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Star Formation in Low Metallicity Gas

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Star Formation in Low Metallicity Gas Star formation in Low Metallicity Gas Charlotte Christensen University of Arizona Tom Quinn, Fabio Governato, James Wadsley 1 Motivation ª How does star formation proceed in low- metallicity gas where H2 is rare? ª Deviations from solar K-S relation ª Theoretical: Pelupessy+ 2006; Robertson & Kravtsov, 2008; Gnedin & Kravtsov, 2010 ª Observational: Bigiel+ 2010, Bolatto+ 2011 ª Potentially reduces of star formation in dwarfs ª Krumholz & Dekel, 2012; Kuhlen+ 2012; Kuhlen+ 2013; Thompson+ 2013 2 Gasoline (Wadsley, et al., 2003) ª SPH code with ª Which reproduces ª Cosmic UV background ª Damped Lyman-α systems (Pontzen et al., 2008, radiation 2010) ª H & He ionization ª Mass-metallicity relation (Brooks et al., 2007) ª Metal line cooling (Shen+ 2010) ª Broken exponential disks in spirals (Roskar et al., ª Metal diffusion 2008) ª Supernovae feedback ª Tully-Fisher relation (Governato et al., 2007) (blastwave) (Stinson+ 2006) ª Realistic rotation curves ª Non-equillibrium in dwarfs (Governato et al., Molecular Hydrogen 2010) (Christensen+ 2012) ª Reduced bulge mass in spiral galaxies ª Molecular Hydrogen- (Guedes et al., 2011) based star formation ª Change the angular (Christensen+ 2012) momentum distribution (Brook et al., 2011, Pontzen et al., 2011) ª … 3 Implementing Molecular Hydrogen Formation Destruction ª Forms primarily on ª Destroyed primarily by LW radiation dust (metals) (Wolfire et ª Flux from local young al., 2008) stars ª Metallicity ª Self-shielding and ª Density shielding by dust (Draine & Bertoldi, 1996) ª Gas clumpiness (McKee & Ostriker et al., 2007) ª Surface density (column length x ª Also forms in gas- density) phase through H- ª Metallicity (Abel+ 97) ª (Dust shielding applied to HI) ª Also destroyed Based on the work of Gnedin & Kratsov, 2009 through collisions 4 HI-H2 Transition Fraction Fraction 2 Log H Gas Surface Density [amu/cm2] Christensen et al., 2012 5 Implementing Molecular Hydrogen Formation Destruction ª Forms on dust ª Destroyed by LW radiation (metals) (Wolfire et al., 2008) ª Flux from local young ª Metallicity stars ª Density ª Self-shielding and shielding by dust ª Gas clumpiness (Draine & Bertoldi, 1996) (McKee & Ostriker et al., 2007) ª Surface density (column length x density) ª Metallicity ª (Dust shielding applied to HI) 6 Star Formation Recipe ª Probabilistic, based on local gas properties p ∝ 1 – e-c*Δt/tdyn -1/2 ª Formation time: tdyn ∝ ρ ª Efficiency: c* ª Threshold density allowed: ρmin ª Threshold temperature allowed: Tmax ª No H2–based SF ª H2-based SF ª c* = 0.1 ª c* = H2/(HI + H2) 0.1 ª ρmin = 100 amu/cc ª ρmin = 0.1 amu/cc ª Tmax = 10000 K ª Tmax = 1000 K 7 Dwarf Galaxy Zoom-in Simulation ª 253 Mpc3 Box ª z = 0 properties ª M = 3.8 1010 M ª Resolution vir × ¤ ª V = 60 km/s ª DM (107): 200 ª log(O/H) + 12 = 7.8 mp = 16,000M¤ ª Mag = -16.4 ª Gas (6 x 106): i ª g – i = 0.42 mg = 3300M¤ ª Star: ms = 1000M¤ ª Force Resolution: 90 pc 8 Reproducing Kennicutt-Schmidt Relation ª HI -2 Log HI Surface Density [M¤ pc ] ª Mock THINGS observation (Walter et al., 2008) 9 Reproducing