Dust Properties in Metal-Poor Environments Observed by AKARI Hiroyuki Hirashita Hiroyuki 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 Observation 3.Dust Mass and Dust Enrichment 4.Implication for High-z Galaxies 5.Summary Outline

1. Why BCDs? BCDs are nearby “laboratories” of high-z primeval galaxies. BCD = Blue Compact Dwarfs  Star 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 J (z = 6.4) M d ~ 10 8 M  Dust enrichment should be efficient even within 1 Gyr of the early galaxy evolution.

UV luminosity FIR luminosity (Dust) 4  dyn Dust enrichment is important even in a metal-poor phase. Importance of Dust Enrichment z form = 10 M vir = 10 9 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 (  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 IRAS catalog (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

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) T d ([65/90]): Temperature from 65 / 90  m color T d ([140/90]): Temperature from 140 / 90  m color (with an emissivity index  = 2; F ∝  B (T d )) Spiral galaxies for comparison contaminated by very small grains Dust Temperature

Color-Color Diagram (1)Higher dust temperatures 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 L FIR SFR = 2.0 × 10 –10 L FIR /L  [M  /yr] (Hirashita et al. 2003)  SF = M H I / SFR: gas consumption timescale Large variation of  SF ⇒ Star formation in BCD is intermittent?

3. Dust Mass and Dust Enrichment F : flux D: distance  : mass absorption coefficient (Hildebrand 1983) T d : dust temperature (estimated from 140/90  m color) Dust mass from FIR flux

Dust-to-Gas Ratio f in = 0.1 with various  SN ⇒ 10% of metals in stellar ejecta are condensed into dust. Gas Metals Dust SF from stars SF Destruction by SN shocks Lisenfeld & Ferrara (1998); Hirashita et al. (2002) 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 observed by AKARI at = 65  m, 90  m, 140  m, and 160  m. a.High dust temperatures support intense star formation in concentrated regions. b.A variety of gas consumption timescale implies intermittent star formation activity. c.Positive correlation between dust-to-gas ratio and metallicity 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.