Reproducing the Observed Universe with Simulations

Qi Guo

Max Planck Institute for Astrophysics

MPE April 8th, 2008

Science problem:

how well does the Millennium Run simulations + L-Galaxies semi-analytic model reproduce the properties of observed galaxies?

• An example: study of high redshift star forming galaxies

• General requirements to VO

Observations

• Keck survey in the filters UnGR, magnitude limit of R<25.5 with a field of view of a few hundred arcmin2

colour-colour selection:

• LBG samples (star forming galaxies at z~3): (Un- G) > (G-R)+1.0 (G-R) < 1.2

• BX systems (star forming galaxies at z~2) :(G-R) > -0.2(Un- G) > (G-R)+0.2(G-R) < 0.2(Un- G)+0.4(Un- G) < (G-R)+1.0

Light Cone Survey to Mimic Observations

N-body simulation (MR)

Galaxy catalogues

Mock catalogueSAM

Lightcone code

• precalculated lightcones in the desired filters? not available

• precalculated SAM galaxy catalogs with desired filters or spectra ? not available

• needed to rerun SAM to generate magnitudes in desired filters

• needed to recalculate light cones in the new filter systems

Producing the right mock catalogs from the MR

How well can we reproduce the high redshift star forming galaxies?

Compare: • color-color diagram

• redshift distribution

• correlation function

• number density

Science analysis

• MR LBG sample selection identical to observed LBG samples

Steidel et al. 2004 Mock catalogue

Science analysis

Basic analysis: redshift distribution

Solid histograms:

from mock catalogue

(red: z~3 black: z~2)

Dashed histograms:

Steidel et al. 2004

Science analysis

Basic analysis: correlation function iterations:

Solid curves:

from mock catalogue

(red: z~3 black: z~2)

Region within dashed curves: correlation function within 1 sigma deviation (Adelberger et al. 2005)

Science analysis

Advanced analysis: Evolution (color-mass)

Science analysis

Iterations:

- adjust dust model, make lightcone, select new LBG sample

4.34 2.39Mock 2

5 2.78Mock 1

5.2 1.8literature

BXs

(n/arcmin^2)

LBGs

(n/arcmin^2)

Data and codes are available for local users:

Through a combination of catalogs stored at MPA, running SAM code in C, C++ programs to generate light cones, IDL scripts for sample selection, plotting, analysis

What would be required for an outside user to perform the similar analysis using VO-like services?

Data and codes are available for local users:

Through a combination of catalogs stored at MPA, running SAM code in C, C++ programs to generate light cones, IDL scripts for sample selection, plotting, analysis

1 - Basic requirements

• main physical parameters in halo and galaxy catalogues from simulations• main observables for a detailed comparison with observed samples of galaxies (e.g. magnitudes)• merger trees both for halo and galaxies• link between galaxies and their dark matter halo

step-by-step overview of VO-requirements 1

For high-level comparison with observations, it is essential to transform from a simulation consisting of discrete, fixed-epoch “snapshots” to a simulation in which both the physical as well as the apparent properties of galaxies evolve

along the “observed” lightcone.

2 - Making Realistic Lightcones

• geometry of light cone• conversion of snapshot magnitudes to observed (apparent) magnitudes, taking into account the proper distance modulus, K-correction, peculiar motion and IGM attenuation towards each object along the light cone• high flexibility of generating multiple lightcones to beat down cosmic variance

(e.g. Blaizot et al. 2005, Kitzbichler & White 2007)

step-by-step overview of VO-requirements 2

step-by-step overview of VO-requirements 3

3 - The simulated data must be available in the same filter system as the observed sample used for comparison

freedom of choice of the output magnitudes of the SAM catalogs for any combination of telescope+instrument+filter. This can be achieved by

• making most common filters available in the VO, OR

• making spectral energy distributions available in the VO, OR

• allowing ``on-the-fly’’ creation of custom filtersets from the SAM

step-by-step overview of VO-requirements 4

4 - Data creation/transfer/storage

• all procedures performed through remote usage of VO

• The generated data need to be either stored and accessible (sql) on the VO site, or easily retrievable without running into server time-outs (within limits)

step-by-step overview of VO-requirements 5

5 - Sample Selection and Basic Analysis

• allowing remote filtering in order to only download data of interest (e.g. by applying magnitude and colour selection criteria to the generated mock light cone observations)

and possibly,• access to online data sets generated by large galaxy surveys (e.g. SDSS, UKIDS, PAN-STARRS, HST legacy archive) with the same filtering as applied to the simulated data

• ``on-the-fly’’ calculation of the main sample diagnostics (e.g. number counts, luminosity functions, redshift distributions, correlation functions) to assess basic quality of the SAM

step-by-step overview of VO-requirements 6

6 - Results and feedback into SAM

And, Start Over !

• report the agreement/disagreement between model galaxies and observed samples

• allowing ``on-the-fly’’ rerun the SAM ( e.g. parameters, models…)