Transcript Epoch of Reionization

How Well Do We Know
Stellar Populations?
Nick Gnedin
Co-starring
Andrey Kravtsov (Chicago)
Kostas Tassis (Crete)
Oleg Gnedin (Michigan)
Sasha Muratov (Michigan)
Where Do Stars Form?
F. Walter &
The HI Nearby
Galaxy Survey
Where Do Stars Form?
 In the local universe stars
only form in molecular gas.
 We know of no example of
a single star forming in
atomic gas.
 On theoretical grounds,
molecular gas is expected
to be a good tracer of
star-forming gas.
(Krumholz, Leroy, McKee 2011)
How Molecular Clouds
Form
 Molecular hydrogen is fragile: it
is destroyed by UV radiation in
the Lyman-Werner band (11.3
– 13.6 eV).
 Molecular clouds only exist
because of shielding.
 Both, shielding by H2 (selfshielding) and shielding by dust
are important.
Atomic-To-Molecular
Transition 101
 Dust shielding for hydrogen molecules is like a
castle wall for defenders: without the wall, they
are not able to
withstand the
assault of the
UV radiation.
 But without the
defenders, the
wall is useless.
Putting It All Together
 ART (Adaptive Refinement Tree) Code
 N-body + gas + SF + RT + NLTE chemistry
 50 pc spatial resolution in the ISM with mesh
refinement
 Star formation recipe from Krumholz & Tan 07
 Optically Thin Variable Eddington Tensor
Approximation (OTVET) for RT
 Non-equilibrium cooling rates and ionization &
chemical balance are computed “on the fly”
 Realistic galaxies in cosmological simulations in a
35 comoving Mpc box (dynamic range of >105)
Sub-cell Model
 Radiative transfer in LW bands can not be done
exactly in realistic simulations: 3D, adaptive in
space, time-dependent.
 Spatial scales over which absorption lines are
coherent are sub-parsec (= unresolvable).
 Unless aliens give us a super-duper
hypercomputer to solve all this, we
need to use a “sub-cell” model.
 RT in LW bands is done in Sobolevlike approximation (“like” because
the velocity field is not resolved).
Multi-phase ISM
All 3 main ISM phases
are there:
• hot coronal gas
• warm diffuse gas
• cold HI / H2 gas
 Picture of M51
Kennicutt-Schmidt
Relation
 Local galaxies
Atomic gas
Molecular gas
All neutral gas
Kennicutt-Schmidt
Relation
 Just like atomic-to-molecular transition scales nontrivially with DMW and UMW, so does star formation.
Does It Work In Real Life?
Local galaxies (THINGS)
Galaxies at z=3
cB58 (Baker et al. 2004)
LBG measurement
from Rafelski (2011)
Does It Work In Real Life?
 Gas in nearby, low metallicity dwarfs appears to
be inert to star formation.
UGC 5288
NGC 2915
Bimodality Prediction
Lot’s of dust, efficient SF
Little dust, inefficient SF
Can’t Kill All Birds With
One Stone Molecule
Sloan galaxies
Local Group dwarfs
Oops – observations
don’t show any
bimodality
A Mystery of the
“Metallicity Floor”
All those heavy elements
came from dust-unrelated
star formation process.
A Mystery of the
“Metallicity Floor”
 There is no place known in the whole universe
(except 2 stars and 2 LL systems) that has
metallicity less than about 0.2% solar.
• Most metal-poor galaxies.
• Lowest metallicity stars in the Milky Way.
• Damped Lyman-a Absorbers.
 These heavy elements might have come from the
metal-free generation of stars (Pop III stars), but
why is it so universal?
 Could there be another population of stars (call it
Pop A)?
Genealogy Tree for
Stars
Primordial gas, no metals
Pop III stars
Metal-enriched gas
Dust is present,
Pop M (Pop I+II) stars
No dust,
Pop A stars
From Pop III to Other Pops
 Pop III episode is brief:
Wise & Abel 2008
Muratov et all 2011
Key Questions #1
 Can dust form fast right after a Pop III episode?
• Little dust is observed in z>5 galaxies & GRBs
(Bouwens et al 2010, Zafar et al 2011).
• Dust mostly forms in the ISM by nucleation;
seeds come from AGB and SNe.
• Nucleation time is long!
(Inoue 2011)
Key Questions #2
 Have all metals in dwarf galaxies (M*<108M8)
come from Pop III stars?
• Pop III episode is brief.
• Chemical abundances of the most metal poor
stars in the MW and stars in LG dwarfs look
normal, unlike those of Pop III ejecta.
(Caffau 2011)
Conclusions
 The existence of the “metallicity floor” is a hard to
understand puzzle in early galaxy formation.
 Metals in galaxies with M*<108M8 came from a
dust-unrelated episode of star formation.
 Pop III a Pop Whatever transition cannot be
properly modelled without understanding how
dust forms at z>10.
 If the dust formation time-scale is long, then
high-z galaxies contain little dust (as observed)
and a population of stars forming in atomic gas
(Pop A) can not be excluded.
The End