COSMIC DUST—NEAR AND FAR
ASP Conference Series, Vol. 414, 2009
Th. Henning, E. Grün, and J. Steinacker, eds.

Origin and Nature of Dust in the Early Universe
T. Nozawa1 , T. Kozasa2 , H. Umeda3 , H. Hirashita4 , K. Maeda1 ,K. Nomoto1,3,5 , N. Tominaga6 , A. Habe2 , E. Dwek7 , T.T. Takeuchi8 ,T.T. Ishii9
1 Institute for the Physics and Mathematics of the Universe, Universityof Tokyo, Kashiwa, Chiba 277-8568, Japan
2 Department of Cosmosciences, Graduate School of Science, Hokkaido
University, Sapporo 060-0810, Japan
3 Department of Astronomy, School of Science, University of Tokyo,
Bunkyo-ku, Tokyo 113-0033, Japan
4 Institute of Astronomy and Astrophysics, Academia Sinica, Taipei 106,
Taiwan5 Research Center for the Early Universe, School of Science, Universityof Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan
6 Division of Optical and Infrared Astronomy, National Astronomical
Observatory of Japan, Mitaka, Tokyo 181-8588, Japan
7 Laboratory for Astronomy and Solar Physics, NASA Goddard Space
Flight Center, Greenbelt, MD 20771, USA
8 Institute for Advanced Research, Nagoya University, Nagoya 464-8601,
Japan
9 Kwasan and Hida Observatories, Kyoto University, Yamashina, Kyoto
607-8471, Japan
Abstract.
We present recent advances in theoretical studies of the formationand evolution of dust in primordial supernovae (SNe) that are considered tobe the main sources of dust in the early universe. Being combined with theresults of calculations of dust formation in the ejecta of Population III SNe, theinvestigations of the evolution of newly formed dust within supernova remnants(SNRs) show that smaller grains are predominantly destroyed by sputtering inthe shocked gas, while larger grains are injected into the ambient medium. The
mass of dust grains surviving the destruction in SNRs reaches up to 0.1–15 M⊙ ,which is high enough to account for the content of dust observed for the hostgalaxies of quasars at z > 5. In addition, the transport of dust formed in theejecta causes the formation of low-mass stars in the dense shell of primordial
SNRs and affects the elemental composition of those stars. We also show thatthe flat extinction curve is expected in the high-redshift universe where SNe are
the possible sources of dust.

1.

Introduction

Recent submillimeter and millimeter observations have detected the continuum
thermal emission of dust from a growing number of quasars at redshifts larger
than z = 5 (Bertoldi et al. 2003; Priddey et al. 2003; Robson et al. 2004; Beelen
et al. 2006; Wang et al. 2008a, 2008b). The estimated mass of dust in the host
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galaxies of these quasars is 108 –109 M⊙ , which implies a rapid enrichment of theinterstellar medium (ISM) with dust in such young systems. In addition, therest-frame near-infrared photometries of quasars at z > 5 with Spitzer (Hines
et al. 2006; Jiang et al. 2006) have confirmed the existence of hot dust heatedby the central active nuclei. This indicates that the fundamental structures ofquasars such as accretion disks and hot-dust tori had been already establishedduring the first billion years of cosmic history. However, the origin and natureof dust in the high-redshift universe are not necessarily the same as those in
our Galaxy. In fact Maiolino et al. (2004a) have found that the rest-frame UV
extinction curve observed for the quasar SDSS J1048+4637 at z = 6.2 is quite
different from those in local galaxies, suggesting that the source and evolutionof dust at high redshifts (z > 5) are different from those at low redshifts (z < 4).
Dust grains in the early universe have great impacts on the formation processes of stars and galaxies. Dust grains control the chemistry of the ISM bylocking up a large fraction of refractory elements and by serving as catalysts
for chemical reactions of gas. Especially they foster the formation of hydrogenmolecules on their surfaces (Cazaux & Tielens 2004; Cazaux & Spaans 2004),
enhancing the star formation rate (SFR) in metal-poor star-forming gas clouds(Hirashita & Ferrara 2002). In dense metal-poor molecular clouds, dust grains
provide efficient cooling pathways of gas through their thermal emission, which
triggers the fragmentation of gas clouds into low-mass clumps of ∼0.1–1 M⊙ formetallicity of 10−6 –10−4 Z⊙ (Schneider et al. 2002, 2003, 2006a; Omukai et al.
2005; Tsuribe & Omukai 2006) and affects the initial mass function (IMF) in themetal-poor universe (e.g., Schneider et al. 2006b). Dust grains also influence thethermal balance in the interstellar space by producing photoelectrons that heat
the gas and by reemitting the absorbed stellar light at infrared wavelengths.
The reprocessing of stellar light by dust at high redshift is also crucial inunderstanding the structure formation history of the universe from relevant observations. Obscuration of stellar light by dust leads to a serious misleading ininterpreting the cosmic SFR and the IMF of the very early generation of stars
from the cosmic infrared background radiation (e.g., Hauser & Dwek 2001).
Thermal emission from high-z dust dominates the far-infrared background radiation and can even distort the cosmic microwave background radiation (Loeb& Haiman 1997). Thus, the investigation of dust in the early universe is oneof the most important subjects to reveal the nature of stellar populations andunderlying physical processes at the dark age of the universe.
In the previous studies that assessed the effects of dust grains at high redshift, the composition and size distribution of dust grains were assumed to be
the same as those in our Galaxy, and their amount was treated as a parameter (e.g., Loeb & Haiman 1997). However, the physical processes involving theinteractions of dust with photon and gas are sensitive to the chemical composition and size of dust. The size distribution and amount of dust are determinedby the competition between its production and destruction and vary with time
according to the star formation activity. Therefore, in order to evaluate theimpacts of high-z dust, it is essential to clarify the evolution of dust in the early
epoch of the universe by taking account of the formation and destruction of dust
self-consistently.

