The First Stars

Abstract

The formation of the first generations of stars at redshifts z > 15-20 signaled the transition from the simple initial state of the universe to one of increasing complexity. We here review recent progress in understanding the assembly process of the first galaxies, starting with cosmological initial conditions and modelling the detailed physics of star formation. In particular, we study the role of HD cooling in ionized primordial gas, the impact of UV radiation produced by the first stars, and the propagation of the supernova blast waves triggered at the end of their brief lives. We conclude by discussing how the chemical abundance patterns observed in extremely low-metallicity stars allow us to probe the properties of the first stars. Comment: 12 pages, 9 figures, proceedings of the IAU Symposium 250 "Massive stars as cosmic engines"

Full text

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arXiv:0802.0207v1 [astro-ph] 1 Feb 2008
??
Proceedings IAU Symposium No. 250, 2007
c ? 2007 International Astronomical Union
DOI: 00.0000/X000000000000000X
The First Stars
Jarrett L. Johnson1,Thomas H. Greif2
and Volker Bromm1
1Department of Astronomy, University of Texas, Austin, TX 78712
2Institut f¨ ur Theoretische Astrophysik,
Albert-Ueberle Strasse 2, 69120 Heidelberg, Germany
Abstract. The formation of the first generations of stars at redshifts z ? 15 − 20 signaled the
transition from the simple initial state of the universe to one of ever increasing complexity. We
here review recent progress in understanding the assembly process of the first galaxies, starting
with cosmological initial conditions and modelling the detailed physics of star formation. In
particular, we study the role of HD cooling in ionized primordial gas, the impact of UV radiation
produced by the first stars, and the propagation of the supernova blast waves triggered at the
end of their brief lives. We conclude by discussing how the chemical abundance patterns observed
in extremely low-metallicity stars allow us to probe the properties of the first stars.
Keywords. cosmology: theory, early universe — galaxies: formation — ISM: HII regions,
molecules — stars: formation, supernovae — hydrodynamics
1. Introduction
One of the key goals in modern cosmology is to study the formation of the first gen-
erations of stars and to understand the assembly process of the first galaxies. With
the formation of the first stars, the so-called Population III (Pop III), the universe was
rapidly transformed into an increasingly complex, hierarchical system, due to the energy
and heavy element input from the first stars and accreting black holes (Barkana & Loeb
2001; Bromm & Larson 2004; Ciardi & Ferrara 2005; Miralda-Escud´ e 2003). Currently,
we can directly probe the state of the universe roughly a million years after the Big Bang
by detecting the anisotropies in the cosmic microwave background (CMB), thus provid-
ing us with the initial conditions for subsequent structure formation. Complementary to
the CMB observations, we can probe cosmic history all the way from the present-day
universe to roughly a billion years after the Big Bang, using the best available ground-
and space-based telescopes. In between lies the remaining frontier, and the first stars and
galaxies are the sign-posts of this early, formative epoch.
To simulate the build-up of the first stellar systems, we have to address the feedback
from the very first stars on the surrounding intergalactic medium (IGM), and the for-
mation of the second generation of stars out of material that was influenced by this
feedback. There are a number of reasons why addressing the feedback from the first stars
and understanding second-generation star formation is crucial:
(i) The first steps in the hierarchical build-up of structure provide us with a simplified
laboratory for studying galaxy formation, which is one of the main outstanding problems
in cosmology.
(ii) The initial burst of Pop III star formation may have been rather brief due to the
strong negative feedback effects that likely acted to self-limit this formation mode (Greif
& Bromm 2006; Yoshida et al. 2004). Second-generation star formation, therefore, might
well have been cosmologically dominant compared to Pop III stars.
(iii) A subset of second-generation stars, those with masses below ≃ 1 M⊙, would have
1

