2014/09/13

高赤方偏移クェーサー母銀河の星間ダスト進化と減光曲線(Evolution of interstellar dust and extinction
curve in the host galaxies of high-z quasars)

野沢 貴也 （Takaya Nozawa）
国立天文台 （NAOJ）
共同研究者
浅野 良輔、竹内 努 (名古屋大学)、平下 博之 (ASIAA)
Reference:
Nozawa, Asano, Hirashita, Takeuchi (2014, submitted to MNRAS Letter)1-1. Extinction curves in high-z quasars
SDSS J1048+4637 at z=6.2
broad absorption line (BAL) quasars

Hirashita+2008

Maiolino+2004, Nature, 431, 533➔ interstellar dust in the early
universe is SN origin?

Gallerani+2010

1-2. A large amount of dust in high-z quasars

Gas mass (Msun)

Gas mass (Msun)

Calura+2014‐Huge amounts of dust grains (>108 Msun, D/G>0.01) are observed
for the host galaxies of quasars at z > 5➜ Grain growth in molecular clouds may be needed to accountfor such massive dust contents(Michalowski+2010; Mattsson 2011; Valiante+2011; Kuo & Hirashita 2012)## it seems only the contribution of dust from SNe II cannot explain## the observed amount of dust grains in high-z quasars➜ 1 Msun of dust per SN and/or top-heavy IMF are required1-3. Aim of our study
How can we explain the massive dust and unusual extinction
curves observed for high-z quasars in a self-consistent way?

In previous works, we construct the evolution model of grain
size distribution by considering the following dust processes:- production of dust in SNe II and AGB stars- destruction of dust by interstellar shocks- grain growth due to metal accretion in molecular clouds- shattering and coagulation due to grain-grain collisions(Asano, Takeuchi, Hirashita, TN 2013)

We apply this dust evolution model to investigate theevolution of grain size distribution and the expected
extinction curves in high-z dusty galaxies

2-1. Dust evolution model in a galaxy (1)‐one-zone closed-box model (no inflow and no outflow)‐SFR(t) = Mgas(t)/τSF (Schmidt law with n = 1)
‐Salpeter IMF: φ(m) = m-q with q=2.35 for Mstar = 0.1-100 Msun
‐dust processes- production of dust in SNe II and AGB stars- destruction of dust by interstellar shocks- grain growth due to metal accretion in molecular clouds- shattering and coagulation due to grain-grain collisions‐two dust species:- graphite (carbonaceous grains)- silicate (grains species other than carbonaceous grains)‐multi-phase ISM- WNM (warm neutral medium): T = 6000 K, n = 0.3 cm-3
- CNM (cold neutral medium): T = 100 K, n = 30 cm-3
- MC (molecular cloud): T = 25 K, n = 300 cm-3

2-2. Dust evolution model in a galaxy (2)‐evolution of dust mass ΔMd(a,t) with radii between a and a+da
xSFR(t), astration
dust production by SNe II and AGB stars

shock destruction
grain growthshattering

coagulation

2-3. Evolution of extinction curves in galaxies
grain size distribution

dust amountextinction curve

MRN

τSF=5 GyrWNM=0.5
CNM=0.5

Asano, Takeuchi, Hirashita, TN+2013, 2014

‐early phase : formation of dust in SNe II and AGB stars➔ large grains (>0.1 μm） are dominant ➔ flat extinction curve

Asano+12‐middle phase : shattering, grain growth due to accretion of gas metal➔ small grains (< 0.03 μm） are produced ➔ steep extinction curve‐late phase : coagulation of small grains➔ shift of peak of size distribution ➔ making extinction curve flatter2-4. Reproducing the MW extinction curve‐two-phase ISM
・ WNM (T = 6000 K, n = 0.3 cm-3)

WNM=0.5
CNM=0.3
MC=0.2

・ CNM (T = 100 K, n = 30 cm-3)‐three-phase ISM

・ WNM (T = 6000 K, n = 0.3 cm-3)
・ CNM (T = 100 K, n = 30 cm-3)・ MC (molecular clouds)➜ T = 25 K, n = 300 cm-3

Nozawa+2014, submitted- three-phase ISM model including the MC phase can reproduce
the average extinction curve in the MW- ISM phase is one of the important quantities in constructing theevolution model of interstellar dust

3-1. Explaining massive dust in high-z quasars
high-z quasar host: starburst galaxies➜ indicating a high fraction of MC
MH2/MH,total ~ 0.7-0.97 (Calura+2014)‐two-phase ISM:
WNM=0.3 and MC=0.7‐τSF = 0.5 Gyr

Grain growth is necessary toachieve the observed high D/GNozawa+2014 submitted

3-2. Explaining the high-z extinction curves
The presence/absence of 2175 A
bump may be related to the dust
composition of dust rather thanthe dust evolution model- graphite and silicate- amorphous carbon & silicate➜ the derived extinction curve
well match the observed highz extinction curve
Nozawa+2014 submitted

The origin of the 2175 A bump is still unclear➜ small size (<0.02 µm) of graphite? (e.g., Draine & Lee 1984)➜ PAHs (polycyclic aromatic hydrocarbon?) (e.g., Joblin+1992)
・ formation site of PAHs
- AGB stars? (bottom-up scenario) (e.g., Cherchneff+1993)- shattering of C grains? (up-down scenario) (e.g., Seok+2014)

4. SummaryWe investigate the evolutions of grain size distribution
and the extinction curves in high-z dusty galaxies
・ our dust evolution model can reproduce the average extinction
curve in the MW by considering- three-phase ISM (WNM=0.5, CNM ~ MC ~ 0.25)- graphite & silicate
・ a large amount of dust grains and the unusual extinction curve
observed for high-z quasars can be explained by considering- a large mass fraction of MC (>0.5) in the ISM➜ efficient growth and coagulation of dust grains- amorphous carbon & silicate➜ different properties of carbonaceous grains## It is possible that the quasar extinction curves reflect the properties## of dust in circumnuclear (AGN) torus, not those of interstellar dust