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Alyson Brooks Fairchild Postdoctoral Fellow in Theoretical Astrophysics Caltech

Modeling Realistic Dwarf Galaxies (and Stellar Halos). Alyson Brooks Fairchild Postdoctoral Fellow in Theoretical Astrophysics Caltech In collaboration with C. Brook (JHI), F. Governato (UW), L. Mayer (ETH, U Zurich), P. Madau (UCSC), and the University of Washington’s N-body Shop ™

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Alyson Brooks Fairchild Postdoctoral Fellow in Theoretical Astrophysics Caltech

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  1. Modeling Realistic Dwarf Galaxies (and Stellar Halos) Alyson Brooks Fairchild Postdoctoral Fellow in Theoretical Astrophysics Caltech In collaboration with C. Brook (JHI), F. Governato (UW), L. Mayer (ETH, U Zurich), P. Madau (UCSC), and the University of Washington’s N-body Shop™ makers of quality galaxies

  2. Disks rotate too fast at a given luminosity Mass (dark and luminous) is too concentrated Log Vrot Disks are too small at a given rotation speed MI The CDM Angular Momentum Problem catastrophe! Navarro & Steinmetz (2000)

  3. N=1,000,000 100,000 30,000 x1019 atoms cm-2 20 kpc 20 kpc 20 kpc What is the role of resolution in forming realistic disk galaxies? Rd=30% smaller • Isolated (non-cosmological) galaxy simulation • MW mass halo after 5 Gyr Low resolution: disks heat and lose angular momentum to halo Kaufmann et al. (2007)

  4. “Zoom in” technique:high resolution halo surrounded by low resolution region Computationally Efficient Cosmic Infall and Torques correctly included 6 Mpc 100 Mpc 1 million particles 3 million particle simulation Katz & White (1993)

  5. What is the role of feedback in forming realistic disk galaxies? No feedback feedback Zavala et al. (2008) Scannapieco et al. (2008)

  6. What is the role of feedback in forming realistic disk galaxies? “Over-cooling” leads to loss of angular momentum similar to low resolution Maller & Dekel (2002)

  7. Sub-grid physics & Blastwave Feedback Model HI Map • Star Formation: reproduces the Kennicutt-Schmidt Law; each star • particle a SSP with Kroupa IMF • Energy from SNII deposited into the ISM as thermal energy based on McKee & Ostriker (1977) • Radiative cooling disabled to describe adiabatic expansion phase of SNe (Sedov-Taylor phase); ~20Myr (blastwave model) • Only Free Parameters: SN & Star Formation efficiencies Stinson et al. (2006), Governato et al. (2007)

  8. Fully Cosmological, • Parallel, • N-body Tree Code, • + smoothed particle hydrodynamics (SPH) • Star Formation Rate 1.5 • UV background radiation (Haardt & Madau 96) • - Compton & radiative cooling • Low temperature cooling (<104K, metal lines) • Supernovae feedback II & Ia (Stinson et al. 2006) • - metal enrichment: H,He,O,Fe Stadel (2001) Wadsley et al. (2004)

  9. The Formation of a Bulgeless Dwarf Galaxy Mvir = 2x1010 M M* = 1.2x108 M 15 kpc on a side Green = gas Blue/Red = age/metallicity weighted stars

  10. What is the effect of resolution on feedback?

  11. “Resolving” Star Formation Regions High threshold Low threshold Feedback becomes more efficient (more outflows per unit mass of stars formed) See also: Ceverino & Klypin (2008) Robertson & Kravtsov (2008) Tasker & Bryan (2008)

  12. Resolving Star Forming Complexes The effect of altering the SF density threshold The effect of altering resolution Governato et al., 2009, Nature, 463, 203 arXiv:0911.2237

  13. Outflows Remove Low Angular Momentum Gas Gas removal from the galaxy center Hot gas perpendicular to disk plane: 100km/sec Cold Gas in shells = 30 km/sec Accreted material J At high z outflows remove low angular momentum gas at a rate 2-6 times SFR(t) Outflows Time See also: van den Bosch (2002), Maller & Dekel (2002), Bullock et al. (2001)

  14. Angular Momentum of Stellar Disk vs DM halo van den Bosch et al. (2001) The Angular Momentum Distribution of baryons must be altered to match observed galaxies

  15. “Observed” Surface Brightness Profile 18 20 22 24 Mag/arsec2 Diffuse Star Formation 0 1 2 3 4 5 6 7 Radius (kpc) 22 24 26 28 “Resolved” Star Formation Mag/arsec2 0 1 2 3 4 Governato et al., 2009, Nature, 463, 203, arXiv:0911.2237 Radius (kpc)

  16. baryonic Tully-Fisher relation (magnitudes from Sunrise) HI W20/2 velocity widths = Vmax Geha et al. (2006) Brooks et al., in prep Governato et al. (2008, 2009)

  17. Size - Luminosity Relation Simulated Galaxies Observed Sample Brooks et al., in prep. Data from Graham & Worley, 2008; van Zee 2000

  18. Comparison to THINGS: the HI Nearby Galaxy Survey Rotation curve rises too slowly to match cuspy NFW profile Walter et al. (2008); Oh et al. (in prep)

  19. Clumpy Gas transfers energy to DM DM expands as gas is rapidly removed Dark Matter with a Central Core e.g., Mashchenko et al. (2007, 2008); El-Zant et al. (2004); Navarro et al. (1996); Mo & Mao (2004); Tonini et al. (2006)

  20. Belokurov et al.

  21. Conclusions I • Simulations are improving! (due to resolution and feedback) • Bulgeless galaxies with shallow DM cores are compatible with CDM! • Strong gas outflows can • selectively remove low angular • momentum gas (but force • resolution < 100pc is required)

  22. Modeling Realistic Dwarf Galaxies (and Stellar Halos) Inner Alyson Brooks Fairchild Postdoctoral Fellow in Theoretical Astrophysics Caltech Primary collaborators: A. Zolotov (NYU), B. Willman (Haverford)

  23. Accreted Stellar Halos N-Body + SAM results of Bullock and Johnston; Cooper et al.: Stellar halos built through accretions alone. Need fully cosmological simulations wit baryons to examine if halo stars can be formed in the main galaxy. Cooper et al. (2010)

  24. Observing the Simulations Zolotov et al. (2009)

  25. Chemical Evolution +Mass – Metallicity – SFR Relation

  26. Chemical Trends 0.4 0.0 0.4 0.0 New Stronger Feedback [O/Fe] [O/Fe] Red: Stars formed in situ Black: Accreted stars -3.0 -2.0 -1.0 0.0 [Fe/H] Zolotov et al. (2010)

  27. Already observed? 78 halo stars (kinematically selected) Nissen & Schuster (2010)

  28. Conclusions II • Galaxies with quiescent merger histories may have inner halos dominated by stars formed in situ • Outer halos always dominated by accreted stars • Chemical trends may give a • clue to the formation origin • of individual stars

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