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2Johnson, Greif & Bromm
Figure 1. Evolution of the HD abundance, XHD, in primordial gas which cools in four distinct
situations. The solid line corresponds to gas with an initial density of 100 cm−3, which is
compressed and heated by a SN shock with velocity vsh = 100 km s−1at z = 20. The dotted
line corresponds to gas at an initial density of 0.1 cm−3shocked during the formation of a 3σ
halo at z = 15. The dashed line corresponds to unshocked, un-ionized primordial gas with an
initial density of 0.3 cm−3collapsing inside a minihalo at z = 20. Finally, the dash-dotted line
shows the HD fraction in primordial gas collapsing from an initial density of 0.3 cm−3inside
a relic H ii region at z = 20. The horizontal line at the top denotes the cosmic abundance of
deuterium. Primordial gas with an HD abundance above the critical value, XHD,crit, denoted by
the bold dashed line, can cool to the CMB temperature within a Hubble time.
survived to the present day. Surveys of extremely metal-poor Galactic halo stars therefore
provide an indirect window into the Pop III era by scrutinizing their chemical abundance
patterns, which reflect the enrichment from a single, or at most a small multiple of,
Pop III SNe (Beers & Christlieb 2005; Frebel et al. 2007). Stellar archaeology thus pro-
vides unique empirical constraints for numerical simulations, from which one can derive
theoretical abundance patterns to be compared with the data.
Existing and planned observatories, such as HST, Keck, VLT, and the James Webb
Space Telescope (JWST), planned for launch around 2013, yield data on stars and quasars
less than a billion years after the Big Bang. The ongoing Swift gamma-ray burst (GRB)
mission provides us with a possible window into massive star formation at the highest
redshifts (Bromm & Loeb 2002, 2006; Lamb & Reichart 2000). Measurements of the near-
IR cosmic background radiation, both in terms of the spectral energy distribution and
the angular fluctuations provide additional constraints on the overall energy production
due to the first stars (Dwek et al. 2005; Fernandez & Komatsu 2006; Kashlinsky et al.
2005; Magliocchetti et al. 2003; Santos et al. 2002). Understanding the formation of the
first galaxies is thus of great interest to observational studies conducted both at high
redshifts and in our local Galactic neighborhood.
2. Population III Star Formation
The first stars in the universe likely formed roughly 150 Myr after the Big Bang, when
the primordial gas was first able to cool and collapse into dark matter minihalos with
masses of the order of 106M⊙(Bromm et al. 1999, 2002; Abel et al. 2002). These stars
are believed to have been very massive, with masses of the order of 100 M⊙, owing to the

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The First Stars3
Figure 2. Characteristic stellar mass as a function of redshift. Pop III.1 stars, formed from
unshocked, un-ionized primordial gas are characterized by masses of the order of 100 M⊙.
Pop II stars, formed in gas which is enriched with metals, emerged at lower redshifts and have
characteristic masses of the order of 1 M⊙. Pop III.2 stars, formed from ionized primordial
gas, have characteristic masses reflecting the fact that they form from gas that has cooled to
the temperature of the CMB. Thus, the characteristic mass of Pop III.2 stars is a function of
redshift, but is typically of the order of 10 M⊙.
limited cooling properties of primordial gas, which could only cool in minihalos through
the radiation from H2molecules. While the initial conditions for the formation of these
stars are, in principle, known from precision measurements of cosmological parameters
(e.g. Spergel et al. 2007), Pop III star formation may have occurred in different envi-
ronments which may allow for different modes of star formation. Indeed, it has become
evident that Pop III star formation might actually consist of two distinct modes: one
where the primordial gas collapses into a DM minihalo (see below), and one where the
metal-free gas becomes significantly ionized prior to the onset of gravitational runaway
collapse (Johnson & Bromm 2006). We had termed this latter mode of primordial star
formation ‘Pop II.5’ (Greif & Bromm 2006; Johnson & Bromm 2006; Mackey et al. 2003).
To more clearly indicate that both modes pertain to metal-free star formation, we here
follow the new classification scheme suggested by Chris McKee (see McKee & Tan 2007;
Johnson et al. 2008). Within this scheme, the minihalo Pop III mode is now termed
Pop III.1, whereas the second mode (formerly ‘Pop II.5’) is now called Pop III.2. The
hope is that McKee’s terminology will gain wide acceptance.
While the very first Pop III stars (so-called Pop III.1), with masses of the order of
100 M⊙, formed within DM minihalos in which primordial gas cools by H2 molecules
alone, the HD molecule can play an important role in the cooling of primordial gas in
several situations, allowing the temperature to drop well below 200 K (Abel et al. 2002;
Bromm et al. 2002). In turn, this efficient cooling may lead to the formation of primordial
stars with masses of the order of 10 M⊙(so-called Pop III.2) (Johnson & Bromm 2006).
In general, the formation of HD, and the concomitant cooling that it provides, is found
to occur efficiently in primordial gas which is strongly ionized, owing ultimately to the
high abundance of electrons which serve as catalyst for molecule formation in the early
universe (Shapiro & Kang 1987).
Efficient cooling by HD can be triggered within the relic H ii regions that surround
Pop III.1 stars at the end of their brief lifetimes, owing to the high electron fraction
that persists in the gas as it cools and recombines (Johnson et al. 2007; Nagakura &
Omukai 2005; Yoshida et al. 2007). The efficient formation of HD can also take place
when the primordial gas is collisionally ionized, such as behind the shocks driven by the
first SNe or in the virialization of massive DM halos (Greif & Bromm 2006; Johnson &

