Toyoaki Suzuki (ISAS/JAXA) Hidehiro Kaneda (Nagoya Univ.) Takashi Onaka (Tokyo Univ.)

Key topics (1)What is the star formation process on a kiloparsec scale ? (2)How is the kinetic energy of collisions released to form H2 gas providing a reservoir of fuel for future star formation ? Infall Star formation activities can be influenced by interactions. (1) Triggered star formation Enrichment of the IGM (2) Intergalactic star formation Stripping Stephan’s QuintetM101 Stephan’s Quintet-A NASA Condensing

Key topic (1)What is the star formation process on a kpc scale?

Are these active starforming regions associated with interactions? → The SFR-Gas relation gives an insight into star-formation process. → However, it is difficult to detect CO emission in the three galaxies … → Mid to far-IR image data can provide both SFR and gas content. 3 arcmin ・ Two prominent spiral arms. 3 arcmin NGC1313NGC1313 NOAO/AURA/NSF 3 arcmin NASA M101M81 ・ Four-giant HII regions in outer spiral arms. outer spiral arms. ・ Star forming regions surrounding the giant super surrounding the giant super shell. shell.

3 μm 4 μm 7 μm 11 μm 15 μm 24 μm 65 μm 90μm 140μm 160μm AKARI 10-band images of NGC1313 Graybody model (β=1) Local SED ■ Spectral decomposition into the two dust components ■ Finer allocation of AKARI mid to far-IR bands ■ Finer allocation of AKARI mid to far-IR bands ・ Continuously covers thermal emission from the two dust emission from the two dust components. components. Cold dust luminosity luminosity Warm dust luminosity ■ Mid to far-IR dust emission from spiral galaxies ・ Cold dust (~20 K) → Gas content ・ Warm dust (~60 K) → SFR Cox & Mezger (1989), Suzuki et al. (2010) Cox & Mezger (1989), Suzuki et al. (2010) Suzuki et al. (2012) to be submitted

Separation between the cold and warm dust components (4) The individual SED constructed from the four-band fluxes at each image bin is fitted with a double-temperature grey body model, in which the temperatures are fixed at the obtained for the SED of a whole galaxy. (3) The flux densities in each image bins are derived with aperture correction. (1)Adjust beam sizes of the N60 and WIDE-S bands to those of the WIDE-L and N160 bands (60 arcsec). (2) The images are resized with the common spatial scale among the four bands (25 arcsec □ ). 10 kpc M101M81NGC1313 Suzuki et al. (2007, 2010, 2012 to be submitted)

■ SFR surface density, Σ SFR ■ Gas surface density, Σ Gas ・ OB stars are instantaneously formed. ・ Initial mass function is constant. ・ OB stars are instantaneously formed. ・ Initial mass function is constant. Assumptions Combined SFR (Calzetti et al. 2007) SFR(Hα) = 5.6x L(Hα) corr -(1) L(Hα) corr = L(Hα) obs L(24μm) -(2) L(Hα) corr - L W relation ∑ Lw (r,θ) → ∑ SFR (r,θ) ∑ gas (r,θ) = GDR(r) ・ ∑ Mcold (r,θ) SFR(L w ) =5.6x log Lw -0.6 log[Lw] =log[L(Hα corr )] ΣLw : warm luminosity surface density Σ Mcold : cold dust mass surface density GDR : gas-to-dust mass ratio Σ SFR, Σ gas can be obtained at each kpc-scale field M ◎ yr -1 kpc- 2 Suzuki et al. (2010)

∑ gas [ M ◎ pc -2 ] ∑ SFR [ M ◎ yr -1 kpc- 2 ] ■ Kennicutt-Schmidt (K-S) Law Aperture diameter = 1 kpc M101 ★ : M101 ■ : M81 ● : NGC1313 Disk-averaged galaxy samples Kennicutt (1998) ● : starburst galaxies ■ : normal galaxies N=1.4 ・ Local K-S law for fields within the galaxies is in agreement with the global K-S law is in agreement with the global K-S law for individual galaxies. for individual galaxies. ・ Power-law index is not always constant within a galaxy. within a galaxy. ∑ SFR ∝ ∑ N gas Difference in “N” may indicate difference in star formation process M101 ∑ gas [ M ◎ pc -2 ] ∑ SFR [M ◎ yr -1 kpc- 2 ] Spiral arms Giant HII regions N=1.0±0.5 N=2.2±0.2 ∑ SFR ∝ ∑ N gas Suzuki et al. (2010) local K-S law by regions Spiral arms Giant HII regions

⇔ obtained N~1.0 for the four-giant HII regions. Gas pools Morris (2006) High velocity gas clouds (150 km/s) NIST ■ Numerical simulations -HVG infall causes the Parker instability. ■ Numerical simulations -HVG infall causes the Parker instability. (Santillan et al. 1999) (Van der Hulst et al.,1998) -SFR ∝ gas density 1 (Elmegreen 1994) ■ Observational results - High velocity gas (HVG) infall (150 km/s) near giant HII regions.. ■ Observational results - High velocity gas (HVG) infall (150 km/s) near giant HII regions.. ■ Theoretical prediction for the star formation law ■ Theoretical prediction for the star formation law Parker instability Galaxy-galaxy interactions can dramatically change in starforming activities in a galaxy. Suzuki et al. (2007) (1) Star formation in the giant HII regions is triggered by gas infall due to the interaction. is triggered by gas infall due to the interaction. (2) Star forming activities in the giant HII regions are highest in M101 are highest in M101

Key topic (2)How is the kinetic energy of collisions released to form H2 gas providing a reservoir of fuel for future star formation ?

