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

Observations of star formation induced by galaxy-galaxy & galaxy-IGM interactions with AKARI

Galaxy-Galaxy & Galaxy-IGM interactions

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

StrippingStephan’s Quintet M101

Stephan’s Quintet-A

NASA

NASA

Condensing

Star formation induced by galaxy-galaxy interactions

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

kpc scale?

Interacted face-on spiral galaxies : M101, M81 & NGC1313

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 arcminNGC1313

NOAO/AURA/NSF

3 arcmin

NASA

M101 M81

・ Four-giant HII regions in outer spiral arms.

・ Star forming regions surrounding the giant super shell.

AKARI observations

3 μm 4 μm 7 μm 11 μm

15 μm 24 μm 65 μm 90μm

140μm 160μmAKARI 10-band images of NGC1313

Cold dust

Wavelength [μm]

Flux

den

sity

[Jy]

Warm dust

Graybody model (β=1)

Local SED

■ Spectral decomposition into the two dust components

■ Finer allocation of AKARI mid to far-IR bands・ Continuously covers thermal emission from the two dust components.

Cold dust luminosity

Warm dustluminosity

■ 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)

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

10 kpc

10 kpc

10 kpc 10 kpc

10 kpc

M101 M81 NGC1313

Cold and warm dust distributions

Suzuki et al. (2007, 2010, 2012 to be submitted)

SFR and Gas surface densities ■ SFR surface density, ΣSFR

■ Gas surface density, ΣGas

・ OB stars are instantaneously formed.・ Initial mass function is constant.

Assumptions

Combined SFR (Calzetti et al. 2007)SFR(Hα) = 5.6x10-42 L(Hα)corr -(1)

L(Hα)corr = L(Hα)obs +0.031L(24μm) -(2)

L(Hα)corr - LW relation

∑Lw(r,θ) → ∑SFR (r,θ)

∑gas(r,θ) = GDR(r) ・∑ Mcold (r,θ)

SFR(Lw) =5.6x10-42 10log Lw -0.6

log[Lw] =log[L(Hαcorr)] + 0.6

ΣLw : warm luminosity surface density

ΣMcold: cold dust mass surface densityGDR : gas-to-dust mass ratio

ΣSFR, Σgas can be obtained at each kpc-scale field

M◎yr-1 kpc-2

Suzuki et al. (2010)

Relation between SFR and Gas1

1000

10-3

10-6

∑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 for individual galaxies.・ Power-law index is not always constant 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 k

pc-

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

What controls active star formation in the four-giant HII regions in M101 ?

⇔ 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.

(Santillan et al. 1999)

(Van der Hulst et al.,1998)

-SFR ∝ gas density1 (Elmegreen 1994)

■ Observational results - High velocity gas (HVG) infall (150 km/s) near giant HII regions..

■ 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. (2) Star forming activities in the giant HII regions are highest in M101

Star formation induced by galaxy-IGM collision

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)

■ Galaxy-IGM collision (ΔV~1000 km/s)

- Ongoing IGM star formation - SQ-A: ΣSFR~8x10-3 M◎ yr-

1 kpc-2

Stephan’s Quintet (SQ, HCG92)

・ X-ray emission (T~107 K, ne=0.03/cc)

Natale et al. (2011)Triggered by compressing preexisting giant molecular clouds with shock.

Xu et al. (2003)

- Collision energy ~1056 erg

-Enrichment of the IGM by stripping of metal-enriched gas 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~102-3 K, nH=102-3/cc) Appleton et al. (2006)

NGC7320(foreground galaxy)

NGC7319

NGC7318bNGC7318a

SQ-A

SQ-B

NGC7317

Shock

Mid-infrared observations of the SQ

(1) Clumpy molecular clouds embedded in plasma (2) Large line width (ΔV~900 km/s ~collision speed)

- H2 line is excited by shocks (Vs=5-20 km/s)

Contour: H2 0-0 S(3) 9.7umImage: X-ray Cluver et al. (2010)

It’s still unclear that H2 gas coexists with dust grains.

(3) Large quantity of H2 mass (~106 M◎ kpc-2)- Shocks induced the formation of H2

gas in dense preshock HI clouds (nH ~102-

3/cc). Appleton et al. (2006) , Cluver et al. (2010)

& Extremely luminous (LH2 ~1042 erg/sec ~3 Lx-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 Appleton et al. (2006) , Cluver et al. (2010)

Shocked region

AKARI Far-IR observations of the SQ

Wavelength [μm]

Flux

den

sity

[Jy]

T=22 K

Gray body(β=1)

■ SED at the shocked region

Far-IR emission at 160 microns clearly shows

good spatial correlation with H2 and X-ray

emissions at the shocked region.

■ AKARI four-band images

-Thermal emission from dust grains (~20K)

Shocked region

65μm

90μm

140μm 160μm

Suzuki et al. (2011)

- Dust sputtering time scale ~ collision age(~106 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-ANGC7319

NGC7320

SQ-B Contour: X-ray

Shock-excited [CII]158μm line ?

Suzuki et al. (2011)Assumption: Dust emission is constant in Flux over WIDE-L(140μm) and N160(160μm) bands

Σ L[CII] = 1.0 x1039 erg s-1 kpc-2+0.4-0.5 ~ΣLH2

> ΣLX-

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

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.

Summary■ 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.-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).

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