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At redshift larger than 5 when the cosmic age is less than 1.2 billion years,
the formation of dust in mass-loss winds from asymptotic giant branch (AGB)
stars is too inefficient to supply a copious amount of dust into the ISM because
most of low-mass stars cannot evolve off the main sequence in such an early
epoch. Hence, the main formation sites of dust are considered to be in theejecta of Type II SNe (SNe II) evolving from short-lived massive stars with theprogenitor masses Mpr = 8–40 M⊙ . Indeed the chemical evolution models pointout that 0.1–1 M⊙ of dust per SN II event is required to form to explain alarge content of dust observed in high-redshift galaxies (Morgan & Edmunds
2003; Maiolino et al. 2006; Dwek et al. 2007). On the other hand, numericalsimulations (e.g., Bromm et al. 2002; Abel et al. 2002; Yoshida et al. 2006) andsemi-analytic models (Nakamura & Umemura 2001, 2002) have shown that thefirst stars formed out of the primordial composition of gas are typically very
massive with masses more than 100 M⊙ . Such metal-free stars as massive as
140–260 M⊙ are considered to explode as pair-instability SNe (PISNe, Fryer et
al. 2001; Umeda & Nomoto 2002; Heger et al. 2002) and pollute the ISM byejecting large amounts of metal and dust at the end of their lives.
Dust grains newly condensed in the SN ejecta are subsequently processed
due to sputtering in the hot gas swept up by the reverse and forward shocks,
and their size and mass can be greatly modified before being injected into theISM. Thus, in order to clarify what kind of and how much dust are supplied to
the ISM by SNe, it is necessary to investigate the formation of dust in the ejecta
and its evolution in the shocked gas within the SN remnants (SNRs). In thisproceedings, we describe the properties of dust in the early universe revealed
by a series of our studies; the calculations of dust formation in the ejecta ofprimordial SNe II and PISNe are presented in § 2, and the evolutions of dustin those SNRs are described in § 3. In § 4 we discuss the role of dust in theearly universe, focusing on the impacts of dust on the formation processes of thenext generation of stars. In § 5 the extinction curve expected at high redshift isderived based on the results of calculations given in § 3 and is compared withthe existing observational results. The concluding remarks are presented in § 6.
2.

Formation of Dust in Primordial SNe

In the expanding ejecta of SNe, dust grains can condense through the formationof seed nuclei and their growth by the collisions with the gas in the metal-rich He
core. Based on the nucleation and grain growth theory by Kozasa & Hasegawa(1987), Kozasa et al. (1989, 1991) demonstrated the formation sequence of dustin the ejecta of SN 1987A. They also discussed that the composition of dust
grains formed in the ejecta is controlled by the elemental composition of gas attheir formation sites and that their radii and number densities strongly dependon the time evolution of gas temperature in the ejecta.
Applying the method of dust formation calculations by Kozasa et al. (1989,
1991), Todini & Ferrara (2001) performed the calculations of dust formation in
SNe II with the progenitor metallicity of Z = 0–0.02. They assumed one-zone
models for the composition and density of gas within the He core, adopting
the elemental yields for SNe II with Mpr = 12–35 M⊙ by Woosley & Weaver(1995). Considering the formation and destruction of CO and SiO molecules,

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Figure 1.
Size distribution function of each grains species summed up overthe formation region in the unmixed ejecta (a) for Mpr = 20 M⊙ and (b) for
Mpr = 170 M⊙ , and in the mixed ejecta (c) for Mpr = 20 M⊙ and (d ) for
Mpr = 170 M⊙ .

Todini & Ferrara (2001) showed that C grains can condense along with silicate
and oxide grains in the O-rich environment and that their size distributions are
lognormal-like with the typical radii of 0.03 µm for C grains, 0.002 µm for Fe3 O4grains, and 0.001 µm for MgSiO3 and Mg2 SiO4 grains. The total mass of dust
formed ranges from 0.08 M⊙ to 2 M⊙ depending on the progenitor mass andmetallicity. It should be noted that the one-zone uniform model for gas densityand elemental composition with the same time evolution of gas temperature
tuned by comparing with the observation of SN 1987A is not always applicable
to clarify the dependence of Mpr and Z on the total mass, chemical compositionand size of dust formed in the ejecta.
We investigated the condensation of dust in Population III SNe II andPISNe (Nozawa et al. 2003), adopting the hydrodynamic models and the resultsof nucleosynthesis calculations by Umeda & Nomoto (2002). We consider SNe
II with Mpr = 13, 20, 25, and 30 M⊙ , all of which have the explosion energy of1051 ergs, and PISNe with Mpr = 170 M⊙ and 200 M⊙ whose explosion energies
are 2 × 1052 ergs and 2.8 × 1052 ergs, respectively. As for the mixing of elements
within the He core, we consider the two extreme cases; the unmixed case withthe onion-like elemental composition and the uniformly mixed case. The gas
temperature is calculated for each model by solving the radiative transfer, taking

