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 offaint 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 starburstKennicutt (1998)

Star formation efficiency mapRevealed by AKARI

■ Star formation efficiency (SFE) (ΣSFR/Σgas [yr-1])

4x10-92x10-9 8x10-96x10-9 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 armSatellite 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.

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