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4Johnson, Greif & Bromm
Figure 3. The chemical interplay in relic H ii regions. While all molecules are destroyed in and
around active H ii regions, the high residual electron fraction in relic H ii regions catalyzes the
formation of an abundance of H2 and HD molecules. The light and dark shades of blue denote
regions with a free electron fraction of 5 × 10−3and 5 × 10−4, respectively, while the shades of
green denote regions with an H2 fraction of 10−4, 10−5, and 3 × 10−6, in order of decreasing
brightness. The regions with the highest molecule abundances lie within relic H ii regions, which
thus play an important role in subsequent star formation, allowing molecules to become shielded
from photodissociating radiation and altering the cooling properties of the primordial gas.
Bromm 2006; Machida et al. 2005; Shchekinov & Vasiliev 2006). In Figure 1, we show
the HD fraction in primordial gas in four distinct situations: within a minihalo in which
the gas is never strongly ionized, behind a 100 km s−1shock wave driven by a SN, in the
virialization of a 3σ DM halo at redshift z = 15, and in the relic H ii region generated
by a Pop III.1 star at z ∼ 20 (Johnson & Bromm 2006). Also shown is the critical HD
fraction necessary to allow the primordial gas to cool to the temperature floor set by the
CMB at these redshifts. Except for the situation of the gas in the virtually un-ionized
minihalo, the fraction of HD becomes large quickly enough to play an important role in
the cooling of the gas, allowing the formation of Pop III.2 stars.
Figure 2 schematically shows the characteristicmasses of the various stellar populations
that form in the early universe. In the wake of Pop III.1 stars formed in DM minihalos,
Pop III.2 star formation ensues in regions which have been previously ionized, typically
associated with relic H ii regions left over from massive Pop III.1 stars collapsing to black
holes, while even later, when the primordial gas is locally enriched with metals, Pop II
stars begin to form (Bromm & Loeb 2003; Greif & Bromm 2006). Recent simulations
confirm this picture, as Pop III.2 star formation ensues in relic H ii regions in well under
a Hubble time, while the formation of Pop II stars after the first SN explosions is delayed
by more than a Hubble time (Greif et al. 2007; Yoshida et al. 2007a,b; but see Whalen
et al. 2008).
3. Radiative Feedback from the First Stars
Due to their extreme mass scale, Pop III.1 stars emit copious amounts of ionizing radi-
ation, as well as a strong flux of H2-dissociating Lyman-Werner (LW) radiation (Bromm
et al. 2001b; Schaerer 2002). Thus, the radiation from the first stars dramatically influ-