Origin and Nature of Dust in the Early Universe

251

Figure 2.
Total mass of dust at the time of dust formation versus progenitor
mass (connected by the solid lines), together with total mass of dust surviving
the destruction in SNRs for various ambient gas densities (see § 3); (a) for theunmixed case and (b) for the mixed case. The results for nH,0 = 0.1, 1, and 10
cm−3 are connected by the dot-dashed, dashed, and dotted lines, respectively.

account of the energy deposition from radioactive elements. The formation of
CO and SiO molecules is assumed to be complete.
Our results of dust formation calculations show that there is a great difference in the species of dust formed in the SN ejecta between the unmixed case andthe mixed case. In the unmixed case, various grain species condense reflecting
the difference of elemental composition in each layer: C grains in carbon-rich He
layer, Mg2 SiO4 , Al2 O3 , and MgO grains in O-Mg-Si layer, MgSiO3 , Al2 O3 , andSiO2 grains in O-Si-Mg layer, Si and FeS grains in Si-S-Fe layer, and Fe grainsin the innermost Fe-rich layer. In the mixed case, where the oxygen atoms are
more abundant than carbon atoms, only silicates (MgSiO3 , Mg2 SiO4 , SiO2 ) andoxide (Al2 O3 and Fe3 O4 ) grains form.
Figure 1 shows the differential size distribution of each dust species summed
up over its formation region in the unmixed and mixed cases for SNe II (Figs. 1aand 1c) and PISNe (Figs. 1b and 1d). In the unmixed case, the size distributionsof C, SiO2 , and Fe grains are lognormal-like with relatively large radii (>∼ 0.01µm), while the other grain species have power-law-like distributions with radii
ranging a few orders of magnitude. In the mixed ejecta, the size distributionfunction of dust formed is almost lognormal-like except for Al2 O3 grains. The
average radius of dust for the mixed case is a little smaller compared with thatfor the unmixed case, although the maximum radius of dust is limited to lessthan 1 µm in both cases. The main species of newly formed dust and their size
distribution functions are almost independent of the progenitor mass.
The total mass of dust formed in the ejecta of primordial SNe is shown in
Figure 2 for the unmixed case (Fig. 2a) and the mixed case (Fig. 2b). In general,
the total mass of dust is higher for the mixed case than the unmixed case because
oxygen atoms are more efficiently locked into dust grains as silicates and oxides
in the mixed case. The total mass of dust formed is 0.1–2 M⊙ for Type II SNe
and 25–60 M⊙ for PISNe and increases with increasing the progenitor mass.
The depletion factor, defined as the mass fraction of metal locked into grains tometal available in the ejecta, is 0.05–0.25 for SNe II and 0.45–0.55 for PISNe.

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Therefore, PISNe can convert a much larger fraction of refractory elements into
dust grains than SNe II. Note that the yields of dust in PISNe are consistent withthe results by Schneider et al. (2004) who extended the model of dust formationin SNe II by Todini & Ferrara (2001) to PISNe, adopting the nucleosynthesis
results by Heger & Woosley (2002).
3.

Evolution of Newly Formed Dust in Primordial SNRs

Before newly formed dust grains in the ejecta are injected into the ISM, they
can be processed in the hot gas swept up by the reverse and forward shocks thatare generated by the interaction between the SN ejecta and the surrounding
medium. Dust grains condensed in the He core initially expand with the gas, but
once they encounter the reverse shock, they acquire the high velocities relative
to gas and are eroded by sputtering in the shocked gas. Dust grains are also
decelerated by the gas drag, and some of them are trapped in the hot plasma andare completely destroyed by sputtering. The deceleration rate and erosion rateof dust depend on the temperature and density of gas and the relative velocityof dust to gas, as well as the composition and size of dust. Therefore, in order toreveal the composition, size distribution, and amount of dust supplied from SNe
into the ISM, we need to investigate the motion and processing of dust within
SNRs, based on the physical properties of dust obtained by the dust formation
calculations.
Bianchi & Schneider (2007) have semi-analytically investigated the destruction of newly formed dust through the passage of the reverse shock, by revisiting
the models of dust formation by Todini & Ferrara (2001) and by employing alittle different size distribution of dust from their results. Bianchi & Schneider(2007) found that the size distribution of dust surviving the reverse shock is
skewed to smaller radius in comparison with the original one. The mass of surviving dust is 2–20 % of the initial dust mass and decreases with increasing thegas density in the ambient medium. However, in the calculations, they did notconsider the motion of dust outwardly penetrating through the shocked hot gas,
which is a very important process to pursue the evolution and survival of dust
5
within SNRs. Their calculations are preformed only up to <∼ 10 years.
We investigated the evolution of dust in primordial SNRs (Nozawa et al.
2007), by adopting the results of dust formation calculations by Nozawa et al.
(2003) and by carefully treating the dynamics of dust and its destruction by
sputtering. Here we briefly introduce the method of calculation; the deceleration
rate of dust due to the gas drag is inversely proportional to grain radius andbulk mass density. The erosion rate by sputtering is evaluated by applying the
sputtering yield for each dust species given in Nozawa et al. (2006). Dust grains
being as test particles, the transport and destruction of dust are calculated ina self-consistent manner by taking account of the size distribution and spatial
distribution of each grain species formed in the ejecta.
The time evolution of gas temperature and density in SNRs is numericallysolved by assuming spherical symmetry and considering the cooling of gas via
the atomic process, the inverse Compton, and the thermal radiation of dust. For
the initial conditions of gas in the ejecta, we adopt the hydrodynamic modelsof Population III SNe II and PISNe by Umeda & Nomoto (2002), which areOrigin and Nature of Dust in the Early Universe