- Large scale shock front (~40 kpc) - Large scale shock front (~40 kpc) ■ Galaxy-IGM collision (ΔV~1000 km/s) - Ongoing IGM star formation - Ongoing IGM star formation - SQ-A: Σ SFR ~8x10 -3 M ◎ yr -1 kpc- 2 - SQ-A: Σ SFR ~8x10 -3 M ◎ yr -1 kpc- 2 ・ X-ray emission (T~10 7 K, n e =0.03/cc) Natale et al. (2011) Triggered by compressing preexisting giant molecular clouds with shock. Xu et al. (2003) - Collision energy ~10 56 erg -Enrichment of the IGM by stripping of metal-enriched gas contained in member galaxies. contained in member galaxies. Trinchieri et al. (2005) ■ Compact group of galaxies -Extreme-high density of galaxies ~ density at the core region of rich clusters. → Unique laboratories to study the effect of enrichment of the IGM and to serve as analogues to clusters in the early universe. ・ H2 emission (T~ K, n H = /cc) Appleton et al. (2006) NGC7320 (foreground galaxy) NGC7319 NGC7318b NGC7318a SQ-A SQ-B NGC7317 Shock

(1) Clumpy molecular clouds embedded in plasma (2) Large line width (ΔV~900 km/s ~collision speed) (2) Large line width (ΔV~900 km/s ~collision speed) - H2 line is excited by shocks (V s =5-20 km/s) Contour: H2 0-0 S(3) 9.7um Image: X-ray Cluver et al. (2010) Contour: H2 0-0 S(3) 9.7um Image: X-ray Cluver et al. (2010) It’s still unclear that H2 gas coexists with dust grains. (3) Large quantity of H2 mass (~10 6 M ◎ kpc -2 ) - Shocks induced the formation of H2 gas in dense preshock HI clouds (n H ~ /cc). dense preshock HI clouds (n H ~ /cc). Appleton et al. (2006), Cluver et al. (2010) & Extremely luminous (L H2 ~10 42 erg/sec ~3 L x-ray ) & Extremely luminous (L H2 ~10 42 erg/sec ~3 L x-ray ) → Reservoir of fuel for future star formation once H2 gas cools. ■ H2 line emission from the shocked region Guillard et al. (2009) - Collision energy ● Kinetic energy of molecular clouds ▲ Thermal energy of hot plasma ▲ Thermal energy of hot plasma Appleton et al. (2006), Cluver et al. (2010) Shocked region

Wavelength [μm] Flux density [Jy] T=22 K Gray body(β=1) ■ SED at the shocked region Far-IR emission at 160 microns clearly shows Far-IR emission at 160 microns clearly shows good spatial correlation with H2 and X-ray emissions at the shocked region. emissions at the shocked region. ■ AKARI four-band images -Thermal emission from dust grains (~20K) Shocked region 65μm 90μm 140μm160μm Suzuki et al. (2011) - Dust sputtering time scale ~ collision age(~10 6 Myr). → Dust grains should coexist with H2 gas clouds. Dusty environment in the IGM is indispensable to form H2 gas H2 line is a dominant cooling channel → Dust grains are destroyed in the hot plasma × × × × × NGC7318b SQ-A NGC7319 NGC7320 SQ-B Contour: X-ray

Suzuki et al. (2011) Assumption: Dust emission is constant in Flux over WIDE-L(140μm) and N160(160μm) bands Σ L [CII] = 1.0 x10 39 erg s -1 kpc ~ΣL H2 > ΣL X-ray F(160μm)/F(140μm) is not very sensitive to the dust temperature (~20K). → Far-IR emission at the shocked region is hard to explain only the dust emission hard to explain only the dust emission Dramatic change in the spatial distribution between 140 and 160 micron images. ■ AKARI four-band images ■ Possibility of [CII]158μm line emission -[CII]158line luminosity surface density The IGM in the SQ is dusty environment (2) [CII] & H2 lines rather than X-ray emission are powerful cooling channels to release collision energy. (1) Warm H2 can be formed on dust grains as fuel for future star formation.

■ Star formation induced by galaxy-galaxy interactions ■ Star formation induced by galaxy-IGM interaction ■ Interacted spiral galaxies: M101, M81, and NGC1313 ■ Stephan’s Quintet: IGM star formation & the large-scale shock Local K-S law (1) gives an insight into association of star formation with interactions (2) shows the variation of “N” by regions in a galactic disk -Local K-S law indicates that star formation is triggered by HVG infall. -At the shocked region, AKARI clearly shows the presence of dust grains that coexist with warm H2 gas. → The dusty IGM environment. -At the shocked region, AKARI clearly shows the presence of dust grains that coexist with warm H2 gas. → The dusty IGM environment. -Single peak emission seen in the 160 μm image indicates the possibility of the luminous [CII]158 μm line emission (L [CII] ~L H2 > L X-ray ). -Single peak emission seen in the 160 μm image indicates the possibility of the luminous [CII]158 μm line emission (L [CII] ~L H2 > L X-ray ). Galaxy-galaxy interactions can dramatically change in starforming activities in a galaxy. In the dusty IGM, H2 can be formed on dust grains as fuel for future star formation. [CII] and H2 lines rather than X-ray emission are powerful cooling channels to release collision energy. For example : the four-giant HII regions in M101 -Star formation activities are highest in M101 despite outer regions.

~4 arcmin ~1 arcmin Ghost from WIDE-L