Star Formation in the Central Molecular Zone Sungsoo S. Kim 金 成洙 Takayuki Saitoh Myoungwon Jeon David Merritt Keiichi Wada (Kyung Hee University & Rochester Institute of Technology) (National Astronomical Observatory of Japan) (University of Texas) (Rochester Institute of Technology) (Kagoshima University)

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

The Central Molecular Zone 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). CMZ ±200 pc

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)x10 7 M ⊙. Optically thin molecular line(C 18 O) maps gives a ~3 times smaller value.

Star Formation in the CMZ The Arches (~Myr, ~10 4 M ⊙ ) Figer et al The Quintuplet (~Myr, ~10 4 M ⊙ ) Figer et al G Cluster of YSOs (~10 2 M ⊙ ) Yusef-Zadeh et al μm(MIPS)/8μm(Spitzer) Ratio Image Yusef-Zadeh et al Typical recent SFR of 0.04~0.08 M ⊙ /yr

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 ~10 9 M ⊙. 2.2 μm emission from COBE/DIRBE (Launhardt et al. 2002) Enclosed mass profiles of different components (Launhardt et al. 2002) GB NSC+NSD 200 pc ~10 9 M ⊙ Bulge = Nuclear Bulge + Galactic Bulge Nuclear Bulge = Nuclear Stellar Cluster + Nuclear Stellar Disk

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 ~10 9 M ⊙. 2.2 μm emission from COBE/DIRBE (Launhardt et al. 2002) Enclosed mass profiles of different components (Launhardt et al. 2002) GB NSC+NSD 200 pc ~10 9 M ⊙ Bulge = Nuclear Bulge + Galactic Bulge Nuclear Bulge = Nuclear Stellar Cluster + Nuclear Stellar Disk Disk log r log ρ Bulge GB NSD NSC -2 ~3 kpc ~200 pc ~100 pc

“Nuclear Bulges” in Other Spiral Galaxies Many (>50%) spiral galaxies have identifiable cores that are photometrically and 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. Hughes et al. (2005) Nuker profile

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)

Transition from Atomic to Molecular Form Morris & Serabyn (1996) Bar potentials generally have (at least) two distinct stable orbit families, X 1 (along the bar) and X 2 (perpendicular to the bar) orbits. Gas is compressed at the cusps or self- intersecting tips of the inner X 1 orbits, loses orbital energy, and falls inward to settle onto an X 2 orbit. This compression and subsequent cooling will cause a transition from atomic to molecular form.

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.

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

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 10–10 8 K from Spaans & Norman (1997) –Heating by uniform FUV radiation (1–30x solar neighborhood values) –Star Formation: Spawns collisionless particles when n > n th T < T th 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.

ASURA Simulations of SF in Galactic Disks Saitoh et al. (2008) successfully reproduced the complex structure of the gas disk. Found that n th is the most important SF criterion, but SFE is not. n th =100cm -3 T th =100 K T cut =10 K C * =0.033 n th =0.1 cm -3 T th =15000 K T cut =10000 K C * =0.033 n th =0.1 cm -3 T th =15000 K T cut =10 K C * =0.033 n th =100cm -3 T th =100 K T cut =10 K C * =0.5 N=10 6 m SPH =3500 M ⊙ ε=10 pc

SF in the CMZ : Standard Run (Run #1) N=100k M gas =5x10 7 M ⊙ m SPH =500 M ⊙ ε=3 pc n th =100 cm -3 T th =100 K T cut =10 K C * =0.033

Gas Influx and Star Formation Rates Yusef-Zadeh+(2009) Equilibrium SFR ~ 0.04 M ⊙ /yr Equilibrium gas mass within ±400 pc ~ M 0 → Equilibrium SFR for the observed gas mass within ±400 pc ~ 0.1 M ⊙ /yr GC Schmidt–Kennicutt with slope 1.4 Yusef-Zadeh et al. (2009)

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

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

Inclination of the Disk (Thinner disk potential) Christopher et al. (2005) 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.

SFR Dependence on the SF Criteria n th =1000 cm -3 G 0 =10 C * =0.01 C * =0.1 n th =100 cm -3 T th =100 K T cut =10 K C * =0.033 Standard Parameters

SFR Dependence on the Bar Elongation b 22 =0.1 b 22 =0.14 b 22 =0.07

SFR Dependence on the Initial Cloud Distribution

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) ~ 10 9 M ⊙. –Consistent with the current NB mass, 1-2x10 9 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.