253

Figure 3.
Structures of gas density (upper panel ) and temperature (lower
panel ) at given times in the SNR that is generated from the explosion of thestar with Mpr = 20 M⊙ and is expanding into the ISM with nH,0 = 1 cm−3 .
The downward-pointing arrows in the upper and lower panels indicate thepositions of the forward and reverse shocks, respectively.

the same as those used in the dust formation calculations. We assume theuniform ambient medium with a gas temperature of 104 K based on the studiesof the radiative feedback from massive primordial stars (Kitayama et al. 2004;
Machida et al. 2005). To examine the dependence of the evolution of dust on
the gas density in the ISM, we consider the hydrogen number densities of nH,0 =0.1, 1, and 10 cm−3 for the ambient gas density. The calculations are carriedout from 10 years up to 106 years.
Figure 3 shows the time evolutions of the structure of gas density (upper
panel) and temperature (lower panel) in the SNRs for Mpr = 20 M⊙ and ambientdensity of nH,0 = 1 cm−3 . The positions of the forward and reverse shocks
are pointed by the arrows in the upper and lower panel, respectively, at given
times after explosion. We can see that the gas swept up by the forward andreverse shocks is heated to the temperature of 106 –108 K, where dust grains are
efficiently eroded by sputtering (see Fig. 2 in Nozawa et al. 2006).
Figure 4 exhibits the trajectory of dust (upper panel) and the time evolutionof dust radius relative to the initial one (lower panel) within the SNR given in
Figure 3. The thick solid curves in the upper panel depict the trajectories of
the reverse and forward shocks. The evolution of dust within the SNR is shown
for C, Mg2 SiO4 , and Fe grains formed in the unmixed case. Initially dust grainsare expanding with gas, and then collide with the reverse shock at different
times depending on their initial positions: at 3650 yr for C grains formed in theoutermost He core, at 6300 yr for Mg2 SiO4 grains in the O-rich layer, and at
13,000 yr for Fe grains condensed in the innermost He core.

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Figure 4.
Trajectories of C, Mg2 SiO4 , and Fe grains (upper panel ) and thetime evolutions of their radii relative to the initial ones (lower panel ) within
the SNR given in Figure 3. The evolutions of dust with aini = 0.01, 0.1, and1 µm are depicted by the dotted, dashed, and solid lines, respectively. In theupper panel, the trajectories of the reverse and forward shocks and the outerboundary of He core are indicated by the thick solid and dashed lines.

Once dust grains intrude into the reverse shock, the evolution of dust heavily
depends on the composition and initial radius aini (see Fig. 4). Grains with theinitial radius of aini = 0.01 µm (dotted lines) are efficiently decelerated by thegas drag and are trapped in the hot gas between the reverse and forward shocks.
These small grains are completely destroyed by sputtering in the shocked gas.
Grains of aini = 0.1 µm (dashed lines) reduce their sizes by sputtering but are
finally trapped in the dense shell formed behind the forward shock at 2 × 105
years without being completely destroyed. Fe grains of aini = 0.1 µm are injected
into the ambient medium because of its high bulk density. Grains with aini = 1µm (solid lines) can maintain their high velocities and pass through the forward
shock to inject into the ambient medium without being eroded significantly.
In Figure 5, we show the size distribution function of each dust species
surviving the destruction in SNRs. Figure 5a presents the size distribution ofdust in the unmixed case after destruction for Mpr = 20 M⊙ and nH,0 = 1 cm−3 ,and Figure 5b for Mpr = 170 M⊙ and nH,0 = 0.1 cm−3 . Because small grains are
efficiently trapped in the hot gas and are predominantly destroyed by sputtering,
the size distribution of surviving dust is greatly deficient in small-sized grains,compared with that at its formation for both cases of SNe II and PISNe (see Figs.
1a and 1b for their initial size distributions). Thus, we conclude that relatively
large-sized grains inhabit the interstellar space in the early universe, where dust
grains are supplied only by SNe II and PISNe.

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255

Figure 5.
Size distribution of each dust species surviving the destruction in
SNRs for the unmixed case with (a) Mpr = 20 M⊙ and nH,0 = 1 cm−3 and(b) Mpr = 170 M⊙ and nH,0 = 0.1 cm−3 .