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The First Stars5
Figure 4. Optical depth to LW photons due to self-shielding, averaged over two different scales,
as a function of redshift. The diamonds denote the optical depth averaged over the entire cosmo-
logical box of comoving length 660 kpc, while the plus signs denote the optical depth averaged
over a cube of 220 kpc (comoving) per side, centered on the middle of the box. The solid line
denotes the average optical depth that would be expected for a constant H2 fraction of 2×10−6
(primordial gas), which changes only due to cosmic expansion.
ences their surroundings, heating and ionizing the gas within a few kpc (physical) around
the progenitor, and destroying the H2and HD molecules locally within somewhat larger
regions (Alvarez et al. 2006; Abel et al. 2007; Ferrara 1998; Johnson et al. 2007; Kitayama
et al. 2004; Whalen et al. 2004). Additionally, the LW radiation emitted by the first stars
could propagate across cosmological distances, allowing the build-up of a pervasive LW
background radiation field (Haiman et al. 2000).
3.1. Local Radiative Effects
The impact of radiation from the first stars on their local surroundings has important
implications for the numbers and types of Pop III stars that form. The photoheating of
gas in the minihalos hosting Pop III.1 stars drives strong outflows, lowering the density of
the primordial gas and delaying subsequent star formation by up to 100 Myr (Johnson et
al. 2007; Whalen et al. 2004; Yoshida et al. 2007a). Furthermore, neighboring minihalos
may be photoevaporated, delaying star formation in such systems as well (Ahn & Shapiro
2007; Greif et al. 2007; Shapiro et al. 2004; Susa & Umemura 2006; Whalen et al. 2007).
The photodissociation of molecules by LW photons emitted from local star-forming re-
gions will, in general, act to delay star formation by destroying the main coolants that
allow the gas to collapse and form stars.
The photoionization of primordial gas, however, can ultimately lead to the production
of copious amounts of molecules within the relic H ii regions surrounding the remnants of
Pop III.1 stars (Johnson & Bromm 2007; Nagakura & Omukai 2005; Oh & Haiman 2002;
Ricotti et al. 2001). Recent simulations tracking the formation of, and radiative feedback
from, individual Pop III.1 stars in the early stages of the assembly of the first galaxies
have demonstrated that the accumulation of relic H ii regions has two important effects.
First, the HD abundance that develops in relic H ii regions allows the primordial gas to
re-collapse and cool to the temperature of the CMB, possibly leading to the formation
of Pop III.2 stars in these regions (Johnson et al. 2007; Yoshida et al. 2007b). Second,
the molecule abundance in relic H ii regions, along with their increasing volume-filling
fraction, leads to a large optical depth to LW photons over physical distances of the order
of several kpc. The development of a high optical depth to LW photons over such short
length-scales suggests that the optical depth to LW photons over cosmological scales may

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6 Johnson, Greif & Bromm
Figure 5. The Pop III star formation rates (left panel) and the corresponding LW background
fluxes (right panel) for three models of the build-up of the LW background by Pop III.1 stars
formed in minihalos. The maximum possible LW background, JLW,max, is generated for the case
that every minihalo with a virial temperature ? 2 × 103K hosts a Pop III star, without the
LW background in turn diminishing the SFR, labeled here as SFRIII,noLW. The self-regulated
model considers the coupling between the star formation rate, SFRLW,crit, and the critical LW
background that it produces, JLW,crit. The minimum value for the LW background, JLW,min, is
produced for the case of a high opacity through the relic H II regions left by the first stars, in
which case the self-consistent SFR, SFRIII,max can approach the undiminished SFRIII,noLW.
be very high, acting to suppress the build-up of a background LW radiation field, and
mitigating negative feedback on star formation.
Figure 3 shows the chemical composition of primordial gas in relic H ii regions, in
which the formation of H2molecules is catalyzed by the high residual electron fraction.
Figure 4 shows the average optical depth to LW photons across the simulation box, which
rises with time owing to the increasing number of relic H ii regions.
3.2. Global Radiative Feedback
While the reionization of the universe is likely to have occurred at later times, as inferred
from the WMAP third year results (Spergel et al. 2007), the process of primordial star
formation can be affected by the build up of a LW background very soon after the
formation of the first stars. This LW radiation, which acts to destroy H2, the very coolant
that enables the formation of the first stars, can, in principle, dramatically lower the
formation rate of Pop III stars in minihalos (e.g. Haiman et al. 2000; Machacek et al.
2001; Yoshida et al. 2003; Mackey et al. 2003).
While star formation in more massive systems may proceed relatively unimpeded,
through atomic line cooling, during the earliest epochs of star formation these atomic-
cooling halos are rare compared to the minihalos which host individual Pop III stars.
While the process of star formation in atomic-cooling halos is not well understood, for
a broad range of models the dominant contribution to the LW background is from Pop
III.1 stars formed in minihalos at z ? 15-20 (Johnson et al. 2008). Therefore, at these
redshifts the LW background radiation may be largely self-regulated, with Pop III.1 stars
producing the very radiation which, in turn, suppresses their formation. Johnson et al.
(2008) argue that there is a critical value for the LW flux, JLW,crit ∼ 0.04, at which
Pop III.1 star formation occurs self-consistently, with the implication that the PopIII.1
star formation rate in minihalos at z > 15 is decreased by only a factor of a few, as
shown in Fig. 5. The star formation rate may be even higher if the cosmological average
optical depth to LW photons through the relic H II regions left by the first stars is
sufficiently high. An analytical model of this effect shows that the SFR may be only

