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Formation and Evolution of Galaxy

Galaxy Formation Supermassive Black Hole The most accepted view on the formation and evolution of large scale structure is that it was formed as a consequence of the growth of primordial fluctuations by gravitational instability. Galaxies can form in a "bottom up" process in which smaller units merge and form larger units. It is referred to as the "Inside-out Theory" or "Merger" as shown in the upper half of Figure 05-08a. In the present epoch, large concentrations of galaxies (clusters of galaxies) are still in the process of assembling. The opposing view is the "top down" process in which large clump breaks up into smaller units. It is referred to as the "Outside-in Theory" as shown in the lower half of Figure 05-08a. The figure also shows the kind of objects the NGST (Next Generation Space Telescope) will detect according to the two opposing theories.

Figure 05-08a BH, Initial Formation [view large image]

Figure 05-08b BH, Supermassive

Protogalaxy Evolution The "bottom up" theory have been given a boost in 2008 by the first ever detection of the infant protogalaxies with an unprecedented 92-hour session on the European Southern Observatory's Very Large Telescope. These protogalaxies were irregularly shaped and with low star-formtion rates, but the stars that did form were massive and consequently exploded as supernovae. The image on the left shows a group of protogalaxies in the process of merging.
(See a movie of "Galaxy Formation")

By 2013 the bottom up theory becomes less sustainable with the discovery that supermassive black hole with mass over billion Msun is common at an age about 750 - 900 million years after the Big Bang. It just did not have enough time to amass for such behemoth. Various schemes have been proposed to resolve the discrepancy, but each one has its own problem (see Figure 05-08b).

Density Fluctuation The difference between the "bottom up" (inside-out) and "top down" (outside-in) point of view is related to whether the universe is composed with cold dark matter (CDM, slow moving) or hot dark matter (HDM, fast moving). In the former scenario there is fluctuation in the power spectrum over a wide range of physical scales as shown in Figure 05-08c. Smaller size have larger fluctuation, therefore structure formed first with small objects, which then merge to form ever larger structures. This is called ``bottom up'' structure formation. The observations strongly favour this scenario over its competitor: "top down" structure formation. The proto-typical ``top down'' scenario is structure formation in a universe dominated by hot dark matter. Hot dark matter cannot support fluctuations on small length scales - they are washed out with the rapid motion of the particles. Thus only large scale fluctuations survive to the present epoch. Structure forms first large scale objects which fragment into smaller objects.

Figure 05-08c Density Fluctuations in Three Models

The early universe was a barren wasteland of hydrogen, helium, and a touch of lithium, containing none of the elements necessary for life as we know it. From those primordial gases were born giant stars a few hundred times as massive as the Sun, burning their fuel at such a
Protogalaxy Evolution prodigious rate that they lived for only about 3 million years before exploding. Those explosions spewed elements like carbon, oxygen and iron into the void at tremendous speeds. By the remarkably young age of 275 million years, the universe was substantially seeded with metals thrown off by exploding stars. That seeding process was aided by the structure of the infant universe, where small protogalaxies less than one-millionth the mass of the Milky Way clustered together into vast filamentary structures. Giant stars form at the intersections of these great filaments of primordial hydrogen, forming the nuclei of the first galaxies - the protogalaxies (Figure 05-08d). The small sizes and distances between those protogalaxies allowed an individual supernova to rapidly seed a significant volume of star forming space. New simulations show that the first, "greatest generation" of stars spread incredible amounts of such heavy elements like carbon, oxygen and iron across thousands of light-years of space, thereby seeding the cosmos with the stuff of life.

Figure 05-08d Protogalaxy Evolution
[view large image]

Mass Assembly Galaxies used to be considered as "island universes" that formed in the distant past and have since evolved in isolation from their surroundings. Such concept was replaced by the hierarchical growth scenario, which enrols galaxy interactions, collisions and mergers to shape the mature galaxies in the current epoch. Lately in the 2010s, observations reveal that the decline in galaxy formation is not matched by the decrease of atomic hydrogen - only half of the latter has been consumed by the galaxies. The latest explanation proposes that the hydrogen fuel is supplied by ionized hydrogen in the intergalactic environment. Such novel idea is supported by the recent (2013) detection of neutral hydrogen in space between the Andromeda and Triangulum galaxies. The detection of 21 cm radio emission from neutral hydrogen is much easier although the neutral variety is only about 1% of the ionized one. The new discovery adds one more element into the process of galaxy assembly.

Figure 05-08e Galactic Mass Assembly [view large image]

Figure 05-08e is the numerical simulation of galaxy formation and evolution, in which green denotes higher ionized hydrogen density and black is lower (a-c are zoom in views).

