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

1. Observations of star formation induced by galaxy-galaxy & galaxy-IGM interactions with AKARI Toyoaki Suzuki (ISAS/JAXA) Hidehiro Kaneda (Nagoya Univ.) Takashi Onaka (Tokyo Univ.)

2. Galaxy-Galaxy & Galaxy-IGM interactions Star formation activities can be influenced by interactions. Stephan’s Quintet M101 Stripping Infall Enrichment of the IGM Condensing (1) Triggered star formation NASA Stephan’s Quintet-A NASA (2) Intergalactic star formation Key topics What is the star formation process on a kiloparsec scale ? How is the kinetic energy of collisions released to form H2 gas providing a reservoir of fuel for future star formation ?

3. Star formation induced by galaxy-galaxy interactions Key topic What is the star formation process on a kpc scale?

4. Interacted face-on spiral galaxies : M101, M81 & NGC1313 3 arcmin 3 arcmin 3 arcmin M101 M81 NGC1313 NASA NOAO/AURA/NSF ・Star forming regions surrounding the giant super shell. ・Four-giant HII regions in outer spiral arms. ・Two prominent spiral arms. 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.

5. AKARI observations ■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) 3 μm 4μm 7μm 11μm ■Finer allocation of AKARI mid to far-IR bands 24μm 65μm 90μm 15μm ・Continuously covers thermal emission from the two dust components. AKARI 10-band images of NGC1313 140μm 160μm Suzuki et al. (2012) to be submitted ■Spectral decomposition into the two dust components Graybody model (β=1) Flux density [Jy] Cold dust Local SED Warm dust Cold dust luminosity Wavelength [μm] Warm dust luminosity

6. Separation between the cold and warm dust components • 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□). (3) The flux densities in each image bins are derived with aperture correction. (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. Cold and warm dust distributions M101 M81 NGC1313 10 kpc 10 kpc 10 kpc 10 kpc 10 kpc 10 kpc Suzuki et al. (2007, 2010, 2012 to be submitted)

7. ・OB stars are instantaneously formed. ・Initial mass function is constant. SFR and Gas surface densities Assumptions ■ SFR surface density, ΣSFR ∑Lw(r,θ) → ∑SFR(r,θ) ΣLw : warm luminosity surface density Combined SFR (Calzetti et al. 2007) Suzuki et al. (2010) SFR(Hα) = 5.6x10-42L(Hα)corr -(1) L(Hα)corr= L(Hα)obs +0.031L(24μm) -(2) L(Hα)corr-LW relation SFR(Lw) =5.6x10-42 10log Lw -0.6 M◎yr-1 kpc-2 log[Lw] =log[L(Hαcorr)] + 0.6 ■Gas surface density, ΣGas ∑gas(r,θ) = GDR(r)・∑Mcold(r,θ) ΣMcold: cold dust mass surface density GDR : gas-to-dust mass ratio ΣSFR, Σgas can be obtained at each kpc-scale field

8. Relation between SFR and Gas M101 Spiral arms Disk-averagedgalaxy samples Kennicutt (1998) ●: starburst galaxies ■: normal galaxies 1000 ∑SFR[ M◎yr-1 kpc-2 ] Aperture diameter = 1 kpc 1 ∑SFR∝ ∑Ngas Giant HII regions ★: M101 ■: M81 ●: NGC1313 10-3 M101 N=1.4 local K-S law by regions 10-6 ∑gas[ M◎ pc-2 ] ∑SFR[M◎yr-1 kpc-2 ] ■Kennicutt-Schmidt (K-S) Law ∑SFR∝ ∑Ngas ・Local K-S law for fields within the galaxies is in agreement with the globalK-S law for individual galaxies. N=1.0±0.5 Spiral arms N=2.2±0.2 Giant HII regions ・Power-law index is not always constant within a galaxy. Suzuki et al. (2010) ∑gas [ M◎pc-2 ] Difference in “N” may indicate difference in star formation process

9. What controls active star formation in the four-giant HII regions in M101 ? ■Observational results - High velocity gas (HVG) infall(150 km/s) near giant HII regions. . (Van derHulst et al.,1998) High velocity gas clouds (150 km/s) ■Numerical simulations -HVG infall causes the Parker instability. (Santillan et al. 1999) ■Theoretical prediction for the star formation law NIST -SFR ∝ gas density1 (Elmegreen 1994) Gas pools ⇔ obtained N~1.0 for the four-giant HII regions. (1) Star formation in the giant HII regions is triggered by gas infall due to the interaction. Parker instability Morris (2006) (2) Star forming activities in the giant HII regions are highest in M101 Suzuki et al. (2007) Galaxy-galaxy interactions can dramatically change in starforming activities in a galaxy.

