Active star formation triggered by galaxy interactionDramatic change in environments of galaxy disks and intergalactic space Suzuki et al. (2007,2010a) M101銀河 Suzuki et al. (2010b) M101 Stephan’s Quintet (HCG92) Suzuki et al. 2007, 2010 Suzuki et al. 2010 (submitted) High-velocity (150 km/s) HI gas infall → Four active star-forming regions Gas & dust stripping from galaxies → Intergalactic star-forming regions AKARI → Investigation of star formation acitivity on a kpc scale
Active star formation triggered by galaxy interaction
Dramatic change in environments of galaxy disks and intergalactic space Suzuki et al. (2007,2010a) M101銀河 Suzuki et al. (2010b) M101 Stephan’s Quintet (HCG92) Suzuki et al. 2007, 2010 Suzuki et al. 2010 (submitted) High-velocity (150 km/s) HI gas infall → Four active star-forming regions Gas & dust stripping from galaxies → Intergalactic star-forming regions AKARI → Investigation of star formation acitivity on a kpc scale in nearby galaxies. Toyoaki Suzuki (ISAS/JAXA)
Barred spiral galaxy NGC1313 Star-forming activity within the disk has never been discussed because of faint CO emission. → AKARI 2 arcmin ESO 1. Transition stage between SBc and SBm ・Optical image → Bar and two spiral arms ・Metal poor : 12+log(O/H) = 8.2 (Hadfield et al. 2007) Cf. 8.1 for SMC, 8.4 for LMC ・No significant gradient of O/H abundance (Walsh & Roy, 1997) 2. Star forming regions ・Star forming regions over a wide field → Satellite HII regions around Supergiant HI shell Ryder et al. (1995) Red : Hα Satellite HII regions (D=3.2 kpc, Vs=42 km/s) Cf. Typical size of HI shell D～100 pc Ryder et al (1995) HI column density map ・HI image→ chaotic morphology. → Tidally disrupted by a companion?? Southern arm 3.2 kpc
→ Enhanced star formation at the supergiant HI shell.
Starburst triggered by expanding supergiant HI shell in southern arm & satellite HII regions ?? SFE : ≦ 10-9 yr-1 for normal spiral ～ 10-8 yr-1 for starburst Kennicutt (1998) Star formation efficiency map Revealed by AKARI ■ Star formation efficiency (SFE) (ΣSFR/Σgas [yr-1]) 4x10-9 2x10-9 8x10-9 6x10-9 1x10-8 Contour: 24 um ■ AKARI observations (24 – 160 um) Cold dust(~20K) → Gas surface density, Σgas Warm dust(~60K) → SFR surface density, ΣSFR Southern arm ～ 2x10-8 yr-1 !! > Nouthern arm Satellite HII reg. ～4-5x10-9 yr-1 Color: SFE
ALMA observations of NGC1313 ■ Super shells have long been suggested as drivers of molecular cloud formation (and then star formation). However, conclusive observational evidence of super shell-associated molecular clouds is just a few of the examples because of poor spatial resolution. e.g. Dawson et al. (2010) Object: reveal evidence of starburst triggered by expanding supergiant HI shell. ■ A kpc-scaled super shell is expected to be capable of changing in ISM environment on galactic scale (→ impact on galaxy evolution). → ALMA gives chance to observe super shells in nearby galaxies. Those in face-on galaxies are less affected by contamination from unrelated emission, which can be problem in the case of our galaxy. → NGC1313 that has the largest super HI shell (3 kpc) is a best candidate for ALMA observation. Very active star formation in the southern arm and satellite HII regions may be triggered by expanding supergiant HI shell.
そもそも、空間分解能の制限で遠くの天体は困難。また系内天体であればコンタミの問題もあり、観測例がすくない。 ALMAによる高空間分解・高感度によって、近傍銀河まで対象範囲になりえる。Face-on galaxyを選べば、コンタミ問題も低減されるであろう。 典型的なsupeshellサイズは100pc程度であり、銀河のローカルな領域での環境変化に影響を及ぼす。 一方、Kpcサイズになると、supeｒshell による影響は銀河スケールまで及ぼし得ると期待され、 銀河進化の過程で無視できないイベントとして位置づけられる。このような観点でNGC1313は、大変良い観測対象である。 AKARIで明らかになったsouthern arm, satellite HII regionでの活発な星形成が super shell に起因しているという裏付けを 得るためには、ALMAの観測が必須となる。 ‹#›
ALMA observing plan of NGC1313 ■ Observations 12CO(J=1-0) : Dynamics and spatial distribution of molecular clouds to associate CO clouds with the supergiant HI shell. Continuum emission@450, 850um: Temperature map of cold dust to identify prestellar (TD~10 K). regions. Stutz et al. (2010) ■ Sensitivity requirement From Swedish ESO Submilimeter Telescope, I12CO(J=1-0)= 810 Jy/sr (ave.) @southern arm (Contursi et al. 2002) → ~40 μJy/beam (ALMA beam size 45”) ■ Target area : Supergiant HI shell in NGC1313 - Southern arm (early science phase) - All of the area along the shell (full science phase) ALMA FOV (45” @115GHz) Southern arm 20 pc/arcsec From AKARI, B(450um) = 14 MJy/sr, B(850um) = 4 MJy/sr @southern arm → Tb ~ 1 mK
しかし、 normal から starburst galaxyに相当する広いレンジにわたる星形成が ‹#›
Flux intensity [ Jy ] Wavelength [ μm ] AKARI Spitzer IRAS Tc = 21 K Tw = 62 K M81 NGC1313 Flux intensity [ Jy ] Wavelength [ μm ] AKARI IRAS ISO T [K] Luminosity [L◎] Mass [M◎] Gas(HI) -to-dust mass ratio Gas(HI+H2)-to-dust mass ratio Cold dust 26 1.0x109 9x106 1400 1700 Warm dust 64 0.7x109 7x103 HI mass surface density = 1.2x107 M◎/arcmin2 (Ryder et al. 1995) H2/HI mass ratio = 0.2 (ave.) (Israel et al. 1997) ～SMC
Cold & warm dust distribution
Cold dust 分布 : HI gas 分布& PAHと良い相関 Warm dust 分布 : HII region と良い相関 5 bands (24, 65, 90, 140, 160 um) を用いて、各bin毎にSED fitting (amplitudes, temperatures: all free) Ryder et al (1995) HI column density
Spiral arms & barとdiffuse ISM に有意な冪の差が見られない。 Cf. N≒2 @ spiral arms (M81, M101) Jeans instability による星形成が ”銀河円盤全体” で支配的か ? (1) N≒1.5 , (2) Spiral arms : 渦巻腕周囲で星形成、 (3) Diffuse ISM: Supershells 周囲で星形成 NGC1313 Diameter:1 kpc Σgas = 1700 Σcold_dust_mass ΣSFR = 5.6x10-4210 (log Lw-0.6/1.04) Suzuki et al. (2010) 一定と仮定 ΣSFR ∝ ΣNgas Kennicutt 1998 NGC1313 Kennicutt-Schmidt law in the disk of NGC1313 ΣSFR ∝ ρgasτgrowth H Gravitational instability (Elmegreen 1994) Jeans instability : τgrowth∝ ρ0.5gas Ρgas: Gas density τgrowth : Instability growth rate H : disk height → ΣSFR ∝ Σ1.5Gas
Comments