How Well Do We Know How Well Do We Know Stellar Populations?Stellar Populations?

Nick Gnedin

Co-starringCo-starring

Andrey Kravtsov (Chicago)

Kostas Tassis (Crete)

Oleg Gnedin (Michigan)

Sasha Muratov (Michigan)

Where Do Stars Form?Where Do Stars Form?

F. Walter &The HI Nearby Galaxy Survey

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)

Where Do Stars Form?Where Do Stars Form?

How Molecular Clouds FormHow 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 (self-shielding) and shielding by dust are important.

Atomic-To-Molecular Atomic-To-Molecular Transition 101Transition 101

Dust shielding for hydrogen molecules is like a castle wall for defenders: without the wall, theyare not able to withstand the assault of the UV radiation.

But without the defenders, the wall is useless.

Putting It All TogetherPutting 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 ModelSub-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 Sobolev-like approximation (“like” because the velocity field is not resolved).

Multi-phase ISMMulti-phase ISMAll 3 main ISM phases are there:• hot coronal gas• warm diffuse gas• cold HI / H2 gas

Picture of M51

Kennicutt-Schmidt Kennicutt-Schmidt RelationRelation

Local galaxies

Atomic gas

Molecular gas

All neutral gas

Kennicutt-Schmidt Kennicutt-Schmidt RelationRelation

Just like atomic-to-molecular transition scales non-trivially with DMW and UMW, so does star formation.

Does It Work In Real Life?Does It Work In Real Life?

LBG measurement from Rafelski (2011)

cB58 (Baker et al. 2004)

Galaxies at z=3

Local galaxies (THINGS)

Does It Work In Real Life?Does It Work In Real Life? Gas in nearby, low metallicity dwarfs appears to

be inert to star formation.

UGC 5288 NGC 2915

Bimodality PredictionBimodality Prediction

Lot’s of dust, efficient SF

Little dust, inefficient SF

Can’t Kill All Birds With Can’t Kill All Birds With One Stone MoleculeOne Stone Molecule

Oops – observations don’t show any

bimodality

Sloan galaxies

Local Group dwarfs

A Mystery of the A Mystery of the “Metallicity Floor” “Metallicity Floor”

All those heavy elements came from dust-unrelated star formation process.

A Mystery of the A Mystery of the “Metallicity Floor” “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- 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 StarsGenealogy 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 PopsFrom Pop III to Other Pops

Pop III episode is brief:

Wise & Abel 2008 Muratov et all 2011

Key Questions #1Key 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

(Inoue 2011)

long!

Key Questions #2Key Questions #2

Have all metals in dwarf galaxies (M*<108M) 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)

ConclusionsConclusions The existence of the “metallicity floor” is a hard to

understand puzzle in early galaxy formation. Metals in galaxies with M*<108M came from a

dust-unrelated episode of star formation. Pop III 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 EndThe End