Two Models of Galaxy Formation and EvolutionThe Classical or Monolithic Model •proposes that galaxies form and evolve as relatively isolated bodies. •In this scenario galaxies evolve in a pre-determined fashion dependant on the initial conditions and with relatively little impact from the surrounding environment
The Hybrid Hierarchical Modelproposes that galaxies form and evolve through successive mergers of smaller bodies and their fate is more dependent on the environment which they inhabit.
Figure 1: The two competing models of galaxy formation and evolution that could produce the galaxies we observe today. The classical ‘top down’ or monolithic model is shown on the left. This involves the collapse of a largecloud over time. The hierarchical or ‘bottom up’ model is shown on the right and involves successive mergers of small bodies.
Figure 2: A simplified schematic of the hybridhierarchical model of galaxy formation and evolution .
The Evidences1.Cold Dark Matter (CDM)It is created in the early stages of the Big Bang and survives to the present time in sufficient numbers to contribute significantly to the present density of the Universe. The term ‘cold’ signifies that these particles move at speeds much less than that of light, usually because they are heavy.
2. Blue GalaxyFigure 5: These 18 small blue objects imaged by the HST could be the precursors to galaxies we see in the universe today.
3. Star Formation Rate (SFR)The history of recent star formation from the recent compilation of Blain (2000). Datapoints are taken from a variety of sources referenced in that article. Thick solid anddashed lines represent trends expected from simple luminosity evolution and hierarchicalmodels, respectively. It is clear there is considerable observational scatter at allredshifts, not just beyond z 1 as often assumed.
Cluster of Galaxiesare the largest known gravitationally bound objects to have arisen thus far in the process of cosmic structure formation. They form the densest part of the large scale structure of the universe. In models for the gravitational formation of structure with cold dark matter, the smallest structures collapse first and eventually build the largest structures, clusters of galaxies. Clusters are then formed relatively recently between 10 billion years ago and now. Groups and clusters may contain from ten to thousands of galaxies. .
Two classes of Clusters in terms of shape 1. Regular Clusters are relatively compact, with highest density near the center. Members are mostly elliptical galaxies. 2. Irregular Clusters including our Local Group, have a looser structure with little central concentration and less very hot gas.
Superclusterslarge groups of smaller galaxy groups and clusters and are among the largest structures of the cosmos. They are so large that they are not gravitationally bound and, consequently, partake in the Hubble expansion
How Did Large-Scale StructureEvolve In The Universe?The Hubble Deep Field will be used to perform a statistical study of the distribution of galaxies on the sky. This is an essential test of models for the structure of the universe and galaxy formation theories. Predicting how clustering should vary with brightness (or other galaxy properties) is a key challenge to models of structure formation. Current observations show that galaxies tend to cluster around other galaxies. However, the faintest galaxies are almost randomly distributed on the sky. The Hubble Deep Field will push such studies to fainter limits.
Active GalaxiesA galaxy emitting unusually high quantities of radiation from an active galactic nucleus at its center. Some active galaxies emit more energy in radio wavelengths than they do visible lightProperties:(1) High Luminosity,(2) Nonthermal Spectra that do not look like the sum of many stellar spectra,(3) Most of the luminosity is in a region of the spectrum other than optical (e.g., radio, UV, Infrared),(4) bright, star-like nucleus,(5) strong emission lines (most),(6) rapid variability, and sometimes(7) radio jets.
Normal galaxyone that does not have material for the supermassive black hole to "feed" on. But even if these normal galaxies are quite in regards to core activity, they still emit in other wavelengths.
Differences betweenNormal Galaxy and Active Galaxy
Quasarsa very energetic and distant active galactic nucleus. Quasars are the most luminous objects in the universe.Quasars were first identified as being high redshift sources of electromagnetic energy, including radio waves and visible light, that were point-like, similar to stars, rather than extended sources similar to galaxies.
The region of the sky containing one of the high-energy quasars, PKS 0528+134, is shownat two different times using the EGRET instrument on the Compton Gamma-RayObservatory These active galaxies are highly variable, strongly emitting gamma-rayssometimes, disappearing at other times.
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