Merger Simulation After the initial phase of galaxy formation, there was an era of cosmic fireworks: galaxies collided and merged (see Figure 05-08f), powerful black holes in quasars sucked in huge whirlpools of gas, and stars were born in unrivaled profusion. The activity of star formation peaked about four to six billion years. Since then galactic mergers became much less common, the gargantuan black holes were replaced by numerous moderate ones, star formation continued but mostly in the low mass variety. In other words, the contents of the universe have transitted from a small number of bright objects to a large number of dimmer ones. Computer simulations suggest that such shift may be a direct consequence of cosmic expansion.

Figure 05-08f Simulation of Galactic Merger [view large image]

As the universe expands, galaxies become more separated and merger become rarer. Furthermore, as the gas surrounding galaxies grows hotter and more diffuse, it does not gravitationally collapse as readily into the galaxy's potential well. A few billion years from now, the
Galaxy Evolution Simulation smaller galaxies that are active today will have consumed much of their fuel, and the total cosmic output of radiation will decline drametically. As the cosmic expansion continues, the dwarf galaxies - which hold only a few million stars each but are the most numerous type of galaxy in the universe - will become the primary hot spots of star formation. Inevitably, though, the universe will darken, and its only contents will be the fossils of galaxies from its past. Figure 05-08g shows the evolution subsequent to the initial phase. Figure 05-08h shows another evolutuon simulation with cold dark matter.

Figure 05-08g Evolution History [view large image]

Figure 05-08h Evolution Simulation

It seems that galaxy formation is a very complicated process involving star-forming history, merger history, mass, size, angular momentum, and external environment. But research in 2008 indicates that the mass is the only dominant factor in determining the properties of individual galaxies. A sample of roughly 200 galaxies has been selected from a large, blind sky survey for neutral hydrogen (HI) emission using the hydrogen spectral line at a wavelength of 21 cm. For each galaxy in the catalogue, they measured a number of quantities: 1. the total hydrogen mass; 2. the width of the hydrogen spectral line; 3. the redshift; 4. the inclination with respect to the line of sight; 5. two radii, containing 50% and 90% of the light, respectively; and 6. the optical luminosity in four different colour bands. It is found that the six independent components that they use to describe the galaxies in their sample are all correlated with each other and with a single principal component - the galaxy's mass. This finding is consistent with the progression of mass among the types of
Mass Range of Galaxies galaxy as shown in Figure 05-08i, where the dwarf galaxies are in the lowest mass range of 107 - 108 Msun, the mass of irregular galaxies is in between 108 - 1010 Msun, the range for spiral galaxies is 1010 - 1012 Msun, while the giant elliptical galaxies is in the range 1012 - 1013 Msun. The mass ranges form a

Figure 05-08i Mass Range of Galaxies

continuous sequence without overlapping. Thus, if the mass of the galaxy is known, the type would follow according to Figure 05-08i. It is not that straight forward conversely, for if we know the type of the galaxy there is a range of mass within that type.

A single observation in 2009 discovered that elliptical galaxies seem to expand in size from epoch as early as 3 billion years after the Big Bang to present day. However, the mass of such elliptical galaxies remains constant in contradiction to the scaling relationship between
Elliptical Galaxy mass and effective radius re for nearby galaxies (Figure 05-08j). The picture on the left of the Figure shows very compact galaxies evolved to bigger, more diffuse objects and became more abundant. Diagram on the right depicts the scaling relationship for nearby galaxies (the black dots) and the high redshift galaxy 1255-0 (at z = 2.186 in red symbol, dynamical mass refers to mass obtained from velocity dispersion of the stars). Current theory predicts that galaxies evolve according to the scaling law perhaps by merger (the blue arrow). The new observation implies another path as shown by the red arrow.

Figure 05-08j Evolution of Elliptical Galaxy

Further research is required to confirm the atypical observation. It remains to be seen whether we need conventional or novel explanations for the evolution of elliptical galaxies.

Table 05-01 summarizes the evolutionary sequence. The time epoch t is computed from a computer program on cosmological evolution with m = 0.26 and = 0.74.

Epoch (109 years) Red- shift Astronomical Objects Activities
~ 0.38
x 10-3
~ 1100 Cosmic Microwave Background Radiation Transparent to light.
> 0.38
x 10-3
< 1100 None. Dark age.
< 0.2 > 20 First stars, supernovea. Formation of black holes, production of heavy elements.
< 0.5 > 10.0 Protogalaxies. Protogalaxies drew in matter.
< 0.8 > 7 Quasar, supermassive black holes. Galaxies collided and merged, bursts of star formation.
< 1.3 > 4.70 Baby galaxies. Galaxies took shape.
< 7.2 > 0.73 ERO (extremely luminous galaxies). Rate of star formation peaked at ~ 5 x 109 year.
< 13.7 > 0 AGN; elliptical, spiral, & irregular galaxies. Small # of bright objects replaced by large # of dimmer ones.
>>13.7 Dwarf galaxies... ... galaxies will disappear with the evaporation of matter.

Table 05-01 Evolution of Galaxies