observations of star formation induced by galaxy galaxy galaxy igm interactions with akari
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Toyoaki Suzuki (ISAS/JAXA) Hidehiro Kaneda (Nagoya Univ.) Takashi Onaka (Tokyo Univ.)

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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.). Galaxy-Galaxy & Galaxy-IGM interactions.

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observations of star formation induced by galaxy galaxy galaxy igm interactions with akari
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.)

galaxy galaxy galaxy igm interactions
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 ?

star formation induced by galaxy galaxy interactions
Star formation induced by galaxy-galaxy interactions

Key topic

What is the star formation process on a kpc scale?

slide4

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.

slide5

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

slide6

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)

sfr and gas surface densities

・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

relation betwee n sfr and gas
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

slide9

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.

star formation induced by galaxy igm collision
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 ?

slide11

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)

slide12

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.

slide13

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]

slide14

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.

slide15

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.

slide16

Ghost from WIDE-L

~4 arcmin

~1 arcmin

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