The behaviors of size distribution of surviving dust do not depend on theprogenitor mass for SNe II because the explosion energy is the same (E51 = 1)
and thus the time evolution of temperature and density of gas in a SNR is similar.
For nH,0 = 1 cm−3 , the critical radius below which dust is completely destroyedis ∼0.05 µm although it weakly depends on the dust species and their initial
positions. For PISNe, the critical radius is approximately three times higher
than that for SNe II as the results of much higher explosion energy (> 1052
ergs) and more massive hydrogen envelope compared to SNe II.
The evolution of dust in SNRs is also affected by the ambient gas density. A
higher ambient density results in denser shocked gas, which causes the efficienterosion and deceleration of dust due to more frequent collisions with the hotgas. Hence, the critical radius for each dust species increases with increasing
ambient gas density: ∼0.01 µm for nH,0 = 0.1 cm−3 and ∼0.2 µm for nH,0 = 10cm−3 . Accordingly, the total mass of surviving dust decreases with increasing
the gas density in the ISM, as shown in Figure 2. For nH,0 = 10 cm−3 , themass of surviving dust is less than 0.1 M⊙ for all models considered here, andall or almost all (>∼ 85 %) of dust grains formed in the ejecta are destroyed. The
mass fraction of dust destroyed is generally larger for the mixed case than theunmixed case because the mass of dust formed in the mixed case is somewhat
weighted toward smaller size than that in the unmixed case. It can be also seenfrom Figure 2 that the mass fraction of dust destroyed in remnants of PISNe ismuch larger than in SNe II. This is because the critical radius is large and theaverage radius of dust formed is relatively small for PISNe. Nevertheless, fornH,0 = 0.1 and 1 cm−3 , the mass of dust surviving the destruction is between 0.1
M⊙ and 15 M⊙ , which are high enough to account for the dust budget observedin the host galaxies of quasars at z > 5 (Maiolino et al. 2006; Dwek et al. 2007).
Thus, we conclude that SNe II and PISNe are the important dust factories inthe early universe.
It should be mentioned here that although we have focused on Population
III SNe, the results of the present study could be useful for evaluating thecomposition, size distribution, and mass of dust ejected from SNe II with highmetallicity of the progenitors. One of the reasons for this is that the species of256

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dust formed in the ejecta of SNe II and their size distribution are insensitive tothe metallicity of the progenitor stars (Todini & Ferrara 2001; Nozawa 2003). On
the other hand, the increase of the metallicity in the ambient medium enhances
the cooling in the shocked gas and affects the efficiency of dust destruction ina SNR. However, even if we calculate the evolution of dust within a SNR for
the ambient medium with solar metallicity, the destruction efficiency of eachgrain species decreases at most by 15 % compared with that for the primordial
gas composition (Nozawa et al. 2007). Therefore, the results of calculations
described here can be also applicable to Type II-p SNe in the local universe,
regardless of the initial metallicity of progenitor stars and ambient medium.
4.

Role of Dust on the Formation Process of Stars

In the previous sections, we have shown that dust grains can condense even in theejecta of Population III SNe II and PISNe evolving from zero-metal progenitor
stars (§ 2) and that a sufficient fraction of newly formed grains can survive thedestruction in the SNRs as long as the gas density in the surrounding medium
is not very high (§ 3). This indicates that dust grains are surely ejected fromprimordial SNe and can quickly enrich the ISM in the early universe. In thissection we describe the fundamental roles that the first cosmic dust plays in theformation processes of the subsequent generations of stars.
4.1.

Critical Metallicity

The theoretical studies investigating the collapse and fragmentation of the primordial gas (e.g., Bromm & Larson 2004 and references therein) have predictedthat the characteristic mass of the first stars is very high, probably in excess of
100 M⊙ . On the other hand, a typical mass of Population I and II stars seen in
the present days is as low as ∼1 M⊙ . Thus, in the course of cosmic history, there
should have been the transition of star formation mode from massive Population
III stars to low-mass Population II stars.
The fundamental quantity responsible for this transition is the initial metallicity of collapsing gas clouds (Yoshii & Sabano 1980; Omukai 2000). The presence of metals largely enhances the cooling efficiency of gas and governs the thermal and chemical evolution of low-metallicity clouds. If the initial metallicity
reaches a critical value Zcr , the protostellar clouds become gravitationally unstable at higher densities where the Jeans masses are correspondingly lower, andundergo fragmentation to produce low-mass gas clumps. Bromm et al. (2001)
computed the evolution of pre-enriched star-forming gas clouds and found thatthe critical metallicity is likely to be Zcr ≃ 5 × 10−4 Z⊙ . Also, Bromm & Loeb
(2003) and Santro & Shull (2006) have derived the critical abundance of eachheavy element such as C, O, Si, and Fe, above which the fragmentation of gas
clouds is caused by a fine-structure line cooling of the individual heavy element.
It should be noted here that if a significant fraction of metals are locked into
dust grains, cooling by thermal emission of dust is more efficient than metal line
cooling in the dense metal-poor molecular clouds. Schneider et al. (2006a) investigated the thermal history of collapsing gas clouds, developing the calculations
by Omukai et al. (2005) and adopting the models of dust formed in primordial
SNe II by Todini & Ferrara (2001) and PISNe by Schneider et al. (2004). They

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257

concluded that the efficient cooling of gas by dust grains enables fragmentation of gas clouds into solar or sub-solar mass scales even at Zcr = 10−6 Z⊙ .
Their calculations are based on one-zone models for the evolution of clouds, but
this prediction is confirmed by the 3-D hydrodynamical simulations. Tsuribe &Omukai (2006) studied the dynamical collapse and fragmentation of gas clouds
with Z = 10−6 –10−5 Z⊙ and showed that the core of prestellar clouds actually
fragments into small gas clumps. Therefore, dust produced in primordial SNe
plays a critical role in the formation of the first low-mass stars in the universe.
4.2.

