GALICS: CAPTURING THE PANCHROMATICITY OF GALAXIES

JULIEN DEVRIENDTUniversity of Oxford, NAPL, Keble Road, Oxford OX13RH, UK

Abstract. This contribution describes results obtained with the GALICS model (for Galaxies InCosmological Simulations), which is a hybrid model for hierarchical galaxy formation studies, com-bining the outputs of large cosmological N-body simulations with simple, semi-analytic recipes todescribe the fate of the baryons within dark matter halos. Designed to predict the overall statisticalproperties of galaxies, with special emphasis on the panchromatic spectral energy distribution emittedby galaxies in the UV/optical and IR/submm wavelength ranges, such an approach can be usedto predict the galaxy luminosity function evolution from the ultraviolet to far infrared, along withindividual galaxies star formation histories.

Keywords: cosmology, galaxies, multi-wavelength

1. The GALICS Model

GALICS combines high resolution N-body simulations� for the dark matter withsemi-analytic recipes that describe the behaviour of the baryons. A complete de-scription of this hybrid model is given in Hatton et al. (2002). Figure 1 illustratesthe principle of the method by presenting a slice through the dark matter simulationat its final output time, with bright galaxies overplotted. One can clearly see fromthis figure that galaxies trace the cosmic web in the simulation, as most of thelight is concentrated in filaments and clusters. The power of the hybrid approachis immediately apparent when one considers that state-of-the-art attempts to modelthe sub-grid physics with smoothed particle hydrodynamics produce around 2000galaxies in a cube with one-third the volume of ours (Pearce et al., 2001), while wehave about 25 000 galaxies in total in our final timestep, thus giving us access to amuch broader range of galaxy mass and merging history.

2. Evolution of Galaxy Luminosity Functions

We then proceed to compute galaxy luminosity functions at various redshifts fromthe model. In Figure 2 we show them at redshifts 0, 1, and 3. The most strikingfeature is the apparent lack of evolution in the optical and near-IR bands, at leastup to z = 1. However, this should not be interpreted as no evolution at all but rather

� Here �� = 0.67, �0 = 0.33, H0 = 67kms−1Mpc−1 and σ8h−1 = 0.88

Astrophysics and Space Science 281: 505–508, 2002.© 2002 Kluwer Academic Publishers. Printed in the Netherlands.

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Figure 1. A slice through our 2563 particle �CDM simulation at redshift 0. Dark matter density isrepresented by a grey-scale, whilst white circles mark galaxies, with their sizes proportional to theB-band luminosity. Only bright galaxies (MB < −19.5) are shown. The slice is 100h−1 Mpc on aside (the entire simulation cube) and 10h−1 Mpc in thickness.

Figure 2. Evolution of the galaxy luminosity function in different wavebands. Open squares witherror bars are local universe data taken from the 2dF (B and K bands; Cross et al. 2001 and Cole etal., 2001, respectively) and IRAS (60 microns; Soifer and naugebauer, 1991) surveys. Solid curves ineach panel represent redshift 0 results whilst dotted and dashed lines are for redshift 1 and 3 respect-ively. Vertical dot-dashed lines mark the formal resolution limit, i.e. the resolution corresponding tothe minimum halo mass in the simulation.

GALICS: CAPTURING THE PANCHROMATICITY OF GALAXIES 507

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Figure 3. Individual star formation and baryonic mass histories for a sample of GALICS galaxies. Thetwo upper and bottom rows of panels are MW-like spiral and early-type galaxies respectively. Solidcurves indicate the total amount of baryonic mass for each galaxy whilst dashed lines are their stellarmass. Dotted curves give the star formation rate.

as an evolution in galaxy luminosity and number which compensate each other.Indeed, in CDM hierarchical scenarios some galaxies get bigger and brighter asthey merge but there also exist a fraction of more isolated galaxies which evolvequasi passively. On the other hand, one can notice a strong evolution in the far-IRgalaxy luminosity function from z = 0 to at least z = 3. This behaviour finds its

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roots in the greater sensitivity of dust emission to the instantaneous star formationrate of galaxies.

3. Individual Star Formation Histories

To better illustrate the previous remarks, i.e. that star formation rate (and thereforeluminosity evolution) depends on environment we show in Figure 3 individualstar formation histories for a sample of galaxies of various morphological types,together with their individual baryonic mass assembly histories. One can see fromthis figure that discontinuities in the baryonic mass assembly histories (solid lines),which are signatures of merger events between galaxies of comparable mass, areaccompanied by massive bursts of star formation (peaks in the dotted curves) andconsequently by equivalent discontinuities in the stellar mass assembly histories(dashed line). It is also clear from Figure 3 that such bursts do not happen with thesame frequency for all galaxies and that there exist extended periods of quiescentstar formation between mergers. Note that present day stellar and baryonic massesare already in place at high redshift (z ≈ 3) for most of the early-type galaxiesplotted in Figure 3. Finally, we point out that summing over all individual starformation histories available for GALICS galaxies one can easily build a Madaudiagram and compare the predicted cosmic star formation history to observations.But this is another story, which will explore in detail in Devriendt et al. (2002).

Acknowledgements

I want to thank all my collaborators without whose help I could never have presen-ted these results.

References

Cole, S. et al.: 2001, The 2dF galaxy redshift survey: near-infrared galaxy luminosity functions,MNRAS 326, 255–273.

Cross, N. et al.: 2001, The 2dF Galaxy Redshift Survey: The number and luminosity density ofgalaxies, MNRAS 324, 825–841.

Devriendt, J.E.G. et al.: 2002, GalICS II: Galaxy evolution in a hierarchical scenario, in preparation.Hatton, S. et al.: 2002, GalICS I: A hybrid N-body/semi-analytic model of hierarchical galaxy

formation, MNRAS, submitted.Pearce, F. et al.: 2001, Simulations of galaxy formation in a cosmological volume, MNRAS 326,

649–666.Soifer, B.T. and Naugebauer, G.: 1991, The properties of infrared galaxies in the local universe, AJ

101, 354–361.