Dust Properties in Metal-Poor Environments Observed by AKARI

　　 Hiroyuki HirashitaHiroyuki Hirashita (ASIAA, Taiwan) H. Kaneda (ISAS), T. Onaka (Univ. Tokyo), T. Suzuki (NAOJ), T. Ichikawa (Univ. Tsukuba)

1. Why BCDs (blue compact dwarfs)?2. Sample of AKARI FIR Observation3. Dust Mass and Dust Enrichment4. Implication for High-z Galaxies5. Summary

Outline

1. Why BCDs?

BCDs are nearby “laboratories” of high-z primeval galaxies.

BCD = Blue Compact DwarfsBCD = Blue Compact DwarfsStar formation (blue)Small (compact)

Low metallicity ⇒ early stage of evolution

II Zw 40 (Vanzi et al. 2008) at 9.2 Mpc Z ~ 1/6 Z

400 pc

Dust Enrichment at z ~ 6

Dwek et al. (2007)a lot of other works on high-z quasars

SDSS J1148+5251 (z = 6.4) Md ~ 108 M

Dust enrichment should be efficient even within 1 Gyr of the early galaxy evolution.

UV luminosity

FIR luminosity (Dust)

4dyn

Dust enrichment is important even in a metal-poor phase.

Importance of Dust Enrichment

zform = 10Mvir = 109 M

Hirashita & Ferrara (2002)

Dust enrichment by supernovae

⇒ FIR ~ UV on a short timescale (at a typical metallicity 1/100 Z).

Aim of This Study

☆ To reveal the dust properties and enrichment in the early phase of galaxy evolution:

AKARI observation of BCDs in FIR (50 - 180 m)(1)SED: dust temperature → interstellar radiation field;

total FIR luminosity → star formation rate(2) Dust Mass: dust enrichment history

In the future:＊ Application of our knowledge to high-z low-

metallicity galaxies → observations with ALMA

2. Sample

7 BCDs are selected from IRAScatalog (II Zw 40, Mrk 7, Mrk 71,UM 439, UM 533, II Zw 70, Mrk 36).1 BCD is occasionally detected (II Zw 71).

metallicity ~ 1/3 – 1/10 Z

(1)Four bands: 65 m, 90 m, 140 m, 160 m (new at > 100 m: important for dust temperature estimate).

(2)These bands cover the wavelength continuously. AKARI/FIS bands

Kawada et al. (2007)

II Zw 40

3 kpc

Mrk 71

3 kpc

= 90 m = 90 m

Photometric Results

■ AKARI (this obs.) ◇ IRAS △ Spitzer

Consistent with IRAS at 100 m.Spitzer data of II Zw 40 are also consistent.

Higher dust temperature than spirals Intense ⇒UV radiation field, supporting intense star formation in a concentrated region

dust in radiative equilibrium (large grains)

Td([65/90]): Temperature from 65 / 90 m color Td([140/90]): Temperature from 140 / 90 m color(with an emissivity index = 2; F ∝ B(Td))

Spiral galaxies for comparison

contaminated by very small grains

Dust Temperature

Color-Color Diagram(1) Higher dust temp

eratures than the Milky Way, LMC and SMC.

(2) The colors are consistent with the extension of the Milky Way, LMC and SMC (Hibi et al. 2006).

⇒ Common wavelength dependence of emissivity among these galaxies.

DIRBE data

Star Formation Rate from LFIR

SFR = 2.0 × 10–10 LFIR/L [M/yr](Hirashita et al. 2003)

SF = MH I / SFR: gas consumption timescaleLarge variation of SF

⇒ Star formation in BCD is intermittent?

3. Dust Mass and Dust Enrichment

F: fluxD: distance: mass absorption coefficient (Hildebrand 1983)Td: dust temperature (estimated from 140/90 m color)

Dust mass from FIR flux

€

Md =Fν D2

κ ν Bν (Td )

Dus

t-to

-Gas

Rat

iofin = 0.1 with various SN ⇒10% of metals in stellar ejecta are condensed into dust.

Gas

Metals

Dust

SF

from stars SF Destructionby SN shocks

€

dMgas

dt= −ψ + E

€

dMZ

dt= −Zψ + EZ

Lisenfeld & Ferrara (1998);Hirashita et al. (2002)

€

dMdust

dt= f in EZ − Dψ −

Mdust

τ SN

from stars

Model equations (one-zone)

Dust Enrichment

Typical error

4. Implication for High-z Galaxies

(1)Dust condensation efficiency in stellar ejecta is ~ 10%: roughly consistent with dust formation in SN II (Todini & Ferrara 2001; Nozawa et al. 2003, 2007; Bianchi & Schneider 2007).

(2)Similar dust temperatures of BCDs to those observed for high-z populations (e.g., Chapman et al. 2005). → FIR observations of BCDs may be useful in making strategies for ALMA observations of high-z galaxies.

(3)Compact star formation in BCDs suggested from high dust temperature is similar to that in submillimeter populations (Tacconi et al. 2006).

5. Summary(1)8 nearby blue compact dwarf galaxies (BCDs) are obser

ved by AKARI at = 65 m, 90 m, 140 m, and 160 m.(1)High dust temperatures support intense star formatio

n in concentrated regions.(2)A variety of gas consumption timescale implies inter

mittent star formation activity.(3)Positive correlation between dust-to-gas ratio and me

tallicity is consistent with a picture that ~ 10% of metals ejected from stars condense into dust grains.

(2)Since the dust temperatures are similar to those observed in high-z populations, BCDs could really be used as “nearby laboratories” of high-z galaxies.