Galaxy Formation: Overview

Transcription

1 Galaxy Formation: Overview Houjun Mo March 30, 2004

2 The basic picture Formation of dark matter halos. Gas cooling in dark matter halos Star formation in cold gas Evolution of the stellar populaion Metal and energy feedback from stars Merger of dark halos Merger of galaxies

3 The basic reason for the two basic morphological types Competition between energy dissipation and star formation If cooling is effective and gas does not possess angular momentum, the result of the collapse is dense point If cooling is effective but gas has angular momentum, the result of the collapse is a thin rotating disk If the rate for the gas to be converted stars is longer than the collapse time scale, then the collapse of the gas (with angular momentum) will first form a gaseous disk. Stars will form on the disk If the rate for the gas to form stars is shorter than the collapse time scale, then most gas can be converted into stars before the collapse; Since the kinetic energy of stars cannot be dissipated, the collapse will for a system supported by the random motions of stars, like elliptical galaxies

4 Another important process: galaxy interaction, important in transforming galaxies from one type to another

5 Physical Processes and Time Scales Dynamical time scale: For a unifom gas cloud with density ρ, this time scale is t dyn = 1/ Gρ, which represents the fastest time scale the cloud can collapse. Dissipation time scale: This is the time scale on which gas dissipates its thermal or turbulent energy. In the case where the dissipatiion is due to radiative cooling, the cooling time scales can be obtained from cooling function. If the cooling time is much longer than the dynamical time, the gas can reach hydrostatic equilibrium, otherwise, it will collapse in a dynamical time. Star-formation time scale: This is the the time scale at which the gas in a protogalactic cloud turns into stars, and hence it depends on the star formation rate in the cloud. If this time scale is much shorter than the dissipation time scale, the gas cannot dissipate much of its energy before

6 turning into stars, a situation which may lead to the formation of hot stellar systems, such as ellipticals and spiral bulges. The opposite situation, where the star formation time scale is much longer than the time scale for gas dissipation, may lead to the formation of cold gaseous disks. Chemical time scale: This is the time scale at which protogalactic gas is enriched in heavy elements by supernova explosions and stellar winds, and so depends on the lifetimes of massive stars. If this time scale is longer than the star formation time scale, the change of metallicity in the star forming gas is not important and all stars will have the initial metallicity. On the other hand, if this time scale is shorter than the star formation time scale, star-forming gas is enriched continuously, and stars formed at different times may have different metallicities and different enrichment patterns. Merging time scale: This is the time scale at which an extended galactic halo merges with another one. When two haloes merge, the gas that has not turned into stars can be shock heated as the energy in the merger is

7 thermalized. In this case, the merging time scale regulates the amount of stars that can form in the progenitors. Dynamic-friction time scale: This is the time scale at which a galaxy moving through an extended halo loses its momentum, and hence it determines how fast two galaxies can merge after the merger of their haloes. As we have discussed, major mergers of galaxies are important in re-shaping galaxies and in triggering starbursts and AGNs. This time scale is therefore relevant to the transform of galaxy types and to the formation of starbursts and AGNs.

8 Galaxy Formation in Hierarchical Models Two aspects of the problem: (1) Formation of individual galaxies; (2) The formation of the galaxy population In CDM models: Formation of dark matter halos. Thus: to understand how galaxies form in individual dark halos, i.e to obtain P(G H ), the probability of forming a galaxy G in a halo H. to understand the properties of the dark halo population in a cosmological model, i.e. to obtain P(H ), the distribution of dark haloes with respect to their intrinsic properties. The properties of the galaxy population can then be obtained through P(G) = P(G H ) P(H )

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