Dust Properties At Extremely Low Metallicity: Herschel’s Views 施勇 ( 南京大学 ) Collaborators: Lee Armus (Caltech, US), Yu Gao (PMO, China), Qiusheng Gu (NJU, China), George Helou (Caltech, US), Sabrina Stierwalt (U.of Virginia, US), Junzhi Wang (SHAO, China) ， Zhi-Yu Zhang (Edinburgh, England). Students: Zhou luwenjia, Jiang Chaofeng, Zhang Peilun

Outline 1. Introduction. 2. Observations and Data 3. Dust Properties At Extreme Low Metallicity. (Lu, W. et al.) 4. Star Formation At Extreme Low Metallicity. (Shi, Y. et al.) 5. Summary. July 16, 2014, 大连星际物理与化学

1. Introduction 极端贫金属星系特指：  是一类恒星形成星系，即具 有发射线，而不是只有吸收线。  氧丰度通过氧离子的发射线 强度来测量。  氧丰度低于 10% 太阳丰度的 矮星系。 July 16, 2014, 大连星际物理与化学 Izw 18: 3% Solar

1. Introduction Extremely Metal Poor Galaxies (Z<Z  /10):  Testing existing theories that are built based on metal rich spirals.  A local laboratory to gain insights into galaxy formation in early Universe. 星际物理与化学 July 16, 2014, 大连

2. Observations And Data A Herschel Space Telescope Program (PI: Yong Shi) : PACS (70 and 160 um) + SPIRE (250, 350, 500 um) Imaging of 3 galaxies: PACS (70 and 160 um) + SPIRE (250, 350, 500 um) Imaging of 3 galaxies: 星际物理与化学 July 16, 2014, 大连

2. Observations And Data 跟其他类似 Herschel 项目相比，我们项目的特 点：  相对较大的样本数目（ 3 个星系）： Fisher 2014 Nature, 505,186 (Izw 18) Hunt 2014 A&A, 561, 49 (SBS 0335−052)  提供空间分解的多波段红外图像。 （在 70 微米， Herschel 的分辨率是 6 角秒）。 July 16, 2014, 大连星际物理与化学

Multi-band IR Images Blue: HI gas, Green: Far-UV, RED: um Sextans A 星际物理与化学 July 16, 2014, 大连 2. Observations And Data

Outline 1. Introduction. 2. Observations and data 3. Dust Properties At Extreme Low Metallicity. (Lu, W. et al.) 4. Star Formation At Extreme Low Metallicity. (Shi, Y. et al.) 5. Summary. July 16, 2014, 大连星际物理与化学

3. Dust Properties  Infrared Color  Single modified black-body (MBB) fitting.  Dust/stellar mass ratio.  Gas/dust mass ratio.  IRX-beta relationship. July 16, 2014, 大连星际物理与化学

3. Dust Properties: IR color July 16, 2014, 大连星际物理与化学 Engelbracht et al What we have learned from Spitzer: 10 % Solar  Above 10% Solar: lower Z, higher T.  The trend reverses below 10% solar.

3. Dust Properties: IR color July 16, 2014, 大连星际物理与化学 Updating the plot with our galaxies:  Above 10% Solar: lower Z, higher T.  Below 10% Solar: a large range of T. 极端金属丰度下，金属丰度不是影响尘埃颜色温度的主要因素。

3. Dust Properties: a single MBB fit 星际物理与化学 July 16, 2014, 大连 T: 温度 β ：尘埃发射系数

星际物理与化学 July 16, 2014, 大连 3. Dust Properties: a single MBB fit

Contours are spirals in Galametz+2012 星际物理与化学 July 16, 2014, 大连 3. Dust Properties: a single MBB fit

星际物理与化学 July 16, 2014, 大连 3. Dust Properties: a single MBB fit

DISKSf-aSf-bSf-cSf-dDiskSf-aSf-bSf-c Χ 2 /dof free β Χ 2 /dof β= Dust Properties: a single MBB fit 星际物理与化学 July 16, 2014, 大连

3. Dust Properties: dust/stellar ratio July 16, 2014, 大连星际物理与化学 Izw 18 (Fisher Nature, 505, 186) DDO 68

3. Dust Properties: gas/dust ratio July 16, 2014, 大连星际物理与化学 ESO 146Sextans A DDO 68 A large scatter in HI-gas/dust mass ratio is observed below 10% Solar.

