Download Mark Rubin

Abstract
• All simulations present common trends. Metal enrichment
is very patchy, with rare, unpolluted regions surviving at all
redshifts, inducing the simultaneous presence of metal-free
and metal-rich star formation regimes.
• As a result of the rapid pollution within high-density
regions due to the first SN/pair instability SN, local
metallicity is quickly boosted above the critical metallicity
for the transition.
• For this reason, pop III stars dominate only during the very
first stages of structure formation, with an average
contribution to the total star formation rate that reaches a
constant value of ~10 -3 at redshift 11-13.
• The primordial (population III) stars are formed out of a
pristine environment, where the cooling agents are limited
to primordial, H-based molecules only which are able to
cool the gas down to temperatures ~ 102 K.
• Therefore the mass of primordial stars should be relatively
large and their spectrum is commonly referred to as ‘topheavy’ initial mass function. These features imply very
short lifetimes (up to ~106 yr only) and final death mostly
into black holes. The only mass range where primordial
stars can explode as pair-instability supernovae (PISN) and
pollute the surrounding medium is [140, 260 Msolar].
• The subsequent star formation events in enriched regions
will happen in completely different conditions, because
metals allow further cooling and fragmentation to smaller
scales.
Fate of Population III stars as a function of initial mass
Conclusions and Discussion
• The main influence of chemical evolution on the
following generations of stars is via metal pollution.
• This event can completely alter the cooling properties
of the gas and thus the modalities of star formation,
inducing a transition from top-heavy to a standard IMF.
• There is assumed to be a threshold metallicity, zcrit,
above which which all further star-forming populations
will transition from an IMF characteristic of Population
III stars to that of Population II stars.
• The values for zcrit chosen in this study include 10-6, 105, 10-4 and 10-3 Z
solar
• The gas is easily enriched above zcrit. For this reason ,
the average contribution from pristine, metal-free stars
to the total cosmic star formation density is dominant
only in the very early phases of structure formation,
while it drops below ~10-3 quite rapidly, after the
explosion of the first PISN and their metal ejection.
• PISN explosions which follow the death of the metalfree or very-metal-poor stars, are responsible for
enriching the surrounding medium up to a minimum
level of ~10-4 Zsolar.
• Only minimal differences are found upon the selection
of differing values for zcrit, differences are only found if
different IMF mass ranges for primordial stars are used.
• Because of the short life and high metal yields of early,
massive SN, the pop III regime contributes slightly to
the global star forming rate since the early pollution
events quickly raise Z above zcrit.
• The results clearly predict that pop III star formation
reaches a constant value of ~10-3 at redshift z ~ 11-13.
This is a solid result, which holds independently from
many poorly constrained parameters.
• The simulations were performed using a standard
∧CDM cosmology, but slightly different results would
not change the general picture.
• After ~2 x108 yr, molecular evolution leads to the very
first bursts of star formation (pop III), but metal
enrichment is extremely fast leading to pop II IMF.
• Metal pollution proceeds from the densest cores of
star formation outwards, because of SN ejections from
high-density to lower-density environments. Rare
unpolluted regions can still survive, determining the
simultaneous presence of two star formation regimes,
and Zcrit can affect the level of residual pop III star
formation.
• As a result, we find that the average contribution of
pop III component to the total star formation rate
density is a few x 10-4 or 10-3 by z ~ 11.
Metallicity maps at redshift z = 11
Maio et al. Figure 1
Comparison of metallicity maps at redshift z = 11, 12 and 13 for Zcrit = 10-3 and 10-5
Metal evolution as a function of redshift, Maio,et al. Figure 4
Because of the short life and high metal yields of early, massive SN, the pop III regime contributes
slightly to the global star forming rate since the early pollution events quickly raise Z above zcrit.
Part II – Looking ahead
• First Light Sources at the End of the Dark Ages:
Direct Observations of Population III Stars, ProtoGalaxies, and Supernovae During the Reionization
Epoch
- A White Paper Submitted to the Astro2010 Decadal Survey
Committee; Cooke et al.
Although the initial epoch of star formation has long been a
topic of theoretical interest, technology is only now beginning
to allow observational insights into this epoch.
• Strongest direct spectroscopic signature of these hot stars in the
observed-frame optical and near-infrared is the likely intense He II (1640
A) emission, which would indicate a hard ionizing radiation field typical of
the top-heavy IMF characteristic of Pop III star formation.