Kennicutt-Schmidt Relation ª HI -2 Log H2 Surface Density [M¤ pc ] ª Mock THINGS observation (Walter et al., 2008) ª H2 10 Reproducing Kennicutt-Schmidt Relation Log SFR Surface Density ª HI -2 -1 [M¤ kpc yr ] ª Mock THINGS observation (Walter et al., 2008) ª H2 ª SFR ª Mock FUV and 24µm observations (Sunrise, Jonsson et al., 2006) 11 Reproducing Kennicutt-Schmidt Law ª HI Obs. data from Bigiel+ 2010 ª Mock THINGS no H2 ] -1 H observation 2 yr (Walter et al., 2008) -2 kpc ª H2 ¤ ª SFR ª Mock FUV and 24µm observations (Sunrise, Jonsson et al., 2006) Log SFR SurfaceLog Density [M -2 Log Surface Density of Hydrogen [M¤ pc ] Christensen et al., 2012 12 Extended Star Formation with H2 No H2 H2 13 Clumpier Gas with H2 Log Surface Log Density no H2 Log Density H2 Christensen et al., 2012 14 Shielding’s Effect on Temperature ª Dust shielding of HI and H2 reduces heating of dense gas from UV background/LW ª Results in the increased formation of cold, dense gas Shielding, no H2 No shielding, no H Temperature [K] Temperature 2 H2 (shielding + cooling) Density [amu/cc] See Glover & Clark, 2012 15 Reproducing Kennicutt-Schmidt Relation ª HI ª Mock THINGS no H2 ] -1 H observation 2 yr (Walter et al., 2008) -2 kpc ª H2 ¤ ª SFR ª Mock FUV and 24µm observations (Sunrise, Jonsson et al., 2006) Log SFR SurfaceLog Density [M -2 Log Surface Density of Hydrogen [M¤ pc ] Christensen et al., 2012 16 Kennicutt-Schmidt Relation at Different Metallicities Dwarf Galaxies: log(O/H) + 12 = 7.8 Spirial Galaxies: log(O/H) + 12 = 8.5 Obs. data from Bigiel+ 2010 17 SFHs 18 Comparing H2 & non-H2 SF ª Dwarf galaxy with H2 and H2-based star formation ª No SN Feedback ª Dwarf galaxy with H2 and non-H2- based star formation ª No SN Feedback 19 SFHs H2-based SF Non-H2-based SF 20 Stellar Mass -- Halo Mass H2-based SF Non-H2-based SF 21 Density of Star Forming Gas H2-based SF Non-H2-based SF 22 23 Mock-Observations at z=0 12 kpc 6 kpc Images from SUNRISE (Jonsson ‘06) 24 Comparing the Evolution of a Dwarf and Spiral Galaxy ª Simulated “similar” galaxies created from scaled-up initial conditions ª Spiral galaxy is scaled up by 2 spatially and 8 (23) is mass ª Same environment, different mass 25 Evolution of Total Mass Spiral Galaxy Dwarf Galaxy Virial Mass (Dwarf Galaxy) VirialMass (Spiral Galaxy) Scaled 26 Evolution of Gas Mass All Gas Scaled Gas Mass (Dwarf Galaxy) Cold Gas Gas Mass (Spiral Galaxy) 27 Evolution of Stellar Mass Spiral Galaxy Dwarf Galaxy StellarMass (DwarfGalaxy) Stellar Mass (SpiralGalaxy) Scaled 28 -1 SFR [M¤ yr ] (Spiral Galaxy) Star Formation Histories Dwarf Galaxy SpiralGalaxy -1 Scaled SFR [M¤ yr ] (Dwarf Galaxy) 29 Molecular Hydrogen Over Time Spiral Galaxy Dwarf Galaxy 30 Molecular Hydrogen Over Time Spiral Galaxy Dwarf Galaxy 31 Summary ª Shielded gas produces clumpier and more extended star formation ª H2-based star formation can reproduced the observed dependency of the resolved K-S relation on metallicity ª Global K-S relation can hide wide variation in the star forming properties ª H2-based star formation does not limit star formation in dwarf galaxies ª Lower average pressure/surface densities strongly correlated with reduced star formation in dwarf galaxies 32 .
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