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The First Stars7
Figure 6. The hydrogen number density and temperature of the gas in the region of the forming
galaxy at z ∼ 12.5. The panels show the inner ∼ 10.6 kpc (physical) of our cosmological box.
The cluster of minihaloes harboring dense gas just left of the center in each panel is the site of
the formation of the two Pop III.1 stars which are able to form in our simulation including the
effects of the self-regulated LW background. The remaining minihaloes are not able to form stars
by this redshift, largely due to the photodissociation of the coolant H2. The main progenitor DM
halo, which hosts the first star at z ∼ 16, by z ∼ 12.5 has accumulated a mass of 9 × 107M⊙
through mergers and accretion. Note that the gas in this halo has been heated to temperatures
above 104K, leading to a high free electron fraction and high molecule fractions in the collapsing
gas. The high HD fraction that is generated likely leads to the formation of Pop III.2 stars in
this system.
negligibly reduced once the volume-filling factor of relic H II regions becomes large, as is
also shown in Fig. 5 (Johnson et al. 2008).
Simulations of the formation of a dwarf galaxy at z ? 10 which take into account the
effect of a LW background at JLW,crit show that Pop III.1 star formation takes place
before the galaxy is fully assembled, suggesting that the formation of metal-free galaxies
may be a rare event in the early universe (Johnson et al. 2008). Figure 6 shows the
temperature and density of the protogalaxy simulated by these authors at z ∼ 12.5.
4. The First Supernova Explosions
Recent numerical simulations have indicated that primordial stars forming in DM
minihalos typically attain 100 M⊙ by efficient accretion, and might even become as
massive as 500 M⊙(Bromm & Loeb 2004; O’Shea & Norman 2007; Omukai & Palla 2003;
Yoshida et al. 2006). After their main-sequence lifetimes of typically 2−3 Myr, stars with
masses below ≃ 100 M⊙are thought to collapse directly to black holes without significant
metal ejection, while in the range 140 − 260 M⊙ a pair-instability supernova (PISN)
disrupts the entire progenitor, with explosion energies ranging from 1051−1053ergs, and
yields up to 0.5 (Heger et al. 2003; Heger & Woosley 2002).
The significant mechanical and chemical feedback effects exerted by such explosions
have been investigated with a number of detailed calculations, but these were either
performed in one dimension (Kitayama & Yoshida 2005; Machida et al. 2005; Salvaterra
et al. 2004; Whalen et al. 2008), or did not start from realistic initial conditions (Bromm
et al. 2003; Norman et al. 2004). The most realistic, three-dimensional simulation to
date took cosmological initial conditions into account, and followed the evolution of the
gas until the formation of the first minihalo at z ≃ 20 (Greif et al. 2007). After the

