Star Formation in the Central Molecular Zone

1. Star Formation inthe Central Molecular Zone Sungsoo S. Kim 金 成洙 Takayuki Saitoh MyoungwonJeon David Merritt Keiichi Wada (KyungHee University & Rochester Institute of Technology) (National Astronomical Observatory of Japan) (University of Texas) (Rochester Institute of Technology) (Kagoshima University)

2. Molecular Gas Distribution in the Milky Way Central Molecular Zone Molecular ring at ~4 kpc Gap! 3-kpc arm Longitude-velocity map of CO emission in -2º < b < 2º (Dame, Hartmann & Thaddeus, 2001).

3. The Central Molecular Zone ±200 pc CMZ l-v diagram of CO emission in the CMZ using data of Bally et al. (1987). Figure from Rodriguez-Fernandez & Combes (2008). Schematic diagram of face-on view of the inner Galactic bulge by Rodriguez-Fernandez & Combes (2006).

4. Total Gas Mass in the CMZ SCUBA Galactic center survey at 450 μm and 850 μm (Pierce-Price et al. 2000) “Thermal dust continuum emission at submillimeters gives optically thin map to trace essentially all the mass in the CMZ.” The total mass in the central 400 pc measured by integrating the total 850 μm emission is (5.3±1.0)x107 M⊙. Optically thin molecular line(C18O) maps gives a ~3 times smaller value.

5. Star Formation in the CMZ The Quintuplet (~Myr, ~104 M⊙) Figer et al. 1999 G359.43+0.02 Cluster of YSOs (~102 M⊙) Yusef-Zadeh et al. 2009 24μm(MIPS)/8μm(Spitzer) Ratio Image Yusef-Zadeh et al. 2009 Typical recent SFR of 0.04~0.08 M⊙/yr The Arches (~Myr, ~104 M⊙) Figer et al. 1999

6. Sustained Star Formation • Serabyn & Morris (1996) argued that star formation has been occurring in the CMZ throughout the lifetime of the Galaxy, and formed the “r-2 central star cluster”. • With a constant SFR of the current value, the inner bulge would have accumulated a total mass of ~109 M⊙. 200 pc Bulge = Nuclear Bulge + Galactic Bulge Nuclear Bulge = Nuclear Stellar Cluster + Nuclear Stellar Disk ~109 M⊙ NSC+NSD GB 2.2 μm emission from COBE/DIRBE (Launhardt et al. 2002) Enclosed mass profiles of different components (Launhardt et al. 2002)

7. Sustained Star Formation -2 NSC Bulge NSD ~100 pc log ρ • Serabyn & Morris (1996) argued that Star formation has been occurring in the CMZ throughout the lifetime of the Galaxy, and formed the “r-2 central star cluster”. • With a constant SFR of the current value, the inner bulge would have accumulated a total mass of ~109 M⊙. GB ~200 pc 200 pc Disk ~3 kpc Bulge = Nuclear Bulge + Galactic Bulge Nuclear Bulge = Nuclear Stellar Cluster + Nuclear Stellar Disk ~109 M⊙ log r NSC+NSD GB 2.2 μm emission from COBE/DIRBE (Launhardt et al. 2002) Enclosed mass profiles of different components (Launhardt et al. 2002)

8. “Nuclear Bulges” in Other Spiral Galaxies • Many (>50%) spiral galaxies have identifiable cores that are photometricallyand morphologically distinct from the surrounding bulge (e.g., Böker et al. 2002; Carollo et al. 2002). • The sizes of these cores range from several tens to hundreds of parsecs. Nuker profile Hughes et al. (2005)

9. Inward Transport of the Gas • Processes for angular momentum loss • shear viscosity • Compression and shocks associated with bar potential • dynamical friction • magnetic field viscosity • dilution of specific angular momentum by stellar mass loss material from outer bulge (Jenkins & Binney 1994)

10. Transition from Atomic to Molecular Form • Bar potentials generally have (at least) two distinct stable orbit families, X1 (along the bar) and X2 (perpendicular to the bar) orbits. • Gas is compressed at the cusps or self-intersecting tips of the inner X1 orbits, loses orbital energy, and falls inward to settle onto an X2 orbit. • This compression and subsequent cooling will cause a transition from atomic to molecular form. Morris & Serabyn (1996)

11. The Questions • Does this model really work? • Is SF really dominant in the CMZ region? • Is SFR consistent with the observations? • We have performed global hydrodynamic simulations for these questions.

