta travel > 1.5 kpc in 72 Myr – Halo virial radius increases from 100 to 350 pc Mass loading of dense gas ablated from cosmic web filaments by blastwaves enables some ejecta retention in the halo Ejecta-rich shell remains clumpy and unmixed Inflow from cosmic web filaments bring metal- free gas to the center of the halo Only a small fraction of the ejecta, preferentially from the latest supernovae, enriches metal- enriched star forming clouds 32

Further Analysis Needed Continue the simulation until enriched gas is able to cool and recollapse to high density Analyze the metallicity structure of metal- enriched cold gas Discern the contributions to the outflow and fallback from individual supernovae Fork a new simulation to track the large scale, long-term evolution of the outflow 33

Future Projects Collaborate with Chalence to simulate the formation of metal-enriched, second generation stars from cosmological initial conditions – Simulate the explosion and inhomogeneous enrichment by a 60 M  star in a minihalo until cold gas in the fallback has reached ~ 10 7 cm −3 – Excise a portion of the cosmological box containing the metal enriched halo and simulate the formation of individual protostellar cores – Compare the metallicity and mass distribution of simulated protostars to the known very metal-poor stars in UFDs and the galactic halo 34

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Scalar Transport Eulerian: diffusion grows a spurious pseudo-exponential tail. If the thermodynamic evolution is sensitive to Z ~ 10 −3 Z  ~ 10 −5 or smaller with dust, a spuriously large gas mass can cool. Turbulence assisted mixing requires several turbulent eddy crossing times. If turbulence is weak, the metals from SN ejecta and the unpolluted gas could remain well separated on small scales. 36

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