CO J =1-0 + J =3-2 map (Oka+ 1999, 2007) Galactic Center Kunihiko Tanaka (1), Tomoharu Oka (1), Shinji Matsumura (1), Kazuhisa Kamegai (2), Makoto Nagai (3), Testuo Hasegawa (4) (1) Keio University (2) ISAS/JAXA(3) KEK (4) Joint Alma Office

Outline Galactic Center as Starburst/AGN nucleus Results of the ASTE Galactic Center Key Science Project What we can do with a Large Aperture Sub-mm Telescope

Central Molecular Zone (CMZ) CMZ : Central ～ 200 pc of the Milky Way – 5 ｘ 10 7 M ☉ Molecular Gas (several % of Milky Way) – 3 Most Massive Stellar Clusters – The Most Active Star Formation Site – Supermassive Black Hole (SMBH) CO J =1-0 + J =3-2 map (Oka+ 1999, 2007) Massive Compact Clusters in GC

GC is Quiescent Sgr A * – Super Massive Black Hole (4x10 6 M ☉ ) – ‘Spectacularly Dark’ Low Luminosity AGN (LLAGN) – L x < L edd Moderate SF activity – SFR = M ☉ yr -1 (Yusef-Zadeh+ 2009) – SFE = 10 -8~-9 yr -1 cf. Galactic Disk = 5x10 -9 yr -1 cf. Galactic Disk = 5x10 -9 yr -1 Yusef-Zadeh GC Galactic Disk

Past Acticity? Fermi Bubbles – Past AGN event? (Zubovas+ 2011) – Past Starburst? (Crocker+ 2011) Past Starburst? – SFR = 0.14 M ☉ yr -1 (Yusef-Zadeh+ 5 yr ago – cf. present SFR ~ M ☉ yr -1

Bar-driven flow? (Binney 1991) Gas Structure & Kinematics in the GC Bar-Driven InflowBar-Driven Inflow – Starburst? Herschel Obs (Molinari+ 2011) – CMZ is a Twisted Ellipitical Ring?

Goals of the GC Key-science Project SF history of the GC – Did starburst took place in the past? – How the physical condition and kinematics of the molecular gas are related to the SF in the CMZ? e.g. Bar-Driven Flow e.g. Bar-Driven Flow Activity of the Central SMBH Sgr A* – Did Sgr A* burst in the past? – How did the SMBH form?

ASTE observations CO J=3-2 Survey (2005 – 2009) Oka+ 2007, Nagai+2007 HCN J=4-3 Survey (2010-) [CI] 3 P P 0 Survey (2011-) Tanaka+ ApJL submitted ( ＾・ ω ・＾ ) KT (1.75 m) ASTE (10 m)

High Velocity Compact Clouds (HVCCs) Molecular Clumps with Very Large Velocity Width (~ 100 km s -1 )

High Velocity Compact Clouds HVCCs – ~100 HVCCs are in the GC? – Very energetic internal motion ( ergs!) – Often has high CO J=3-2/J=1-0 ratio > 1 (~50 %; Nagai PhD Thesis 2008 ) – Intense [CI] 3 P P 0 emission (Tanaka+ ApJL submitted) What are the energy sources? – Shocked feature – Dissipating (= unbound) – Tidal Shear? – Cloud-Cloud Collision ? – SN interaction

High Velocity Compact Clouds HVCCs are often related to Expanding Shells Multiple Expanding Molecular Shell Multiple Expanding Molecular Shell E kin ~ erg : driven by SN explosions in stellar cluster of > 10 5 M ☉ Tanaka+2007 E kin ~ erg : driven by SN explosions in stellar cluster of > 10 5 M ☉ Tanaka+2007 Galactic equivalent to ‘Molecular Superbubbles’ in the Starburst galaxies ?(NGC253, M82) Galactic equivalent to ‘Molecular Superbubbles’ in the Starburst galaxies ?(NGC253, M82)

Study of HVCCs will tell us : SF History of the Galactic Center – Cluster formation in the past ( ~ 10 Myr) – Total Kinetic energy of HVCCs > 5x10 52 erg – N SN = E kin /(10 51  SN [erg])  SN : conversion efficiency to E kin ⇒ SFR ~ ( )  SN -1 M ☉ yr –1 Cf. IR : 0.01 M ☉ yr -1 IR : 0.14 M ☉ yr -1 (Yusef-Zadeh+ Myr ago X-ray: ( ) M ☉ yr –1 (Yamauchi+ Myr ago HVCCs identified in the CO J =1-0 map (Matsumura 2011)

HVCCs as wombs of IMBH Intermediate mass blackholes (IMBHs) – Formed in massive stellar clusters (Ebisuzaki+ 2001) – IMBHs merge to form a SMBH at the nucleus? – IMBH candidates are found in nearby galaxies (Ultra Luminous X-ray Objects) IMBHs may have been formed in the HVCCs or Molecular Bubbles – Most energetic HVCCs may be remnant of clusters with mass M ☉ cf. NIR/FIR idensified GC clusters M ☉ cf. NIR/FIR idensified GC clusters M ☉ – IMBHs of similar mass can be formed. The formation process of the SMBH may be studied by observations of HVCCs … ?

Current Problems Origin of HVCCs – We want direct evidence that they were really created by SNe! Do they have expanding motion? Do they have expanding motion? … or they may be rotating disks around IMBHs … or they may be rotating disks around IMBHs – This observation requires high resolution (< 0.1 pc) ⇒ ALMA Complete List of HVCCs – Statistical Study – How many HVCCs? How much energy? Where they are? ⇒ SFR in the past 10 Myr ⇒ Formation process of massive clusters ⇒ Counterparts in radio continuum/X-rays – Requires a survey covering the entire CMZ

Why do we need a Large Aperture Submillimeter telescope To make reliable detection of HVCCs: – High resolution (< 1 pc) is required ASTE GHz 0.7 pc ASTE GHz 0.7 pc – We have to avoid confusion by the ambient gas HCN/HCO + line are good tracers HCN/HCO + line are good tracers Only small fraction of HVCCs are detectable with the current ASTE observation. Only small fraction of HVCCs are detectable with the current ASTE observation. Deep & high resolution survey – HCN/HCO + J=4-3, CS J=7-6, [CI] 3 P P 0 … ASTE HCN J=4-3 survey Correlation between Clusters and HVCCs ?

How Much Time is Required for the GC survey ? Impossible with ALMA Needs better resolution/sensitivity than ASTE I would be happy with Multi-Beam receiver

IMBHs might be detectable … ? IMBHs might be detectable? – Sgr A* often flares up in wide wavelength (radio – X-ray) ~ 1  m ~ 1  m – IMBHs may have similar time-variable emission ~ mJy order? ~ mJy order? …. If they really are in the GC ! …. If they really are in the GC ! ?

[CI] 3 P P 0 Observation C 0 traces : – Chemically Immature Molecular gas – SNR-Molecular Cloud interaction – Cosmic-ray/X-ray ionization rate [CI] 3 P P 0 (500 GHz) Survey Tanaka+ ApJL submitted – Evidence of GMC formation – Gas inflow (bar-driven?) ⇒ Cluster Formation in future! [CI]-excess

Conclusions GC survey in the Submillimeter is still important – SF in the past, at present and in future – Evolution of Sgr A* Singledish Telescope is necessary for the GC survey – Multibeam Heterodyne Receiver – 230, 350, 500 GHz