10. Star formation induced by galaxy-IGM collision Key topic How is the kinetic energy of collisions released to form H2 gas providing a reservoir of fuel for future star formation ?

11. Stephan’s Quintet (SQ, HCG92) ■ Compact group of galaxies • Extreme-high density of galaxies ~ density at the core region of rich clusters. • Enrichment of the IGM by stripping of metal-enriched gas • contained in member galaxies. SQ-A →Unique laboratories to study the effect of enrichment of the IGM NGC7319 and to serve as analogues to clusters in the early universe. NGC7318b ■ Galaxy-IGM collision (ΔV~1000 km/s) NGC7318a SQ-B - Collision energy ~1056 erg - OngoingIGM star formation - SQ-A: ΣSFR~8x10-3M◎ yr-1 kpc-2 Shock Natale et al. (2011) Triggered by compressing preexisting giant molecular clouds with shock. Xu et al. (2003) NGC7320 (foreground galaxy) NGC7317 - Large scale shock front (~40 kpc) ・X-ray emission (T~107 K, ne=0.03/cc) Trinchieri et al. (2005) ・H2 emission (T~102-3 K, nH=102-3/cc) Appleton et al. (2006)

12. Mid-infrared observations of the SQ ■ H2 line emission from the shocked region (1) Clumpymolecular clouds embedded in plasma Guillard et al. (2009) (2) Large line width (ΔV~900 km/s ~collision speed) & Extremely luminous (LH2 ~1042 erg/sec ~3 Lx-ray ) - Collision energy ●Kinetic energy of molecular clouds ▲Thermal energy of hot plasma - H2 line is excited by shocks (Vs=5-20 km/s) Appleton et al. (2006) , Cluver et al. (2010) Shocked region (3) Large quantity of H2 mass(~106 M◎kpc-2) Contour: H2 0-0 S(3) 9.7um Image: X-rayCluver et al. (2010) -Shocks induced the formation of H2 gas in dense preshock HI clouds (nH ~102-3/cc). → Reservoir of fuel for future star formation once H2 gas cools. Appleton et al. (2006) , Cluver et al. (2010) It’s still unclear that H2 gas coexists with dust grains.

13. AKARI Far-IR observations of the SQ SQ-A ■ AKARI four-band images NGC7319 × × SQ-B Far-IR emission at 160 microns clearly shows good spatial correlation with H2 and X-ray emissions at the shocked region. × × × NGC7318b NGC7320 65μm 90μm Contour: X-ray ■ SED at the shocked region -Thermal emission from dust grains (~20K) - Dust sputtering time scale ~ collision age(~106Myr). 140μm 160μm → Dust grains are destroyed in the hot plasma → Dust grains should coexist with H2 gas clouds. Suzuki et al. (2011) Shocked region Gray body(β=1) Dusty environment in the IGM is indispensable to form H2 gas Flux density [Jy] T=22 K H2 line is a dominant cooling channel Wavelength [μm]

14. Shock-excited [CII]158μm line ? ■ AKARI four-band images Dramatic change in the spatial distribution between 140 and 160 micron images. 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 ■ Possibility of [CII]158μm line emission -[CII]158line luminosity surface density Σ L[CII] = 1.0 x1039erg s-1 kpc-2 +0.4 -0.5 ~ΣLH2 > ΣLX-ray Assumption: Dust emission is constant in Flux over WIDE-L(140μm) and N160(160μm) bands Suzuki et al. (2011) The IGM in the SQ is dusty environment (1) Warm H2 can be formed on dust grains as fuel for future star formation. (2) [CII] & H2 lines rather than X-ray emission arepowerful cooling channels to release collision energy.

15. Summary ■ Star formation induced by galaxy-galaxy interactions ■Interacted spiral galaxies: M101, M81, and NGC1313 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 For example : the four-giant HII regions in M101 -Local K-S law indicates that star formation istriggered by HVG infall. -Star formation activities are highest in M101 despite outer regions. Galaxy-galaxy interactions can dramatically change in starforming activities in a galaxy. ■ Star formation induced by galaxy-IGM interaction ■Stephan’s Quintet: IGM star formation & the large-scale shock • 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] ~LH2 > LX-ray). In the dusty IGM, H2 can be formed on dust grains as fuel for future star formation. [CII] and H2lines rather than X-ray emission are powerful cooling channels to release collision energy.

16. Ghost from WIDE-L ~4 arcmin ~1 arcmin