Impact of Dust on Population II.5 Stars

In the halo of our Galaxy, many extremely metal-poor (EMP) low-mass stars
with [Fe/H] < −3 have been discovered so far. In particular, two hyper-metalpoor (HMP) stars, HE 0107–5240 (Christlieb et al. 2002) and HE 1327–2326(Frebel et al. 2005) with [Fe/H] < −5, and an ultra-metal-poor (UMP) star, HE0557–4840 (Norris et al. 2007) with [Fe/H] < −4, are considered to be very early
generation of stars. The most iron-deficient HMP stars show extreme (≥100
times) overabundances of C and O and modest (1–100 times) enhancements of
Si and Mg, and several scenarios have been proposed to explain the origin of
their peculiar elemental compositions (Beers & Christlieb 2005 and references
therein). Here we would suggest one possibility that dust grains newly formed inprimordial SNe are responsible for the elemental compositions of these low-massstars, considering that they are the second generation of stars formed in thedense shell of primordial SNRs, which are referred to as Population II.5 stars(Mackey et al. 2003; Salvattera et al. 2004; Machida et al. 2005).
As demonstrated in § 3, the results for the evolution of dust within a SNR
show that the dust grains surviving the destruction but not injected into theambient medium are accumulated in the dense shell behind the forward shock
after 105 to 106 years. The existence of dust in the shell may make possible
the formation of stars with solar mass scales through its thermal emission if its
metallicity is above 10−6 Z⊙ , as shown in the last subsection. In that case, it isexpected that the elemental composition of piled-up grains reflects the elemental
abundance of low-mass Population II.5 stars. Thus, we analyze the metallicity
and metal abundance in the SN shell and compare with observations of HMP
and UMP stars.
Table 1 presents the elemental composition and metallicity in the dense SN
shell for various progenitor masses and gas densities in the ambient medium. We
show only the results calculated for the unmixed case of SNe II since Fe-bearing
grains are not significantly piled up in the shell of SNRs for the mixed case of
SNe II and both cases of PISNe. Table 1 shows that metallicity of the gas in
the shell is in the range from 10−6 to 10−4 Z⊙ , which satisfies the condition
to cause the formation of stars with solar mass scales (Schneider et al. 2006a;
Tsuribe & Omukai 2006). We can also see that most of calculated [Fe/H] spans
the range from −6 to −4.5 and are in good agreement with those for HMP and
UMP stars. Thus, we can conclude that the transport of dust segregated frommetal-rich gas can be an important process to form the iron-poor low-mass stars
such as HMP and UMP stars.
In Figure 6a, we also plot the abundances of C, O, Mg, and Si in the denseshell relative to solar value for the ambient densities of 0.1 and 1 cm−3 . The

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Table 1.
Metallicities, [Fe/H], and abundances of C, O, Mg, and Si relative
to Fe in the dense shell of primordial SNRs for various progenitor masses andambient gas densities. The results calculated for the unmixed case of SNe II
are only presented (see text).

Mpr

log(Z/Z⊙ )[Fe/H]

13
20
25
30

−5.89
−5.44
−5.55
−5.33

−6.43
−5.20
−5.90
−5.56

[C/Fe][O/Fe]−3nH,0 = 0.1 cm−0.274
0.117
1.11
0.566

−0.699−0.595−1.42−0.043[Mg/Fe][Si/Fe]−0.2300.034−0.500
0.739

1.92
0.410
−0.5520.866−0.459
1.16
0.407
1.09

1.01
1.87
0.989
1.26

−3.89
−1.80
0.232
−1.23

0.599
2.14
2.34
2.66

nH,0 = 1 cm−3
13
20
25
30

−4.72
−4.68
−4.79
−4.60

−5.15
−5.53
−5.23
−5.11

1.11
0.992
1.09
0.797

−0.5550.585−0.4120.242

nH,0 = 10 cm−3
13
20
25
30

−4.40
−4.09
−3.91
−3.84

−4.13
−4.92
−5.10
−5.11

0.284
0.946
1.60
−0.207

−2.54
−2.15
0.122
0.375

Origin and Nature of Dust in the Early Universe

259

Figure 6.
Abundances of C, O, Mg and Si relative to solar value in thedense shell of primordial SN II remnants for nH,0 = 0.1 and 1 cm−3 (filled
triangles); (a) derived from the elemental composition of the grains piled up
in the shell, and (b) derived from the elemental composition of the piledup grains and the gas outside the innermost Fe layer. For the observational
data of HMP and UMP stars, the 3-D corrected abundances are adopted andare denoted by plus (HE0107–5240 with [Fe/H] = −5.62, Collet et al. 2006),
asterisk (HE1327–2326 with [Fe/H] = −5.96, Frebel et al. 2008), and cross(HE0557–4840 with [Fe/H] = −4.75, Norris et al. 2007).

results are denoted by triangles, while the observational data of HMP and UMP
stars are plotted by other symbols. The calculated abundances of Mg and Si also
reproduce 1–100 times overabundances in HMP stars. Therefore, the elemental
composition of dust piled up in the shell can influence the abundance patterns ofmetals such as Fe, Mg, and Si in HMP and UMP stars if they are Population II.5
stars. However, more than 100 times excesses of C and O cannot be reproduced
by any models considered here. For one of the reasons, we assumed that themetal-rich gas in the ejecta does not mix with the gas in the shell.
Hence, we then examine the abundance patterns in the shell by consideringthat in addition to the piled-up grains, the gas outside the innermost iron layer of
the SNe is incorporated into the shell until the time of star formation. The results
are shown in Figure 6b. This case can produce the extreme overabundances of
C and O, but leads to unreasonable excesses (≥ 100 times) of Mg and Si as
well, which is not in agreement with the observations. It might be possible toreproduce the elemental abundances of HMP stars if the Si-Mg-rich layer is not
mixed with the gas in the shell, although it is necessary to clarify what extent of
the gas in the ejecta can be actually incorporated into the shell when Population
II.5 stars form.
We also mention here that Venkatesan et al. (2006) have proposed that thepeculiar abundances of refractory elements in EMP stars can be reproduced by
the transport of newly formed dust decoupled from the metal-rich gas in primordial SNe II and PISNe with Mpr = 10–150 M⊙ . However, they consideredthat the transport of dust is driven by the UV radiation field from the stellar
cluster within a SNR, which seems to be an impracticable assumption. Furthermore, they have not made a detailed comparison between their results and theabundance data of EMP stars.

