Numerical Simulations of Galaxy Formation in a LCDM Universe

Mario G. AbadiObservatorio Astronómico De La Universidad Nacional De Córdoba

CONICET, Argentina

Collaborators:

Julio Navarro: University of Victoria, Canada

Matthias Steinmetz: Astrophysikalisches Institute, Postdam, Germany

Vincent Eke: University of Durham, United Kingdom

Andres Meza: Universidad de Chile, Santiago, Chile

Amina Helmi: Kapteyn Astronomical Institute, Groningen, Netherlands

●WMAP and LSS results have established LCDM model as the new paradigm of hierarchical structure formation●Low mass density but flat scenario●Fully specified by the following cosmological parameters:● Constituents 70% dark energy, 26% dark matter and 4% baryons●Amplitude of mass fluctuation in spheres of 8 Mpc/h is given by RMS=0.9 and a Hubble´s constant h=0.7with no tilt in the initial power spectrum ●Success in LS (>1mpc) closer to linear regime

LCDM Universe

Galaxy Formation

● Observed disk at odds with ”natural” trends of hierarchical models

● Difficult to reconcile the early collapse and eventful merging history with dynamical clues

which point to a smooth assembly of disks

● Fragility of disks to rapid fluctuations of the gravitational potential such as those stirred by

mergers or satellite accretion events

● Dominant, cold, thin, stellar disks points to a histoty of mass accretion where major mergers

have player a minor role

● Age of oldest disk stars used to estimate the epoch of the last major merger (14 gyrs in the

solar neighborhood)

● Milky Way’s thick disk has its origin in an early thin disk of velocity and size comparable to

today’s but “thickened” by the accretion of a satellite

Numerical Simulations

• Initial conditions given by the lambda CDM model

• Astrophysics: gravitation, hydrodynamics, radiative cooling, star

formation, feedback and metals

• Initially only dark matter and gas particles

• Gas particles transformed in star particles

• 8 simulations finished with M~ 1-2x10^11 solar masses and N~0.6-

1.8x10^5 star particles inside 20 kpc @ z=0

The Formation and Evolution of a Disk Galaxy

Luminous GalaxyRadius ~ 20 kpc

Dark Matter HaloVirial Radius ~ 300 kpc

Luminous Stellar HaloVirial Radius ~ 300 kpc

Observational and Theoretical Approach

● Observational

● Photometric (luminosity, isophotes, surface brightness profile

decomposition, bulge to disk ratio, fundamental plane, colors, star

formation)

● Kinematics (gas and stars rotation curves, velocity maps, Tully fisher and

Faber Jackson relation)

● Theoretical

● Dynamics (dark matter, gas and stars properties, evolution, mass

distribution, velocity support, dynamical decomposition, in-situ vs

accretion, origin of different components)

All Stars

Spheroid

Thick Disk

Thin Disk

Dynamical Decomposition

The Formation and Evolution of a Disk Galaxy

Disk vs Halo Formation

• Disk• Young (90%) + old (10%)• Rotational velocity supported• Outcome of a smooth

dissipative deposition (and transformation into stars) of gas cooling more or less continuously off the intergalactic medium

• Eggen Lynden-bell & Sandage (1962)

• Correlations between metallicity and kinematics of 221 stars in the solar neighborhood

• Halo• Old (predate the last major

merger)• Velocity dispersion supported• Build up over an extended

period of time through a number of early mergers

• Searle & Zinn (1978)• Wide range of metal

abundances independent of radius for 177 red gigants in 19 globular clusters

Halo Evidence of Accretion Events

● Tidal streams of the Sagittarius dSph galaxy (Ibata el al. 1994)

● Substructure in the galactic halo (Helmi et al. 1999)

● Giant stream of metal-rich giants around Andromeda (Ibata el al. 2001)

Disk Evidence of Accretion Events

• Monoceros ring in the outer Galaxy (Yanny et al. 2003)

• Canis Major dwarf (Martin et al. 2004)

• Arcturus stream (Navarro et al. 2004)

• Debris from omega Cen parent galaxy in the solar neighborhood (Meza et al. 2005)

• Substructure in the Galactic disk (Helmi et al. 2005)

Conclusions

● Galaxy componets in cosmological context: spheroid, thin disk, thick disk, but also

stellar halo, satellites and dark matter halo

● Simulated galaxies resemble observed galaxies, surface brightness, colors, etc

● Different implementation of astrophysical effects in order to avoid efficient star

formation at early times and massive spheroid and stellar halos

● Stellar halos form from mergers

● Disk form from dissipative collapse

● There is growing evidence that the hierarchical models are correct

The Inner Milky Way

Inner bright components: spheroid (or bulge), thin disk and thick disk

• Infrared Milky Way image (DRIBE COBE NASA)

The Outer Milky Way

Inner bright components: spheroid (or bulge), thin disk and thick disk

Outer faint components: satellites and stellar halo both difficult to detect in other galaxies

Outer dark components: dark matter halo and substructure

• Infrared Milky Way image (DRIBE COBE NASA)