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8 Johnson, Greif & Bromm
Figure 7. Temperature averaged along the line of sight in a slice of 10/h kpc (comoving) at
1, 10, 50, and 200 Myr after a Pop III PISN (Greif et al. 2007). In all four panels, the H ii
region and SN shock are clearly distinguishable, with the former occupying almost the entire
simulation box, while the latter is confined to the central regions. (a): The SN remnant has just
left the host halo, but temperatures in the interior are still well above 104K. (b): After 10 Myr,
the asymmetry of the SN shock becomes visible, while most of the interior has cooled to well
below 104K. (c): The further evolution of the shocked gas is governed by adiabatic expansion.
(d): After 200 Myr, the shock velocity approaches the local sound speed and the SN remnant
stalls. By this time the post-shock regions have cooled to roughly 103K.
gas approached the ‘loitering regime’ at nH≃ 104cm−3, the formation of a primordial
star was assumed, and a photoheating and ray-tracing algorithm determined the size
and structure of the resulting H ii region (Johnson et al. 2007). An explosion energy of
1052ergs was then injected as thermal energy into a small region around the progenitor,
and the subsequent expansion of the SN remnant was followed until the blast wave
effectively dissolved into the IGM. The cooling mechanisms responsible for radiating away
the energy of the SN remnant, the temporal behavior of the shock, and its morphology
could thus be investigated in great detail (see Figure 7).
The dispersal of metals by the first SN explosions transformed the IGM from a simple
primordial gas to a highly complex medium in terms of chemistry and cooling, which
ultimately enabled the formation of the first low-mass stars. However, this transition
required at least a Hubble time, since the presence of metals became important only
after the SN remnant had stalled and the enriched gas re-collapsed to high densities
(Greif et al. 2007; but see Whalen et al. 2008). Furthermore, the metal distribution
was highly anisotropic, as the post-shock gas expanded into the voids in the shape of
an ‘hour-glass’, with a maximum extent similar to the final mass-weighted mean shock
radius (Greif et al. 2007).
To efficiently mix the metals with all components of the swept-up gas, a DM halo of

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The First Stars9
Figure 8. Transition discriminant, Dtrans, for metal-poor stars collected from the literature as
a function of [Fe/H]. Top panel: Galactic halo stars. Bottom panel: Stars in dSph galaxies and
globular clusters. G indicates giants, SG subgiants. The critical limit is marked with a dashed
line. The dotted lines refer to the uncertainty. The detailed references for the various data sets
can be found in Frebel et al. (2007).
at least Mvir≃ 108M⊙had to be assembled (Greif et al. 2007), and with an initial yield
of 0.1, the average metallicity of such a system would accumulate to Z ≃ 10−2.5Z⊙, well
above any critical metallicity (Bromm & Loeb 2003; Bromm et al. 2001a; Schneider et
al. 2006; see also Wise & Abel 2007). Thus, if energetic SNe were a common fate for
the first stars, they would have deposited metals on large scales before massive galaxies
formed and outflows were suppressed. Hints to such ubiquitous metal enrichment have
been found in the low column density Lyα forest (Aguirre et al. 2005; Songaila & Cowie
1996; Songaila 2001), and in dwarf spheroidal satellites of the Milky Way (Helmi et al.
2006).
5. The Chemical Signature of the First Stars
The discovery of extremely metal-poor stars in the Galactic halo has made studies
of the chemical composition of low-mass Pop II stars powerful probes of the conditions
in which the first low-mass stars formed. While it is widely accepted that metals are
required for the formation of low-mass stars, two general classes of competing models for
the Pop III – Pop II transition are discussed in the literature: (i) atomic fine-structure line
cooling (Bromm & Loeb 2003; Santoro & Shull 2006); and (ii) dust-induced fragmentation
(Schneider et al. 2006). Within the fine-structure model, C ii and O i have been suggested
as main coolants (Bromm & Loeb 2003), such that low-mass star formation can occur in

