Dependence of galactic clumpiness on
ISM models for high-density gas incosmological simulations
MNRAS submitted

Shigeki Inoue (Tsukuba / NAOJ)
Naoki Yoshida (Tokyo)

Clumpy galaxies
Disc galaxies in their formative stages
Typically at z~1-3

Clumpy galaxies
 Giant clumps
 Gas-rich (fgas>30%)

Guo et al. (2014)

Law et al. (2012)

 Toomre instability?
 Spiral-arm instability?
 Mergers?

Elmegreen& Elmegreen (2014)

Clumpy galaxies in simulations
Gravitationally bound structures
If feedback is weakGasStar

Mandelker et al (2017)Unbound structures and/or disrupted soonIf feedback is strong
Buck et al. (2017)

U-bandGasStar

Clumpy galaxies in simulations

Is feedback the only key factor?
➡ ISM model can also affectclump formation
Purpose:We show how ISM model affects cosmological simulations,Examine three ISM models that are widely used.
“Two-phase model” (Illustris-TNG)
“Polytrope model”(EAGLE)
“Single-phase model” (FIRE)

What is ISM model?
Wiersma et al. (2010)

ICM

ISM is gas inside a galaxy
i.e. star-formingISM properties are uncertain

WHIM？？？

ISM

IGM(inside galaxy)

Halo gasVarious physics involves:
star formation, supernovae,radiation, rapid cooling,turbulence etc…

Simulations often assume ISM
properties.
EOS by modelling ISM

Two-phase ISM model
For dense gas that can form stars (𝜌𝑔𝑎𝑠 > 𝜌𝑆𝐹 )Yepes et al. (1997), Springel & Hernquist (2003)
Illustris / Illustris-TNG simulations
Auriga simulations• This model gives a barotropic EOS.
HotCold
Quiescent SF&Instantaneous SNe

SN ejecta
evaporation
thrm. Inst.
A gas element• Considering pressure equilibrium,• with only a few parameters,• a cooling function givenTwo-phase ISM model
For dense gas that can form stars (𝜌𝑔𝑎𝑠 > 𝜌𝑆𝐹 )Yepes et al. (1997), Springel & Hernquist (2003)

Cold
Quiescent SF&
Instantaneous SNe

SN ejecta
evaporation

PressureHot

TemperatureIllustris / Illustris-TNG simulations
Auriga simulations

thrm. Inst.
A gas element

Density

SF criterion

Polytrope ISM model
For dense gas that can form stars (𝜌𝑔𝑎𝑠 > 𝜌𝑆𝐹 )Schaye & Dalla Vecchia (2008)

TemperatureIn order to suppress fragmentation within aJeans unstable cloudPressure

OWLS / EAGLE simulations
APOSTLE simulations
Horizon-AGN simulations

SF criterion

Density

Single-phase ISM modelThis model does not enforce EOS on gas
The most standard and common method

Gas can cool according to cooling functionBut, a pressure floor is applied to prevent
artificial fragmentation.

Temperature

FIRE simulations
NIHAO simulations
VELA simulations
Etc…

SF criterion

Pressure

Truelove-Ceverino condition (Ceverino et al. 2010)• 𝑁𝑐 = 7• Δ : spatial resolution5• 𝛾=3Density

ISM models
Polytrope modelDensityDensity

Single-phase model

Pressure

Temperature

Two-phase model

Cosmological zoom-in simulations• Arepo (moving-mesh code, Springel 2010)• Resolution: ~105 M for baryon• Target: log 𝑀𝑠𝑡𝑎𝑟 Τ𝑀 = 10.5 − 11.5 at z=1

Density

Clumpiness of galaxies
Polytrope model

Single-phase modelNclump= 8

Nclump= 49

Young stars(<100Myr)

Stars

Gas

Two-phase model# of clumps(>108 M)Nclump= 0

z=1.51log 𝑀𝑠𝑡𝑎𝑟 Τ𝑀 ≅ 11.3

Clumpiness of galaxies
Polytrope model

Single-phase modelNclump= 6

Nclump= 35

Young stars(<100Myr)

Stars

GasTwo-phase model# of clumps(>108 M)Nclump= 0

z=1.04log 𝑀𝑠𝑡𝑎𝑟 Τ𝑀 ≅ 11.5

Global properties (stellar mass)
Two-phase Polytrope Single-phaseGlobal properties (gas fraction)
Two-phase Polytrope Single-phaseGlobal properties (SFR)
Two-phase Polytrope Single-phase

Stellar angular momentum
at z=1the Kennicutt-Schmidt law
In simulations

EOS?

Toomre Q

Two-phase

Polytrope

Single-phaseTwo-phase

Polytrope

Single-phase

Conclusions ①
ISM models can dramatically change “clumpiness” of galaxies.
All of the models are currently used in recent cosmological simulations.
Two-phase ISM model, e.g. Illustris-TNG simulations
Polytrope ISM model, e.g. EAGLE simulations
Single-phase ISM model, e.g. FIRE simulations

Not only feedback but ISM model is also important!

Two-phase model

Polytrope model

Single-phase model

Conclusions ①
ISM models can dramatically change “clumpiness” of galaxies.
All of the models are currently used in recent cosmological simulations.
Two-phase ISM model, e.g. Illustris-TNG simulations
Polytrope ISM model, e.g. EAGLE simulations
Single-phase ISM model, e.g. FIRE simulations

Not only feedback but ISM model is also important!

However, the ISM models hardly change global properties.
E.g. Stellar mass, gas fraction, SFR, angular momentum, K-S law
Simulations are usually calibrated to match these properties with observations.

Problem: Which ISM model should we accept / reject?
ISM model (EOS of ISM) should be calibrated with observations.

Conclusions ②
An ideal test to investigate influence of clump formation.
Changing ISM models can control clump formation
With the same initial conditions and feedback

Clump formation little affects galaxies.
e.g. SFRs are enhanced only by a factor ~2-3.
High cosmic SFR density at z~2 is NOT due to clump formation

Clump formation would not cause starburst or quenching.