• However, this feature could also be excited by AGN activity or by accretion
on to mini-black holes, which could confuse the interpretation.
Model spectral energy distributions of massive, metal-free (Population III) stars from Schaerer & Pello
showing the wealth of signatures of the “hard” radiation field produced by these massive and hot stars.
Pair-Production Supernovae Detection
• Second method involves detection of Pop III pair-instability
supernovae (PISNe; e.g. Heger & Woosley 2002); these might
be visible in their rest-frame ultraviolet continua with JWST.
• PISNe energies on the order of 1051 – 1053 ergs may allow
them to be distinct and readily detectable with the GSMTs.
• Spectroscopy of the resultant supernovae with ground-based
Giant Segmented Mirror (infrared) Telescopes (e.g., GSMT)
should reveal metal abundance patterns typical of these
supernovae providing unambiguous confirmation of the Pop
III nature of the progenitor stars.
Observing Pop III PISNe
• At least a fraction of Type IIn supernovae (SNe IIn) are
energetic PISNe, providing the only observable examples of
this process. They have potential to yield enormous insight
into the behavior of high-mass (> 140 Msolar) Pop III stars.
• Moreover, SNe IIn are the most luminous SN type in the restframe UV, rendering them easier to detect at high redshift
than any other SN type.
• SNe IIn are defined by the presence of extremely luminous,
long-lived emission lines that are dominated by Lyman α,
MgII, and Hα.
• The combination of sensitive broad-band near-infrared
imaging and spectroscopic follow up with JWST, JDEM or
ground-based GSMTs will provide the means to distinguish
PISNe from core-collapse SNe by their light curve rise times,
overall energy output, and relative emission-line
characteristics.
Emission-line strength evolution of local SNe IIn redshifted to z~6
from Cooke (2008). At z~6 and beyond, the emission lines of SNe IIn
may be detectable using 25-30-meter class GSMTs.
Observations require a baseline of 2-10 yrs – not target of
opportunity observations.
Direct Detection of Population III “Dwarf” Galaxies
• At “modest” redshifts (z<11) where Lyman α emission appears
in the J-band, one of the best environments to discover Pop III
dwarf galaxies may be the chemically unevolved surroundings
of a large galaxy or proto-cluster of galaxies.
• The central sources are likely to have ionized a local “bubble”
through which strong Lyman α emission can escape from
surrounding dwarf galaxies.
• The near-infrared observations of these dwarfs could reveal
strong Lyman α and He II emission detectable by large groundbased telescopes, and possibly a rest-frame ultraviolet
continuum observable from the ground and/or with the JWST.
• When strong Lyman α emitters are found, both the Lyman α
and He II lines can be observed with R>3000 spectroscopy
using a GSMT.
Direct Detection of Pop III “Dwarf Galaxies
• In Population III sources, direct detection of the He II emission
line, expected with equivalent widths that exceed ~10 A, is
within the reach of ground-based 25-30-meter telescopes.
• The figure on the following page illustrates simple predictions
for the He II counts of various star-forming scenarios
described in Barton et al. (2004). The “optimistic” scenario
corresponds to pure Pop III, metal-free, 300-1000 solar-mass
stars. Both the optimistic and “plausible” scenarios can be
detected with next-generation large ground-based telescopes.
Simple model estimates of the He II (1640 A) “luminosity function at z ~ 8.
Following Barton et al. (2004), we plot the expected source counts in He II
per square arcminute per redshift increment near z ~8 for fully Population III
(“optimistic”), low-metallicity (“plausible), and heavy Population II (“heavy
Salpeter”) scenarios. We also indicate show the approximate sensitivity
range of the GSMTs. The presence of strong, detectable HeII emission is a
clear indication of a drastic change in the stellar initial mass function and
metal content of high-redshift galaxies.
We can use the results from JWST and/or ground-based
infrared surveys to search for intense Lyman α sources.
• When strong Lyman α emitters are found, both the Lyman α
and the He II lines can be observed with R > 3000
spectroscopy using a GSMT. Observations can focus on the
region in which He II is expected (e.g., at 1.44 microns or Hband, for the z ~ 7.7 window).
• If a He II (1640 A) feature is discovered that is strong relative
to the UV continuum limits from JWST and the Lyman α
feature (e.g., rest frame EW (He II) > 10-20 A), the strength of
the feature provides direct evidence that the source is not a
“standard” Pop II star formation region similar to what is
observed in the local universe. (as opposed to Wolf-Rayet
stars with EW of a few Angstroms)