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10Johnson, Greif & Bromm
β
aγγ (Integr. frac. of PISN−domin. stars)
Observational upper limit
Predicted range
10
−3
10
−2
10
−1
10
0
10
−6
10
−5
10
−4
10
−3
10
−2
10
−1
Figure 9. The predicted integrated (total) fraction of PISN-dominated stars below [Ca/H] = −2
as a function of β, corresponding to > 90% (big blue dots), > 99% (medium sized, blue dots),
and > 99.9% (small, dark blue dots) PISN-enrichment. The dashed (red) lines indicate the obser-
vational upper limit of β, assuming that none of the ∼ 600 Galactic halo stars with [Ca/H] ? −2
for which high-resolution spectroscopy is available show any signature of PISNe (N. Christlieb,
priv. comm.). The dotted (blue) line and shaded (blue) area denote the predicted range of aγγ
anticipated from the calculations of Padoan et al. (2007) and Greif & Bromm (2006), respec-
tively.
gas that is enriched beyond critical abundances of [C/H]crit≃ −3.5±0.1 and [O/H]crit≃
−3 ± 0.2. The dust-cooling model, on the other hand, predicts critical abundances that
are typically smaller by a factor of 10 − 100.
Based on the theory of atomic line cooling (Bromm & Loeb 2003), a new function, the
‘transition discriminant’ has been introduced:
?
Dtrans≡ log10
10[C/H]+ 0.3 × 10[O/H]?
,(5.1)
such that low-mass star formation requires Dtrans> Dtrans,crit≃ −3.5 ± 0.2 (Frebel et
al. 2007). Figure 8 shows values of Dtrans for a large number of the most metal-poor
stars available in the literature. While theories based on dust cooling can be pushed
to accommodate the lack of stars with Dtrans< Dtrans,crit, it appears that the atomic-
cooling theory for the Pop III – Pop II transition naturally explains the existing data
on metal-poor stars. Future surveys of Galactic halo stars will allow to further populate
plots such as Figure 8, and will provide valuable insight into the conditions of the early
universe in which the first low-mass stars formed.
The abundance patterns observed in the most metal-poor stars can also provide infor-
mation about the types of SN that ended the lives of the first stars, as the metals that
are emitted in these explosions will become incorporated into later generations of stars,
some of which are observed in the halo of the Galaxy. Interestingly, while detailed numer-
ical simulations of the formation of the first stars suggest that they were often massive
enough to explode as PISN, no clear signature of such a PISN has yet been detected in
a metal-poor star. Does this imply that the first stars did not explode as PISN, or that
they were not very massive (see also Ekstrom in this proceedings)?
PISNe may have ejected enough mass in metals to enrich the IGM to a metallicity well
above those of the most metal-poor stars. Therefore, one possible explanation for the
apparent lack of Pop III PISNe is that the few stars which might have formed from gas
dominantly enriched by a PISN may have relatively high metallicities, and so may have
eluded surveys seeking such true second generation stars at lower metallicity (Karlsson

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The First Stars11
et al. 2008). Karlsson et al. (2008) developed a model for the inhomogeneous chemical
enrichment of the gas collapsing to become a dwarf galaxy at z ∼ 10 in which the
formation of both Pop III stars from metal-free gas and the formation of Pop II stars
from metal-enriched gas were tracked self-consistently. These authors find that the lack of
the discovery of a metal-poor star showing signs of enrichment dominated by PISN yields
in the existing catalog of metal poor stars is not inconsistent with theories predicting that
the first stars were very massive, as shown in Figure 9. It is hoped that future surveys of
stars in the Galactic halo will test this model by searching for PISN-enriched stars with
metallicities [Fe/H] ? -2.5.
6. Conclusion
Understanding the formation of the first galaxies marks the frontier of high-redshift
structure formation. It is crucial to predict their properties in order to develop the optimal
search and survey strategies for the JWST. Whereas ab-initio simulations of the very first
stars can be carried out from first principles, and with virtually no free parameters, one
faces a much more daunting challenge with the first galaxies. Now, the previous history
of star formation has to be considered, leading to enhanced complexity in the assembly
of the first galaxies. One by one, all the complex astrophysical processes that play a role
in more recent galaxy formation appear back on the scene. Among them are external
radiation fields, comprising UV and X-ray photons, and possibly cosmic rays produced
in the wake of the first SNe (Stacy & Bromm 2007). There will be metal-enriched pockets
of gas which could be pervaded by dynamically non-negligible magnetic fields, together
with turbulent velocity fields built up during the virialization process. However, the goal
of making useful predictions for the first galaxies is now clearly drawing within reach,
and the pace of progress is likely to be rapid.
Acknowledgements
V.B. acknowledges support from NSF grant AST-0708795. The simulations presented
here were carried out at the Texas Advanced Computing Center (TACC).
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