260
5.

Nozawa et al.
Extinction Curve Expected at the High-redshift Universe

The properties of dust at high redshift have been probed by the dust extinction
curves for quasars at z > 4.9 (Maiolino et al. 2004a). In particular, broad
absorption line (BAL) quasars are good targets on this matter because they areassociated with powerful outflows of dense gas and are significantly reddened bydust. It has been also known that the extinction properties for low-z (z < 4)
BAL quasars are well described by the extinction curve in the Small Magellanic
Cloud (SMC) (Reichard et al. 2003; Richard et al. 2003; Hopkins et al. 2004).
However, Maiolino et al. (2004a) found that the UV extinction curve for the
low-ionization BAL quasar SDSS J1048+4637 at z = 6.2 is flat at wavelengths
λ>∼ 1700 Å and is rising at λ <∼ 1700 Å , which is quite different from that in the
SMC. Thus, the difference in the extinction curve indicates the different origin
and evolution of dust at high redshifts (z > 5). Stratta et al. (2007) find thatthe spectral energy distribution of afterglow of gamma-ray burst (GRB) 050904
at z = 6.3 can be reproduced by adopting the extinction curve for the z = 6.2
quasars. Willott et al. (2007) also show that the extinction curve for the quasar
CFHQS J1509–1749 at z = 6.12 is similar to that for the z = 6.2 quasar. These
are additional evidence implying that the dust production mechanism at z > 5
is different from that at z < 4.
In this sense, the extinction curve for the z = 6.2 quasars allows us to examine the hypothesis that the formation of dust in the early universe is dominatedby SNe II and/or PISNe. Maiolino et al. (2004b) have suggested that the extinction curve for the z = 6.2 quasar is in excellent agreement with that derived
by using SNe II dust models by Todini & Ferrara (2001). Models of dust formedin the unmixed SNe II by Nozawa et al. (2003) can also successfully reproduce
the extinction curve observed for the quasar at z = 6.2 by weighting the progenitor mass with the Salpeter IMF (Hirashita et al. 2005). However, these studies
apply the composition, size distribution, and mass of dust at the time of its formation and do not take into account the modification on the physical propertiesof dust due to the destruction in SNRs. Recently, Bianchi & Schneider (2007)
consider the processing of dust by the reverse shock within SNRs and show thatthe extinction curve by the dust surviving the destruction is still consistent withthe extinction curve for the z = 6.2 quasar.
Here we present the extinction curve expected at the high-redshift universe(Hirashita et al. 2008) by using the size distribution and mass abundance of each
dust species surviving the destruction in SNRs by Nozawa et al. (2007). The
results of dust evolution within SNRs show that small-sized grains are destroyed
more efficiently and that the typical size of dust injected from SNe to the ISM
is biased toward relatively large size (> 0.02 µm). Furthermore, for the higher
ambient gas density, the efficiency of dust destruction is higher, and the average radius of surviving dust shifts toward further large radius. As a result, thederived extinction curve becomes flatter as the ambient gas density increases,
regardless of types of SNe and mixing of elements within the He core, as shownin Figure 7. This leads to the conclusion that at high redshift when SNe are thepossible sources of dust, the dust extinction curves are very flat. We also compare our results with the extinction curves for the quasars at z = 6.2 (Maiolino
et al. 2004b). It can be seen that the derived extinction curves can be also
allowed by the current observational constrains.

Origin and Nature of Dust in the Early Universe

Figure 7.
Dust extinction curves derived for the mixed ejecta with (a)Mpr = 20 M⊙ and (b) Mpr = 170 M⊙ , and the unmixed ejecta with (c)Mpr = 20 M⊙ and (d) Mpr = 170 M⊙ . The dotted and solid lines indicate
the results obtained from the dust surviving the destruction in SNRs for theambient hydrogen number densities of 0.1 and 1 cm−3 , respectively. The casewithout the reverse shock destruction is also shown by the dashed lines. The
shaded area in each panel shows the range observed for the quasar SDSS
J1048+4637 at z = 6.2 (Maiolino et al. 2004b). The extinction is normalized
to the value at λ = 0.3 µm.

261

262

Nozawa et al.