12. Previous Hydrodynamic Simulations of the CMZ Region Jenkins & Binney (1994) 2D Sticky Particle, N=8k, no self-grav • Mostly 2-dimesional, isothermal, and/or no SF/feedback/self-gravity. • Not many studies specifically for the CMZ region. Rodriguez-Fernandez & Comes (2008) 2D hydro grid, 3D motion, no self-grav, N=1M Englmaier & Gerhard (1999) 2D isothermal SPH, N=10–200k Namekata et al. (2009) 2D isothermal grid, no self-grav/viscosity Double-bar potential Regan & Teuben (2003) 2D isothermal grid, no self-grav ±5 kpc

13. Our Simulations • Code developed by T. Saitoh: ASURA • 3D SPH with self-gravity • Parallelized tree scheme • Uses GRAPE boards or Phantom-GRAPE (accelerated gravity routine on a machine-language level) • Cooling function for 10–108 K from Spaans & Norman (1997) • Heating by uniform FUV radiation (1–30x solar neighborhood values) • Star Formation: Spawns collisionless particles when • n >nth • T < Tth • Converging flow • No heat increase by nearby SNs • SN feedback in forms of thermal energy addition • Galactic Potential • Power-law bulge with a m=2 bar • Miyamoto-Nagai disk • Performed a series of simulations for the central ±1.2 kpc region for up to 700 Myr.

14. ASURA Simulations of SF in Galactic Disks • Saitoh et al. (2008) successfully reproduced the complex structure of the gas disk. • Found that nth is the most important SF criterion, but SFE is not. N=106 mSPH=3500 M⊙ ε=10 pc nth =0.1 cm-3 Tth =15000 K Tcut=10000 K C* =0.033 nth =0.1 cm-3 Tth =15000 K Tcut=10 K C* =0.033 nth =100cm-3 Tth =100 K Tcut=10 K C* =0.033 nth =100cm-3 Tth =100 K Tcut=10 K C* =0.5

15. SF in the CMZ : Standard Run (Run #1) N=100k Mgas=5x107 M⊙ mSPH=500 M⊙ ε=3 pc nth =100 cm-3 Tth =100 K Tcut=10 K C* =0.033

16. Gas Influx and Star Formation Rates GC Equilibrium SFR ~ 0.04 M⊙/yr Equilibrium gas mass within ±400 pc ~ 0.3-0.5 M0 → Equilibrium SFR for the observed gas mass within ±400 pc ~ 0.1 M⊙/yr Yusef-Zadeh+(2009) Schmidt–Kennicutt with slope 1.4 Yusef-Zadeh et al. (2009)

17. The l-v Diagram Rodriguez-Fernandez & Comes (2008)

18. Inclination of the Disk (Standard Potential) Disk Evolution in Bar-like Triaxial Potential Jeon, Kim & Ann (2009)

19. Inclination of the Disk (Thinner disk potential) Circumnuclear Disk in HCN Response of the inner disk to the bar-like triaxial potential may be responsible for the apparent inclination of the CND. Christopher et al. (2005)

20. SFR Dependence on the SF Criteria nth =1000 cm-3 nth =1000 cm-3 G0 =10 C* =0.01 C* =0.1 Standard Parameters nth =100 cm-3 Tth =100 K Tcut=10 K C* =0.033

21. SFR Dependence on the Bar Elongation b22=0.14 b22=0.1 b22=0.07

22. SFR Dependence on the Initial Cloud Distribution

23. Summary • Main results from the first hydrodynamic simulations for the CMZ with SF/feedback/self-gravity are • SF takes place mostly in the inner ±200 pc. • Obtained SFR, ~0.1 M⊙/yr, is consistent with recent YSO observations. • These support the suggested connection between the CMZ and the stellar nuclear bulge. • (Obtained SFR) x (Lifetime of the Galaxy) ~ 109 M⊙. • Consistent with the current NB mass, 1-2x109 M⊙. • Triaxial bulge potential can cause the CND to be inclined from the Galactic plane, and may also be responsible for the inclined stellar disks in the central parsec.