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Hydro simulations and SAMs: two sides of the same coin

arXiv:1109.4635

Eyal NeisteinTMoX group, MPE Garching

Collaborators: Sadegh Khochfar, Claudio Dalla-Vecchia, Joop Schaye

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Hydrodynamic simulations (HYDs)

• How each process affects the galaxy mass? (feedback, SF, Cooling)

• Why changing the parameters of a given process (e.g. SF) do not affect the galaxies? What is `self-regulation’?

• What is the physics missing in order to reproduce the observational results (e.g. the LF)?

• Can we extrapolate the results of one zoom simulation into a large volume easily?

• Can we easily quantify the differences between different codes/parameters?

We need a tool to post-process a HYD !

Typical questions:

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Semi-analytic models (SAMs)

Is this description accurate? Unique?Do we need only hot & cold gas?

Is it the right tool ??

What is a SAM?• Each galaxy is described by only 3 numbers• Use the same underlying DM merger trees as in the HYD

fs – star formation fc – cooling; fd – feedbackfa - accretion

Can we use a SAM in order to interpret HYDs?

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Project outline

1. We take a HYD that was run over a cosmological box (OWLS, Schaye+10)

2. We extract the rates of: accretion, cooling, feedback, and SF directly from the HYD

3. We save these rates as function of halo mass and redshift only

4. We run our SAM using the above recipes over the merger trees that are extracted from the HYD

HYD

Recipes:fs, fc, fd, fa

Dark matter merger trees

SAM

),( zMff

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The history of one galaxy

fs – star formation fc – cooling; fd – feedbackfa - accretion

We follow all the particles, and checkwhich got heated/cooled/SF/accreted

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How similar are the galaxies?

Galaxies are compared on an object-by-object basis:

The HYD & SAM are similar at a level of ~0.1 dex

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Star-formation rates

The SFR includes more scatter than integrated properties (like stellar mass)

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Efficiency values

The HYD physics is different from typical SAMs

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Works well!!Bouche +(2010), Krumholz & Dekel (2011); Khochfar & Silk (2011); Dave+ (2011)

One-phase modelAssume that each galaxy is composedfrom ‘gas’ and ‘stars’ only:

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SAM based on only one galaxy?We chose 3 random galaxies from the box ( )

The SAM recipes are extracted from one galaxy only

The SAM runs over the entire volume

Works well!!

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Happy birthday Avishai !

Summary

•HYD and SAM agree quite well, once efficiencies are matched on average

•The SAM can be very simple, even one phase model is enough

•All the recipes are summarized as a function of the host halo mass and redshift

•The evolution of one galaxy includes enough information to extract the baryonic behavior

•We have a common language for both SAMs & HYDs