It should be noted here that there are some pieces of evidence supporting theflat extinction curve in high-redshift systems. Two high-z BAL quasars, SDSS
J1044–0125 (z = 5.8) and J0756+4104 (z = 5.1) are detected at submillimeter
wavelengths (Priddey et al. 2003), which indicates the presence of dust grains
in excess of 108 M⊙ . However, it is reported that these quasars do not show
any reddening in their UV spectra (Maiolino et al. 2004a). The absence ofreddening and the detection of dust emission seem to be mutually exclusive,
but this contradiction can be resolved by the flat extinction curve. In addition,even at low redshift, the extinction curves of active galactic nuclei (AGNs) aresuggested to be flat at UV wavelengths and are quite different than those in the
diffuse local ISM (Maiolino et al. 2001a; Gaskwel et al. 2004, 2007). This alsofavors the existence of large grains in the circumnuclear dense region of AGNs
where the coagulation of grains and/or the dominant depletion of small grains
by sublimation are expected to take place (Maiolino et al. 2001b). However, theextinction curves extracted from SDSS quasar samples are similar to that in the
SMC and do not show flat behavior in the UV. This discrepancy is considered tobe due to the systematic difference in the method used in determining extinction
laws from composite quasar spectra (e.g., Willott 2005).
On the other hand, it has been recognized that the afterglows of GRBs are
obscured and reddened by dust (Li, A. et al. 2008 and references therein) andare powerful tools to derive the extinction curves in their host galaxies (e.g.,
Starling et al. 2007; Heng et al. 2008). The spectrum of afterglow is givenby the power-law formula over the broad range from X-rays to near-infrared
wavelengths. Thus, comparing the actually observed spectrum with the powerlaw spectrum extrapolated from the X-ray bands, we can obtain the intrinsic UVoptical extinction curve. Using this method, some studies (Stratta et al. 2005;
Chen et al. 2006) have reported the flat properties for the GRB extinction curves,
almost independent of wavelengths. Li, Y. et al. (2008) also find that the very
flat extinction curves are identified in the host galaxies of four GRBs with a wide
range of redshifts (z = 0.5–4). These facts indicate that the size distribution ofdust is biased toward large grains in the environments surrounding the GRBs,
which can be produced by the coagulational growth of dust in the dense clouds,
the evaporation of small grains due to the intense X-ray and UV emission fromthe GRBs, or the preferential destruction of small grains by SN blast waves.
The latter case is consistent with the destruction of newly formed dust in theshocked gas within SNRs considered here.

6.

Concluding Remarks

In this paper, we briefly outline recent advances in theoretical studies of theformation and evolution of dust in the early universe, with emphasis on thecomposition, size distribution, and amount of dust ejected from primordial SNe.
The investigations of the evolution of newly formed dust in SNRs show that thetransport and destruction of dust grains within SNRs heavily depend on theirinitial radii. Small grains are efficiently decelerated by the gas drag and are
predominantly destroyed by sputtering in the shocked gas. A fraction of large
grains surviving the destruction are finally trapped in the dense shell behindOrigin and Nature of Dust in the Early Universe263

the forward shock, and the larger grains are injected into the ISM without
undergoing significant deceleration and erosion.
Dust grains surviving the destruction in SNRs bring a few important conclusions. The dust grains accumulated in the dense SN shell can enrich theprimordial gas gathered by the forward shock and cause the formation of lowmass Population II.5 stars in the dense shell. Those dust grains can also affectthe elemental compositions of the Population II.5 stars and produce the metalabundance patterns resembling those seen in HMP and UMP stars. In addition,
the large grains ejected from SNe prescribe the extinction property of the ISM
and lead to a flat extinction curve at the high-redshift universe where dust grain
are mainly supplied by primordial SNe. Similar situations appear in the circumnuclear regions of AGNs and the GRB host galaxies at low redshifts. Finally,
the mass of dust surviving in primordial SNRs reaches up to 0.1–15 M⊙ , whichis consistent with the dust yield per SN required to explain a large quantity of
dust observed in high-redshift systems. Thus, SNe play the dominant roles inthe enrichment history of dust as dust factories in the early universe.
Recently, Cherchneff & Lilly (2008) have suggested that primordial SNe are
the first molecule providers to the early universe. They show that the mixingof hydrogen from the progenitor envelope facilitates the formation of moleculesin the He core, which acts as a bottleneck process against the formation of dustin the SN ejecta. However, it has been still debated how large the mixing ofelements extends into the ejecta and whether the mixing is at the knotty levelor at the atomic level. We also note that although the mass of dust formed in
nearby SNe has been considered to be less than 10−3 M⊙ , the estimated massis derived for a limitted number of observations by assuming that the ejecta is
optically thin (see Kozasa et al. in this volume, see also Sugerman et al. 2006;
Miekle et al. 2007; Nozawa et al. 2008).
In order to reveal the evolution of dust grains in the early universe, we needfurther investigations of the fundamental processes of formation and destructionof dust expected at high redshift. Elvis et al. (2002) have proposed the possible
condensation of a large amount of dust in quasar winds, while it is suggested
that the formation of dust in AGB stars can dominate over the contributions
from SNe even at z = 5 (Valiante, R. et al. in preparation). Hence, the formationof dust and its survival in these possible formation sites in the early universeshould be examined. In addition, modellings of the spectral energy distributionof high-z galaxies (e.g., Takeuchi & Ishii 2004) as well as nearby young blue
compact dwarfs (e.g., Takeuchi et al. 2003, 2005; Galliano et al. 2005) are crucial
to specify the properties of dust in the primeval universe. Such sophisticatedstudies that treat the chemical evolution and the radiation transport calculation
self-consistently can be powerful probes to elucidate the origin and nature of thecosmic dust in the early universe, in particular when their results are comparedwith the future observations with Herchel, JWST, and ALMA.

Acknowledgments. This work has been supported in part by World Premier International Research Center Initiative (WPI initiative), MEXT, Japanand by the Grant-in-Aid for Scientific Research of the Japan Society for the
Promotion of Science (18104003, 19740094).

264

Nozawa et al.

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