3. Dust Properties: IRX-beta July 16, 2014, 大连星际物理与化学 Boquien et al  A large scatter among EMP galaxies in IRX vs. beta plane.  Does not simply follow the extrapolation.

3. Take-home message Dust properties at extreme low metallicity are different from spirals but show large scatters among themselves.  Metallicity plays crucial roles in dust properties; but other factors also matter! July 16, 2014, 大连星际物理与化学

4. Star Formation at Low Metallicities Stellar Mass Growth Chemical Enrichment Gas heating and cooling ……. Star Formation 星际物理与化学 July 16, 2014, 大连

4. Star Formation at Low Metallicities Cosmic Re-ionization: one of the most important modern astrophysical problems. Pop. III stars initiate the re-ionization; Pop. II stars complete the re- ionization. July 16, 2014, 大连星际物理与化学

4. Star Formation at Low Metallicities However, the basic properties of formation of Pop. III and Pop. II stars are far from understood:  Transition metallicity: Pop. III vs. Pop. II stars.  Star formation efficiency (SFR/gas-mass).  Lyman photon escape fraction.  Structures of ISM and IGM.  …. July 16, 2014, 大连星际物理与化学

4. Star Formation at Low Metallicities Severe limitations of current capabilities in probing the above problems. Using nearby galaxies as local laboratories to get insights into star formation in primordial galaxies. July 16, 2014, 大连星际物理与化学

4. Star Formation at Low Metallicities July 16, 2014, 大连星际物理与化学 Local Laboratories Extremely low metallicity:  Extremely low metallicity galaxies Extremely low metallicity:  Extremely low metallicity galaxies

4. Star Formation at Low Metallicities Existing studies of star formation efficiencies are limited to galaxies with Z > 20%Z  : Kennicutt et al Bigiel et al Bolatto et al …. July 16, 2014, 大连星际物理与化学

Pop. II Stars: 1%-10% Z  4. Star Formation at Low Metallicities However, 星际物理与化学 July 16, 2014, 大连 Wise et al. (2012)

4. Star Formation at Low Metallicities SFR: Relatively Easy! Gas Mass: CO is a poor tracer of H 2 in EMP galaxies: faint and unknown CO-to-H 2 factor.  Employ dust to trace the cold gas content. 星际物理与化学 July 16, 2014, 大连

4. Star Formation at Low Metallicities Spatially Resolved Dust Map! Diffuse Region! GDR=HI-gas- mass/dust-mass Star Forming Regions! GDR x Dust-Mass=Gas Mass 星际物理与化学 July 16, 2014, 大连

Draine-Li Model Fitting 星际物理与化学 July 16, 2014, 大连 4. Star Formation at Low Metallicities

星际物理与化学 July 16, 2014, 大连 If looking at HI alone, SFEs seem normal! Kennicutt-Schmidt relation

4. Star Formation at Low Metallicities July 16, 2014, 大连星际物理与化学 This is in sharp contrast with models of star formation (Ostricker et al. 2010, Krumholz et al. 2013).

4. Star Formation at Low Metallicities July 16, 2014, 大连星际物理与化学 If looking at dust-based total gas masses, the SFEs are much lower than spiral galaxies.

4. Star Formation at Low Metallicities July 16, 2014, 大连星际物理与化学 Low SFEs based on measured dust-based gas. Low SFEs predicted by models. Fundamental differences between what is observed and what models predict!

4. Star Formation at Low Metallicities July 16, 2014, 大连星际物理与化学 Much higher H 2 fractions than model’s predictions!

4. Summary  The properties of dust grains at extreme low metallicity are different from spirals, and show large scatters among themselves.  Using dust to trace total gas, we found that the star formation efficiencies are much lower than spiral galaxies. July 16, 2014, 大连星际物理与化学